Recent Developments in Condition Assessment, Repair Materials and Repair - Retrofitting Technique

Recent Developments in

Condition Assessment, Repair Materials

and Repair / Retrof itting Techniques for

Concrete Structures9-11 February, 2011

Editors

P. Srinivasan

Dr. J. K. Dattatreya

Dr. B. H. Bharatkumar

CBA Publishers

© CSIR-Structural Engineering Research centre February 2011

No part of the material, protected by this Copyright notice, may be reproduced or utilized in any form or by any means, electronic or mechanical including pho-tocopying, recording or by any information storage and retrivel system, without prior written permission from the Copyright owner.

ISBN: 978 93 80430 03 4

Printed by Betaprint, Chennai.

Published by CBA Publisher, Chennai.

Foreword

A sound and effective built environment is critical for socio-economic development and economic growth in the country. Expand-ing and improving infrastructure such as roads, rail networks, bridges,ports, airports, buildings and other facilities is a national priorityand must be achieved without forfeiting environmental sustainabil-ity. Concrete is widely used for the construction of structures such asbuildings, infrastructures such as bridges, dams, power plant struc-tures, harbour structures, etc., Defects such as cracks, honeycombsand voids are likely to be present in the hardened concrete due to con-struction deficiencies. The concrete also undergoes degradation due tounfavourable environment, ageing of materials, overloading etc., Theinfrastructure which are becoming older are to be strengthened orrepaired for extending its service life. There are approximately 125000bridges of Indian Railways. Of these, around 45% are more than 100years old. Infrastructure such as Power plants structures (Thermaland Nuclear), bridges, etc., which are more than 40 to 50 years oldare to be strengthened/ repaired for extending the service life. Even,one day of shutdown in a thermal/nuclear power stations will causea loss of power in the order of few crores of rupees. Non Destruc-tive Testing and Evaluation has become a regular feature in assessingnew concrete structures for their quality and structural integrity andalso the condition assessment of aging structures. The advancementin Nondestructive Testing and Evaluation (NDTE) for concrete struc-tures has led to methods such as Impact Echo, Ultrasonic Pulse Echoand Ground Penetrating Radar besides the commonly used reboundhammer and ultrasonic pulse velocity tests. With these techniques,critical features such as voids, cover thickness, delamination, locationof reinforcement and ducts, can be obtained, which enables betterassessment of structural integrity and more accurate identification ofdefects. In the recent past, fibre optic sensors have been used forhealth monitoring of concrete structures. The residual prestress inPSC members can be obtained by core drilling technique. In additionto the advancements in condition assessment techniques, consider-able progress has also been made in developing new repair materials,enhancing the performance of existing repair materials and repair tech-niques to produce durable and sustainable repair of existing reinforcedand prestressed concrete structures. Protecting the civil engineeringstructures is essential for a sustainable building that is likely to expe-rience high-consequence natural hazard over its lifetime. CSIR-SERC,Chennai has acquired considerable expertise in the latest NDTE tech-niques for condition assessment of reinforced and pre-stressed concrete

Recent Developments in Condition Assesment, Repair Materials and Repair...

structures, and performance evaluation of new/improved repair mate-rials and techniques. This course addresses recent developments andadvances on non destructive techniques and evaluation, repair materi-als and retrofitting techniques. My scientist colleagues at CSIR-SERC,who have first hand experience and expertise due to their involvementin various field problems, have documented the technical notes.

I congratulate the coordinators Shri. P. Srinivasan, Dr. B.H.Bharatkumar and Dr. J. K. Dattatreya for their excellent efforts inbringing out this course volume for the advanced course on “Recentdevelopments in condition assessment, repair materials and repair/ retrofitting techniques for concrete structures”. I also thank CBAPublisher, Chennai, for the excellent cooperation in bringing out thiscourse volume in time.

Dr. Nagesh R. IyerDirector,

CSIR-SERC, Chennai.February, 2011

iv

Contents

Foreword iii

1 Need for Non-Destructive Testing and Evaluation 1Nagesh R. Iyer

2 Radar and Ultrasonic Pulse Echo for NonDestructive Evaluation of Concrete Structures 9P. Srinivasan

3 Use of Impact Echo Method for Determination ofThickness and Defects in Concrete Elements 23S. Bhaskar

4 Advanced Cement Composites (ACCS)- Productionand Application to Repair 35J. K. Dattatreya

5 Polymer Concrete Composites for Repair andRehabilitation of Concrete 59Meyappan Neelamegam

6 Investigations on Geopolymer Concrete and itsApplication for Repair 79P. S. Ambily and J. K. Dattatreya

7 Advances in Fibre Reinforced Concrete and itsApplications 109T. S. Krishnamoorthy and S. Sundar Kumar

8 Fibre Reinforced Polymer (FRP) in CivilEngineering Applications 135B. H. Bharatkumar and G. Ramesh

9 Self-Compacting Concrete as a Repair Material 159J. Annie Peter

10 Mechanism of Corrosion and Repair of CorrosionDamaged Concrete Structures 177J. Prabakar

11 Repair and Retrofitting of RC Structures - CaseStudies 199K. Balasubramanian and V. Rajendran

Recent Developments in Condition Assesment, Repair Materials and Repair...

12 Fire-Affected Concrete Structures and itsRehabilitation 211P. Srinivasan

13 Condition Assessment of Concrete StructuresSubjected to Vibration 223K. Muthumani

14 Application of Fiber Optic Sensors for PerformanceAssessment of Concrete Structures 241K. Ravisankar

15 Evaluation of Residual Pre-stress in ConcreteStructures 259S. Parivallal and K. Kesavan

16 Risk Informed Inspection Planning for RCStructures 275K. Balaji Rao and M. B. Anoop

17 Distress in Prestressed Concrete Members and theirRehabilitation 293K. Ramanjaneyulu

vi

1 Need for Non-Destructive Testing and

Evaluation

Dr. Nagesh R IyerDirector,

CSIR-SERC, Campus, Taramani, Chennai-600 113, India.Email: [email protected]

1.1 INTRODUCTION

The vast majority of structures and buildings consist of concrete andmasonry structures. Construction activities account for a major com-ponent of the budget in our country. Cement Concrete is the mostextensively used material for construction of different types of struc-tures/components such as buildings, bridges, etc., A very large part ofthe infrastructure in most countries is made of concrete, providing thebasis of economical and social development. These are often affectedby damage due to ageing, environmental agents, overloading, vibra-tions and other causes. A great variety of damage situations can occur,as micro cracking and cracking due to material and structural dam-age, material discontinuity, and surface degradation. Maintenance andrepair of constructed facilities/infrastructures is presently the mostsignificant challenge facing the country. As part of the sustainabil-ity, it is necessary to extend the service life of these structures. Fordistressed concrete structures, it is necessary to evaluate its presentcondition so as to select proper choice of repair material and repairtechniques.

Non-destructive testing methods can play a supporting role in thedecision making process of the structure assessment. Not all defects ordeteriorations can be found by visual inspection. Some may only bevisible when it is already too late to avoid major repair. Based on reli-able quantitative measurements, the engineer can grade the structurewith more certainty. Especially in cases, where processes are hiddeneven to the experienced eye of an inspector, e.g., corrosion of strandsinside ducts, testing methods are very much needed. Non-destructive

Recent Developments in Condition Assessment, Repair Materials andRepair / Retrofitting Techniques for Concrete Structures

9-11, February 2011, CSIR-SERC, Chennai-113, India. pp 1–7

2 Recent Developments in Condition Assesment, Repair Materials and Repair...

methods are preferred because they will not alter the appearance orfunctionality of the structure. Durability of concrete structures is themain objective for the assessment of existing structures. Safety is anissue where a sudden collapse of a structure might occur. This kind ofaccidents fortunately is very unusual, structure do usually show visi-ble signs of distress before collapse. However, post tensioned concretestructures may collapse without warning and endanger lives.

Non-destructive testing can be applied to both old and new struc-tures. For new structures, the principal applications are likely to befor quality control or the resolution of doubts about the quality ofmaterials or construction. The testing of existing structures is usuallyrelated to an assessment of structural integrity or adequacy. In eithercase, if destructive testing alone is used, for instance, by removingcores for compression testing, the cost of coring and testing may onlyallow a relatively small number of tests to be carried out on a largestructure which may be misleading. Non-destructive testing can beused in those situations as a preliminary to subsequent coring.

Typical situations where non-destructive testing may be useful are,as follows:

• quality control of pre-cast units or construction in situ

• removing uncertainties about the acceptability of the materialsupplied owing to apparent non-compliance with specification

• confirming or negating doubt concerning the workmanshipinvolved in batching, mixing, placing, compacting or curing ofconcrete

• monitoring of strength development in relation to formworkremoval, cessation of curing, prestressing, load application orsimilar purpose

• location and determination of the extent of cracks, voids, honey-combing and similar defects within a concrete structure

• determining the concrete uniformity, possibly preliminary to corecutting, load testing or other more expensive or disruptive tests

• determining the position, quantity or condition of reinforcement

• increasing the confidence level of a smaller number of destructivetests

• determining the extent of concrete variability in order to help inthe selection of sample locations representative of the quality tobe assessed

Need for Non-Destructive Testing and Evaluation 3

• confirming or locating suspected deterioration of concrete result-ing from such factors as overloading, fatigue, external or internalchemical attack or change, fire, explosion, environmental effects

• assessing the potential durability of the concrete

• monitoring long term changes in concrete properties

• providing information for any proposed change of use of astructure for insurance or for change of ownership

1.2 CAUSES OF DISTRESS IN CONCRETE STRUCTURES

Distress in concrete structures may arise from a variety of causes. Thefollowing are the major causes of distress in concrete structures1:

• Structural deficiency arising out of faulty design and detailing aswell as wrong assumption in the loading criteria

• Structural deficiency due to defects in construction, use of inferiorand substandard materials

• Damages caused due to fire, floods, earthquakes

• Chemical deterioration and marine environments

• Damages caused due to abrasion, wear and tear, dampness

• Damages due to impact, vibration, fatigue

• Settlement of foundation, thermal expansion

Distress in concrete structures due to faulty design and/or defi-ciency in detailing and its effect on durability of concrete couldbe prevented through proper training and understanding of designconcepts, detailing and adhering to codes of practice.

Factors such as complication in geometric/structural form of thestructure leading to difficult execution, congested reinforcement detail-ing, and difficult access for concrete to flow, increase the risks ofinferior insitu quality.

Deficiencies in construction practices in transportation, placing, fin-ishing and curing of concrete affect durability of concrete. A goodconcrete mix from a sound design can have its durability severelyimpaired by improper placement and curing.

Excessive vibration can create internal bleeding resulting in weaktransition zones around coarse aggregate, weak bonding to reinforcingsteel and a porous skin at the contact of formwork. This results inthe development of a network of pathways starting from the concretesurface and penetrating to the interior and these pathways are excel-lent channels for transport of aggressive agents through the hardenedconcrete which adversely affect the durability of the concrete.


Curing is critical from durability point of view. A concrete thatdries very rapidly will be weakened forever and would permit aggres-sive agents to penetrate easily.

Chemical Attack on ConcreteChemical attack on concrete can be classified as follows- Acid attack- Alkali attack- Carbonation- Chloride attack- Leaching- Salt attack- Sulphate attackDamage in many cases dependent on the permeability of the surface

layers and not on the body of the concrete.

1.2.1 Damage due to corrosion of reinforcement

Under marine conditions and in other land-based structures wherechloride ions are deposited on the surface of concrete in substan-tial amounts, rapid deterioration of poor quality reinforced concreteoccurs. The chloride ions tend to destroy the passivating film on thesteel even in uncarbonated concrete. The surface of the steel, there-fore, becomes activated locally forming a small anode, while the rest ofthe passive surface serves as the cathode. Since the latter (cathode) ismuch larger, the dissolution of the iron in the anode is highly localized(rather than the entire surface of the steel) and a pit is formed. Thechloride (Cl−) ions combines with water forming hydrogen chlorideand hydroxyl ions. The hydrogen chloride formed produces an acidicenvironment which prolongs the corrosion causing the pit to increasein depth. In the presence of chloride ions, more generalised corrosionoccurs.

The voluminous corrosion product formed during corrosion of thesteel exerts a tensile stress on the concrete cover. As the corrosionproduct grows, the tensile stresses increase until they become highenough to crack the concrete cover. The effects of corrosion are usuallythreefold: (1) cracking of the concrete along the line of the reinforce-ment, (2) rust staining of the concrete surface, and (3) spalling of theconcrete away from the rebar, leaving it exposed to the environmentand to further corrosion2.


1.2.2 Cracking in Concrete

Cracking in concrete indicates the presence of disruptive forces withinconcrete which exceed its tensile strength. In concrete, they may becaused due to application of external load or by internal changes orby a combination of the two. Cracking in concrete can occur in theprehardened or hardened state.

Cracking accelerates the penetration of aggressive substances intothe concrete, which in turn aggravates any one or a number of othermechanisms of deterioration. For guidance, the acceptable limits oncrack widths are less than or equal to 0.1mm for the severe exposure(industrial or marine environment), 0.1mm to 0.2mm for normal exter-nal exposures or internal exposures in humid atmosphere, 0.2mm to0.3mm for internal and protected members.

A list of some factors causing cracking is given below:

• Poor quality of concrete - too high a water content and use ofexcessively high cement contents

• Poor structural design

• The development of differential thermal stresses due to high heatof hydration

• The tensile stresses developed due to restrained thermal expan-sion and concentration from temperature changes, and ensu-ing dimensional changes as a result of diurnal and seasonaltemperature cycles

• Dimensional expansion and contraction caused by cycles ofwetting and drying

• Errors, negligence, or bad workmanship

• Corrosion of steel by chloride ions

• Rapid evaporation of moisture due to dry, hot, and windyconditions prevailing at the time of placing

• Structural adjustment due to foundation movement by settlementor due to expansive soils

• Chemical attack of concrete both internally(alkali-aggregate) andexternally (sulphate attack)

• Improper use or altered use of a structure

• Aging and weathering

• Plastic settlement and heavy loading


1.3 IDENTIFICATION OF DISTRESS IN CONCRETE

STRUCTURES

A correct diagnosis establishing the nature, cause, intensity and extentof the damage in the structure is essential. Further, it is necessary todetermine if the major portions of the structure are of suitable qual-ity to support a sound repair. Determination of material propertiesof the concrete in the structure and assessment of safety and service-ability of the structure have to be made to formulate a suitable repairstrategy. Undertaking initial site inspection followed up by detailedcondition survey of the distressed structure are important to collectsufficient data to pinpoint the cause and source of the problem andto determine the extent of the damage. Interpreting the results of thecondition survey requires expert knowledge and experience. A correctand appropriate damage assessment is often the key to viable andeconomical repair.

1.4 NDT FOR QUALITY CONTROL

NDT can play a very effective role as a quality assurance managementtool. Nowadays it has become mandatory that the Turbogeneratorfoundations are to be tested for its integrity before commissioning.For new tunnels with concrete in liners, a mandatory quality controlprocedure was established in 2003 in Germany. Using NDT pulse echomethods, the top part of the in liner has to be tested for voids afterconstruction.

1.5 LIFE EXTENSION OF STRUCTURES

The life of the major infrastructures such as power stations, bridgesetc., are to be extended. A large portion of the transportation infras-tructure has been built around the mid of the last century and is nowapproaching its designed service life. In addition to repair and main-tenance, service life extension becomes a necessity. Input to the lifecycle analysis procedures is needed from quantitative measurements,preferably from NDT. NDT will play an important role in providingdata on corrosion, quality of the structure, dimensions, state of com-ponents and durability factors. The advanced NDT methods, such ascore meters, GPR or ultrasonic pulse echo do have the potential toimprove the inspection results.


1.6 NDT FOR DISASTER MITIGATION

Damage due to natural disasters such as earthquake, cyclone, etc., andterrorist attacks may not be prevented, but the consequential damageto concrete structures may be minimized through a proper design andquality control during construction. The structures can be regularlychecked for any developing defects which may alter their resistivityagainst mechanical forces. NDT methods such as Radar, UltrasonicPulse Echo corrosion meters or gammagraphy are valuable tools forthis task3,4. After an event rescue teams need information about thesafety of the remains of an affected structures. Remote sensing tech-niques would be extremely valuable under such circumstances. NDTcan be helpful in mitigating the effect of disasters.

1.7 CONCLUSION

Non destructive testing and evaluation is adopted for concrete struc-tures during its entire life to assess its health. NDT is used for anumber of tasks to locate and quantify a certain damage in a struc-ture. Basic instruments, advanced methods and combined methods areavailable for this task. Proper use and qualified interpretation needs tobe ensured through training and education. Beyond damage detection,integrated quality control uttilizing NDT techniques is the applicationwith far reaching benefits. Disaster mitigation is an area where NDTis of potentially great value. Research is needed to develop the righttools for such applications.

1.8 REFERENCES1. ACI manual of Concrete Practice, 2009, Part 6 ACI 506 R.05 to

AC II TG- 5-1-07.2. Bhaskar S., Srinivasan P., Prabakar J., Neelamegam M., Nagesh

R. Iyer “Corrosion damage studies in cracked RC componentssubjected to aggressive chloride environment”, CSIR-SERC -Research report No. OLP-15241-RR-01, December 2010.

3. Srinivasan P., Murthy S.G.N., Bhaskar S., Wiggenhauser H.,Ravisankar K., Nagesh R. Iyer and Lakshmanan N., “Applica-tion of radar and pulse echo for testing concrete structures”, 7th

International Symposium on Non Destructive Testing in CivilEngineering, Nantes (France), June 30th to July 3rd 2009.

4. Lai W. L., Kind T., Wiggenhauser H., “Using ground penetratingradar and time-frequency analysis to characterize constructionmaterials” NDT & E International, Volume 44, Issue 1, January2011, pp 111–120.

.

2 Radar and Ultrasonic Pulse Echo for

Non Destructive Evaluation of

Concrete Structures

P. SrinivasanAssistant Director,

CSIR-SERC Campus, Taramani, Chennai-600 113, India.Email: [email protected]

2.1 INTRODUCTION

Concrete is widely used for the construction of infrastructures such asbridges, power stations, dams, etc., In the hardened state concrete maycontain defects such as voids/honeycombs, cracks etc., The presence ofvoids particularly in the cover zone of a reinforced concrete structureleads to early corrosion of the reinforcement. Non-destructive testingin reinforced concrete structure plays a very important role for thecondition assessment of reinforced concrete structures. This includesidentification of defects such as honeycombs, voids, cracks, etc., and,thickness measurement, location of reinforcements, ducts, etc., TheGround Penetrating Radar(GPR) technique is a very effective methodfor investigating the integrity of concrete, thickness measurement,reinforcement identification in concrete structures (Krause et al.,1995,Maierhofer C. et al., 2003, Hevin G., 1998, Johannes Hugenschmidit,et al., 2006) The Ultrasonic Pulse Echo is a one-sided techniquewhich can be used effectively for the thickness measurement, local-ization of reinforcement and ducts, and the characteristics of surfacecracks(Krasue et al., 1997, Christoph Kohl, 2006, Wiggenhauser,2008).This paper describes the test methods, its advantages and the lim-itations. Both the methods have been adopted for the evaluation ofdifferent parameters on the large scale NDT test specimen constructedat CSIR-SERC and the results are presented in this paper.




2.2 INTRODUCTION TO GROUND PENETRATING RADAR

(GPR)

The ground penetrating radar (GPR) method, originally used forgeophysical surveys such as sub-grade investigations, is a very effec-tive technique for investigating the integrity of concrete structures. Itis particularly suited for the assessment of large structures such asprestressed concrete bridges, non-ballasted railway tracks, highways,and tunnels. GPR is an electromagnetic investigation method. It isalso known as surface penetrating radar or electromagnetic reflectionmethod. Radar principle works in Reflection mode where a signal isemitted through an antenna into the structure under investigation.The transmitting antenna sends a diverging beam of energy pulses into the structure and the receiving antenna collects the energy reflectedfrom interfaces between materials of differing dielectric properties. Astrong reflection will be received from the air/concrete interface atthe surface whilst other, generally weaker reflections will occur wher-ever distinct boundaries occur beneath the surface. Electromagneticpulses of frequency 500MHZ to 3000MHZ from radar transmitter aredirected into the material having a pulse duration of ≤ 1 ns. Thewaves propagate through the material until a boundary of differentelectrical characteristics is encountered (i.e.,) reflected at interfaceof different layer and reinforcement along its travel path (Fig. 2.1)Reflected energy caused by changes in material properties is recordedand analyzed .The signal recorded is usually referred to as a scan ortrace. The vertical axis gives time axis or calibrated depth and thehorizontal axis corresponds to the length in the X-direction. Both thepropagation velocity of the pulses and the intensity of the reflectionsare a function of the dielectric properties of the materials, which aredefined by the complex permittivity e of the material

ε = ε′ − iε′′

where ε = complex permittivity; ε’ = real part of complex permit-tivity; and e” = imaginary part of complex permittivity. For virtuallylossless materials, such as materials with very low electric conductivity,which mostly applies to concrete and masonry in a dry condition, theimaginary part can be neglected. Then the following relation betweenthe propagation velocity v of the electromagnetic impulses and thepermittivity e can be established by approximation.

v =c√ε

Radar and Ultrasonic Pulse Echo for Non Destructive Evaluation of Concrete Structures 11

where v = propagation velocity of electromagnetic impulse; and c =speed of light in vacuum (2.99792458 × 108 m/s). If the permittivityof the material under investigation is known, the depth of the reflec-tors, and thus their position, can be determined from the propagationtime. The fact that the permittivity is influenced by the followingparameters must be taken into account:

• Temperature of material;

• Moisture content of material;

• Salt content of material (only dissolved salt ions are important);

• Pore structure; and Pulse frequency

GPR has been put to a variety of application in the concreteindustry, such as

• Estimation of the thickness elements from one surface;

• Localization of reinforcing bars and metallic ducts and estimationof the concrete cover depth;

• Determination of most important features construction;

• Localization of moisture variations;

• Localization and the dimensions of voids;

• Localization of cracking;

• Estimation of bar size.

• Location of moisture in the surface near region in concrete andbrickwork

• Location of voids and other in homogeneities in concrete

The advantages are as follows

• It can rapidly and effectively investigate large areas.

• Equipment is portable.

• Immediate continuous graphic display of results is possible.

• Requires only one accessible surface.

• No coupling medium is required.

• Sensitive to materials changes and features of structural interest.

• No special safety precaution is required.

GPR equipment contains three basic units.

1. Antennas2. Control units3. Recorder and display unit


Fig. 2.2 shows the GPR equipment setup.The system of GPR can be classified based on the recording devices

as

2.2.1 Antennas

Converts the driving power into a radiated signal and convertsreturned signal from the material investigated into electrical infor-mation. Mostly for structural investigations a single antenna is usedas transmitting/ receiving antenna (monostatic antenna). Generallyantennas at higher frequency range from 500MHz to 3000MHz andare used for work on concrete. But 1.6 GHz frequency antennas areused for structural concrete, roadways and bridge deck investigations.High frequency units are small and suitable to work in formwork fromscaffolding. Lower frequency units may be effectively limited to use onhorizontal surfaces. Fortunately these factors do not affect the com-monest uses of GPR in building surveys for shallow targets using highfrequencies. The choice of antenna type is selected based on the depthof investigation and the waves to be penetrated. Normally for smallerthick sections higher frequency antennas are used and for greaterthickness very low frequency antennas are used. Table 2.1 gives theappropriate antenna frequencies to be chosen based on depth range.

2.2.2 Control Units

It manages the antennas and processes the transmitted and receivedsignals and output them to the recording / display media.

Key controls available are usually

• Maximum depth of penetration.

• Amplification of the signals to the data recorder or display.

• Filters used on the data to cut unwanted signals or enhance thedesired signals, before they are recorded or displayed.

• Rate at which measurements are taken.

• Digital system may simplify control of the above factors and aidrepeatability.

2.2.3 Standard test method for determining the thickness ofbound pavement layers using Short- Pulse Radar (ASTMD 4748-98)

A test method and the procedure are given in ASTM D 4748 for thenondestructive determination of thickness of bound pavement layersusing short-pulse radar. This test method permits accurate and non-destructive thickness determination of bound pavement layers. This


test method is widely applicable as a pavement system assessmenttechnique.

2.3 THE TEST SPECIMEN

The test specimen is a unique reinforced concrete specimen, designedand constructed at Structural Engineering Research Centre (SERC),exclusively for the data generation and validation of different NDTtechniques. It consists of two slabs of sizes 4.15m × 4.15m (bottomslab) and 3.0m × 3.0m (top slab with cantilever projection at oneend) with beams and columns. The entire block is supported on fourpedestals at a height of 1.2m to have access for the bottom slab. Thetop slab is made with two different thicknesses (150mm and 250mm)and bottom slab with three different thicknesses (150mm, 300mmand 400mm) for validating the thickness measurements using NDTmethods. Top slab is provided with construction joints, different sizesand shapes of honeycombs, PVC conduits, cracks for their identifica-tion and quantification. Columns are provided with different diameterof reinforcements with different spacing of lateral ties and differentcover thicknesses. Different grades of concrete are used in casting thebeams, columns and slabs. Fig. 3 shows the completed large scale testspecimen.. Radar measurements

For the radar measurements, SIR-20 model of GSSI has been usedwith 1.60 GHz antenna. For data collection the bottom slab wasdivided into grids of size 50 mm × 50 mm. A portion of 2.0 m ×2.0 m within the beams was considered for scanning. The data wascollected from the top face on the bottom slab. Dielectric constantof 6.25 was used. Fig. 2.4 shows the radargram for the bottom slabbefore and after migration. The data which was collected in both thedirections were processed using RADAN software and the 3-D anima-tion view was obtained. Fig. 2.5 shows the reinforcements present inthe bottom slab. The spacing of the reinforcements obtained in theline scan was matching with the actual. The sloping portion of thebottom slab, i.e., the back wall reflection was obtained and is shownin Fig. 2.6. The top slab was also divided into grids of 50 × 50 mmover an area of 2.0 m × 2.0 m between the beams. The radar datawas collected on the top and bottom side of the slab. The data wasprocessed using RADAN software. Fig. 2.7 shows the C-scan whichgives the presence of steel box and the PVC pipe. The column C1 ofsize 300 mm × 450 mm was scanned in the 450 mm direction. Radar


data was obtained over a grid spacing of 50 mm in both the directions.Fig. 2.8 show the reinforcemnts present in the column.

2.4 ULTRASONIC PULSE ECHO TECHNIQUE

Ultrasonic-echo needs only one side access with transmitter andreceiver at one side. Longitudinal waves or transverse waves can beused for measurement. For longitudinal waves wet coupling is requiredand for transverse waves dry point contact array system withoutany coupling agent is adopted. For concrete, lower frequencies of 50KHz is used because of the sound attenuation from absorption (porestructure) and scattering (aggregates). Concrete is an inhomogeneousmaterial and the aggregates are nearly the same size as the ultrasonicwavelength and hence several transmitters and receivers in array ispreferred to reduce the structural noise from its inhomogeneous struc-ture. Low frequencies from 25 kHz allow thickness measurement frommore than 1 m but with limited resolution of objects, e.g. single rebars.Higher frequencies from about 150 KHz allow high resolution of objectsbut limited penetration. Thickness measurement with higher frequen-cies can be limited with less than 50 cm. Fig. 2.9 shows the commercialequipment namely A1220 monolith - ultrasonic Pulse Echo for con-crete structures. The transmitter and receiver is housed in the sameunit which consists of A 24 element (6 × 4) matrix antenna array. Theantenna array elements construction allows to test without using anycontact liquid, i.e. with dry-point-contact. All of the elements have anindependent spring load, which allows to test on uneven surfaces.

An interface with a great impedance change (e.g. concrete / air)produces a clear reflection signal like shown in Fig. 2.10 (a). Thereflected signal is attenuated by absorption and scattering due tothe inhomogeneous concrete structure. If wave speed c is known orestimated the thickness can be calculated as follows.

A: Thickness/geometryd = c/2 ∗ t

wheret = measured transit time; c = known or estimated wave speed,d = calculated thickness/depth position

Fig. 2.10(b) shows the case of integrity testing for good and badconcrete quality or workmanship. Bad quality results from decreaseddensity and E modulus. The wave speed is calculated as follows

B: Integrityc = 2.d/t


t = measured transit time; d = known thickness/depth positionc = actual calculated wave speed has to compared with the expected

wave speed

The measurement of intensity Vs time at a point is called A-scan.The signals are processed of all the points along a line using a softwareand the details are obtained for a particular line. These are calledB-scans. The sectional information parallel to the surface is calledC-scans. Fig. 2.11 shows a typical A- scan.

2.4.1 Pulse Echo Measurements

Measurements were made on the slabs of the large scale NDT specimenconstructed at SERC. The slab is divided into grid markings from thebottom side of 50 × 50 mm in both horizontal and vertical directions.The data is obtained over each point. Fig. 2.12 shows the measurementwith A1220 equipment from the bottom side of the slab. The datawas transferred from the instrument to the computer and the datawas analysed using the Introvisio Software. Fig. 2.13 shows the B-scan and the back wall reflection and the thickness of different slabscan be seen.

Fig. 2.14 shows the C- scan (parallel to the surface of the top slab)and the steel plate buried in the concrete is being located.

2.5 CONCLUSIONS

The application of radar and ultrasonic pulse echo have been demon-strated for the thickness measurement, identification of reinforce-ments, steel embedment, and honeycombs. The B-scans and C-scansas obtained for the radar measurements gives the reinforcement distri-bution. The depth slice also provide useful information in identifyingthe steel embedment and the PVC conduits. For the radar measure-ments it was observed that the spacing of the reinforcement affectsthe penetration of the waves in to the concrete. The ultrasonic pulseecho technique provide information on the exact thickness of the con-crete member. In addition, the embedments such as steel plate or PVCpipe can be identified. With the radar method, additional research isrequired for the effect of spacing and the size of the reinforcement onthe penetration of radar waves in concrete.


2.6 ACKNOWLEDGEMENT

The author acknowledge the technical support given by Prof. HerbertWiggenhauuser, BAM for the preparation of specimen and also for theanalysis of test results during his stay at SERC, Chennai under theCSIR- Humboldt Fellowship.

2.7 REFERENCES

1. Krause M., Maierhofer C., Wiggenhauser H., (1995) “Thicknessmeasurement of concrete elements using radar and ultrasonicimpulse echo techniques”, 6th International conference on struc-tural faults and repair, Edited by Forde MC, 1997, London, UK,vol. 1, pp. 17–24.

2. Maierhofer C., (2003) “Nondestructive Evaluation of ConcreteInfrastructure with Ground Penetrating Radar”, Journal ofMaterials In Civil Engineering, ASCE, May-June 2003, pp.287–297.

3. Hevin G., Abraham O., Pedersen HA., Campillo M., (1998)“Characterization of surface cracks with Rayleigh waves: anumerical model”, Nondestructive testing and evaluation inter-national, 31, 1998, pp. 289–97.

4. Johannes Hugenschmidit., Roman Mastrangelo., (2006)“GPRinspection of concrete bridges”, Cement & Concrete Composites,28, 2006 pp. 384–392.

5. Krause M., Barmann R., Friedlinghaus R., Kretzschamar F.,Kroggel O., Langenberg K., Maierhofer Ch., Mu ller W., NeiseckeJ., Schickert M., Schmitz V., Wiggenhauser H., Wollbold F.,(1997), Comparison of pulse echo methods for testing concrete’NDT & E International 4 (special issue), 1997 pp. 195–204.

6. Christoph Kohl., Doreen Streicher., (2006), “Results of recon-structed and fused NDT-data measured in the laboratory andon-site at bridges”, Cement & Concrete Composites, 2006,pp.402–413.

7. Summary Report of the 2nd Phase Visit of Prof. Wiggenhauser,Head of Division, Federal Institute for Material Research andTesting (BAM), Berlin, Germany to SERC, Chennai under CSIR- Humboldt Reciprocity Research Award for 2006, Report No.MLP- 12241- CSIR HUMBOLDT 2006, May 2008


Table 2.1 Appropriate antenna frequency for various applications.

Frequency Field of application Max depth (m)1.6 GHz Structural concrete, 0.50

Roadways, Bridge decks1.0 GHz Concrete structures, 1.00

Archaeology, shallowsoils

400MHz Geological field 4.00

Fig. 2.1 Principle of Radar Surveying


1.6 GHz antenna

Control unit

Display unit

Fig. 2.2 GPR Equipment Setup

Fig. 2.3 Large Scale Test Specimen

First Floor – Migration

Rebars

SERC NDT SPECIMEN

Fig. 2.4 Reinforcements before and after Migration


Fig. 2.5 Reinforcements in First Floor slab - 3D view

Fig. 2.6 Radargram in sloping portion of C- scan of first floor slab

Fig. 2.7 C-scan at 70 mm form top face


Fig. 2.8 C- scan for the column

Fig. 2.9 Ultrasonic Pulse Echo instrument - A1220


Fig. 2.10 Concrete members and typical recordings forultrasonic-echo for (a) sound concrete member (b) member with

good and bad concrete quality

Fig. 2.11 A-Scan measured on a concrete slab showing areflection from a duct

Fig. 2.12 Measurement in Top slab


Thickness - 150 mm Thickness - 250 mm

Fig. 2.13 Back wall reflection from the bottom slab.

Steel Plate at a depth of 70mm

Test results on Top Slab

C - Scan

Fig. 2.14 Location of steel plate in the top slab

3 Use of Impact Echo Method for

Determination of Thickness and Defects in

Concrete Elements

S. BhaskarScientist

CSIR-SERC, Taramani, Chennai-600 113, India.Email: [email protected].

3.1 INTRODUCTION

Non-destructive testing (NDT) techniques are inscreasingly gainingpopularity for the quality assessment of important structures such asbridges, roadways, tunnel linings etc. Impact echo was developed inthe mid-1980s is a method based on impact generated stress waves1−2.Use of long wavelength low-frequency stress waves of impact-echodistinguishes with other traditional ultrasonic methods3−4. In impact-echo testing, low frequency stress waves from about 1 to 30 kHz areintroduced by a short duration of impact by tapping a hammer orsmall steel sphere against a concrete or masonry surface. The wavespropagate into the structure and are reflected by flaws and externalsurfaces. Surface displacements, at the impact surface caused by thearrival of reflected waves due to the generation of a standing waveare recorded by a transducer, located adjacent to the impact posi-tion. Both the waveform and frequency spectra will be plotted on thecomputer screen. The dominant frequencies that appear as peaks inthe spectrum are associated with multiple reflections of stress waveswithin the structure, and they provide information about the thicknessof the structure, its integrity, and the location of flaws5−6. This paperinvestigates the application of impact echo in manual scanning modein determining the thickness and also in identifying the flaws/defects.The specimen used for the determination of thickness and flaws isan R.C slab which is a part of large NDT model test specimen atCSIR-Structural Engineering Research Centre (SERC), Chennai.




3.1.1 Impact Echo Method

The IE system consists of i) a hand held unit containing an impacthammer (steel ball) for producing low frequency stress waves (soundwaves), ii) a piezoelectric transducer that detects surface displace-ments caused by reflected waves, iii) data acquisition system thatreceives and digitises the analogue voltage signal from the transducer.Fig. 3.1 shows the typical impact echo system.

3.1.2 Basic Principle

In the impact-echo technique (IE) a transient stress pulse is intro-duced into a test object by mechanical impact on the surface. Thestress pulse propagates into the object along spherical wavefronts asP- and S-waves. In addition, a surface wave (R-wave) travels alongthe surface away from the impact point. The P- and S- stress wavesare reflected by internal interfaces or external boundaries. The arrivalof these reflected waves at the surface where the impact was gen-erated produces displacements which are measured by a receivingtransducer. If the receiver is placed close to the impact point, thedisplacement waveform is dominated by the displacements caused byP-wave arrivals.

If the receiver is close to the impact point, the round trip traveldistance is 2T, where T is the distance between the test surface andthe reflecting interface. The time interval between successive arrivalsof the multiple reflected P-wave is the travel distance divided by thewave speed. The frequency, f, of the P-wave arrival is the inverse thetime interval and is given approximately by the relationship:

f =Cpp

2T(3.1)

Where Cpp = P wave speed through thickness of the plateT = the depth of the reflecting interface.

In frequency analysis of impact-echo results, the objective is todetermine the dominant frequencies in the recorded waveform. Thisis accomplished by using the fast Fourier transform technique (FFT)to transform the recorded waveform into the frequency domain. Thetransformation results in an amplitude spectrum that shows the ampli-tudes of the various frequencies contained in the waveform. Generallyfor intact plate-like structures, the thickness frequency will usually bethe only dominant peak in the spectrum. The value of the peak fre-quency in the amplitude spectrum can be used to determine the depth

Use of Impact Echo Method for Determination of Thickness and Defects in Concrete ... 25

of the reflecting interface by expressing the Eq.(3.1) as follows:

T =Cpp

2f(3.2)

In the case where the wave encounters a flaw, a part of that wavereflects back to the surface of the slab. Here two distinct peaks will beobserved: one large amplitude peak at a lower frequency, correspondingto the slab bottom, and another smaller amplitude peak at a higherfrequency corresponding to the flaw7.

3.1.3 Test Specimen

The specimen used for IE scanning is the bottom and top slab of NDTmodel test specimen constructed exclusively for NDT data collectionat CSIR-SERC, Chennai. Fig. 3. 2 shows the photograph of NDTmodel test specimen. Both, bottom and top slabs are resting on fourcolumns and beams. The bottom slab is of different thicknesses andthe thicknesses are about 200mm, 300mm and 400mm as per draw-ings. The top slab is of 2.4m × 2.4m and is of two different thicknesses,150mm and 250mm. Also, defects in the form of PVC pipes, honey-combs, cracked specimen and a steel plate are introduced in the topslab during casting. The slab surfaces are polished/ground to get auniform and smooth surface that is essential for scanning

3.1.4 Impact echo (IE) scanning

The IE technique is a punctiform test method. It means one mea-surement only gives information about one point of the structure. Toget more detailed information about the structure scanning techniquesmeasuring at multiple points are more useful. The combination of mea-surement results of several points to a line (B-Scan) or, measurementsin two different orientations, to an area representing a surface of astructure (C-Scan) will give a better idea of the structure.

The impact scanning on bottom slab is carried out on a 2m × 2marea covering the three regions of slab thickness. For scanning, the gridlines are marked at a spacing of 50mm × 50mm. Figs. 3.3 and 3.4 showsthe grid marking for scanning and cross section details of bottom andtop slab. A calibrated wave velocity of 4200 m/s is used during thedata collection. Scanning has been carried out systematically alongeach line and average of two impacts that are repeatable in responsehave taken at each grid point. For simplicity and easy understanding,bottom slab is analysed to determine the thickness and top slab isanalysed to predict flaws/defects.


3.1.5 Thickness Determination

Bottom slab data is analysed for the determination of thickness. Therecorded waveform data is transformed into frequency spectra by FFT.Fig. 3. 5 shows the typical frequency spectra of a point in the 200 mmthick regions. The frequency corresponding to the maximum peak is10.53 kHz. The thickness of the slab can be obtained by using theEq.(3.2) Using this, the average thickness of the slab of that regionis found to be 199 mm, which is almost equal to the actual thicknessof the slab. Similarly, Fig. 3. 6 shows the typical frequency spectraof a point in the 300 mm thick portion. For the other two regions,the average thickness of the slab is found to be 287 mm and 362mm corresponding to the expected thickness of 300 mm and 400 mm.The difference in estimation is found to be 0.5%, 4.3% and 9.5%. Thehigher difference in estimation for 400mm could be due to geometri-cal changes, scattering of signals, multiple reflections, etc8−9. B-scanimage showing different frequencies (thicknesses) along a typical gridline is presented in Fig 3.7.

3.1.6 Detection of Defects

For studying the applicability of impact echo in identifying thedefects/flaws, observations are made on the top slab along the selectedlines passing over the defects and the solid potion. Fig. 3.8 representsthe frequency spectra for the honeycombed portion, which is char-acterized by multiple peaks, whereas a single dominant peak shownin Fig. 3.8 corresponds to the solid portion. Successful identificationof the defects relies on identifying changes in the frequencies in thefrequency-amplitude spectra. Attempts are also made to identify thelocation of flaws, buried objects using the B-scan image, which is acombination of the frequency results from the spectra at several pointsof a line. Fig. 3.9 shows the B-scan image obtained over a portion ofthe slab. In the B-scan image, a shift in frequency is observed (markedinside the dotted line) at positions which corresponds to the locationof the buried pipes. Fig. 3.10 represents the B-scan image along a line18 which is passing over the steel plate and the cracked specimen.From Fig.3.10, it can be assumed that region in the dotted circle indi-cates the steel plate and the region in the dotted square represents thecracked specimens present in the slab. However, the average thicknessobserved for the two halves of the slab is found to be 145 mm and 246mm corresponding to the expected thickness of 150 mm and 250 mm,respectively.


3.2 SUMMARY

This chapter presents the application of impact echo technique for thedetermination of thickness and identification of flaws/defects. The testspecimen used is a slab with simulated variabilities such as differentthicknesses, intentionally created defects, etc. From the analysis ofexperimental data, it is observed that the thickness obtained is foundto be in close agreement with the actual value. The technique is alsosuccessful in identifying the location of buried pipes/ducts and theidentification of defects. Further, number of studies is needed for theexact identification of voids, their size, etc.

3.3 REFERENCES

1. Carino, N.J., Impact-Echo Principle, http://ciks.cbt,nist.gov/carino/ieprin.html

2. Carino, N.J., (2001), “Impact-Echo Method: An Overview”, Pro-ceedings of the 2001 Structures Congress & Exposition, NationalBureau of Standards.

3. Jennifer R.B. (2001), “Detection of Thickness and Tension Ductsof Bounded Elements Using Impact-Echo Method”, University ofthe Philippines.

4. Sansalone, M., and Carino, N.J., (1989), “Detecting Delamina-tions in Reinforced Concrete Slabs with and without AsphaltConcrete Overlays Using the Impact-Echo Method,” MaterialsJournal of the American Concrete Institute, March/April, 1989,pp. 175-184.

5. Chiamen, H., Chia-Chi, C., Tzunghao, L., and Yuanting Juang,(2007), “Detecting Flaws in Concrete Blocks Using the Impact-Echo Method”, NDT & E International 41, pp. 98-107.

6. Ertugrul, C., Sadettin, O., and Murat, L., (2005), “An Analysis ofCracked Beam Structure Using the Impact-Echo Method”, NDT& E International 38, pp. 368-373.

7. Martyn, H., John, M., and John, D.T., (2000), “Cross-SectionalModes in Impact-Echo Testing of Concrete Structures”, Journalof Structural Engineering, February, 2000, pp. 228-234. Yajai,T., Miller, P. K., and Olson, L. D. (2008), ’Internal void imagingusing impact-echo’, NDE/NDT for Highways and Bridges, Struc-tural Materials Technology (SMT), 8-12, Sept. 2008, Oakland,USA (CD format).

8. Bhaskar, S., Murthy, S.G.N., Srinivasan, P., Wiggenhauser, H.,Ravisankar, K., Nagesh R. Iyer and Lakshmanan, N., “Reliability


of the impact-echo method on thickness measurement of concreteelements”, International Conference on Non-Destructive Testingin Civil Engineering NDTCE-2009, Nantes, France, June-July,2009 (CD format).

9. Bhaskar, S., Srinivasan, P., Murthy, S.G.N., Nagesh R. Iyer andRavisankar, K., “Application of Impact-echo Method for theEvaluation of Thickness and Defects in Concrete Structures”,ACTEL-OLP131-RR-06, March 2010.

Data acquisition system

Transducer

Steel ball

Fig. 3.1 Impact Echo System

Fig. 3.2 Model Test Specimen


C4C3C2 C1

AA

N

SECTION A-A

300

200100

100

1 5 10 20 25 30 35 401

5

10

15

20

25

30

35

40

15

24003000

TOP VIEW

2400 3000

B1

B2

2000

2000

all dimensions are in mmgrid spacing 50mm x 50mm

Fig. 3.3 Bottom slab details with grid marking

PLATESTEEL

HONEY COMB B

SPECIMEN -2CRACKED

112m

mØ

PV

C (2

.3m

LO

NG

)

50m

mØ

PV

C P

IPE(

3m L

ON

G)

C3

C2 C1

C4

HONEY COMB AB4

B1

B3

B2

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

111

12 14 16 18 20 22 24 26 28 44 4642403836343230

11

2 4 6 8 10

X

Y

250150

1500 15002400

X

TOP VIEW

SECTION X-X

X

Fig. 3.4 Top slab details with grid marking


Fig. 3.5 Typical frequency spectra at a point in 200 mm thickslab portion

12

10

8

6

4

2

00 1 2 3 4 5 6 7

× 104

Frequency = 10528.5645

Fig. 3.6 Typical frequency spectra at a point in 300 mm thickslab Portion


Fig. 3.7 B Scan Image along a typical grid line

3.5

3

2.5

2

1.5

1

0

0.5

0

Am

plitu

de

Frequency in Hz

Frequency Spectra


1 2 3 4 5 6 7× 104

Fig. 3.8 (a) Frequency spectra at the honeycombed portion


60

50

40

30

20

0

10

0

Am

plitu

de

Frequency in Hz

Frequency Spectra


1 2 3 4 5 6 7× 104

Fig. 3.8 (b) Frequency spectra at the solid portion

Fig. 3.9 B-scan image along a typical line passing over buriedpipes (along x-dir)


Fig. 3.10 B-scan image along a line passing over buried pipes anddefects

.

4 Advanced Cement Composites (ACCS)-

Production and Application to Repair

J. K. DattatreyaScientist

CSIR-SERC, CSIR Campus Tharamani, Chennai-600 113, India.email: [email protected]

4.1 INTRODUCTION

The world’s infrastructure is largely built of concrete. For today’s con-crete structures, we look for materials with four distinctive properties:strength, workability, durability and affordability. Since ancient time,mankind has been searching for construction materials with higherand higher performance so they can build taller, longer and betterstructures. The definition ’high performance’ is meant to distinguishstructural materials from the conventional ones, as well as to optimizea combination of properties in terms of final applications. The needfor new materials with improved properties, which can provide higherperformance, is as imperative now as ever before. To address the prob-lem of rapid deterioration of infrastructures and massive utilization ofconstruction materials and in turn natural resources, an exciting alter-native has emerged in the form of advanced cementitious composites,which include Slurry Infiltrated Cementitious Composites (SIFCONand SIMCON), Engineered Cementitious Composites (ECC) andUltra High Performance Fiber Reinforced Concrete (UHPFRC). Theyare engineered in such a way that the contribution of each constituentis optimized and results in a synergetic composite performance withemphasis on strength or strain capacity or energy absorption as thecase may be. The target properties can reach levels unattainable withconventional concretes. The ultra high strength materials result inreduced material consumption by virtue of their enhanced strengthcharacteristics and provide a possibility of using thin sections, slen-der elements and new geometries. Two distinct approaches have beenexplored in an attempt to improve mechanical performance The first




involved concrete with a dense granular matrix known as DensifiedSmall Particle (DSP) concrete (Bache, 1987 ) when the use of sub-micron particles in cementitious materials was conceived(Fig.4.1) andMacro Defect Free (MDF) cements/mortars (Kendall et al 1983)in conjunction with special processing techniques. In recent yearsthe principles of both are combined to produce UHPCs[Richard andCheyrezy, 1995, Guerrini, 2006]. An increase of mechanical strength isalways associated with an increase in brittleness. The use of reinforc-ing fibers leads not only to the increase of tensile/bending strengthand specific fracture energy, but also to reduction of brittleness and,consequently, to production of non-explosive ruptures. Besides, fiber-reinforced materials are more homogeneous and less sensitive to smalldefects and flaws. Therefore, with growing emphasis on improvementof cement matrix to achieve enhanced strength, there has been a par-allel development towards addition of fibers in order to improve theductility. The development of several types and geometries of fibers hascontributed immensely to this development. The culmination of thesetwo efforts is today’s ultra-high-performance fiber-reinforced concrete(UHPFRC) as illustrated in Fig.4.2. Fig.4.3 provides a pictorial viewof the evolution in cementitious composites towards achievement ofhigh strength and ductility.

High and Ultra High Performance Fiber Reinforced cement Com-posites fall under the more general category of Fiber ReinforcedCement Composites (FRCC). FRCCs are further classified into LowVolume FRCCs and Ductile FRCCs (DFRCs) (Fig.4.4), High Perfor-mance FRCCs and Ultra High Performance FRCCs In the last fewdecades or so, a new class of DFRCCs, generally labeled as high per-formance FRC, or simply HPFRC, has been introduced for specificapplications, for which toughness, ductility, and energy absorption arefundamental properties HPFRC exhibits apparent strain-hardeningbehavior by employing high fiber contents. The tensile strain capacityof HPFRC is typically about 1.5% or more. These HPFRCCs includeSIFCON (slurry infiltrated fibrous concrete) with 5-20% of steel fibers,SIMCON (slurry infiltrated mat concrete) with 6% steel fibers. A spe-cial type of HPFRCCs is the Engineered Cementitious Composites(ECCs). Table.4.1 compares the characteristics of different types ofFRCCs with conventional concrete.

Advanced Cement Composites (ACCS)- Production and Application to Repair 37

4.2 UHPFRCS

UHPFRCCs have a DSP matrix and moderate to high volume offibers and possess compressive strength generally exceeding 150 MPa.The Association Franaise de Gnie Civil Interim Recommendationsfor Ultra High Performance Fiber-Reinforced Concretes 2002 statesthat UHPC tends to have the following characteristics: Compressivestrength that is greater than 150 MPa, internal fiber reinforcementto ensure non-brittle behavior, and high binder content with specialaggregates. Further, UHPFRC tends to have very low water contentand can achieve adequate rheological properties through a combina-tion of optimized granular packing and the addition of high-rangewater reducing admixtures.

The recent history of UHPFRC development has been marked byseparate approaches. In chronological order of their appearance, theseare:

1. Compact Reinforced Composites: UHPFRC containing 5 to 10%of 6 mm (0.2 in.) long and 0.15 mm (6 mils) diameter metal fibers.This type of concrete was developed by Aalborg Portland (Den-mark) and has been marketed as Compact Reinforced Composites(CRC).

2. Reactive Powder Concrete: UHPFRC containing mainly fine reac-tive powders, such as, silica fume, quartz powder and cement andquartz sand or other hard aggregates with particle size less than600μ and a maximum of 2.5% metal fibers which are 6-13 mm(0.5 in.) long and 0.16 mm (6.2 mils) in diameter. This type ofconcrete was developed by Bouygues (France) and has been mar-keted as Reactive Powder Concrete (RPC). Other UHPCCs ofthis type currently being marketed are:• BSI “Bton Spcial Industriel” (special industrial concrete)

developed by Eiffage, which technology is evolving in asso-ciation with cement manufacturer Sika ( Ceracem),

• Different kinds of Ductal concrete, including BPR (reactivepowder concrete) resulting from joint research by Bouygues,Lafarge and Rhodia, and marketed by Lafarge and Bouygues

• BCV being developed by Vinci group in association with Vicat3. Multi-Scale Fiber-Reinforced Concrete[Rossi, 1997] : UHPFRC

containing mixtures of short and long metal fibers(6-20mm). Thiswas developed by the Laboratoire Central des Ponts et Chausses(LCPC, France) and CEMTEC.


4. UHPC with Coarse Aggregates : UHPC produced with crushedbasalt with the particle size from 2 to 5 mm by Ma and Orgassand by Coppola and others. The cementitious paste volume frac-tion is about 20% lower than that in RPC for achieving the sameorder of compressive strength and fluidability. The mix more flow-able and homogenized even with a shorter mixing time. There isa decrease in autogenous shrinkage by about 60%. The highermodulus and lower strain at peak stress under compression areclaimed to be the other advantages.

Durability of UHPC[Fehling et al, 2004a and b, Acker et al,2004, Resplendino et al, 2004]RPC has ultra-high durability characteristics resulting from itsextremely low porosity, low permeability, limited shrinkage andincreased corrosion resistance. In comparison to HPC, there is nopenetration of liquid and / or gas through RPC [Roux et al ,1996].Tables.4.2 - 4.3 present a comparison of durability of RPC and HPC.It has been shown that the total shrinkage of sealed UHPC with fineaggregates amounts to 0.7 mm/m under isothermal conditions in thefirst seven days after pouring. Until an age of 28 days, the total shrink-age increases to about 0.9 mm/m. The influence of steel fibers onthe autogenous shrinkage is of minor importance. The developmentof drying shrinkage of UHPC is similar as of HPC. For heat treatedUHPC, drying shrinkage can practically be neglected after the endof the heat treatment. The creep of UHPC is generally less than forconcrete with lower strength. For UHPC with fine aggregates, 12 μm/mK have been recorded. This value is in the same range as forNSC (about 11.0 μm/mK). The high strength of UHPC with fibersdoes not lead to disadvantages with regard to fatigue. Due to the highcompressive strength and the high density, UHPC enables very highbond strength. For smooth fibers (l = 13 mm, φ = 0.15/0.2 mm),Behloul [1997] reports a value of fb = 11.5 MPa. For ribbed reinforc-ing bars, very high bond stresses in the range of 40 to 70 MPa havebeen reported.

4.3 ENGINEERED CEMENTITIOUS COMPOSITE, (ECC)[LI,

1998]

A new type of FRC which combines the favorable characteristics of thevarious classes of FRCs in use today viz., flexible processing suitable


for pre-cast or cast-in-place applications[ short fibers of moderate vol-ume fraction to facilitate flexible processing, reduce cost and weight;Isotropic properties with no weak planes under multi-axial loadingconditions; high performance with improvements in strength, ductility,fracture toughness and exhibiting pseudo strain-hardening[Table.4.4].

ECC is an easily moulded and shaped reinforced cementitious mor-tar based with short random fibers, usually polymer fibers. It is amicro-mechanically designed such that the mechanical interactionsbetween fiber, matrix and its interface are taken into account by amodel which utilizes these constituent properties to compute the com-posite response. As a result, guidelines for selection of fiber, matrixand interface characteristics advantageous for composite propertieshave been made available.

4.4 TEXTILE REINFORCED ULTRA HIGH PERFORMANCE

CONCRETE

The past decade has seen an increased use of prefabricated cement-bonded fiberboard around the world. Such elements are used for wallpanels, exterior siding, pressure pipes, and roofing and flooring tiles.The use of reinforcement in these elements is essential to improve thetensile and flexural performance. The reinforcements can be eithershort fibers or continuous reinforcements in a fabric form. The useof reinforcement in thin cement-based elements is essential to improvethe tensile and flexural performance. The reinforcements can be eithershort fibers or continuous reinforcements, in a fabric form. Practicaluse of fabric-cement composites requires an industrial, cost-effectiveproduction process. Woven fabrics made from low-modulus polyethy-lene and glass meshes are used to produce the composite by pressuremoulding or pultrusion.

In addition to ease of manufacturing, fabrics provide benefits suchas excellent anchorage and bond development. The flexural strength ofTFRC with low-modulus polyethylene (PE) fabrics is almost two timeshigher than the strength of composites reinforced with straight con-tinuous polyethylene yarns. In addition, they exhibit strain-hardeningbehavior Cement composites containing 5% alkali resistance (AR) uni-directional glass fibers achieved tensile strengths of 50 MPa, comparedwith an average tensile strength of approximately 6 to 10 MPa ofconventional glass fiber-reinforced cement (GFRC) composites. Pul-trusion products reinforced with polyacrylonitrile (PAN)-based carbon


continuous filaments achieve superior flexural strength of approxi-mately 600 MPa with 16% content by volume and 800 MPa with23% content by volume.

Production of UHPC In order to achieve sufficient ductility andstrength, ultra high performance concrete (UHPC) is produced with

• w/b-ratios near 0.25 or less

• silica fume contents up to 30 wt.% w.r.t. cement

• between 1.0 and 3.5 vol.% steel fibers and up to 0.65 vol.%. PPfibers

• maximum aggregate size < 1mm

The homogeneous distribution of steel fibers in concrete is one ofthe most important demands of UHPC. Moreover, the fibers shouldthen be aligned in the direction of the main tensile stress The followingrequirements are placed on the mixer for UHPC production:

• Short mixing duration

• Homogeneous blending of small quantities of additives andadmixtures

• Homogenization of materials having different densities

Generally high shearing action type of mixers as shown in Fig.4.5have been used for UHPC production. Planetary mixers with eccen-trically mounted turning and dividing paddles, mounted very nearthe bottom of the drum, as well as the drum wall scraper results inthree dimensional turning of the mix are used for UHPC mixing. . Inan intensive mixer Fig. 4.6 (Make Eirich with vacuum periphery) bythe optimum combination of drive and geometry, high mixture speedsof up to 40 m/s (counter currents) are achieved and the tilt of thedrum produces optimum homogenization of materials with large dif-ferences in density. The vacuum accessory permits evacuation downto pressures of 50 mbar in a closed system. Depending on the requiredperformance the turning geometry can be varied.

Heat curing (low pressure steam or autoclaving) may be necessaryand depending on the mix composition, type of structural element andthe facilities and turnover time, the curing regime has to be workedout by trials.

Production of Engineered Cementitious Composites (ECC)

Several types of processing routes have been developed for ECC viz.,casting, extrusion and shotcreting. For casting, normal casting and


self-compacting casting are available. Extrusion of ECC has also beendemonstrated. Spray ECC, equivalent to shotcreting, but replacingthe concrete with ECC, is under development at the University ofMichigan.

ECCs can be formed with a variety of fibers, including polymeric,steel and carbon fibers. The matrices used are mostly cement pasteand mortar. So far, most research has been conducted with a highmodulus polyethylene fiber (Trade name Spectra 900) in a cementmatrix. Typical material composition and mix proportions of a PEbased ECC are given in Table.4.5.

Normal ECC processing adopts the casting method for conventionalemendations materials that generally requires high-frequency vibra-tion to place the fresh mix into molds. The efficiency of fibers can besignificantly reduced if fibers are not uniformly distributed due to thelow workability of fresh ECC mix. The polyethylene fibers are suppliedby the manufacturer in bundle-like form. Prior to mixing, the fibersare dispersed using air pressure. Then the mixing is carried out using athree speed (Hobart) mixer with a planetary rotating blade. The totalmixing time is between 15 to 30 minutes depending on the batch sizeand the amount of fibers used (fiber volume fraction). After the mix isready, the specimens are cast under high frequency vibration (150 Hz).Subsequently, they are covered with a polyethylene sheet and allowedto harden at room temperature for one day prior to demoulding. Thespecimens are then cured in water tank for 4 weeks.

Li and co-workers [1998] developed self-compacting ECC via a con-stitutive rheological approach. In this approach, the ingredients ofthe mortar matrix were tailored so that high flowability is achieved,while respecting the conditions of strain-hardening for the compos-ite as described earlier. The high flowability mortar matrix resultsfrom an optimal combination of a strong polyelectrolyte (a super-plasticizer) and a non-ionic polymer with steric action in maintainingnon-aggregation of the cement particle in the dense suspension. Sil-ica sand with size ranges from 0.2 to 0.3 mm was used. Melamineformaldehyde sulfate and Hydroxypropyl methylcellulose (HPMC)with molecular weight of 150,000 as a viscosity agent were used. Tocharacterize and quantify the self-compactability of fresh ECC, a num-ber of tests were conducted, including deformability tests using slumpcone or flow cone, flow rate test using a funnel device, and self-placingtest using a box vessel with reinforcing bars as obstacles to ECC flow.


4.5 PROPERTIES OF ECC

A compendium of mechanical properties obtained far for the ECC inuniaxial tension is given in Table. 4.8. The table also compares similardata for FRC tested under the same conditions. Fig. 4.7 shows typicalcomparison of ECC and FRC behavior.

Real-time observation shows that under uniaxial tension, multiplecracking occurs with many sub-parallel cracks across the specimenduring strain-hardening phase. Beyond peak stress, localized crackextension occurs accompanied by fiber bridging. Fig. 4.8 shows anexample of a damage record at four different stages of loading. Thecompressive strength of this ECC, about 68.5 MPa, is not significantlyhigher than that of the FRC (55 MPa). The compressive strain capac-ity has been observed to increase by approximately 50%-100% overnormal concrete and FRCs. Post-peak ductility of ECCs are expectedto be similar to that of normal FRCs. The modulus of ECC has beenmeasured by strain gages as 20.3 GPa

4.6 APPLICATIONS OF UHPFRC TO REPAIR

Beams of Cattenom and Civaux Power Plants (Acker andBehloul, 2004): Two important precasting jobs involving in replace-ment of cooling tower’s steel beams by UHPC beams was carried out inCattenom (with BSI and Ductal) and Civaux power plants (with BSIbeams) in France The extremely aggressive environment of the coolingtowers induced corrosion of the steel structures. UHPC with its out-standing qualities in terms of durability allows to replace steel beamswith light elements with very long lifetimes without maintenance orrepair. At the end of year 3 years, the AFGC-SETRA working groupon UHPCs visited the cooling tower at the Cattenom power plant.Under a normal layer of sediment, no damage of UHPC was noticed(Fig. 4.9).

Anchor Plates(Resplendino, 2004]: UHPC anchor plates wereused for a post-tensioned soil anchor retaining wall system. 6,300anchor plates with polymer fibers and 200 plates with steel fiberswere used on the sea-front on La Runion island. This solution withUHPC was chosen for its durability performances and the anchorplates closely matched the existing concrete retaining wall sectionsand replaces the traditional steel anchor plate/ concrete bearing padsystem.


Non-metallic anchorage for prestressing (Ehab Shaheen andShrive, 2006] : A new nonmetallic anchorage system consisting ofUHPC and suitable for CFRP post tensioning tendons has been devel-oped by Shaheen and Shrive [2006]. It consists of a CFRP-wrappedbarrel and four wedges. The anchors were tested for fatigue static capa-bilities. The new CFRRPC anchorage system will provide a completelymetal-free environment, with similar dimensions to the previouslydeveloped steel anchorage with 67% mass reduction.

Bond Durability (Ming Gin lee et al, 2007): The importantproperty of RPC controlling its utility as a repair material is the bondwith existing concrete substrate. Ming Lee et al [2007] evaluated thebond strength and bond durability of three materials RC, HSM andRPC wrt to old concrete. Accelerated test viz., namely the freeze-thaw cycle acceleration deterioration used as per ASTM C666 (1997),The specimens were subjected to 0, 300, 600, or 1000 freeze-thaw andevaluated before and after freeze-thaw cycling for their abrasion coeffi-cient, compressive strength, bond strength (slant shear test), steel pullout strength, and relative dynamic modulus. The study showed thatRPC displays excellent repair and retrofit potentials on compressiveand flexural strengthening (200 and 15% increase). The abrasion coef-ficient of RPC is about 8 times higher than that of normal strengthconcrete and RPC is much more durable under free-thaw tests thanHSM and RC. The strength and durability of bond of RPC to steelare much better.

Composite elements: Wuest[2006] investigated composite elementswith a reinforced concrete central core with two UHPFRC layerswith the objective to increase the load carrying capacity and toimprove Durability. The study showed that the UHPFRC layers pro-vide an increased stiffness under service conditions and the high tensilestrength of UHPFRC produces a significant increase in ultimate forceof composite elements as compared to conventional concrete elements.The composite elements structural behavior was not influenced byvarying the interface roughness.

Permeability of Cracked UHPC; Jean Charron and Brhwiler[2008] tested RPC for water and glycol under tensile loads in crackedcondition. The experimental results demonstrated that permeabilityand absorption increased steadily until a residual tensile deformationof 0.13% is reached and later then water seeping rises distinctly. The


test results revealed the high capability of the material to seal cracksand improve its water-tightness with time.

RPC Overlay: Katrin Habel et al [2007] investigated to assess therehabilitation potential of RPC for r existing concrete structures. 12full-sized flexural beams with UHPFRC layer in tension were tested.UHPFRC significantly improved the composite member structuralresponse, including the ultimate force, stiffness, and cracking behavior.Composite UHPFRC-RC elements behaved monolithically under ser-vice conditions. Interface cracks developed only once localized flexuralmacrocracks had propagated through the UHPFRC layer and inter-sected the interface zone near the ultimate load. The interface cracksdeveloped into localized debonding cracks for composite elementswithout reinforcing bars in the UHPFRC layer (NR beams). Interfacecracks remained sufficiently small and did not cause UHPFRC layerdebonding in elements with reinforcing bars in the UHPFRC layer; 3.Composite UHPFRC-RC element stiffness and resistance was furtherincreased when reinforcing bar was embedded in the UHPFRC layer.A 2 Vol.-% of reinforcing bars were embedded in the UHPFRC layerand increased the composite element’s apparent hardening magnitudeby three times and significantly delayed the formation of localizedmacrocracks. In the UHPFRC softening domain, the force transferthrough the reinforcing bar enhanced the composite element structuralresponse by preventing debonding.

4.6.1 Applications of ECC to Repair

ECC can find variety of applications. A number of investigationsinto the use of ECC in enhancing structural performance have beenconducted in recent years. These include the repair and retrofit ofpavements or bridge decks; the retrofit of building walls to withstandstrong seismic loading and the design of new framing systems Thesestudies often reveal unique characteristics of ECC and R/ECC (steelreinforced ECC) in a structural context. These include high damagetolerance, resistance to shear load, energy absorption, delaminationand spall resistance, and high deformability and tight crack widthcontrol for durability.Deck Slab in Michigan[Li et al, 2003]: A jointless bridge deck iscreated by the replacement of the expandable mechanical joint witha slab of deck material that is usually called a link slab. In 2001, theMichigan DOT and University of Michigan (MDOT) assessed the fea-sibility of implementing an ECC link slab. The Grove Street bridge


renovation project was selected as a demonstration site. ECC was useddirectly over the gap between the beams of two adjacent simple bridgedecks, in the location where an expansion joint would be installed. TheECC material was placed 5 percent of the span length into each adja-cent span. By removing the expansion joint and replacing a portion ofthe two adjacent decks with a section of ECC material over the joint,a continuous deck surface was constructed.Concrete Elements Subjected to Shear [Kanda et al, 1998]:Since shear failure often involves diagonal tensile cracks, it is expectedthat ECC structural members should exhibit improved ductility undershear. This was established by testing the Ohio beam configurationas shown Fig. 4.11. The pseudo strain-hardening behavior of ECCrevealed itself in the form of multiple diagonal cracks (Fig. 4.12) withsmall crack widths of less than 0.1 mm even up to ultimate load.In contrast, the FRC beam failed shortly after first crack load witha single crack opening as the crack width increased at continuouslysoftening load. It is clear from Fig. 4.12 that the ductility of the ECCbeam is extensive in both pre-peak and post-peak phases. Indeed Liet al showed that the ductility of this ECC beam is even better thana similar beam with conventional shear reinforcement in the form ofa welded steel wire fabric.Crack Width Control in RC Beam: Maalej and Li [2000] proposedreplacement for the concrete material that surrounds the main rein-forcement in a regular reinforced concrete member. With this designit was shown that crack widths under service load conditions can belimited to values that could never be achieved using conventional steelreinforcement and commonly used concrete and prevent the migra-tion of aggressive substances into the concrete or the reinforcement.Furthermore, accelerated corrosion due to longitudinal cracking orspalling will be reduced if not eliminated, and spalling and delam-ination problems common to many of today’s reinforced concretestructures will be prevented [Fig. 4.13].Energy Absorption in Plastic Hinge of Beam-Column Con-nection [Kesner et al, 2001]: The damage tolerance of a structureis the ability for the structure to sustain load-carrying capacity evenwhen overloaded into the inelastic range. In general, however, it maybe expected that the following properties of the concrete material inthe plastic hinge should be advantageous: (i) high compression straincapacity to avoid loss of integrity by crushing, (ii) low tensile firstcracking strength to initiate damage within the plastic hinge, (iii)


high shear and spall resistance to avoid integrity loss by diagonal frac-tures, and (iv) enhanced mechanisms that increases inelastic energydissipation. In a recent study, the use of a strain-hardening ECC toachieve these objectives instead of increased shear steel reinforcementwas investigated [Fig. 4.14].

The hysteretic behavior showed that for the PC hinge, the displace-ment ductility factor is about 4.8. For the ECC hinge, the displacementductility factor increases to 6.4, with less amount of pinching and amuch reduced rate of stiffness degradation. The damage is mostlyin the form of diagonal multiple cracking in perpendicular direction.Unlike the control specimen which fail in a predominantly shear diag-onal fracture, the ECC specimen fails by a vertical flexural crack atthe interface between ECC plastic hinge zone and the plain concreteat the column face.

Resistance to Delamination and Spalling in Repaired ConcreteStructures[Lim et al, 1997]: In patch repairs, the common failuremodes are spalling and/or delamination between the new and oldconcrete. In bridge deck or pavements overlay repairs, reflective crackand spalling in the concrete overlays and/or delamination between thebonded overlay and the old concrete substrate are often observed. Leefound that the delamination and spalling modes can be both elimi-nated by means of a kink-crack trapping process (Fig. 4.15) As theload increases, the initial interface crack extends slightly but quicklykinks into the ECC overlay. The kink crack was subsequently trappedin the ECC so that further load increase forces crack extension intothe interface. The kinking-trapping process then repeats itself, result-ing in a succession of kink cracks in the ECC. However, spalling of theECC was not observed since the kink crack does not propagate to thespecimen surface. Delamination of the interface was also eliminatedsince the interface crack tip repeatedly kink into the ECC. In contrast,the specimen with a regular FRC overlay shows the expected kink-spall brittle fracture behavior. Fig. 4.15 illustrates the improvementin load-deflection characteristics.

4.7 CONCLUDING REMARKS

Advanced cementitious composites, such as RPC, UHPFRC, CRC andECC are slowly gaining acceptance for many interesting applicationsand are likely to be strong candidate materials for infrastructure con-struction and repair in the years to come. Their outstanding propertiesin terms of strength, stiffness, ductility and durability have contributed


to their superior performance. Added to this, the optimum utilizationof resource materials provides a very attractive feature. CSIR-SERChas been working on the development and utility of ACCs over the lastfew years and the technology and for production is currently available.Although the materials are costly in the present context, the cost willcome down with increase in usage over the years.

4.8 REFERENCES

1. Acker P., and Behloul M., “Ductal Technology: a Large Spectrumof Properties, a Wide Range of Applications”, Proc. Int. Symp.on UHPC, Kassel, Germany, 2004, pp 11–25

2. Arnon Bentur and Sidney Mindess, “Fiber reinforced Cemeti-tious composites”, Modern concrete technology series, Taylor andFrancis, Oxon, 2007

3. Bache H. H., Introduction to Compact Reinforced Composite,Nordic concrete research, No.6, pp 19–33, 1997

4. Bickley J. A., and Mitchell D., (2001), “A state-of -the - ArtReview of High performance Concrete structures Built in Canada:1990-2000”, pp.96–102

5. Dauriac C., “Special Concrete may give steel stiff competition,Building with Concrete”, The Seattle Daily Journal of Commerce,May 9., 1997

6. Ehab Shaheen and Nigel G., Shrive, “Optimization of mechan-ical properties and durability of reactive powder concrete”, ACIMaterials Journal, Nov. - Dec 2006, pp. 444–451.

7. Fehling E., Bunje K., Schmidt M., Schreiber W., 2004a, “UltraHigh Performance Concrete Bridge across the River Fulda inKassel - Conceptual Design, ”, Design Calculations and Invi-tation to Tender” Proceedings of the International Symposiumon Ultra High Performance Concrete, Kassel University Press,Kassel, Germany, pp 69–76

8. Fehling E., and Bunje K., Leutbecher T., 2004b, “Design rele-vant Properties of hardened Ultra High Performance Concrete,”Proceedings of the International Symposium on Ultra High Per-formance Concrete, Kassel University Press, Kassel, Germany, pp327–338.

9. Guerrini G. L., “Applications of High-Performance Fiber-Reinforced Cement-Based Composites”, Naaman A. E., Rein-hardt H. W., ” Proposed classification of HPFRC composites


based on theirtensile response”, Materials and Structures 39,2006pp. 547–555

10. Jacques Resplendino, “Ultra-High-Performance Concrete : FirstRecommendations and Examples of Application”, Proceedings ofthe International Symposium on Ultra High Performance Con-crete, Kassel University Press, Kassel, Germany, 2004, Part 2pp79–89

11. Jean-Philippe Charron, Emmanuel Denari, Eugen Brhwil,“Transport properties of water and glycol in an ultra highperformance fiber reinforced concrete (UHPFRC) under high ten-sile deformation”, Cement and Concrete Research 38 2008 pp689-698

12. John Wuest, “Structural behaviour of reinforced concrete Ele-ments improved by layers of ultra high Performance reinforcedconcrete”, 6th international phd symposium in civil engineering,Zurich, August 23-26, 2006, pp 1–8

13. Katrin Habel, Emmanuel Denari, and Eugen Brhwiler, ’Exper-imental Investigation of Composite Ultra-High-PerformanceFiber-Reinforced Concrete and Conventional Concrete Members’,ACI Structural Journal/January-February 2007, pp 93–101

14. Kanda T., Watanabe S., and Li V. C., “Application of PseudoStrain Hardening Cementitious Composites to Shear ResistantStructural Elements”, in Fracture Mechanics of Concrete Struc-tures Proc. FRAMCOS-3, AEDIFICATIO Publishers, D-79104Freiburg, Germany, 1998 pp 1477–1490,.

15. Kendall K., Howard A. J., Birchall J. D., The relation betweenporosity, microstructure and strength, and the approach toadvanced cement-based materials, Philosophical Transactions ofthe Royal Society of London, A 310, London, England, 1983, pp139–153.

16. Kesner K. E., and Billington S. L., “Investigation of Duc-tile Cement-Based Composites for Seismic Strengthening andRetrofit,” in Fracture Mechanics of Concrete Structures, de Bostet al (eds), A.A. Balkema, Netherlands, 2001, pp 65–72,

17. Ming-Gin Lee, Yung-Chih Wang and Chui-Te Chiu, “A prelimi-nary study of reactive powder concrete as a new repair material”,Construction and Building Materials 21 2007 pp 182–189

18. Li V. C., “Engineered Cementitious Composites - Tailored Com-posites Through Micromechanical Modeling,” in Fiber ReinforcedConcrete: Present and the Future. Eds. N. Banthia et al, CSCE,Montreal, 1998, pp 64–97, .


19. Li V. C., Kong H. J., and Chan Y. W., “Development ofSelf-Compacting Engineered Cementitious Composites,” in Pro-ceedings, International Workshop on Self-Compacting Concrete,Kochi, Japan, 1998 pp 46–59, .

20. Li V. C., Fischer G., Kim Y., Lepech M., Qian S., WeimannM., and Wang S., Durable Link Slabs for Jointless Bridge DecksBased on Strain-Hardening Cementitious Composites, Universityof Michigan, Ann Arbor, Michigan, 2003.

21. Lim Y. M., and Li V. C., “Durable Repair of Aged InfrastructuresUsing Trapping Mechanism of Engineered Cementitious Com-posites” J. Cement and Concrete Composites, 19(4) 1997 pp373–385, .

22. Maalej M., and Li V. C., “Introduction of Strain HardeningEngineered Cementitious Composites in the Design of Rein-forced Concrete Flexural Members for Improved Durability,” ACIStructural J., 92(2), 1995. 2000. pp 167–176,

23. Parra-Montesinos G. J., and Wight J. K., “Seismic Responseof Exterior RC Column-to-Steel Beam Connections,” ASCE J.Structural Engineering, pp 1113–1121

24. Richard P., and Cheyrezy M., “Composition of Reactive PowderConcretes”, Cement and Concrete Research, Vol.25, No.7, 1995,1995 pp 1501–1511.

25. Rossi P., High Performance Multimodal Fiber Reinforced CementComposites (HPMFRCC)The LCPC Experience, ACI MaterialsJournal, Vol. 94, No. 6, November - December, 1997, pp 478–483.

26. Roux N., Andrade C., Sanjuan M. A., Experimental Studyof Durability of Reactive PowderConcretes, ASCE Journal ofMaterials in Civil Engineering, Vol. 8, No. 1, February, 1996,pp 1–6


Table 4.1 Comparison of important properties of ACCs

Material Young’s Tensile Fracture Ductilitymodulus E strength, σt energy, EG σ2

t

MPa MPa GF,N/m (mm)F

Cement 7000 4 20 10

DSP paste 15000 20 20(*) 0.8

Concrete 30000 3 60 200

DSP mortar 50000 20 100 12.5

DSP mortar 60000 40 16000 600+6% vol. of fiber

CRC 100000 10 1.2 × 106 8300

RPC 50000 20 1200 150-2000

Table 4.2 Durability of RPC compared to HPC [Dauriac, 1997]

Abrasive wear 2.5 times lowerWater absorption 7 times lowerRate of corrosion 8 times lower

Chloride ions diffusion 25 times lower

Table 4.3 Durability comparison: HPC (80MPa) and RPC200[Bickley and Mitchell, 2001]

Property HPC (80MPa) RPC200Freeze - thaw, 90 RDF** 100RDF**ASTM C666ASalt scaling 80 g /cm2 < 10 g/cm2

Carbonation Depth: 36 2 mm 0 mmdays in CO2

Abrasion 275 *10-12 m2/s 1.2*10-12 m2/s


Table 4.4 Comparison of different types of FRCCs

Design N.A. Use high Vf MicromechanicsMethodology based,

minimize Vf forcost and

processibility

Fiber Any type, Mostly Vf Tailored,Vf usually steel, usually >

polymer fibers, Vf

less than 2%; 5%; df < 50μmdf for df 150μm

df < 50μmsteel 500 μm

Matrix Coarse Fine Controlledaggregates aggregates

for matrixtoughness, flaw

size; finesand

Interface Not controlled Not Chemical andcontrolled frictional

bonds controlled forbridging properties

Mechanical Strain Strain Strain

Properties -softening: -hardening: hardening:

Tensile 0.1% ¡1.5% >3% (typical);strain 8% max

Crack Unlimited Typically Typicallyseveral < 100

width hundredmicrometers, micrometers

unlimited during strain-beyond 1.5% hardening

strain

Table 4.5 Material mix proportions of ECC

Materials Cement SF SP w/c Aggregates,FA/CA

ECC 1 0.10-020 0.01-0.03 0.30-0.32 -FRC 1 - - 0.45 1.73/1.73


Fig. 4.1 Mechanism of DSP

Fig. 4.2 Principle of UHPFRC

Fig. 4.3 Evolution of of ACCs


Fig. 4.4 Classification of cement Composites

Fig. 4.5 Lancaster Intensive Mixe

Fig. 4.6 Eirich Intemsive Mixer with Planetary Action


Fig. 4.7 Tensile Stress-strain Behaviour of ECC and FRC

20 mm (a)

(c) (d)

(b)

Fig. 4.8 Damage Evolution in ECC Uniaxial Tensile Specimens at


Fig. 4.9 UHPC Beams in Cattenom Power Plant

Fig. 4.10 UHPC Anchor Plates


Fig. 4.11 Ohio Shear Critical Beam Application of UHPC

Fig. 4.12 Load deflection behaviour and crack pattern


φ = 5

φ = 10

102

114

127

152

152

305305305

20

15

10

5

00 0.1 0.20.05 0.15 0.25

Unit = mm

Mo

men

t (kN

/m)

1.6

1.2

0.8

0.4

0C

rack

wid

th (m

m)

Control RC BeamsRC Beams with ECC layer

16

16

13

25

Fig. 4.13 RC Beam with ECC Cover and Load-deflectionBehaviour

Fig. 4.14 ECC Hinge at Beam -column Joints


Fig. 4.15 Illustration of Performance Characteristics of ECCOverlay

5 Polymer Concrete Composites for Repair and

Rehabilitation of Concrete

Meyappan Neelamegam,Scientist-G

CSIR-SERC, CSIR Campus, Taramani, Chennai-600 113, India.Email: [email protected]

5.1 INTRODUCTION

In a tropical country like India that has more than 3000 KM of coastalline and where approximately 80% of the annual rainfall takes placein the two monsoon months, corrosion related problems are alarming.In metro cities, the carbon and nitrogen oxide emissions aggravatethe situation further by neutralizing the concrete cover. Typically, aR.C. Structures require major restoration work within 15 years of itsconstruction. With the ageing of nation’s infrastructures, many of theexisting concrete structures have outlasted their useful life and it israther dangerous to continue to use them without any strengthening,keeping in view the present day requirements. In recent years, theconcrete construction industry has faced a very significant challengein view of the rapid rate of deterioration of infrastructure. One of themajor reasons is that infrastructure is required in such severe exposurecondition where construction activity was not even imaginable earlier.A large number of bridges, buildings and other structural elementsrequire repair, rehabilitation and retrofitting. Effect of environment,increase in both traffic volume and truck weights and re-design andstrengthening of old structures, which may have been adequate as perold codes of practice but are not structurally adequate as per thecurrent codes of practice, are all the factors that contribute to theinfrastructure becoming either structurally deficient or functionallyobsolete. Because of the dwindling of resources and serious economiccrunch faced by the construction industry, abandoning of existingstructures/ or replacement by a new construction fulfilling the presentneeds, does not seems to be an economical agenda. Hence, the current




trend all over the world seems to be to rehabilitate a existing structuresrather than building a new one.

There is currently a range of techniques available for extend-ing the useful life of structurally deficient and functionally obsoletestructures. One such technique is adding fibre reinforced polymercomposites (FRPC’s) as external reinforcement in conjunction withconcrete-polymer composites as repair materials. Since 1970‘s researchand development work on concrete polymer composites have beencarried out in many research centres, academic institutions and pri-vate organizations in India. Considerable work on concrete polymercomposites has also been carried out by Council of Scientific andIndustrial Research (CSIR) laboratories in India, especially, at theStructural Engineering Research Centre (SERC), Chennai, CentralBuilding Research Institute (CBRI), Roorkee, Central Road ResearchInstitute (CRRI), New Delhi and laboratories at Bhopal, Jorhat andThiruvanthapuram in India. This paper briefly presents studies onthe use of fibre reinforced polymer composites for repair, rehabilita-tion and retrofitting of reinforced concrete structural elements. At theLaboratories of CSIR, India, Indian Institutes of Technologies (IIT’s),Anna University, Annamalai University, etc.

5.1.1 Repair Methodology

A basic understanding of the causes of concrete deficiency is essentialto perform meaningful evaluations and successful repair. If the causesof deficiency is understood, it is much more likely that an appropriaterepair system will be selected and, consequently, that the repair will besuccessful and the maximum life of the repair will be obtained. Symp-toms or observations of a deficiency should be differentiated from theactual cause of the deficiency, and it is imperative that causes and notsymptoms be dealt with wherever possible or practical. For example,cracking can be symptom of distress that may have variety of causessuch as, drying shrinkage, thermal cycling, accidental over-loading,corrosion of embedded metal or inadequate design or construction.Only after the cause or causes of deficiency are determined can rationaldecisions be made regarding the selection of a proper repair materialsystem and implementation of the repair process.

5.2 SELECTION OF REPAIR MATERIALS

The selection of repair materials is a predictive effort to maximizefuture performance or durability. Therefore, selection must be based

Polymer Concrete Composites for Repair and Rehabilitation of Concrete 61

on the knowledge of the physical and chemical properties, the functionthe designers plans to impose on them, and the nature of the environ-ment in which they will be placed. Also, in choosing a material, thedesigner must be aware that it will posses same properties other thanthose required for the basic function. Frequently, these will have agreater influence on its durability in service than the properties thatdictated its choice. Consequently, all the properties of material mustbe considered in the light of both function of requirements and theeffects of the microenvironments.

Durable repairs can be obtained only by matching the propertiesof the base concrete with those of the repair material indented for use.Therefore, the selection of appropriate material is imperative for thepurpose. Some of the material properties that should be consideredwhen selecting a repair material include:

• Dimensional stability

• Effective adhesion with parent concrete

• Development of positive grip with rebars

• Coefficient of thermal expansion

• Modulus of elasticity

• Permeability

• Chemical compatibility

• Electrical properties

• Fast gain in strength

• Durability even under adverse atmosphere conditions

• Easy of application

In addition to the material properties, the choice of the rightproduct also depends on the anticipated service conditions and theprevailing conditions at the time of application of the products.

5.3 POLYMERS

All matter in this world is composed of extremely small units calledmolecules. They are too small to be seen even under the most powerfulmicroscope and are a complex association of atoms. Molecules come indifferent sizes and shapes. Molecules of plastics are much larger thanthe ordinary molecules. They are giant molecules in the form of longchain which are called polymers. ‘Poly‘ means many and ‘meros‘ meansparts. Thus polymer means ‘composed of many parts or many units‘.Each polymer chain is made up of thousands of smaller molecules like


a string of glass beads. The small parts or beads in the string are calledmonomers (mono means single). They are the building blocks of thepolymer chain. These monomers are organic molecules consisting ofcarbon atoms as their base with the atoms of some other elementslike hydrogen, oxygen, chlorine or sulphur sticking to them. All themonomers in a polymer chain are identical but the monomers of twodifferent polymers differ in their chemical composition.

5.3.1 Types of Polymers

The distinction between types of polymers is based on their reactionto heating and cooling.

5.3.2 Thermoplastic polymers

Thermoplastic polymer soften upon heating, and can be made to flowwhen a stress is applied. When cooled again, they reversibly regaintheir solid or rubbery nature. Continued heating of thermoplasticswill lead ultimately to degradation, but they will generally soften attemperatures below their degradation points.

5.3.3 Thermosetting polymers

Thermosetting polymers are materials which can be heated to thepoint where they would soften and made to flow under stress. How-ever, they do not revert to the original solid state as the heatingcauses the material to undergo a curing reaction. Often, these poly-mers emerge from their synthesis reaction in a cured state. Furtherheating ultimately leads only to degradation and not softening andflow.

5.3.4 Applications of Polymers

In building construction the application of polymers can be classifiedin various ways, for example:

• Nonstructural polymers

• Structural and semi-structural polymers

• Auxiliaries to other materials

The first group constitutes, by far, the greatest volume andnumber of different uses. The second group include patch repair,overlays, linings to concrete/ steel products, injection to structuralcracks, strengthening of structural elements, etc. Auxiliaries includeadhesives, bonding agents, sealants, and decorative and protectivecoatings.


5.4 TYPES OF CONCRETE POLYMER COMPOSITES

Depending on the manner in the polymeric materials are incorporated,concrete polymer composites can be classified under the followingthree major types:

• Polymer Modified Cement Composites (PMCC): InPMCCs, polymeric materials are incorporated into cement com-posites (cement concrete or cement mortar)during the mixingstage. The composite is then cast to the required shape in theconventional manner and is cured in a manner similar to the cur-ing of cement concrete. The hydrated cement and the polymerfilm, formed due to the curing of the polymeric material, form aninter penetrating network that binds the aggregates .

• Resin Concrete (RC) also called polymer Concrete (PC):In these, polymers are used as the binders of the aggregates, inlieu of the cement water binder system adopted in cement com-posites. Monomers or pre polymers are mixed with the aggregatesand the mix is cast to the required shape or form. The mix is thenpolymerized either at the room temperature. The polymer phasebinds the aggregates to give a strong composite.

• Polymer Impregnated Concrete (PIC): In PIC, monomersor pre polymers are impregnated into the pore system of hard-ened cement composites and are then polymerized. A very strongcomposite viz., PIC, results, in which cured polymer fills almostall the pores.

5.5 POLYMER MODIFIED CEMENT REPAIR MATERIALS

5.5.1 Concrete Crack Repair Systems

The success of many crack repair applications depends on repair mate-rials that have significantly different properties, such as high elasticityand low modulus of elasticity, from that of substrate, and that willperform better than the base concrete in the service environment. Ingeneral, slight concrete cracks due to drying shrinkage, heat of hydra-tion or poor placing joints in concrete structures are repaired by thefollowing three methods:

• Coating or lining using polymer modified pastes over concretecracks with widths of 0.20mm or less.

• Injection using polymer-modified pastes into concrete cracks withwidths of 0.20 to 1.00mm.


• Grouting polymer modified mortars into concrete cracks withwidths of 1.00mm or more. The polymer-modified pastes andmortars such as styrerne butadiene rubber (SBR) latex, poly-acrylic easter (PAE) emulsion and poly(ethylene-vinyl acetate)(EVA)- modified pastes and redispersible polymer powderssuch as poly(vinyl acetate-vinyl-versatate-acrylic ester) (VA/VeoVa/AE), poly(ethylene-vinyl acetate) (EVA) and poly(vinylacetate-vinyl versatate) (VA/Veo Va) powders are used for suchconcrete crack repair systems.

5.5.2 Polymer-Cement Grout

Polymer-cement grout is a mixture consisting of primarily of cement,fine aggregate, water and a polymer such as acrylic, styrene-acrylic,styrene-butadiene, or a water-borne epoxy. The consistency of thismaterial may vary from a stiff material suitable for hand-packinglarge cracks on overhead, and vertical surfaces to a pourable consis-tency suitable for gravity feeding cracks in horizontal slabs. Typicalproperties of polymer cement grouts are presented in Table 5.1.Polymer-Modified Mortar/Concrete for Patch Repair Sys-tems

5.5.3 Polymer Modified Concrete (PMC)

PMC has been of considerable interest to engineers because of itssimilarity in process technology to conventional concrete. Most of themonomers used successfully with PIC and PC have not worked wellwhen added to fresh concrete. However, polymer latexes have beenused very successfully to make latex modified concretes(LMC) andmortars (LMM). Polymer latexes are usually copolymer systems suchas vinyl acetate, vinyl chloride, and butadiene, besides elastomericsystems like acrylonitrile butadiene (NBR), neoprene, and styrenebutadiene(SBR). Polymer cement ratio is generally 6 20% by weight.Epoxies are also available which can be added to fresh concrete toimprove properties of hardened concrete. Table 5.2 gives the typicalproperties of polymer modified mortar and ordinary cement mortar.

PMC and PMM are increasingly used for rehabilitation becausethey are cement based and therefore, give homogeneity to the sys-tem and the repair materials, and are more compatible with concretecompared to all other PC composites. The simple process technologyand low cost due to comparatively lower polymer content are addedattractions. Further, the alkaline nature of the repair material restoresthe alkalinity of deteriorated concrete and arrests further corrosion of


rebars. After patch repairing with PMC or PMM, it is a common prac-tice to coat the entire repaired surface with a protective coating usingelastomeric membrane forming materials like.

Acrylics in order to arrest the diffusion of harmful CO2 and Cl,while at the same time, permitting escape of moisture, and thusenabling the repaired structure to breathe.

Nowadays, PMC used for repair works generally consists of one dryand one or two liquid components. The dry component is a readymixed mortar containing cement, gravel, and additives like redis-persible polymer powder, shrinkage compensators, etc. The liquidmay be pure water or water mixed with acrylic or epoxy emulsion.An advantage of solvent free PMC/PMM is the ease of adjustingthe working rhythm as against the pot life and film forming resis-tance of polymer solutions. They are economical while maintainingthe technical value .

Several case histories on the use of PMC/PMM for the repair ofbuildings and bridges have been documented. It has been estimatedthat about 60000 m3 of SBR based LMC is used in US every year fornew as well as old construction. Fig. 5.1 shows the typical applica-tions methods for repair materials for deteriorated reinforced concretestructures.

5.5.4 Crack Repair Resin Materials

EpoxiesCrack width less than .05mm are generally not treated or consideredtreatable. Very thin cracks may seal themselves due to autogeneoushealing, which occurs when the cement continues to hydrate and car-bonates, forming calcium carbonate and calcium hydroxide crystalsthat can seal the cracks. Epoxies are used to repair cracks rangingfrom 0.05 to 6.00mm in width. The most common method of appli-cation in the range of 0.05 to 0.12mm is pressure injection methodinto the cracks. Epoxy resins are the most common materials used inpressure injection to repair cracks in this width range. Cracks in hori-zontal slabs that are between 0.01 and 6.00mm. may filled by groutingor ponding the epoxy over the crack. The depth of penetration is deter-mined by the viscosity, pot life and surface tension of the epoxy resin.The standards classifies into seven different types of epoxies dependsupon the applications. Typical properties of epoxy resins are given inTable 5.3.


5.5.5 High-Molecular Weight Methacrylate (HMWM)

HMWM is an ester of methacrylic acid that contains carbon atomsseparated by double bonds through which the material polymerizes toa solid. High molecular weight is a term used to differentiate methary-lates by high and low volatile content and flash point; this molecularweight has been arbitrarily chosen as 150. High adhesive strengthsmake these materials suitable for structural repairs. Low viscosity(25cP and less) and a more forgiving mixing ratio than epoxies makethese materials easy to mix. HMWMs are available in many mod-uli and reaction rates, makes them versatile materials appropriate formany application requirements. HMWMs are typically used as a struc-tural bonding, waterproofing repair or both, for cracks 0.12mm andgreater in width. Because of their low viscosities, HMWMs are oftenused on horizontal surfaces to flood the surface and fill the cracks withthe adhesive. Table 5.4 gives the Typical Properties of HMWM.Polyurethane Chemical Grout: Polyurethane chemical grouts arewidely used to repair cracks that are both wet and active, or thatare leaking a significant amount of water. These grouts are semi-flexible; thus, they may tolerate some change in crack width. Thereaction time to form the foam may be controlled from a few sec-onds upto several minutes using different catalyst and additives. Thesegrouts penetrates effectively, and the technique of chemical groutingis a water-proven method of repairing cracks. Polyurethane chemicalgrouts may be used to treat cracks that are 0.12mm and greater inwidth. These materials are pressure injected at the high pressure. Incontrast to epoxy resins that are suitable for dormant, dry or dampcracks, polyurethane chemical grouts are suitable for injection of ver-tical, overhead, and horizontal cracks that are active or leaking. Thesecharacteristics make them particularly suited for vertical, overheadand horizontal applications, and it is their ability to stop active leaks,that makes them particularly well suited for tanks for the storageof liquids, dams, tunnels, sewers and other water-containment struc-tures. Typical properties of polyurethane chemical growth are given inTable 5.5.

5.5.6 Silicone sealants

Silicone sealants are based on polymers where the polymer backbone consists of alternating silicon and oxygen atoms with carbon-containing side groups. They have different curing mechanisms,depending on the end group of the polymer. Typically, fumed silica,plasticizers, calcium carbonate fillers, and silanes for adhesion. Sealant


performance life typically is 3 to 10 years. Silicon sealants are gener-ally used to seal cracks that are from 2.5 to 50mm in width.Typicalproperties of silicon sealant are shown in Table. 5.6a and 5.6b

5.5.7 Polymer Grout

Polymer grout is a mixture where the polymer, such as an epoxy resin,serves as the binder, and where sand, usually an oven-dried silica witha grading from 0.8 to 0.4mm is the filler. The consistency of this mate-rial may vary from a stiff material suitable for hand-packing largecracks on overhead and vertical surfaces to a pour able consistencysuitable for gravity feeding cracks in horizontal slabs. Polymer groutsbond extremely well to concrete and have low shrinkage, resultingin a liquid tight repair in dormant cracks. Similar to epoxy resins,polymer grouts are suitable cracks requiring structural repairs. Mate-rials of varying consistencies are readily available to repair cracks invertical, overhead, and horizontal applications. Some polymer grouts,depending on the binder used, are moisture tolerant, and will cure inthe presence of moisture. While a few polymer grouts may effectivelybond to concrete with some moisture present in the concrete pores,moist polymer grouts marketed in the engineering and constructioncommunity will not bond to the concrete in the presence of moisture.The chemical resistance of polymer grout is generally much betterthan the substrate concrete. Finally, these materials may be designedfor a fast cure to minimize the downtime because of repairs.

5.5.8 Polymer Concrete(PC)

Polymer Concrete Patching Materials: PC can provide a fast curing,high-strength patch material suitable for use in the repair of Portlandcement concrete structures. Many PC patching materials are primarilydesigned for the repair of highway structures where traffic conditionsallow closing of a repair area for only a few hours. PCs are not lim-ited to that usage: however, and can be formulated for a wide varietyof application needs. For any patching, the following aspects of therepair should be given consideration by the user; a) evaluating thesurface to be repaired, b) preparing the surface, c) materials selection,d) PC formulations, e) placement techniques, f)cleanup of tools andequipments, and g) safety.

Initial use of PC was almost exclusively for repair of ordinary PCC.The excellent bond of PC to concrete and very rapid cure time(30-90minutes) make PC an ideal repair material, especially in urban areaswhere fast, permanent repairs are essential. Added to these advantages


are the possibility of tailoring its properties to suit any particularsituation, excellent chemical resistance, and high bond strength. Themost common types of monomers used to produce PC are methylmethacrylate (MMA), unsaturated polyester(UP) resin, and epoxies,besides furan, urethane, furfuryl alcohol, and vinyl ester which are alsooccasionally used.

In carrying out PC repairs, it is recommended that all unsoundconcrete be removed and all surfaces to which PC will bond to becleaned, preferably by sand/steel shot blasting, and dried. Corrosivescale should be removed from reinforcing steel. The monomer systemis added just prior to the mixing and placing of PC. The PC repaircan be carried out in two ways: (i) Dry pack system in which theaggregates are prepacked and vibrated into the crater location andthen infiltrated by a low viscosity monomer like MMA. The repaircan then be finished and levelled by a more viscous monomer system;(ii) Premixed PC in which the aggregates and monomer are mixedtogether in a wheel barrow or a conventional concrete mixer and thendirectly applied to the surface and levelled. The dry pack system,although simple in principle, results in segregation of aggregates and incase of wet aggregates or sudden rains , the initial moisture may affectproper coating of aggregates. The premixed system on the other handresults in a more cohesive and uniform mix, and is more popularlyused in practice. Many repairs have been carried out in a number ofbridges, pavements, foundations, and hydraulic structures using PC,generally with excellent results. A typical resin mortar mix consists of1 part of resin and hardener to 3 parts of sand. The aggregates arepredried and may be graded to impart unique surface properties.

Polyesters require accurate control of proportions and mixing. Theycure faster than epoxies and less sensitive to lower temperatures. How-ever they shrink more and at a faster rate and therefore can be appliedin very thin sections only.

Vinyl esters combine resiliency, impact resistance, and excellentchemical resistance. They are generally used for severe climatic envi-ronments encountered in the paper and pulp, food, and beverage, andchemical industries. Typical applications areas are floors, trenches,and pickling and plating tanks.

The most widely used patching materials are based on acrylicmonomers. Two types of monomers are used: methyl methacry-late(MMA), which has been used for over three decades and highmolecular weight methacrylates(HMWMA), a relatively new material.Because of the disagreeable odour and high inflammability, there has


been a general reluctance in adopting MMA based PCs. The devel-opment of HMWMA seems to have solved this problem to a greatextent. They have low viscosities and can be poured or sprayed ontoconcrete and brushed on concrete surfaces. They are especially suit-able for sealing of narrow cracks because of their excellent wettabilityand can fill cracks as narrow as 0.2mm in width. They are odourless,possess higher flash point (> 100◦C), higher solvent resistance, andare non toxic. They can be cured by ultra violet radiation in 2 5 hourseven at low temperatures.

5.5.9 Polymer Impregnated Concrete (PIC)

PIC was developed in the 1950s and received wide publicity in the 60sand 70s. However, full depth PIC never became a commercial realityin US, although partial depth PIC (PD PIC) was used for providingdurability to floors, bridge decks, and hydraulic structures in 1970s.When it was discovered that some bridges had developed high chlo-ride contents beneath the impregnated zone, apparently due to crackscaused by the high temperature required for drying and/or polymer-ization, the wide scale applications of PD PIC also received a set back.Besides this, the complicated process technology for impregnation cre-ates an undesirable balance between their performance and cost forvarious practical applications. However, interest in this technique hasnot completely subsided and quite a few applications continue to bereported in the recent literature.

Recently, concrete sealing compounds like alkoxy silanes, alkoxysiloxanes, and metallic stearates have entered the market with claimsof providing surface protection like surface impregnation. However,they do not provide the same extent of surface penetration and abra-sion resistance, and their long term durability and performance aresuspect, due to possibility of removal from surface due to shallow depthof penetration.

The process technology of PIC, particularly for insitu applications,needs further improvement to make it economically viable in orderthat a process of rethinking may occur with regard to its large scalecommercial applications.

5.6 APPLICATION AREAS OF FFMC

In several important industrial installations, often damaged or dis-tressed reinforced concrete structural components may have to bereplaced or encased within shortest possible time. On account of high


early strength of these “free flow concrete”, such a replacement orencasement is feasible without any risk. As mentioned above, it iscompatible with conventional concrete. It has excellent adhesive prop-erties and develops positive grip with reinforcements or embedment.The restraint is that it has to be restrained from expansion for at least48 hours to get the optimum results. In one of the paper plants, atBadravathi, in Karnataka, corrosion affected areas in reinforced con-crete gables had to be restored without affecting the running of theplant below. The job could be done effectively with this material.

5.7 INTELLIGENT REPAIR MATERIALS

Polymer-modified mortar with nitrite-type hydrocalumite:Nitrite-type hydrocalumite [3CaO.Al2O3.Ca(NO2)2.nH2O(n =11-12)]is a corrosion-inhibiting admixture or anti-corrosive admixtureswhich can observe the chloride ions (Cl−) inhibiting the corro-sion as expressed by the following formula, 3CaO.Al2O3.nH2O+2Cl+ 3CaO.Al2O3.CaCl2.nH2O+2NO2 and provides excellent corrosion-inhibiting property to the reinforcing bars in reinforced concrete.Polymer-modified mortars using polymer dispersions and redispersiblepolymer powders with the nitrite-type hydrocalumite (calumite) havesuperior corrosion-inhibiting property and durability, and attractnotice as effective repair materials for deteriorated reinforced concretestructures. A calumite content of around 5-10% is recommended tomake effective repair mate4r5ials for deteriorated reinforced concretestructures.

5.8 HARDENER-FREE EPOXY-MODIFIED MORTARS WITH

AUTOHEALING OR SELF-REPAIRING FUNCTION

Ohama et. al. developed a hardener-free epoxy resin-hydraulic cementsystem with a new concept in the early 1990s. In this system, hardener-free epoxy resin can harden in the presence of alkali or hydroxideions produced by the hydration cement, the unhardened epoxy resinphase may be sealed with the hardened epoxy resin forms self-capsuledepoxy resin phase has an autohealing or self-repairing functions formicrocracks is shown in Fig. 5.2.

There have been many recent developments in the production ofmore durable concrete. Self-healing of concrete provides a valid andpractical solution to the problems. Even a combined model of auto-genic and autonomic principle may be incorporated for better solution.


These systems will be highly applicable in the remote and physicallyunreachable portion of the structure, where direct repair is not possi-ble from outside. The self-healing technology may be also used in thatportion of concrete structure where the reinforcing bars are in dangerof corrosion. The chemical in the microcapsule, micro tube or sporesshould be then a corrosion inhibitor that will delay the corrosion byreleasing corrosion inhibiting chemicals. There by the life span of thereinforced concrete structure will be substantially increased.

5.9 FIBRE REINFORCED POLYMER COMPOSITES

Fibre reinforced polymer (FRP) is a composite material generally con-sisting of carbon, aramid or glass fibres in a polymeric matrix. FRPcomposites are, as the name suggests, a composition of two or morematerials which, when properly combined, from a different materialwith properties not available from the ingredients alone. Dependingon the ingredients chosen and the method of combining them, a largevariety of properties can be achieved. A brittle material can be mademore ductile by adding a softer material; conversely a soft materialcan be made stiffer. Fig. 5.3 shows the typical application procedure.


The selection of appropriate types of polymers and concrete polymercomposites is one of the most important steps in their applications,such as, new construction, specific products and repair and rehabili-tation works. The civil engineer is confronted with an infinite numberof proprietary materials and products available in the market and isliable to err on this count. Commercial literature speaks abundantlyabout the advantages of the materials and products but is highly defi-cient regarding necessary technical data and suitability for specificapplications. Concrete polymer composites are being used extensivelyin India for repair of damaged RC structural elements. With the fastgrowing knowledge about the advantages of other applications, suchas, PC floorings and overlays and specific products, such as, floor tiles,insulators, etc., the usage is expected to steadily increased.

5.11 ACKNOWLEDGEMENT

This paper is published with the kind permission of the Director,SERC, Chennai. The authors sincerely thank their colleagues and


technical staff in the Concrete Composites Laboratory of SERC fortheir help and encouragement.

5.12 REFERENCES

1. Neol Mailavaganam, “Repair and Production of Concrete Struc-tures”, CRC Press, 1991.

2. Dorel Feldman,“Polymeric Building Materials”, Elsevier AppliedScience, London-New York, 575 pp.

3. Satish Chandra and Yoshihiko Ohama, “Polymers in Concrete”,CRC Press, 1994.

4. Yoshihoko Ohama, “Hand Book of Polymer-Modified Concreteand Mortars, Properties and Process Technology”, Noyes Publi-cations, 236 pp.

5. Rajamane N. P., Neelamegam M., Peter J. A., Dattatreya J. Kand Gopalakrishnan S., “Development and Applications of Natu-ral Rubber Latex Modified Concretes”, Internal Technical Report,No. MLP 06641/1/97, SERC, March 1997.

6. Bentur A., “Properties of Polymer Latex-Cement Steel FibreComposites”, International Journal of Cement Composite andLightweight Concrete, Vol. 3, No. 4, 1981, pp 283–289.

7. Viswanatha C. S.,“ Restoration Materials for ConcreteStructures- a recent Trend”, Proceedings of ICI-Asian Confer-ence on Escasy in Concrete (ICI-ACECON-2000), Nov. 2000,Bangalore, India, pp 393–400.

8. Oshiro T., Yamada Y., Tanigawa S. and Goto N., “Deteriorationof R.C Buildings Under Marine Environment”, Concrete UnderSevere Conditions and Loading, Vol. 1, E & FN Spon, pp 523–532.

9. Fowler D.W., “Status of Concrete-Polymer-Materials”, proceed-ing of the vi International Congress on Polymers in Concrete,Shangai, China, 1990, pp 10–27.

10. Shaw, J.D.N., “Concrete Decay: causes and Remedies”, proceed-ing of the Seminar on Corrosion and deterioration in Concrete,1991.

11. “New Millinium New Material, FRPs”, Concrete EngineeringInternational, Vol. 2, No. 8, Nov-Dec. 1998, pp 29–31.

12. Ir. Bart Herrelen, Triconsult N. V. and Ir.Kris Brosens, “CFRCRoof Repair” Concrete Engineering International, Vol. 2, No. 2,March 1998, pp 55–56.


13. Subrahmanyam B.V, Neelamegam .M, Rajamane N. P, JosephG.P, Pandian .N, Karim E.A And Rao E.U. Modular Light-beacon Tower of Polymer Impregnated Ferrocement, Journal ofFerrocement, Vol.16, No.3, July 1986, pp 263–271.

14. Neelamegam M., Parameswaran V. S. Durability of Glass FibreReinforced Polymer Composites, Proceeding of the IV RILEMInternational Symposium on Fibre Reinforced Concrete, Sheffield,UK., pp 802–821.

15. Neelamegam M., Dattatreya J. K., Parameswaran V. S. PC Com-posite Laminates for Strengthening RC Beams, Proceeding of theVIII International Congress on Polymers in Concrete, July 1995,pp 149–154.

16. Neelamegam M., Dattatreya J. K. Behaviour of Concrete Beamswith Externally Bonded Polymer Impregnated Highly ReinforcedFerrocement Plates, Proc., Second East Asia Pacific Symposiumon Polymers in Concrete, Nihon University, Koriyama, Japan,May 11–13, 1997, pp 493–502.

17. Neelamegam M., Dattatreya J. K ,Parameswaran V. S. PC Com-posite Laminates for Strengthening RC Beams, Proceeding of theVIII International Congress on Polymers in Concrete, July 1995,pp 149–154.


Table 5.1 Typical Properties of Polymer Cement Grout

Description Test Specimen ValuesMethod Age Typical Recommended

Value Value

Polymer -cementgrout

Bond strength (MPa) ASTM C 1042 28 Days 10 to 21 >10Direct tensile bond ACI 503 R 28 Days 0.69 to 2.1 >0.86

(MPa)Tensile strength (MPa) ASTMC496/C 496M 28 Days 2.1 to 6.9 >2.1Modulus of elasticity ASTM C 469 28 Days 6.9 to 38 -

(GPa)Thermal expansion ASTM C 531 28 Days 1.37 to 6.4 -

∗10−5◦CDrying shrinkage (%) ASTM C 596 28 Days 0.05 to 0.15 ¡0.1

Flexural strength ASTM C 293 28 Days 8.3 >3.4Compressive strength ASTMC109/C 109M 28 Days 28 to 85 >20.7

(MPa)

Table 5.2 Ordinary mortar and PAE mortar, SBR mortar physicsmechanics performance

S.No Physical & Mechanical Ordinary PAE SBR Remarkproperty mortar mortar mortar

1 Compressive strength 50.1 47.7 42.5(MPa)

2 Flexural strength (MPa) 8.8 10.4 9.5

3 Tensile strength (MPa) 3.5 4.6 4.9 Specimen size7.07 cm ×7.07 cm

× 7.07 cm4 Bonding strength (MPa) 1.4 3.4 4.2

5 Anti-permeability 9 2 2.6 Anti-permeability(mm) Height of water test machine

seepage, under constantpressure 1.5 MPa, 24 h

6 Frost-resistance Grad - F300 F300

7 Modulus in tension 2.56 2.29 2.19(x104 MPa)

8 Ultimate Tensile 220 318 306 Specimen size 10 cm ×Elongation (x10−6)

9 Dry-shrinkage 580 166 188 10 cm × 515 cmdeformation (x10−6)

10 Tear factor (×10−5) 5.2 38.5 36.4 -

11 Wear resistance (%) 5.47 3.95 1.65 -

12 Weight loss by 10.7 8.9 - Water blastingwater blasting (%) gun

13 Fast carbonation 3.6 0.8 - -depth (mm)

14 Penetration depth >20 1 - Immersionof Cl− (mm)

15 Water absorption 12 0.8 3.3 -rate (%)


Table 5.3 Typical properties of Epoxy Resin

Description Test Method Specimen Age ValuesTypical RecommendedValue Value

Slant shear ASTM 14 Days 6.9 to 21.0 >10bond (MPa) C 882

Tensile strength ASTM 7 Days 28 to 55 >35(MPa) D 638

Elongation at ASTM 7 Days 1 to 10 % 1 to 10break (%) D 638

Modulus of ASTM 14 Days 1.4 to 4.1 2.1 to 3.4elasticity (GPa) D 638

Deflection ASTM 7 Days 43 to 71 >49temperature (◦C ) D 648

Flexural ASTM 14 Days 35 TO 105 >6.9strength (MPa) D 790

Compressive ASTM 7 Days 35 TO 105 >21strength (MPa) D 695

Compressive ASTM 7 Days 0.52 to 3.4 >1modulus (GPa) D 695

Shear ASTM 14 Days 17 TO 70 >14Strength (MPa) D 732

Gel time ASTM - 5 minutes to >30 minutesC 881 3 hours

Water ASTM D 570 24 hours 0.25 to 1.5 <1absorption (%)

Coefficient linear ASTM - 0.002 to 0.01 <0.005shrinkage D 2566

Viscosity (cP) ASTM Immediately 50 to 2000 <1000D 2393

Table 5.4 Typical properties of High-Molecular WeightMethacrylate (HMWM)


Slant shear ASTM 14 Days 6.9 to 21 >10bond (MPa) C 882

Compressive ASTM 7 Days 21 to 70 >21strength (MPa) D 695

Viscosity (cP) ASTM Immediately 20 to 200 < 100D 2393

Gel time ASTM - 5 minutes to > 10 minutesC 881 1 hour


Table 5.5 Typical properties of Polyurethane chemical grout

Gel time ASTM C 881 - 5 minutes to 1 hour >10 minutes

Shear strength ASTM C273 - - - -Tensile strength ASTM D 1623 - - -

Elongation (%) ASTM D 1623 14 Days 25 to 400 >15

Shrinkage (%) ASTM D 2126 14 Days 0 to 10

Table 5.6a Typical properties of Silicon Sealant


Adhesion in peel ASTM 21 Days 2.3 to 11 >2.3(concrete) (Kg) C794

Tensile strength ASTM 21 Days 0.69 to 2.1 0.69 to 2.1(MPa) D 412

Elongation at ASTM 21 Days 400 to 1000 >400

break (%) D 412

Shore A ASTM 21 Days 5 to 15 5 to 15hardness (%) C 661

joint ASTM 21 Days 50 to 100 50 to 100movement (%) C719

Tack free ASTM - 1 to 2 <72(hours) C 679

Artificial weathering ASTM 21 Days 500 to 2000 >100and staining (hours) C510

Tear strength ASTM 21 Days 0.36 to 0.71 >0.89(Kg/mm) D 624

Table 5.6b Typical properties of Silicon Sealant


Slant shear ASTM 14 Days 6.9 to 21 > 10bond (MPa) C 882

Tensile strength ASTM 14 Days 3.4 to 10 >5.2(MPa) D 638

Modulus of ASTM 14 Days 1.4 to 6.9 1.4 to 6.9elasticity (GPa) D 638

Deflection ASTM 7 Days 43 to 71 > 49temperature (◦C) D 648

Flexural ASTM 14 Days 14 to 35 >6.9strength (MPa) D 790

Compressive ASTM 7 Days 21 to 85 >21strength (MPa) D 695

Compressive ASTM 7 Days 0.69 to 6.9 >1.0modulus (GPa) D 695

Shear ASTM 14 Days 14 to 35 >14strength (MPa)

Gel time ASTM Immediately 5 minutes to >30 minutesC 881 3 hours

Thermal expansion ASTM - 4.1 to 5.1 / ◦C Note 3expansion C 531 ∗10−5 / ◦C


Reinforcing Bar

Con

cret

e

Coating Material forFinishing and Protection

Coating Material forSurface Protection

Patch Material

Corrosion-InhibitingCoating Material

Impregnant

Grout for Cracks

Fig. 5.1 Typical applications methods for repair materials fordeteriorated reinforced concrete structures

Cement Hydrate Matrix

UnhardenedEpoxy Resin

OH–

OH–


Self-capsuledepoxy resin

After mixing of epoxy-modifiedmortar without hardener


Curing

Curing

Loading

HardenedEpoxyResin


Partially breaking of self-capsuledepoxy resin and microcracking of

cement hydrate matrix


HardenedEpoxyResin

Microcracks


Self-repair of microcracks withhardened epoxy resin


HardenedEpoxyResin

Self-RepairedMicrocracks


Filling of microcracks withunhardend epoxy resin


HardenedEpoxyResin OH– OH–

Fig. 5.2 Simplified model for self-repair mechanism formicro-cracks in epoxy-modified mortars


Protective Coating

2nd Resin Coat

Carbon Fiber

1st Resin Coat

Epoxy Putty Filter

Primer

Concrete Substrate

Fig. 5.3 Typical Application Procedure for Repair andRetrofitting of RC Structural Members

6 Investigations on Geopolymer Concrete and

its Application for Repair

Mrs. P. S. Ambily and Dr. J. K. DattatreyaCSIR-SERC, CSIR Campus, Taramani, Chennai-600 113, India.

Email: [email protected]

6.1 INTRODUCTION

Concrete is the most widely used man-made material in the world.The production of cement, the main active ingredient of concrete,releases approximately one ton of CO2 for one ton of Portland cementconsumed. As one of the most energy-intensive materials and its expo-nential growth in production and utility in the developing countries, itis incumbent on the concrete manufactures to arrest further damageto the environment by drastically reducing or eliminating OPC con-sumption. The Conservation of rapidly dwindling natural resourcesand promotion of sustainable development through gainful utilizationof industrial byproducts are the primary objectives of the Constructionindustry today. Efforts are underway all over the world to develop envi-ronmentally friendly construction materials, which make minimumutility of fast dwindling natural resources and help to reduce green-house gas emissions. In this connection, geopolymer cement concretesshow great promise.

6.2 GEOPOLYMER CONCRETE

Geopolymer concretes (GPCs) are a new class of building materialsthat have emerged as an alternative to Ordinary Portland cement con-crete (OPCC) and possess the potential to revolutionize the buildingconstruction industry. The term geopolymer was first introduced byDavidovits1 in 1970s to name the three-dimensional alumino-silicatebased binding material produced from the reaction of a source materialor feedstock rich in silicon (Si) and aluminum (Al) with a concen-trated alkaline solution. The source materials include Fly ash (FA),Ground granulated blast furnace slag (GGBS), metakaolin or other




natural/industrial byproducts that are rich in silicon(Si) and alu-minium(Al). Since then considerable research has been carried outon development of Geopolymer concrete and its applications in civilengineering by several researchers2−7. Fig. 6.1 and Table 6.1 summa-rized the difference in features between Ordinary Portland cement(OPC) and Geopolymer (GP) binder and the advantages of GPover OPC. The majority of GP production technologies necessitatethermal/hydro-thermal curing.

The CSIR-Structural Engineering Research Centre(SERC), Chen-nai has been working on room temperature curing Geopolymer Con-crete (GPC) for the past five years. Extensive research has been carriedout at SERC to structural grade GPCs with compressive strengthranging from 20 to 70 MPa6−19. The mechanical and durability char-acteristics of these materials have been studied in detail18,19. Somepilot studies were also carried out on the feasibility of using GPC forthe production of building blocks and pavers6.

6.3 APPLICATIONS

Fig 6.2 shows the successful applications carried out since 1979 withgeopolymer cements of different types20

Geopolymer cementitious products are currently being developedin the following areas21:

• civil construction applications -stabilized fill, pavement materials,and soil stabilization;

• building materials - bricks, blocks, tiles, pavers, lightweight/fireretardant/acoustic panels, pipes, precast concrete products andready mixed concrete products;

• mining–paste back-fill, tailings; dams,-liners, capping media;shotcrete, and acid resistant concrete;

• environment / waste management–impermeable barriers, encap-sulation of domestic, hazardous, radioactive and contaminatedmaterials in a very impervious, high strength material; and

• specialist applications–rapid set binders, very high strengthbinders, lightweight products, super flat floors, low shrinkage,and acid resistant storage facilities.

Geopolymer cements have been around since quite a few decadesand some trace it to the time of the Ancient Egyptians and yet are

Investigations on Geopolymer Concrete and its Application for Repair 81

still considered a relatively new material, given the limited commer-cial applications in recent history. Australia is currently leading theworld in the research and development of geopolymer applications,with interest in the technology growing from within the building,mining and quarrying industries21.

6.4 WORK CARRIED OUT AT CSIR-SERC ON

GEOPOLYMER CONCRETE8

The CSIR-SERC initiated the studies on GPCs with the aim of theirutility in structural concrete, both cast insitu and precast. Since thereactivity and physical characteristics of Indian fly ashes do not com-pare favorably with that from Canada and Australia, it is difficultto achieve this target without heat treatment using fly ash aloneas binder. Therefore, a judicious combination of FA and GGBS wasadopted as both the materials are available in plenty.

Following materials were used to produce GPCs:

• Fly ash,

• Ground Granulated Blast Furnace Slag,

• Fine aggregates and

• Coarse aggregates

• Alkaline activator system (AAS) for GPC. It is a combination ofalkali silicates and hydroxides, liquids and additives. The role ofAAS is to dissolve the active ingredients of fly ash and GGBSand promote polymerization.

Formulation of GPC Mixes: Unlike conventional cement con-cretes, GPCs area new class of materials and hence, conventionalmix design approaches for cement concrete are not directly applicable.The formulation of the GPC mixtures requires systematic experimen-tal investigations on the source materials available and the recipesdeveloped are more specific to the materials being used as the sourcematerials are not standard synthesized products.

Preparation of GPC Mixes10−12

The production of GPCs can be carried out using conventional con-creting machinery and tools used for conventional cement concretes.The mix recipes developed at CSIR-SERC need moist gunny curingfor about a day and set and harden within this period and the strip-ping time and formwork removal time are rather short compared to


OPCs. The products need only a shaded Exposure or cover againstdirect sunlight and there is no need for moist/hydrothermal curing.

Mechanical Properties

Compressive Strength: With proper formulation of mix ingredients,24 hour compressive strengths of 25 to 35 MPa can be easily achievedwithout any need for any special curing. Such mixes can be consideredas self curing. However, GPC mixes with 28 day strengths exceeding50-60 MPa have also been developed at CSIR- SERC. The rate ofstrength development is generally faster compared to OPCs.

Elastic Modulus and Stress Strain Characteristics: The stress-strainrelationship depends upon the ingredients of GPCs and the curingperiod. The elastic modulus is generally 10-30% less than that ofOPCCs for the same order of compressive strength depending on themix composition. The strain at peak stress ranged from 1.5 to 1.75times higher while the failure strain is about 20% to 30% higher.

Reinforced GPC Beams17:

Reinforced Geopolymer concrete (RGPC) beams were cast and testedunder two point static loading to evaluate the performance underconditions critical in flexure and shear and the behavior of RGPC spec-imens were satisfactory and matched or exceeded the performance ofcorresponding OPCC beams in terms of ultimate moment capacities.However, the cracking and service load moments were lower (10-30%)compared to OPCC beams while the post yield ductility was somewhatlower.

Reinforced GPC Columns

The concrete compressive strength and longitudinal reinforcementratio influence the load capacity of columns. The load carrying capac-ity increases with the increase in concrete compressive strength andlongitudinal reinforcement ratio as in case of OPCC columns. Crackpatterns and failure modes of GPC columns are similar to those ofOPCC columns but they show lower buckling strength and greaterlateral deflection.

Bond Strengths of GPC with Rebars16

The bond strengths of GPCs with rebars are marginally higher com-pared to OPC due to better adhesion. Thus developmental lengthof steel bars in reinforced GPC can be kept same, as in the case ofreinforced CC.


Durability Aspects of GPCs13,18−19

The GPC specimens have chloride permeability rating of ’low’ to ’verylow’ as per ASTM 1202◦C but high fly ash content can take it high tovey high range. Water absorption and porosity can range from slight tosignificantly higher depending on the mix recipe. GPCs offer generallybetter protection to embedded steel from corrosion as compared toOPCC. The GPC were found to possess very high acid resistancewhen tested under exposure to 2% and 10% sulphuric acids.

6.4.1 PRODUCTION OF GEOPOLYMER PAVER/BUILDINGBLOCKS6

On the basis of experience gained from the production of geopolymerbuilding/paver blocks at CSIR-SERC large scale production of theseblocks were taken up under the sponsorship of AEON’S ConstructionProducts Limited (ACPL), Chennai. Extensive studies were carriedout in the laboratory to develop mixture proportions and finalize pro-duction technology for geopolymer concrete paver blocks and buildingblocks. Based on these trials, two mix compositions one incorporat-ing high volume GGBS (75% GGBS) and other one (high volumefly ash 80% FA), which can acquire the target strength by ambienttemperature curing alone were finalized. About 1200 building blocksof geopolymer concrete consisting of 950 solid paver blocks of size100 × 200 × 90 mm, 100 solid blocks with fly ash based light weightaggregate 100 × 200 × 90 mm and 150 hollow building blocks of size190× 390× 190 mm were produced at the AEON’S factory [Fig. 6.3].Analysis of the test results shows that the blocks made with bothGGBS, fly ash and fly ash aggregate based hollow and solid block willsatisfy the codal provision as per IS 2185 (Part I & II). The paverblocks made with different GGBS mix is suitable for use in heavy,medium, light and Non traffic application as per IS 15658:2006. Thisis the first time in India a factory scale production of geopolymerblocks have been made.

6.4.2 GEOPOLYMER CONCRETES AS JOINTINGMATERIAL FOR PREFABRICATED CONSTRUCTION

An investigation was taken up at CSIR-SERC to study the struc-tural behaviour of large panel floor and wall elements, the connectionsand the performance of joint assemblies. The performance was eval-uated by means of experimental testing of large panel prefabricatedassemblages. In order to speed up erection of prefabricated buildingcomponents, a quick setting binder would be a promising material


for in-situ jointing of prefabricated elements. Geo-polymer concreteshaving compressive strength of more than 30 MPa in 24 hours wouldbe a best alternative in this regard. They have excellent resistanceto sulphate attack and good acid resistance13,18−19 and excellent fireresistant20 and hence ideal for use in building constructions. A com-prehensive testing programme on joint assemblages to evaluate theability of joint/ connections to transfer moments and lateral loadsfrom floor to wall panels and from wall to wall panels at service condi-tions was undertaken. In the present study, behaviour of GGBS basedgeo-polymer concrete [30Mpa in 24 hrs] as jointing material for largepanel prefabricated systems where wall to wall and roof to roof pan-els need to be jointed in- situ. Two foamed concrete panels of size1200mm × 1200mm × 100 mm with RC ribs on all the four sideswere cast separately. After 28 days the two precast RC ribbed foamedconcrete panels were jointed using GGBS based geo-polymer.

After 24 hours the jointed RC ribbed foamed concrete panel wassubjected to flexural load test.

Flexural Load TestThe flexural test on the geo-polymer jointed panels was conductedusing a reaction frame and 100 t capacity hydraulic jacks (Fig. 6.4).The span of the jointed panels was kept at 2400 mm. The panels werekept in a horizontal position and supported on the steel pedestal andsimply supported boundary condition was adopted. A line load wasapplied on the joint potion of the panel through two 30 mm rollerskept on the top of the panel. The load was applied gradually througha hydraulic jack and the deflection at the centre of the joint and atother six points was measured at regular load intervals. The load wasapplied gradually till failure. The first crack and failure loads wererecorded. The deflection measurements were taken at seven points.Steel strain on the four U-bars in the joint portion was recorded.The load was applied gradually and strain and deflection measure-ments were measured. From the study the following observations weremade:

• Geopolymer concrete hardens and attains high strengths in oneday and hence finds application as jointing material.

• GPC was used to join two precast foamed concrete slab elements(with RC grids) of size 1.2m×1.2m×0.1m. The assemblage hada size of 2.6m× 1.2m. The jointed slab was tested for continuity


by simply supporting at the two ends and applying a line loadalong the joint.

• The joint performed well in the test and withstood a load of 27kN.

• The maximum deflection recorded was 17.65 mm.

Hence, geo polymer concretes have great potential for use in prefab-ricated constructions as it facilitates speeder construction and savingsin cost of construction.

6.5 GEOPOLYMER FOR REPAIR APPLICATIONS

6.5.1 Geopolymer for Repair and rehabilitation of reinforcedconcrete beams

Balaguru et al29 have carried out an experimental investigation ofthe behaviour of reinforced concrete beams strengthened with carbonfiber fabrics bonded using geopolymer adhesive in lieu of conventionalorganic polymers for fastening the carbon fabrics to concrete. Themajor disadvantage of composite is their lack of fire resistance anddegradation under UV light leading to long-term durability problems.The inorganic polymer (geopolymer) used in this study was an aluminosilicate which can sustain up to 1000 C, durable and does not degradeunder UV light. Three beams were strengthened using 2, 3, and 5layers of unidirectional carbon T 300 carbon fibre fabrics after thebottom surface of the beams were roughened by dry grinding and sandblasting. The fabrics were impregged with the adhesive and affixed tothe bottom surface of the beam. The beam with two layers was allowedto dry for 24 hrs while the beams with 3 and 5 layers were subjectedto a vacuum of about 711 mm of mercury for better adhesion. All thebeams were subsequently heat cured at 80◦C.

The beams were instrumented to measure the beam deflections andthe strains in concrete, tension steel, and the fabric using bondedstrain. The simply supported beams were tested over a span of 3000mm and two one third point loading. All the strengthened beams failedby rupturing of the composite demonstrating the effective bond pro-vided by geopolymers adhesive even when five layers of fabric wereused. As the number of layers increased, the length of compositethat rupture also increased. Hence, if the repair system is properlyapplied, failure by delaminating of composite can be eliminated. Thestrengthened beams showed higher service and ultimate loads (Fig.6.5).


The primary difference between the organic and the geopoly-mer adhesives is the failure pattern. In the Sherbrooke study30

using organic adhesive, the composite peeled off, whereas with GPCadhesive, the composite ruptured in this study (Fig. 6.6). Delami-nation failure not only underutilizes the composite strength, but isalso extremely brittle. The deflections and crack patterns of beamswith organic and geopolymers were comparable. The composite inthis study recorded larger strains than the strains reported in theSherbrooke study.

Field implementation of geopolymer coating31

The primary objectives of the current study was to

• Establish a temperature range in which the coating can beapplied, given the requirement being that the coating should beable to withstand rain after 24 hours of curing.

• Establish the surface condition and requirements.

• Make field demonstration applications at Rutgers Universitycampuses and on actual transportation structures

Durability: Wet-Dry ConditionsEpoxies and other organic matrices have been utilized as a protec-tive coating for several decades because they seal the surface of theconcrete. Their main drawback is their inability to release vapor pres-sure buildup that causes damage in the concrete and delamination ofthe dried epoxy. The inorganic matrices that comprise the next gen-eration of barrier and strengthening systems are less permeable thanconcrete, thus slowing the flow of water through the weakened exte-rior surfaces. Vapor pressure is released because the matrices are nottotally impermeable. In strengthening applications, the matrices forma strong bond between the surface of the concrete and the fiber rein-forcement. A study was undertaken31 to evaluate the effect of wet-drycycles found in marine environments on the coatings and the durabil-ity of coated concrete. In strengthening applications, the effectivenessof the carbon reinforced (tows or fabrics) coatings with Geopolymermatrix based on potassium alumino silicate solution and silica fumewas studied with no carbon contamination. The pure silica fume wasneeded to obtain a matrix that could be used to wet the carbon fibers.The formulation consisted of Liquid: 100g, Silica fume: 125g and Wet-ting Agent: 1g. The coating was applied in 2 layers to high strengthconcrete prisms 50 × 50 × 330 mm after surface preparation by sand


blasting and cured for 24 hours at room temperature, followed by 24hours at 80◦C.

The specimens were exposed to wetting and drying in a wet drychamber under 3% saline water) for 50 and 100 cycles. The variablesconsidered for the study were fiber volume fraction in discrete fibers(2 and 4%) and no. of tows/layers in case of tows/fabrics (1, 2 and 3).The response was measured in terms of maximum strength, flexuralstiffness and toughness and obtained from the load deflection response.The failure loads are presented as a factor of the failure load of theunexposed control sample. Flexural strength of the control samplesimproved after exposure to wet-dry conditions [Figs. 6.7-9]. In somecases, after 100 cycles of wet-dry, the failure load of the control sampleswas found to increase by increase by approximately 50 percent. Inall cases, the strengthened samples were durable up to 100 cycles ofwetting and drying. The strength and ductility of the concrete sampleswas increased by the application of the carbon composite system. Peakload and toughness factor values increased as the area of the carbonreinforcement was increased. Effectiveness of the strengthening systemwas not diminished by exposure to wet-dry conditions.

Durability: Scaling ConditionsOne possible solution to the problem of scaling in concrete is to applya protective coating that will cover existing micro-cracks. The coatingshould have a lower permeability than the concrete. Scaling resistancestudy was conducted using inorganic matrices and carbon fibers.

Experimental studyThe effectiveness of the inorganic geopolymer matrix as a surface pro-tector for concrete was evaluated. The matrices were applied to botha high and low strength mortar and subjected to scaling conditions.Specimens of size 50×50×330mm were cast and cured. These prismswere coated with the various matrices or strengthened with carbonreinforcement. A special set-up was built for exposing the test samplesto scaling conditions as shown in Fig. 6.10. The scaling test describedin ASTM C672 was followed.

Test results for strengthTest results for strength evaluation were made at the completion offifty scaling cycles. The data obtained from the flexure testing of sam-ples strengthened with carbon reinforcement is shown in Figs. 6.11 to6.13.


The results obtained from the flexure testing of these samples indi-cate that the system is resistant to scaling conditions. The flexuralstrength and ductility of the specimens were determined before andafter exposure to the scaling conditions. Comparable results wereobserved regardless of the type of carbon reinforcement used.

Evaluation of plain concrete strengthened with an inorganicgeopolymer coating and subjected to wetting and drying and scalingconditions led to the following conclusions:

• The inorganic matrix in combination with carbon tows andcarbon fabrics can be used to strengthen plain concrete members

• Wetting and drying conditions (100 cycles) do not decrease thestrength of samples coated with carbon reinforced geopolymer.

• Strengthened samples exposed to scaling conditions had a small(about 3%) decrease in strength from their exposed strength.

Field durability and demonstration application:

Field durability

Durability under field conditions was evaluated using two locationsat the Rutgers University Campus in New Brunswick, New Jersey. Atotal of 18 test applications were made. Most of the surfaces were onvertical walls and some of them were on relatively smooth concretesurface, Fig. 6.14 a-e. In the case of vertical walls, the surface deterio-ration varied from a weathered but good concrete surface to completelyspelled surface. In addition to surface deterioration, a second majorvariable was fiber type and fiber volume content. Both micro and dis-crete fibers were evaluated at volume fractions ranging from 0.5 to20%. All but two coatings were applied using paint brushes while theother was applied using sprayers.

These applications served as demonstration projects confirmingthat very little surface preparation is needed. All these surfaces werecleaned with low pressure water and allowed to dry to saturatedsurface dry conditions before applying the coating. All but two coat-ing were applied using paint brushes. One coating was applied usinga custom made sprayer and another coating was applied using aninexpensive sprayer. The coating was applied between March andNovember to evaluate the influence of temperature range.

The second set of coatings was applied on the parking lot on BuschCampus, Fig. 6.15. These curbs had a good surface except in one case;part of the curb was broken. The surface was simply wetted before


the application of the coating. In these applications, the coating wassubjected to snow exposure and abrasion of snow removal equipment.There was also abrasion due to sand or dust particles blown by thewind.

Experience gained during these applications was used to formulatean application procedure.

Field Applications on Transportation Structures

Field applications consisted of: (1) Coating a New Jersey Barrier inTrenton, N.J., (2) Coating a guide rail near Trenton, (3) coating aretaining wall on Route 18 in New Brunswick, (4) Coating a NewJersey Barrier near an ocean front in Rhode Island, and (5) Coating ofcurbs and a retaining wall on Route 1 and Route 295 near Providence,Rhode Island.

The coating application on Route 1, Trenton was carried out withthe cooperation of NJDOT (New Jersey Department of Transporta-tion) engineers and field personnel Fig. 6.16.

The retaining wall coating on Route 18 was applied in November2000 and covered about 10 square feet. This coating contained onlymicro fibers and was applied using paint brushes on the pre-wettedsurface.

The coatings in Rhode Island were applied in October 1998 andApril 1999 (Fig. 6.17). Coatings on NJ barriers and curbs were appliedusing paint brushes and the coating on the retaining wall was appliedusing a power sprayer. The retaining wall on which the coating waspower sprayed covered several hundred square feet.

This study focused on the development of a two component inor-ganic geopolymer matrix (the liquid Component was mixed with apowder component using a high shear mixer to achieve a thick paintconsistency, which can be applied by brush, roller, or sprayer) that canbe used both as a protective coating and also as a strengthening coat-ing with the addition of micro, discrete, and continuous carbon fibersand carbon fabrics. This matrix, which is water based, is non toxic,cures at room temperature, were evaluated for working time and cur-ing temperatures ranging from 40◦F to 70◦F, durability under wet-dryand scaling conditions. The application was demonstrated both in thelaboratory and in the field. The durability under field (outside expo-sure) conditions was evaluated using two locations. A total of 18 testapplications were made. Most of the surfaces were on vertical wallsand some of them were on a relatively smooth concrete surface. In the


case of vertical walls, the surface deterioration varied from a weath-ered but good concrete surface to completely spalled (with exposedaggregate) surface.

Evaluation of the geopolymer matrix for field applications led tothe following conclusions:

• The geopolymer coating can be applied in the ambient temper-ature range of 40 to 90◦ F. At temperatures higher than 80◦ F,the pot life might be less than 2 hours.

• The coated surface should be protected from direct rain orrunning water for the first 24 hours.

• The coating should not be subjected to freezing in the first 24hours.

• The geopolymer coating can be applied to new or weatheredconcrete surfaces that have exposed aggregates.

• The surface should be pre-wetted. Loose and oily materials shouldbe removed. Light dust will not reduce the adherence of thegeopolymer coating material.

• The geopolymer coatings are durable in field conditions. The old-est application, under saltwater exposure conditions in RhodeIsland, is 9 (in 2008) years old.

Balaguru32,33 has also demonstrated the viability of coating anexisting 300 ft. parapet wall with inorganic (geopolymer) coating(Figs. 6.18-19) and column wrapping (Figs. 6.20a-d) of a bridge.

Geopolymer Coating of 300ft parapet wall

This project carried out to prove the viability of coating an existing300 ft. parapet wall with inorganic (geopolymer) coating. This wall,located at the Scenic Overlook on I-295 South near Trenton, N.J. (milepost 58.5), was coated with Geopolymer tinted with pigments. Thewall surface was pressure washed before applying the coating. Wash-ing of the wall was needed to obtain as uniform a finish as possible.The performance of the Geopolymer coating was monitored. The fielddemonstration project shows that the inorganic-polymer coating canbe easily applied to large surfaces. The application system was easy towork with and the geopolymer coating was applied using paint rollersand brushes. Extensive surface preparations are not needed prior to theapplication of the coating. Finished surfaces provide an aestheticallypleasing appearance.


Geopolymer column wrapping

The coating was originally developed for use in aircraft structures andmodified for use as a coating material and adhesive for brick, concrete,wood, and steel. The constituents of the coating include nanosilicatesand other nano-size activators and fillers. The demonstration projectconsisted of wrapping of columns with carbon fibers and inorganic-polymer, which is located in Maryland State. The studies showed thatthe inorganic-polymer coating can be applied with and without con-tinuous fiber reinforcement. The system is easy to work with and theapplications can be carried out with paint brushes or rollers. Theoldest application is about 7 years old and is performing well. Thecoated surfaces have been exposed to a number of snow storms, freezethaw cycles, salts used to melt snow and abrasion by snow removingequipment. The self cleaning and de-polluting properties are beingevaluated.

An experimental investigation was conducted Pasco et al [435] toevaluate bond strength between OPCC substrate and three repairmaterials. Tungsten mine waste geopolymeric binder and two com-mercial repair products were used as repair materials.

This study indicates that:

• Tungsten mine waste geopolymeric binders possess much higherbond strength than current commercial repair products.

• Commercial repair products gain no bond whatsoever to sawnconcrete specimens. Scanning electron micrographs reveal thattungsten mine waste geopolymeric binders chemically bond tothe concrete substrate.

• Cost comparisons between tungsten mine waste geopolymericbinder and current commercial repair products are also madeshowing that geopolymeric ones are by far the most cost efficientsolution


From the studies conducted by CSIR-SERC and the field demonstra-tion projects, other strengthening and repair applications presented inthis paper show that:

• Geopolymer concrete hardens and attains high strengths in oneday and hence finds application as a jointing material.


• The geopolymer coating can be easily and successfully applied toconcrete surfaces.

• Geopolymer coating can be applied using paint rollers andbrushes.

• Extensive surface preparations are not needed prior to the appli-cation of the geopolymer coating. Finished surfaces provide anaesthetically pleasing appearance.

• The geopolymer coating is durable in wetting and drying andscaling conditions.

• Geopolymer provides as good or better adhesion in comparisonwith organic polymers. In addition, geopolymer is fire resistant,does not degrade under UV light, and is chemically compatiblewith concrete. Hence it can be successfully developed for use inrepair and retrofitting of concrete structures

6.7 REFERENCES

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to Portland Cement Based Structural Concretes”, Keynote paper,ACECON 2010, IIT Chennai, Dec 2010., pp 243-255

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10. Rajamane, N. P., Sabitha, D., and Sajana Mary James, (2005),Potential of industrial wastes to produce geo-polymeric mortarof practical utility - a study, Indian Concrete Institute Journal,Vol. 5, No 4, Jan-Mar, pp 9–20.

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14. Rajamane, N. P. Nataraja M C, N Lakshmanan, and J.K Datta-treya, “Flexural Behaviour of Reinforced Geopolymer ConcreteBeams”, International Seminar on Waste to Wealth, conductedby BMPTC, 12th-13th, India Habitat Centre, New Delhi.

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Engineering”, March, Department of Civil Engineering, MepcoSchlenk Engineering College, Sivakasi.

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18. Sathish E., J. K. Dattatreya, N. P. Rajamane, D. Sabitha and R.Srinivasa Raghavan, [2009], “Sulphuric acid attack on geopolymerconcrete and Portland plain cement concrete”, Proceedings of theNational Conference on “Innovation in civil engineering”, 19-20March, Department of Civil Engineering, B.S. Abdur RahmanCrescent Engineering College, Chennai.

19. Sathish E., J. K. Dattatreya, N. P. Rajamane, D. Sabitha andR. Srinivasaraghavan, [2009], “Studies on sulphuric acid resis-tance of geopolymer concretes”, Proceedings of the NationalConference on “Recent trends in concrete composites for struc-tural systems”, April, Department of Civil Engineering, KonguEngineering College, Erode.

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22. Rangan, B.V. (2008a). “Fly Ash-Based Geopolymer Concrete”,Research Report GC4, Faculty of Engineering, Curtin Uni-versity of Technology, WA, available at espace@curtin orwww.geopolymer.org.

23. Rangan, B.V. (2008b). “Studies on Fly Ash-Based GeopolymerConcrete”, Malaysia Construction Research Journal, 3 (2), 1–20.


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26. Palomo A., Grutzeck, M.W. and Blanco, M.T. (1999). “Alkali-activated Fly Ashes: A Cement for the Future”, Cement andConcrete Research, 29, 1323–1329.

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28. Sofi, M., van Deventer, J. S. J., Mendis, P and Lukey, G. C.(2007a). “Engineering properties of Inorganic Polymer Concretes(IPCs)”, Cement and Concrete Research, 37 (2), pp 251–257.

29. P. Balaguru, Stephen Kurtz, and Jon Rudolph, Report on“Geopolymer for Repair and Rehabilitation of Reinforced Con-crete Beams”, www.geopolymer.org

30. M’Ba Zaa, I., Missihoum, M., and Labossiere, “Strengthen-ing of Reinforced Concrete Beams with CFRP sheets”, FiberComposites in Infrastructure, 1996, pp 746–759.

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36. American Society for Testing and Materials (1993). “StandardTest Method for Scaling Resistance of Concrete Surfaces Exposedto De-icing Chemicals.” Standard C-672, Section 4, Volume 2, pp345–347.

37. American Society for Testing and Materials (1993). “Resistanceof Concrete to Rapid Freezing and Thawing.” Standard C-666A,Section 4, Volume 2, pp 326–331.Web Site (WS)1. http://www.civil.canterbury.ac.nz/events/pandp/03McSaveney2. www.geopolymer.org

Table 6.1 Comparison of OPC vs GP Binder6

Sl No OPC Binder GP BinderReactants / Calcium Sodium / potassium

monosilicate oligo-sialate-siloxofeedstockRaw materials Lime, clay, Metakaolin/fly ash/

gypsum GGBS/red mud/otherslags

Reaction Hydration presence of alkalineactivators and subsequentpolymerization bypolycondensation

Water Essential for Required as a carrier forhydration), promotes activators and mediumrheology of paste, for dissolution of siliconhydrated water is and aluminium ionschemically bound, from the feedstock,excess water promotes rheology offorms capillary pores paste.

Reaction- Ca-disilicate hydrate, Sodium / potassiumProducts lime, Ca-sulpho Poly-sialate-siloxo

aluminatesTime scale of Relatively longer ShortreactionsProcess 1450◦C 750◦C

calcining of coal /kaolinite clay /fusionof lime


Sl No OPC Binder GP BinderRole of alkalies Formation of undes Soluble alkali

irable silicate compounds speed upand aluminous phases dissolution andof the type condensationKC12 S23, and polymerizationNC8 A3 consequentproblems withworkability andASR

Mixing and More or less Depends on thecuring procedure standardized chemistry of source

material and theactivator, thermalcuring is oftennecessitated,Variability offeedstock

Microstructure structure 3-D Al-Si-a 2-D chain or network forminglayered molecular amorphous (gel-like)structure, or partiallybonding network. amorphous orDiscontinuous crystalline substancesand inhomogeneous depending on thestructure in a character of raw3-D, restricts material materials and on theperformance concentration of theand durability, pores activator. Relativelyranging from dense and less porousnanometers to than HCP. Themicrometers Geopolymer gel is

constituted from anarray of non-sphericalaluminosilicate particleswith mesopores 2-50 nm

CO2 emission 90-100% 20%


Sl No OPC Binder GP BinderShortcomings CSH, CH, CA, CF aluminosilicate

and CSA that do binding phasenot occur as extremelynatural durable in anminerals are aggressive environmentsusceptible to and mechanically strong,degradation in the alkali cation (Na,certain K) is presentenvironments, in the structureCSH is in a solvated formthermodynamically and bonded moreunstable, and weakly than in thetends to revert to crystalline zeolites andsilica gel and possible occurrence ofcalcium carbonate efflorescence, residualin the natural alkali can easilyenvironment and carbonate, highereven faster in electrical conductivity,aggressive susceptibilityenvironment, to sulphateCH and attack and sulphuricCSH are prone to acids especially withsulfate attack. binders containing Ca,

water held in the poresreduces strength

Rheological High percentage Static andproperties of fly ash dynamic viscosity

improves the of the geopolymersrheological concrete are substantiallybehaviour, little higher, longeradhesion in early processing time andstages careful selection of

aggregates required,pronounced adhesionability of the freshcompositioncoating even thesmallest grains of theaggregate, higher airentrainment due to lowmobility of the paste,leaving closedunconnected voids


Sl No OPC Binder GP BinderShrinkage Higher No shrinkage

shrinkage due tosusceptibility as hydration, however largehydration mechanism residual water can causeitself results drying shrinkagein shrinkage

ITZ Present at No transition zone couldpaste-aggregate be detected eitherinterface, thickness morphologically or by a20 to 100 µm, direct measurement inpresence of oriented pure gel, noCH and compositional gradientettringite, ITZ porosity at aggregate interfacehigher thanmatrix porosity

Miscellaneous Time scale Time scaleof strength shorter anddevelopment extends over severalextends up to year, day, higher temperaturerelatively poor stability, resistance totemperature chemical degradationstability, low and freeze thawresistance to chemical resistancedegradationand poor freeze thawresistance


Fig. 6.1 Difference in the Chemistry of OPC and GP Binders WS1

Fig. 6.2 Geopolymer types involved in successful applications


Fig. 6.3 (a) and (b) An Inside view of ACPL Production Yardand Stacking of GPC blocks Produced on Steel Shelves

Fig. 6.4 Flexural test on geopolymer joint


12.5

10

7.5

5

2.5

2.5 500 1000Deflection

5 layers

3 layers2 layers

Control

Failure load(tonnes)

Failure load with 2, 3, 5 layers ofGeopolymer-Carbon composite

on concrete beam

Fig. 6.5 Load vs deflection

Fig. 6.6 Failure of geopolymer-Carbon composite

32.5

2

1.5

1

0.5

0

Cycles of wet-Dry

Failu

re lo

ad a

s a fr

actio

n of

unex

pose

d co

ntro

l sam

ple

0 50 100

Control2 percent4 percent

Fig. 6.7 Comparison of Failure Loads: Control, 2 Percent and 4Percent Discrete Carbon Fibers


3

2.5

2

1.5

1

0.5

0

Cycles of wet-Dry

Failu

re lo

ad a

s a fa

ctor

of

unex

pose

d co

ntro

l sam

ple

0 50 100

Control1 ToW2 Tows3 Tows

Fig. 6.8 Comparison of Failure Loads: Control, 1, 2 and 3 Carbontows

3

2.5

2

1.5

1

0.5

0

Cycles of wet-Dry

Failu

re lo

ad a

s a fa

ctor

of

unex

pose

d co

ntro

l sam

ple

0 50 100

Control2 Tows3 Tows

Fig. 6.9 Comparison of Failure Loads: Control, 1 and 2 carbonfabric layers

0.25 in

13 in

2 in

Dam1 in

Saline solutionConcretespecimen

Fig. 6.10 Schematic of Scaling Test Specimen


3

2.52

1.51

0.5

0

Cycles of Scaling

Failu

re lo

ad a

s a fa

ctor

of

unex

pose

d co

ntro

l sam

ple

0 50

Control2 percent4 percent

Fig. 6.11 Comparison of Failure Loads: Control, 2 and 4 PercentDiscrete Carbon Fibers

3

2.52

1.51

0.5

0Cycles of Scaling

Failu

re lo

ad a

s a fa

ctor

of

unex

pose

d co

ntro

l sam

ple

0 50

Control1 Tow2 Tows3 Tows

Fig. 6.12 Comparison of Failure Loads: Control, 1, 2, and 3Carbon Tows

2.52

1.51

0.5

0Cycles of Scaling

Failu

re lo

ad a

s a fa

ctor

of

unex

pose

d co

ntro

l sam

ple

0 50

Control1 layer2 layers

Fig. 6.13 Comparison of Failure Loads: Control, 1 and 2 CarbonFabric Layers


Fig. 6.14 Application of geopolymer coating on different surfaces

Fig. 6.15 Coating on Concrete Curb on Busch Campus


Fig. 6.16 Coating on Route 1 South, Close-Up View

Fig. 6.17 Application of Coating on Curb in Rhode Island

Fig. 6.18 Uncoated Concrete Block Next to a Coated One


Fig. 6.19 Close-up View of Coated Surface

Fig. 6.20 (a) Column after cleaning (b) Column wrapped withcarbon tape (c) During final coating application (d) Column after

final coating

7 Advances in Fibre Reinforced Concrete and

its Applications

T. S. Krishnamoorthy and S. Sundar KumarScientist

CSIR-SERC, CSIR Complex, Taramani, Chennai-600 113, India.Email: [email protected]

7.1 INTRODUCTION

Random oriented fibre reinforced concrete is one of the most promisingcomposites used in the construction. Generally, for structural applica-tions, steel fibres should be used in a role supplementary to reinforcingbars. Steel fibres relatively inhibit cracking and improve resistance tomaterial deterioration as a result of fatigue, impact, and shrinkageor thermal stresses. In applications where the presence of continuousreinforcement is not essential to the safety and integrity of the struc-ture (e.g., floors on grade, pavements overlays and shotcrete linings),the improvements in flexural strength, impact resistance, and fatigueperformance associated with the fibres can be used to reduce sectionand to enhance performance or both. Some full-scale tests have shownthat steel fibres are effective in supplementing or replacing the stirrupsin the beams.

The mechanical properties of fibre reinforced concrete (FRC) areinfluenced by: the type of fibre; fibre length to diameter ratio(aspectratio); the amount of fibre; strength of matrix; the size, shape andmethod of preparation of the specimen; and the size of the aggregate.Fibres influence the mechanical properties of concrete and mortar inall failure modes. The commonly available shapes of steel fibres arestraight, crimped, hooked, trough shaped. The strengthening mecha-nism of the fibres involves transfer of stress from the matrix to the fibreby interfacial shear or by interlock between the fibre and matrix, if thefibre surface is deformed. Besides the matrix itself, the most importantvariables governing the properties of FRC are the efficiency factor andthe fibre content. Fibre efficiency is controlled by the resistance of the




fibres to pullout, which in turn depends on the bond strength at thefibre matrix interface. Also, since pullout resistance is proportional tointerfacial area, non round fibres offer more pullout resistance per unitvolume than larger diameter fibres. Therefore, for a given fibre length,higher aspect ratio is more beneficial. Most mixes used in practiceemploy fibres with an aspect ratio less than 100, and failure of com-posites, therefore is, due primarily to fibre pullout. However, increasedresistance to pullout without increasing the aspect ratio is achievedin fibres with deformed surface or end anchorage; failure may involvefracture of some of the fibres, but it is still usually governed by pullout.

7.2 BEHAVIOUR OF STEEL FIBRE REINFORCED

CONCRETE

7.2.1 Compression

The effect of steel fibres on the compressive strength of steel fibrereinforced concrete (SFRC) varies with fibre content1. It is interestingto note that both increase and decrease in compressive strength withdifferent fibre types have been experimentally observed. Even for thesame material, there is mounting evidence to show that compressivestrength may first rise, then drop, with increasing fibre volume frac-tion. These observations suggest that the addition of fibres in a cementcomposite leads to a likely manifestation of increased resistance tomicrocrack sliding and extension, whereas strength degradation is alikely manifestation of increase in either pore or microcrack density,as a result of fibre addition. The pores may be caused by insufficientcompaction and the additional microcracks may be related to poorfibre/matrix bonding, or poor adhesion between filaments within fibrebundles.

Krishna Raju et al2 and Narayanan and Kareem3 observed a signifi-cant increase in the compressive strength with increasing fibre content.The test results showed a more or less linear relationship between thepercentage increase in the compressive strength and the fibre content.

Fanella and Naaman4 concluded that the presence of any typeof fibre in a concrete matrix changes the basic characteristics of itsstress strain characteristics. While the ascending portion of the curveis only slightly modified, the descending portion of the curve is modi-fied significantly (Fig. 7.1). A higher fibre content produces a less steepdescending portion, which results in high ductility and toughness ofthe material. They concluded that except for the case of steel fibres,adding fibres to a concrete matrix does not improve its compressive

Advances in Fibre Reinforced Concrete and its Applications 111

strength. However, the strain at the peak stress is increased by thepresence of any type of fibre. The strength improvement with steelfibres ranged from 0 to 15%.

Ramakrishnan et al5 showed that the addition of fibres (hookedend) seemed to have no effect on the compressive strength of concrete.Based on their investigation on normal and light weight concretes withfibres, Balaguru and Ramakrishnan6 have shown that there was only amarginal improvement in the compressive strength of concrete by theaddition of steel fibres. Oh7 also found that the cylinder compressivestrength was increased by about 17%, when the fibres were introducedin the concrete upto 2% by volume.

7.2.2 Direct Tension

Because of the brittle nature of concrete, valid direct tensile testingof concrete and FRC is always difficult to carry out. Presently, nostandard methods are available for the direct tensile test. Due to theimportance of the tensile behaviour of steel fibre reinforced concreteand concrete, many direct tensile tests of these materials have beenattempted, using different designs of loading grips. Indirect methodsof measuring the stress strain curves have been attempted.

SFRC has superior tensile properties, particularly ductility, overplain concrete. Studies have indicated that the tensile stress crackseparation curve is the best alternative to characterise the tensilebehaviour of SFRC. The observed stress crack separation curve ofSFRC depends on the size of the specimen, method of testing, stiffnessof the testing machine, gauge length and whether single or multiplecracking occurs in the gauge length used. The ascending part of thecurve up to first crack is similar to that of unreinforced concrete. Thedescending part depends on the fibre reinforcing parameters, namelyshape, volume and aspect ratio of the fibre. The strength of SFRC intension is generally of the same order as that of unreinforced concretefor lower volume percentage of fibres. The direct tensile strength ofSFRC can be predicted by the law of mixtures applicable to compositematerials as under:

ft = fm(1vf) + 2(l/d)vf

where, ft and fm are tensile strength of the composite and the matrix,respectively, vf the percentage of fibres by volume, l/d the aspectratio, and, the average interfacial bond strength. Tensile strengtheningoccurs at all fibre contents as long as 2τ(l/d) > fm.


7.2.3 Flexure

According to ACI Committee Report 544(4R) 1, the influence of steelfibres on the flexural strength of concrete and mortar is much greaterthan for direct tension and compression. Two flexural strength valuesare commonly reported. One corresponds to the first crack and theother corresponds to the maximum load. For large amounts of fibres,the two loads are quite distinct, but for very small fibre volumes, thefirst crack load may be the maximum load as well.

Ultimate flexural strength generally increases in relation to theproduct of fibre volume concentration and the aspect ratio l/d. Con-centrations less than 0.5 volume percent of low aspect ratio fibreshave negligible effect on the static flexural strength properties. Pris-matic fibres, or hooked or enlarged end fibres, have produced flexuralstrength increases over unreinforced matrices of as much as 100%. Apost cracking load deformation characteristic depends greatly on thechoice of fibre type and volume percentage of the specific fibre typeused.

Crimped fibres, surface deformed fibres and fibres with end anchor-age produce strengths above smooth fibres of the same volumeconcentration, or enable same strength to be achieved with lower fibreconcentration.

The first crack composite flexural strength (σcf ) and ultimatecomposite flexural strength (σcu) of SFRC are given by 1:

σcf = 0.843frVm + 2.95Vf .l/df

σcu = 0.97frVm + 3.42Vf l/df

where, fr is the stress in the matrix (MPa); Vm is the volume fractionof the matrix; Vf is the volume fraction of the fibres; and l/df is theaspect ratio

Hughes and Fattuhi8 examined the effect of addition of varioustypes of steel fibres upon the flexural strength and fracture toughnessof basic concrete matrix at three different ages. It was seen that maxi-mum increase in the first crack flexural strength and ultimate flexuralstrength were 15% and 85% respectively.

Craig9 investigated the elastic and inelastic behaviour of SFRCbeams. Thirteen beams consisting of normal concrete, high strengthconcrete, and light weight concrete with and without fibres weretested. The test results were verified by theoretical analysis. It wasreported that there is an increase in first cracking load, the stiffness ofthe beam and ductility of the beams with the presence of the fibres.


Swamy and Al Noori10 showed that the fibre reinforcement alone inthe form of discrete fibres cannot be used as direct replacement of con-ventional steel in reinforced and prestressed structural members. Thesuperior resistance of fibre concrete to cracking and crack propagationmay, however, be utilised to improve the resistance of structural mem-bers to cracking, deflection and other serviceability conditions. Testswere also carried out on the flexural behaviour of reinforced concretebeams with fibre content in the tension or compression zone or as atensile skin. It was found that fibre content in the tension zone enabledhigh strength steels to be used in practice with characteristic strengthof 70 MPa. Both crack width and deflection were found to be withinacceptable limits, and the beam was able to develop plastic defor-mation characteristics at failure. The use of a single layer of tensileskin of fibre concrete transforms a conventional over reinforced beamto behave in a ductile manner. Fibre concrete can thus enable highersteel percentages to be used in practice without the fear of brittle typeof failure.

Johnston and Skarendahl11 evaluated the flexural performanceof steel fibre reinforced beams with varying amounts and types offibre. They concluded that the first crack strength depends primar-ily on matrix characteristics that influence matrix strength, notablythe degree of consolidation and water/cement ratio. It is minimallydependent on fibre parameters such as type, size, and amount.

A limited number of tests carried out by Hannant12 showed that theincreased deflections of lightweight concrete beams due to the reducedelastic modulus of the lightweight material can be significantly reducedby the addition of steel fibres. It was reported that the load at whichcracks were first seen for the fibre beams was approximately twice thatfor the beams without fibres.

Kormeling et al13 tested a series of concrete beams with a size of100 × 153 × 2200mm. The beams were tested in four point load-ing with a span of 2000mm and a constant bending moment zone of800mm. Three different reinforcement ratios were used 0.17, 0.75, and2.09 percent. Contribution of steel fibres to the strength of reinforcedconcrete beams was moderate.

Oh7 investigated the flexural behaviour of reinforced concretebeams containing steel fibres. It was reported that the crack widthsincreased almost linearly with the increase of steel stress and the crackwidths at the same loading stages were greatly reduced as the contentof steel fibres increased. The ductility and ultimate resistances were


found to be enhanced due to the addition of fibres. A method forincorporating fibre effects in the flexural analysis of singly and doublyreinforced concrete was discussed.

Krishnamoorthy et al14 investigated the behaviour of SFRC withthree different types of fibres, namely straight, crimped and troughshaped fibre. The results of the investigation are given in Table 7.1and Fig. 7.2.

7.2.4 Flexural Toughness

Toughness is an important characteristic for which SFRC is noted.Under static loading, flexural toughness may be defined as the areaunder the load deflection curve in flexure, which is the total energyabsorbed prior to complete separation of the specimen. The test pro-cedures for measurement of flexural toughness indices given in thecodes of practice, such as ASTM C 1018, JCI SF4, JSCE S4, and ACI544, help one to obtain information on the qualitative performanceof different materials and mix proportions. The procedure given inASTM C 101815 involves determining the amount of energy requiredto deflect a beam to a specified multiple of the first crack deflection.The toughness indices I5, I10, and I30 are determined, respectively, asratios of the area of the load deflection curve up to deflections of 3,5.5, and 15.5 times the first crack deflection divided by the area of theload deflection curve up to the first crack deflection.

Values of the ASTM C1018 toughness indices depend primarily onthe type, concentration and aspect ratio of the fibres and essentiallyindependent of whether the matrix is mortar or concrete. Thus, theindices reflect the toughening effect of the fibres as distinct from anystrengthening effect that may occur. Toughness is expressed as indexas per ACI and as absolute energy as per Japan Concrete Institute.These index values indicate a composite with plastic behaviour afterfirst crack that approximates the behaviour of mild steel after reachingits yield point. Lower fibre volumes or less effectively anchored fibresproduce correspondingly lower index values. The flexural toughnessvalues for SFRC are shown in Table 7.2.

7.2.5 Fatigue Strength

The behaviour of SFRC in cyclic fatigue, despite its importance, hasreckoned relatively little attention. FRC improves the dynamic prop-erties like energy absorption, behaviour under fatigue loading overthe plain concrete16. Batson et al17 conducted experimental investi-gation to determine the effectiveness of steel fibre reinforcement for


resisting fatigue loads. It shows that the fatigue strength generallyincreases with the volume percentage for different fibre sizes. It wasalso observed that the post fatigue static strength is greater than thepre fatigue static strength. Romualdi18 also observed this and pro-posed an explanation based on the shrinkage of the mortar duringcuring and relaxing of the residual tensile stress due to shrinkage bythe action of the cyclic loading. A comprehensive evaluation of fatigueproperties has been investigated by Ramakrishnan et al5 among plainconcrete and FRC with four different types of fibres. They observedthat the fatigue strength increased with the fibre content for all thefibre types. The largest increase was found in the hooked end fibresand the smallest increase was found with polypropylene and straightsteel fibres. The endurance limit expressed as a percentage of modulusof rupture of plain concrete increased with increasing fibre content.

7.2.6 Behaviour under cyclic loading

The objective of subjecting the plain and SFRC specimens to cyclicloading is to investigate whether the specimens after subjecting themto cyclic loading would continue to possess their original integrity (i.e.without suffering damage). Since the peak strain for plain concreteis around 0.002, and these specimens fail suddenly, it is possible tosubject them to cyclic loading only at very low strain levels. Theperformance of SFRC is found to be far superior to plain concreteeven with 0.75% fibre volume fraction. The SFRC specimens, whenloaded monotonically after cyclic loading at a strain of 0.003, reachedalmost the same peak load as was obtained under monotonic loading.All the SFRC specimens were able to sustain higher strain even afterbeing loaded cyclically for fifteen cycles at a high strain of 0.007 (i.e. inthe post-peak stress region). It is clear from the Fig. 7.3 that the SFRCspecimens did not suffer damage even after loading them cyclically ata strain of 0.00719. This particular characteristic of SFRC could bebeneficially used in the design of seismic resistant structures.

7.2.7 Shear and Torsion

Studies in the last few decades indicate that use of steel fibres asshear reinforcement in reinforced concrete beams helps in enhancingthe tensile strength, resulting in increase in shear strength and possi-ble prevention of shear failure. Studies carried out so far have shownthat steel fibres upto about 1.5% by volume are effective as shearreinforcement either by themselves or in combination with verticalstirrups.


The first study on shear behaviour was reported by Batson et al20

where the fibres have been used with and without stirrups. Jindal21

tested 44 beams to study the effect of steel fibres as shear reinforce-ment and found that the increase in the shear capacity of the beamwas substantial. Kaushik et al22 shown that a strength ratio of 1.67can be achieved with the addition of 1.5% fibres with an aspect ratio of100. Batson23 evaluated the effectiveness of hooked fibres in T beams;Narayanan and Darwish24 have shown that the shear cracks in FRCbeams are not significantly different from the ones observed in con-ventional beams. However, the spacing of cracks in the former is seento be closer than the later due to a more uniform stress distribution.

The studies carried out on the torsional behaviour of SFRC haveshown that there is an improvement in the torsional strength of con-crete on addition of steel fibres of various types in varying volumefractions.

7.2.8 Impact

Impact is a complex dynamic phenomenon involving crushing, shearfailure and tensile fracturing. It is also associated with penetrationperforation and fragmentation end scaling of the target being hit. Theaddition of fibres improved the impact resistance of the plain concreteto a great extent. The improvement in the strength is dependent onthe fibre type and fibre volume fractions.

As there is no acceptable standard method for determining theimpact resistance of SFRC, several tests have been used, namely,weighted pendulum Charpy type impact test, drop weight test,rotating impact test, blast impact test, projectile impact test andinstrumented impact test. The simplest of the impact tests is thedrop weight test. This test yields the number of blows necessary tocause prescribed levels of distress in the test specimen. The test canbe used to compare the relative merits of different fibre concrete mixesand to demonstrate the improved performance of FRC compared toconventional concrete25.

A simple, portable, and economical test has been devised bySchrader26. This impact test equipment and procedure has been pub-lished in the report by ACI committee 544. The test is currently underconsideration for inclusion as an ASTM standard. Ramakrishnan etal27 have done a comparative evaluation of concrete reinforced withthree different types of fibres. The Schrader’s drop weight impact test-ing equipment was used. The test results showed considerable scatter,


possibly because no redistribution of stresses was possible during thevery short period of deformation. Hence, local weakness has a greatinfluence on the relative strength of the specimen. SFRC has shownbetter impact resistance than plain concrete and it increases as fibrevolume percentage increases. It is also observed that the impact resis-tance of hooked fibres is higher compared to plain or crimped fibres.

7.2.9 Abrasion/Cavitation/Erosion

Both laboratory tests and full scale trials have shown that SFRC hashigh resistance to cavitation force resulting from high velocity waterflow and the damage caused by the impact of large water borne debrisat high velocity. Tests at the Waterways Experiment Station (USA)indicate that steel fibre addition do not improve the abrasion/erosionresistance of concrete caused by small particles at low water velocities.This is because adjustments in the mixture proportions to accom-modate the fibre requirements reduce coarse aggregate content andincrease paste content.

7.2.10 Creep and Shrinkage

There has been little work on the creep of steel fibre reinforced con-crete. Fibres generally reduce the compressive and tensile creep. Testby Mangat and Azari 28 have shown that steel fibres restrain thecreep of cement matrices at all stress strength ratios. The restraint isfound to be more at lower stress and at higher fibre content. Swamyet al found that steel fibres are more effective in controlling com-pressive creep than tensile creep and the reason for this is not fullyunderstood. Tests have shown that steel fibres have little effect onfree shrinkage of SFRC. However, when shrinkage is restrained, steelfibres can substantially reduce the amount of cracking and mean crackwidth.

7.2.11 Freeze Thaw Resistance

Steel fibres do not significantly affect the freeze thaw resistance of con-crete, although they may reduce the sensitivity of the visible crackingand spalling as a result of freezing in concrete with inadequate airvoid system. The freeze thaw resistance of non air entrained concreteis similar for SFRC and control concrete, whereas SFRC was found tobe better in the case of air entrained concrete29.


7.3 BEHAVIOUR OF FRC WITH OTHER TYPES OF FIBRES

7.3.1 Glass Fibres

Since 1960, glass fibres have been explored as a possible alternative toother fibres in high pH content system. Glass fibres possess high ten-sile strength and modulus of elasticity, but serious concern is expressedregarding their durability in an alkaline environment. Majumdar andhis co workers developed an alkali resistant zirconia glass containingapproximately 16 percent by weight of ZrO2. While Zirconia glassappears to provide a measure of resistance to alkali attacks, per-formance and durability aspects of these composites remain to beascertained.

For low w/c pastes, compressive strength is reduced by about 20%and for higher w/c ratio the decrease can be as high as 30%. Uniaxialtensile strength increases with age and amount of fibre. Aggregategrading does not influence the strength. Also, the increase of tensilestrength in the early stages of hydration is dependent on the typeof fibre. In glass reinforced mortar, the ultimate tensile and flexuralstrengths are not linear function of the term vf(l/d); this is not truefor steel fibre composites. Increasing the length and volume fractionof fibres creates mixing problem. In spite of the enhanced mechanicalproperties, question of the durability of alkaline resistant glass fibreconcrete composite in alkaline environment remain unresolved.

7.3.2 Polypropylene Fibres

Investigations on the use of polypropylene(PP) fibres in concretestarted around 1965 by the Shell Chemical Co. A yarn with a netstructure of fibrillated fibres designed to enhance mechanical keyingwith cement matrix was produced and successfully marketed. PP fibreshave high tensile strength and low modulus of elasticity.

The exact nature and properties of fibre cement interfaces, whichcontrol the behaviour of most cement composites are not well estab-lished. A porous contact zone rich in calcium hydroxide has beenidentified through microscope. The presence of small amount of ettrin-gite and C S H in the contact layer which is a few micron thick hasbeen confirmed. The growth of the calcium hydroxide crystals is essen-tially complete after 24 hour hydration time. A transmission region (10to 20 micron) containing calcium silicate hydrate crystals grow out-ward from the contact zone and mesh with the transmission zone. Thetransmission zone moves in to a region of dense, less porous cementhydrates.


The elastic properties would be influenced by the extent to whichcalcium hydroxide interacts with fibre at the interface and thereforedependent on the fibre type. The brittleness of the composite is prob-ably also affected by the amount and size of the calcium hydroxidecrystals present. Further crystallisation of the calcium hydroxide inthe contact zone may actually result in a weakening of the bondbetween the fibre and matrix. Surface modification of PP can resultin improving interfacial bond.

The decrease in stress at first cracking is dependent on the volumeconcentration of fibres. In general, most works confirmed that incor-poration of discontinuous fibres does not improve flexural or tensilestrength. Reinforcement of cement matrices with continuous fibres,fibrillated filaments, fibrillated films, tape or woven fabric generallyresults in increased flexural and tensile strength. Use of collated fib-rillated fibres increases the flexural strength of matrix by about 15 to20%. Compressive strength of concrete decreases by about 5 to 10%when collated fibrillated mesh is used. PP degrades when exposed toultraviolet radiation. PP fibres do not modify significantly the abrasionresistance of concrete.

7.3.3 Natural Fibres

There has been a growing interest in utilising natural fibres for makinglow cost building materials in recent years. Some investigations havealready been carried out on the use of natural fibres from coconut husk,sisal, sugar cane bagasse, bamboo, akara, plantain and musamba incement paste, mortar, and concrete. These investigations have shownencouraging results.

Flexural strength increases with fibre addition to a maximum andthen decreases. The decrease at higher fibre content is due to incom-plete compaction and increased porosity. A decrease in maximumstrength occurs with increase in sand cement ratio. A similar flexu-ral and tensile strength dependence on fibre volume fraction and fibrelength has been observed for coconut fibre reinforced mortars. Thedecrease in the strength for longer fibres was mainly due to ballingeffects of the fibres.

Impact strength depends on curing period and fibre volume fractionfor both jute and coir fibre reinforced concretes. After 90 days of moistcuring, concrete made at a w/c ratio 0.5 has an impact strength morethan 3 times that of the control concrete. Incomplete compaction andgreater porosity contribute to a decrease in toughness at higher volume


fractions. Jute FRC requires a longer curing period to attain equaltoughness to that of coconut FRC.

Durability is of major importance in evaluating the suitability ofnatural fibres for inclusion in cement matrix. Coir fibre exhibits ductilefailure characteristics, while most of the other fibres exhibit brittlefailure. Coir also shows greater resistance to alkali attacks. It has beenshown by many researchers that concrete reinforced with vegetablefibres loses strength in an alkaline environment. Resin coatings providea reasonable measure of protection against alkali attack.

7.3.4 Carbon Fibres

Widespread use of carbon fibres in cement has been limited dueprimarily to cost consideration. Initially it was used in the pipe man-ufacturing only. Alternative uses are now being exploited as a resultof the development of less expensive discontinuous fibres in Japan.Although discontinuous randomly distributed carbon fibres are lessefficient than continuous aligned fibres, the properties of compositescontaining these carbon fibres are significantly improved. Tensile andflexural strengths increase with fibre content and they are generallyless than those with continuous fibres. At low water cement ratios, thestrengths are similar. Compressive strength of carbon fibre reinforcedcements generally decreases with fibre addition.

7.3.5 Hybrid Fibre Reinforced Concrete

The use of two or more types of fibres in a mix has been exploredto arrived at specific requirements. There are two main categories ofhybrid FRC, 1) Fibres of different sizes and/or shapes mix togetherto achieve better packing and stability 2) Fibres of about the samedimensions, but with different elastic moduli mixed together to providebetter toughness over a wide range of crack opening. Mazin Burhan et.al have investigated the performance of steel-nylon hybrid FRC. 0.5%,1% and 1.5% fiber percentage by volume of concrete were used in thestudy with five different mixes of 100-0%, 70-30%, 50-50%, 30-70% and0-100% for each fibers percentage (nylon to steel). It has been reportedthat the optimum performance in terms of compressive strength waswith a fibre percentage of 0.5% for various combination of steel andnylon fibres. But in terms of split tensile strength the best performancewas at 1% fibre, whereas the modulus of rupture increased with theincrease in the fibre volume. Piti et. al., investigated a hybrid FRCwith different sizes of steel fibres. Two macro fibres and one micro


fibre were mixed together at a combined volume fraction of 2% andsubjected to flexural loading. With micro fibre as a secondary fibre, theperformance was poorer than the single fibre system, when a macrofibre itself was used as a secondary fibre the results were similar to thatof the single fibre system, as the aspect ratio of both the macro fibreswas similar. In case of the hybrid system with three types of fibres,the lack of macro fibre did not affect the performance much as betterpacking played an important role in the performance. Zhean et. al.,investigated the mechanical properties of layered steel fibre (LSFRC)and hybrid fibre reinforced concrete (LHFRC). Experimental resultsshowed that LSFRC and LHFRC can improve the flexural strength ofconcrete by 20 to 50%

7.4 APPLICATIONS OF SFRC

7.4.1 Precast Products

One of the largest applications of SFRC in India has been in the pro-duction of precast concrete manhole covers and frames. It has beenestimated that every kilometer of urban road may require 15 to 20manhole chambers. Presently, grey cast iron is being used for themanufacture of these covers. Cast iron covers are expensive and aresusceptible to pilferage. They are also liable to break easily as thematerial is brittle. The SFRC manhole covers and frames possess highductility and impact resistance and cost relatively less as compared tocast iron manhole covers and frames. Manhole covers, in general, areclassified as heavy, medium and light-duty, based on the intensity ofthe vehicular traffic and their usage. The technology for production ofSFRC manhole covers developed by SERC, Chennai has already beentransferred to more than forty agencies in the country for commercialexploitation. Thus, SFRC is being used extensively in our country forthe production of manhole covers and manhole frames and has muchpotential for use in other precast concrete products such as lost forms,dolosses, wall panels, etc.

Central Building Research Institute (CBRI), Roorkee used bothsteel and vegetable fibres in the development and production of build-ing components, such as, precast doubly-curved roofing tiles (1000 ×1000× 20mm and 700 × 700× 20mm), precast lintels (120 × 230×75mm) and precast planks (1200 × 400 × 25 or 50mm). In the early1980s, corrugated roofing sheets made out of coconut fibre reinforcedconcrete have been used in a major leprosy settlement in a village near


Titilagarh in Orissa, and have withstood many monsoon seasons. Sim-ilar FRC roofing is also now being used in various villages in AndhraPradesh.

7.4.2 Steel Fibre Reinforced Shotcrete

One of the most important applications of SFRC is in the shotcrete,popularly known as ’Steel Fibre Reinforced Shotcrete (SFRS)’. Theinclusion of steel fibres in shotcrete improves many of the mechanicalproperties of the basic material, viz., the toughness, impact resistance,shear strength, flexural strength, ductility factor, and the fatigueendurance limits. An important improvement is evident in the modeof failure, i.e., the material continues to carry a significant load aftercracking and failure takes place only after considerable deformation.While the failure of plain shotcrete under flexure is essentially brit-tle at the occurrence of peak load, SFRS continues to support loadswell beyond cracking of the cement matrix upto large deflections. It isgenerally accepted that Steel Fibre Reinforced Shotcrete (SFRS) canbe designed in thinner sections than that required by conventionalshotcrete to resist the same load. By enabling mesh reinforcement tobe replaced by steel fibres, the use of SFRS can offer considerable timesavings to contractors in executing tunnel lining jobs. In the Srisailamhydropower (A.P) and Uri hydropower projects (J & K), steel fibrereinforced shotcrete has been used.

At SERC, Chennai, the investigations on SFRS were mainlydirected towards studying the flexural strength, toughness indices ofbeam specimens and to establish the load deflection curves and eval-uate the energy absorption characteristics of panel specimens. Testswere also conducted on the companion specimens, which were castusing conventional shotcrete with weld mesh reinforcement. It was seenfrom the investigations that the addition of steel fibres in shotcreteimproves the ductility and energy absorption of SFRS panel speci-mens. The peak load obtained with SFRS panels increases upto twotimes and their energy absorption at 25mm deflection increases uptothree times when compared with that of weld mesh shotcrete panels.These improvements, as reflected in the flatter post peak response,were due to the contribution of steel fibres in controlling cracking andholding the material together even after extensive cracking. The energyabsorption at 25mm deflection for 100mm thick panels increases upto2 to 3 times over that of panels with weld mesh, as the fibre volumeincreases [Table 7.3]. It was noted that the energy absorption of 500


N-m, withstood by the 100mm thick panels with weld mesh at 25mmdeflection is obtained with 50mm thick SFRS panels having fibre vol-ume of 0.5 percentage. Since the energy absorption of SFRS panels ismuch higher than that of weld mesh shotcrete panels, to match theenergy absorption of 100mm thick weld mesh shotcrete panels, it wouldbe sufficient to provide 50mm thick SFRS panels resulting in savingsin concrete. As already pointed out, with fibre shotcreting, shotcretecan be placed to follow the exact contours of the tunnel which wouldresult in additional savings in materials and due to elimination of weldmesh placement, time of execution could be considerably reduced.

7.4.3 Beam-Column Joint

Ductility at beam-column joints or connection is desirable in reinforcedconcrete frames under seismic loading. Ductility at joints is generallyachieved by providing closely spaced horizontal or diagonal ties ofhoops, but this causes difficulty in placing concrete in densely rein-forced portions, which results in bad concreting, leading to failure ofcore concrete under seismic type of loading. Steel fibre reinforced con-crete, which possesses high ductility, toughness and tensile strength,can be considered to replace the plain concrete in the portion of thejoint. Hence, investigations were carried out at SERC, Chennai tostudy the influence of fibres to eliminate the congestion of reinforce-ment in the joint portion of the exterior beam-column joint understatic as well as cyclic loading. A constant axial load of 300 kN wasapplied on the column having both its ends hinged and the beam wasloaded at the free end. It was found that:

• The SFRC is very effective in the beam column connections andthe replacement of shear reinforcement at the joint portion bySFRC did not decrease the shear capacity.

• There is increase in the strength capacity of joint by 20% in thecase of SFRC specimens.

• The SFRC joints behaved better under cyclic loading and with-stood 5 cycles (for 1.0% fibres) and 7 cycles (for 1.5% fibres)against one cycle of loading of joints without fibres before failure.

• From the investigations, it is recommended that the spacing ofstirrups at the beam-column joint can be increased to twice thatof design spacing with the addition of 1.0% fibres in the jointportion for exterior beam-column joint.


7.4.4 Pavement and Industrial Floors

Cement concrete, in general, is being used for pavement and pavinglarge areas of industrial floors. To improve the wear resistance qualityof concrete industrial floors, the concrete base of the floor is providedwith a topping or overlay material, such as quartz, emery or metallicaggregates. However, such concrete floors are found to be adequatein terms of trouble free performance with minimum disruption toactivities on the floor, especially, in aggressive environments such aschemical factories, dairy and food processing industries and when sub-jected to heavy impact loads and abrasion. With the use of materialhandling equipments/machines, such as forklifts, trucks and the useof robots in production, the performance specification in terms of flat-ness, levelness and dust free surface for concrete floors have becomeimportant.

Use of SFRC in the place of plain concrete, for laying the base ofthe floor results in many advantages. Since the flexural strength ofSFRC is more than that of plain concrete, with the use of SFRC, it ispossible to reduce the thickness of concrete floor upto 30% and spacingof contraction joints could be increased by 50%. Further, due to highertensile strength of SFRC, shrinkage cracks and warping cracks due tothermal stresses are minimised. Due to higher abrasion resistance ofSFRC, scaling of concrete is prevented. In case of thin overlay applica-tions, the specified location of continuous reinforcing steel in concreteis literally impossible to achieve, given the minimum cover, variablethickness of overlay and construction difficulties. SFRC because of itspre-crack and post-crack load carrying capacities has better resistanceto development and propagation of cracks originating from underly-ing pavement. This delayed propagation of cracks provide a two tothree fold increase in the life of overlay. Thus, SFRC is ideally suitedfor providing overlays for pavements and industrial floors. SFRC hasbeen used abroad for pavement and industrial floor toppings and hasmuch potential for laying industrial floors in heavy vehicle factories,boiler plants, thermal power plants, where very heavy machinery andtools are to be moved on tracked vehicles.

7.4.5 Application of SFRC to Repair of Distressed Structures

The applications of SFRC fall in two categories repairs and newconstruction. Repairs are invariably required to tackle problems ofabrasion, cavitation or impact damage in various components of


hydraulic structures, such as, spillways, stilling basins, baffle blocks,outlet conduits, etc.

7.5 SLURRY INFILTRATED FIBROUS CONCRETE

(SIFCON)

Slurry infiltrated fibrous concrete is a relatively new material in Indiaand can be considered as a special type of fibre reinforced concrete.It is different from normal fibre reinforced concrete in two aspects.In FRC, the fibre content usually varies from 1 to 3% by volumewhereas in SIFCON, the fibre content may vary between 5 to 20%.The matrix of SIFCON consists of cement paste or flowing cementmortar as opposed to regular concrete in fibre reinforced concrete.The process of making SIFCON is also different because of the highfibre content. In FRC, the fibres are added to the wet or dry mix ofthe concrete during mixing but SIFCON is prepared by infiltratingcement slurry into a bed of preplaced fibres. SIFCON has been suc-cessfully used for refractory applications, pavements and overlays, andstructures subjected to blast and dynamic loading30. Because of highductility and impact resistance, the composite has excellent poten-tial for constructing structural components which need to resist highimpact force and exhibit high ductility, such as explosive storage cab-inets, blast resistant doors, high security vaults, repair of concretebridge decks, test track for heavy vehicles, missile silo structures andprecast shapes, where standard modes of reinforcement are ineffective.

At CSIR-SERC investigations have been carried out on SIFCONwith different types, volume and cement to sand ratio. Two mix pro-portions (1:1 and 1:1.5) and two w/c ratios (0.40 and 0.35) wereinvestigated. Sulphonated Naphthalene Formaldehyde (SNF) basedsuperplasticizer was used for higher w/c ratio (0.40) and Polycar-boxylic(PC) based superplasticiser was used for lower w/c ratio (0.35).In order to arrive at the optimum dosage of superplaticizer, MarshCone test was used. Marsh Cone test consisted of the evaluation oftime required to collect 400ml of paste through a standard MarshCone. During the casting of the test specimens, sand that was retainedin a 1.18mm sieve was used. Specimens were cast to evaluate the com-pressive strength (100 ×100× 100mm cubes) and split tensile strength(100mm dia × 200mm height cylinders) at 28 days. The details of var-ious mixes and the test results are given in Table 7.3. The test resultsrevealed that the mix proportion 1:1 with a water cement ratio of0.35 and polycarboxilic based superplasticiser and Viscosity Modifying


Agent (VMA) gave the best performance in terms of compressive andsplit tensile strength hence was choosen for further studies. Similarlytests very conducted to determine the suitable fibre type and optimumdosage. With the addition of 8% fibres the compressive strength wasin the range of 70-80 MPa and the split tensile strength was around15 to 18 MPa. Figure 7.4 is the stress-strain plot for the various typesof fibres at 8% fibre volume. The aspect ratio of straight and crimpedfibres was 66 whereas that for the hooked fibre was 48. In order toshow the enhancement in the stress - strain characteristics achievedwith SIFCON a type stress-strain plot of a traditional SFRC mix with1% fibre volume has been plotted.

7.6 SLURRY INFILTRATED MAT CONCRETE (SIMCON)

One promising new development called SIMCON (Slurry infiltratedmat concrete) uses steel fibre mats to reinforce the concrete matrix.SIMCON produces concrete components with extremely high flexuralstrength31.

SIMCON can also be considered a preplaced fibre concrete, theonly difference between SIMCON and SIFCON being that the fibreis placed in a mat rather than as discrete fibres. The advantage ofsteel fibre mats over a large volume of discrete fibres is that the matconfiguration provides inherent strength and utilises fibres with muchhigher aspect ratios. The fibre volume is less than half that requiredfor SIFCON (slurry infiltrated fibre concrete), while achieving similarflexural strength and energy absorbing toughness.

SIMCON is a non-woven steel fibre mat that is infiltrated withconcrete slurry. The steel fibre is directly cast from molten metal usinga chilled wheel concept, then interlayed into a 1/2 to 2 in. thick mat.This mat is then rolled and coiled into weights and sizes convenient toa customer’s application, and can range upto 48 in. wide and 500 lb.

A variety of factors such as, aspect ratio and fibre volume influ-ence the performance of SIMCON. Higher aspect ratios are criticalto obtain increased flexural strength in the concrete composite. SIM-CON utilizes fibers with aspect ratios exceeding 500. Since the mat isalready in a preformed shape, handling problems are minimized andballing does not become a factor. Hackman et.al carried out investi-gations on SIMCON using manganese carbon steel mat having fibresapproximately 9.5 in. long with an equivalent diameter of 0.010 to0.021 in. and stainless steel mats produced using 9.5 in. long fibres


with an equivalent diameter of 0.010 to 0.020 inches and compared theperformance of SIMCON with SIFCON having fibres 14% by volume.

7.7 CONCLUSION

The most significant influence of incorporation of fibres in concreteis to delay and control the tensile cracking of concrete. Thus, inher-ently unstable tensile crack propagation in concrete is transformed intoa slow and controlled crack growth. The addition of fibres improvesthe static flexural strength, flexural fatigue strength, impact strength,shock resistance, ductility, and flexural toughness of concrete. Thedesigner may best view FRC as a concrete with improved mechanicalproperties. However, the increase in these properties will vary fromsubstantial to nil depending on the quality and types of fibres used;in addition, the properties will not increase at the same rate as fibresare added.

Steel fibre reinforced concrete has been used with considerablesuccess in paving, hydraulic, and shotcreting applications, and theindications are that its use, at least in paving and shotcreting, islikely to increase. There are also signs of increasing interest in usingsteel fibres in a variety of precast products. There has been grow-ing interest in utilizing natural fibres for making low cost buildingmaterials in recent years. Alkali resistant glass fibres have generatedworld-wide interest and are considered as a possible replacement forasbestos fibres.

7.8 ACKNOWLEDGEMENT

The author is thankful to Director, CSIR-SERC for granting permis-sion to deliver the lecture.

7.9 REFERENCES

1. Report by ACI committee 544, ‘Design Consideration for SteelFibre Reinforced Concrete’, ACI Structural Journal, Sep Oct1988, pp 563–580.

2. Krishna Raju N., Basavarajaiah B. S., and Janardhan Rao,K., ‘Compressive Strength and Bearing Strength of Steel FibreReinforced Concrete’, Indian Concrete Journal, June 1977, pp183–188.


3. Narayanan R. and Kareem Palajim, “Effect of Fibre Additionon Concrete Strengths”, Indian Concrete Journal, April 1984, pp100–103.

4. Fanella D. A. and Naaman, A.E., “Stress Strain Properties ofFibre Reinforced Mortar in Compression”, ACI Journal, Vol. 82,No. 4, July Aug 1985, pp 475–433.

5. Ramakrishnan V., Oberling G., and Tatnall P., “Flexural FatigueStrength of Steel Fibre Reinforced Concrete”, ACI Special Pub-lications, SP 105, 1987, pp 225–245.

6. Balaguru P., and Ramakrishnan V., “Properties of Light WeightFibre Reinforced Concrete”, ACI Special Publications, SP 105,1987, pp 305–322.

7. Oh, B.H., “Flexural Analysis of Reinforced Concrete Beams Con-taining Steel Fibres”, Journal of Structural Engineering, Vol. 118,No. 10, Oct. 1991, pp 2821–2836.

8. Hughes, B. P., and Fattuhi, N. I., “Load Deflection Curves ofFibre Reinforced Concrete Beams in Flexure”, Mag. of ConcreteResearch, Vol. 29, No. 101, Dec.1977, pp 199–206.

9. Craig R, “Flexural Behaviour and Design of Reinforced FibreConcrete Members”, ACI Special Publication, SP 105, 1987, pp517–563.

10. Swamy, R. N., and Al Noori K. A., “Flexural Behaviour of FibreConcrete with Conventional Steel Reinforcement”, Rilem Symp.on Fibre Reinforced Cement and Concrete, 1975, pp 187–196.

11. Johnston, C. D. and Skarendahl, A., “Comparative Flexural Per-formance Evaluation of Steel Fibre Reinforced Concrete Accord-ing to ASTM C 1018 Shows Importance of Fibre Parameters”,Material and Structures, Vol. 25, 1992, pp 191–200.

12. Hannant, D. J., “Steel Fibres and Lightweight Beams”, Concrete,Vol. 6, No. 8, Aug. 1972, pp 39–40.

13. Kormeling, H. A., Reinhardt, H. W., and Shah, S. P., “Staticand Fatigue Properties of Concrete Beams Reinforced with Con-tinuous Bars and with Fibres”, ACI Journal, Vol. 77, No. 1, Jan.Feb.1980, pp 36–43.

14. Krishnamoorthy, T. S., Parameswaran, V. S., and Bharatkumar,B. H., “Flexural Behaviour and Toughness of Steel Fibre Rein-forced Concrete”, Proc. of the Int. Symp. on Innovative Worldof Concrete (ICI IWC 93), Bangalore, Vol. 1, Aug. 1993, pp2.163–2.174.


15. American Society for Testing and Materials, “Standard Methodof Test for Flexural Toughness of Fibre Concrete”, ASTM Stan-dards for Concrete and Mineral Aggregates, Vol. 04, No. 02,Standard Number C-1018, August 1984, pp 637–644.

16. Balasubramanian, K., Santhi Gangadar, and Parameswaran, V.S., “Fatigue Performance of Fibre Reinforced Concrete A stateof the art report”, Technical report, SERC, Madras.

17. Batson, G., Ball, C., Bailey, L., Landers, E., and Hooks, J., “Flex-ural Fatigue Strength of Steel Fibre Reinforced Concrete Beams”,ACI Journal, Vol. 69, No. 11, November 1972, pp 673–677.

18. Romualdi, J. P., “The Static Cracking Stress and FatigueStrength of Concrete Reinforced with Short Pieces of Steel Wire”,The Structure of Concrete, Cement and Concrete Association(London), 1968, pp 190–216.

19. Balasubramanian, K., Krishnamoorthy, T. S., Bharatkumar,B. H., and Gopalakrishnan, S., “Study of the Behaviour ofSteel Fibre Reinforced Concrete under Cyclic Loading” ResearchReport No.CCL-FRC-97-1, SERC, Madras, October 1997.

20. Batson, G. B., Jenkin, E., and Spathey, R., “Steel Fibres as ShearReinforcement in Beams”, ACI Journal, Vol. 69, No. 10, 1972, pp640–647.

21. Jindal, R. L. “Shear and Moment Capacities if Steel Fibre Rein-forced Concrete Beams”, Fibre Reinforced Concrete, SP 81, ACI,Detroit, 1984, pp 1–16.

22. Kaushik, S. K., Gupta, V. K., and Tarafdar, N. K., “Behaviourof Fibre Reinforced Concrete Beams in Shear”, Proc. of the Int.Symp. on Fibre Reinforced Concrete, 1987, pp 1.253–1.132.

23. Batson, G. B. and Alguire, C., “Steel Fibres as Shear Reinforce-ment in Reinforced Concrete T beams”, Proc. of the Int. Symp.on Fibre Reinforced Concrete, 1987, pp 1.113–1.123.

24. Narayanan, R. and Darwish, I. Y. S., “Use of Steel Fibres asShear Reinforcement”. ACI, Structural Journal, Vol. 84, No. 3,1987, pp 216–227.

25. Balasubramanian, K., Bharatkumar, B. H., Gopalakrishnan, S.,and Parameswaran, V. S., “Impact Resistance of Steel Fibre Rein-forced Concrete”, The Indian Concrete Journal, Vol. 70, No. 5,May 1996, pp 257–262.

26. Schrader, E. K., “Impact Resistance and Test Procedure forConcrete”, ACI Journal, Vol. 78, No. 2, March-April 1981, pp141–146.


27. Ramakrishnan, V., Brandshaug, T., Coyle, W. V., and Schrader,E. K., “A Comparative Evaluation of Concrete Reinforced withStraight Steel Fibres with Deformed Ends Glued Together inBundles”, ACI Journal, Vol. 77, No. 3, May-June 1980, PP135–143.

28. Mangat, P. S. and Azari, M. M., “A Theory for the Creep ofSteel Fibre Reinforced Concrete Matrices under Compression”,Journal of Material Science, Vol. 20, 1985, pp 1119–1133.

29. Beaudoin, J. J., “Hand Book of Fibre Reinforced Concrete:Principles, Properties, Developments and Applications”, NoyesPublication, New Jersey, USA, 1990.

30. Lankard, D. R., “Slurry infiltrated fibre concrete (SIFCON)”Concrete International, Vol. 6, No. 12, 1984, pp 44–47.

31. Lloyd, E. Hackman, Mark, B. Farrell and Orville O. Dun-ham, “Slurry Infiltrated Mat Concrete (SIMCON)”, ConcreteInternational, 1992, pp 53–56.

32. Erdem Dogan and Neven Krstulovic-Opara, “Seismic Retrofitwith Continuous Slurry Infiltrated Mat Concrete Jackets”, ACIStructural Journal, Vol. 100, No. 6, 2003, pp 713–723.

33. Piti Sukontasukkul, “Hybrid Steel Fibre Reinforced Concrete Cir-cular Plates under Bending”, the Journal of KMITNB, 2004, Vol.14, No. 4.

34. Mazin Burhan Adeen and Alya’a Abbas Al-Attar, “Determi-nation of Mechanical Properties of Hybrid Steel-Nylon FiberReinforced Concrete”, Modern Applied Science, Vol. 4, No. 12,2010, pp 97–109.

35. L U Zhean, FAN Xiaochun, CHEN Yingbo, “Mechanical Prop-erties of Layered Steel Fiber and Hybrid Fiber Reinforced Con-crete”, Journal of Wuhan University of Technology, Vol. 23, No.5, 2008, pp 733–737.

36. Mehmet zcan, D., “Experimental and finite element analysison the steel fiber-reinforced concrete (SFRC) beams ultimatebehavior”, Construction and Building Materials, Vol. 23, Issue2, February 2009,pp 1064–1077.

37. Wang, Z. L., “A study of constitutive relation and dynamic failurefor SFRC in compression”, Construction and Building Materials.Vol. 24, Issue 8, August 2010, pp 1358–1363.

38. Sun, M., “Bending Toughness of Zinc Phosphate Steel Fiber Rein-forced Concrete before and after Corrosion”, Advanced MaterialsResearch, 1762, pp 168–170.


39. Semsi Yazici, “Effect of aspect ratio and volume fraction of steelfiber on the mechanical properties of SFRC”, Construction andBuilding Materials. Vol. 21, Issue 6, June 2007, pp 1250–1253.

40. Wang, X. W., “Research on Fracture-CMOD Toughness of SteelFiber Reinforced Concrete”, Advanced Materials Research, Vol.168, No. 70, pp 1784–1787.

41. Xu, B. W., “Correlations among mechanical properties of steelfiber reinforced concrete”, Construction and Building Materials,Vol. 23, Issue 12, December 2009, pp 3468–3474.

42. Piti Sukontasukkul, “Post-crack (or post-peak) flexural responseand toughness of fiber reinforced concrete after exposure to hightemperature”, Construction and Building Materials Vol. 24, Issue10, October 2010, pp 1967–1974.

43. Kazuo Watanabe, “Effect of Elevated Temperatures on FlexuralBehaviour of Hybrid Fibre Reinforced High Strength Concrete”,Journal of Structural Fire Engineering, Vol. 1, No. 1, 2010 pp17–27.

Table 7.1 Results of Static Flexural Tests on SFRC Beams 14

Load at (kN) ApparentFibre % Volume Ave. Cube First Maximum (Ultimate)Type of Fibre Compressive Crack m Flexural

Strength Strength(MPa) (MPa)

Crimped 0.5 31.03 12 16.4 4.91 29.45 12.5 18.75 5.62

1.5 32.16 15 22.35 6.72 28.2 20 31.9 9.57

Trough- 0.5 31.13 13 17.9 5.37Shaped 1 33.76 14.5 25.5 7.65

1.5 36.25 20 32.75 9.822 32.35 20 34.75 10.42

Straight-1 0.5 30 13.7 17.37 5.211 28.17 15 19.37 5.81

1.5 29.12 17.5 22 6.6

Straight-2 0.5 32.5 12.5 19.38 5.811 32.7 15 26.75 8.02

1.5 31.11 17.5 32.25 9.68

Plain – 28.6 12 12 3.6


Table 7.2 Results of the Tests on the Shotcreted PanelsSpecimen Weld Avg. Measured Experimental peak Energy

ID mesh/Fibre specimen thickness load (kN) absorbed up to(mm) 25 mm

deflection (N-m)

WS WELD MESH 47.5 18.08 331

WS WELD MESH 49.8 19.35 341WS WELD MESH 94.4 40.94 410WS WELD MESH 99.4 58.23 502

F1 FIBRE 0.5% 49.7 33.33 504F1 FIBRE 0.5% 50.8 25.36 511F1 FIBRE 0.5% 97.5 79.37 1021F1 FIBRE 0.5% 100.8 67.97 1270



Table 7.3 Details of Various Sifcon Mixes and the Test Results

Mix Proportion Compressive Split Tensile(Cement : Sand: Strength (28 days), Strength,w/c: SP:VMA) MPa MPa

1 : 1: 0.40 : 0.5% 29.375 2.398SNF : 0.125%

1 : 1: 0.35 : 0.3% 38.945 2.557PC: 0.125%

1 : 1.5 : 0.40: 0.7% 29.012 2.49SNF : 0.125%

1 : 1.5: 0.35 : 0.4% 33.648 2.456PC: 0.125%

8000

6000

4000

2000

5000 10000

Control

15000 20000

Vf = 3%

Vf = 2%

Vf = 1%

(1 psi = 0.0068 MPa)0

0

Com

pres

sive

stre

ss, p

si

Axial strain, millionths

Smooth steel fibersl/df = 83

Fig. 7.1 Stress strain Curve for steel fiber reinforced concrete


0.00 10 20

Deflection (mm)

Load

(kN

)

30 40

COM6TA1SA2CA1

10.0

20.0

30.0

40.0

50.0

60.0

Fig. 7.2 Load deflection plot for reinforced concrete beams withdifferent steel fibers (1% by Volume)

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02Strain

Stre

ss [M

Pa]

C_080 peak C_080 mon

Fig. 7.3 Comparison of monotonic loading and peak loading ofOPC based concrete mixtures for a fiber content of 80 kg/m3


90

8070

60

50

4030

2010

00 0.005

Strain (mm/mm)

Stra

ss (M

Pa)

0.01 0.015 0.02

StraightCrimpedHookedFRC 1%

Fig. 7.4 Stress-Strain plot in compression for 8% fibres

8 Fibre Reinforced Polymer (FRP) in Civil

Engineering Applications

B. H. Bharatkumar and G. RameshScientist


8.1 INTRODUCTION

Majority of the civil engineering structures in the world are beingbuilt using Reinforced concrete (RC) and Prestressed concrete (PSC).However, structures constructed in aggressive environments, such as,structures in marine and coastal regions, chemical industries, waterand waste water treatment facilities and bridges often undergo deteri-oration in the form of cracking and spalling of concrete due to corrosionof steel reinforcement. Repair of such structures are always costly andrequire much effort and time. Also, after the earthquake in Bhuj,on 26th January 2001, there has been a concerted effort to addressthe seismic vulnerability of existing buildings in India. Large areas ofour country have been reclassified and upgraded to higher zones inthe recent revision of IS codes, which essentially requires undertakingnational programme on evaluation and retrofitting of existing multi-storied buildings. Common conventional techniques for strengtheningstructural elements include, concrete jacketing, shotcreting/gunitingand steel plate bonding. The limitations posed by conventionalstrengthening techniques have given an impetus to researchers toinnovate and develop new materials/techniques for structural reha-bilitation. The quest for new materials to replace the reinforcing steeland for structural rehabilitation has led to the development and appli-cation of man-made fibre reinforced plastic (FRP). Until recently, theuses of FRP were limited to aerospace and defense industries due tothe high cost. With recent developments in the manufacturing processof FRP, it can now compete with conventional concrete constructionsmaterials.




FRPs have excellent corrosion resistance in addition to high tensilestrength and low modulus of elasticity. FRP products were first usedto reinforced concrete structures in the mid 1950s (ACI 440R (1996)).Today, these FRP products take the form of bars, cables, 2-D and 3-Dgrid, sheet materials, plates etc. FRP products may achieve the sameor better reinforcement objective of commonly used metallic products,such as, steel reinforcing bars, prestressing tendons, bonded plates, andconcrete or steel jacketing.

The common link among all FRP products is the use of contin-uous fibre (glass, aramid, carbon etc) embedded in a resin matrix,the glue that allows the fibres to work together as a single element.Resins used are thermoset (polyester, vinyl ester etc.) or thermoplastic(nylon, polyethylene terephthalate etc.). The mechanical characteris-tics of FRPs are much dependent on the type of binding agent andthe manufacturing process. Also, the low modulus of elasticity of FRPmay lead to increase in deflection and cracking, particularly in largespan flexural members. Further characteristics and behaviour of FRPreinforced structural members under various loading and environmen-tal condition, viz, flexure, compression, creep, fatigue, impact anddurability are to be evaluated before using FRP in civil engineeringapplications. This paper briefly presents the investigations carried outat Structural Engineering Research Centre(SERC), Chennai on theperformance of FRP wrapping and FRP rebars as reinforcement.

8.2 CONSTITUENT OF FRPS

Different types of fibres, such as, carbon, aramid, glass and polyvinylalcohol are being used as an alternative to steel reinforcement inconcrete structures. The fibers are usually bonded together with bind-ing agents, such as, resins having widely different composition. Steelreinforcement are likely to have almost identical tensile strength andother mechanical characteristics, but the mechanical characteristicsof FRPs are much dependent on the type of binding agent and themanufacturing process.

8.2.1 Fibres

Glass fibres: Glass fibres are the most common of all reinforcingfibres for FRPs (Majumdar 1985). Two types of glass fibres, namely,E-glass fibre and alkali-resistant glass fibre are commercially available.They are low cost and possess high strength and excellent insulationproperties. The disadvantages are low tensile modulus, sensitivity to

Fibre Reinforced Polymer (FRP) in Civil Engineering Applications 137

alkaline environment and low resistance to moisture and sustainedand cyclic loads. Even though alkali resistant glass fibres have beendeveloped specifically to minimize weight and strength loss in alkalineenvironment, this problem has not been entirely eliminated to date.

Carbon fibres: Carbon fibre is made from either petroleum or coalpitch and polyacrylinitril (PAN). Its characteristics depend on thecomposition and orientation of the graphite crystals in the fibre. Car-bon fibres can be classified into four types based on the modulus: low,intermediate, high and ultra high. In general, low-modulus fibres havelower density, lower cost, higher tensile strength than higher modulusfibres. The transverse and shear stiffness and strength of carbon fibresare typically quite low in comparison with the longitudinal properties.Water, solvents, bases and weak acids at room temperature do notaffect the carbon fibres much.

Aramid fibres: Aramid fibres are manufactured by extruding poly-mer solution through a spinnernet resulting in a fibre with thermalstability, high strength and high stiffness. The aligned polymer chainresults in high strength in the longitudinal direction than in trans-verse direction. Aramid fibre is fibrillar in macrostructure, which resultin poor flexural and compressive properties. Since ultra-violet radia-tion degrades aramid fibres, they should be embedded in a protectivematrix.

Polyvinyl Alcohol Fibre: The high strength polyvinyl alcohol fibreis spun by a wet process using polyvinyl alcohol of high degree ofpolymerization and rolled to provide added strength and elasticity toconventional fibres. The fibre also remains stable in the presence ofalkalis.

8.2.2 Matrices

The primary role of the matrix in FRPs is to provide lateral sup-port to the fibres and protect fibres from physical and chemical effectsdue to the surrounding. Some of the important characteristics to beconsidered in selecting a matrix for a structural FRP are: stiffness,strength fracture toughness, thermal and electrical conductivity, abil-ity to impregnate and bond of fibres, flame resistant and sensitivityto ultraviolet radiation. The important factor to be considered in theselection of a matrix is the relative mismatch in shrinkage or expan-sion between the fibre and matrix that can occur during processing.Some of the matrices used in FRP are briefly discussed below.


Polyester resin: Commercial polyester usually consists of an unsat-urated ester polymer dissolved in a cross-linking monomer such asstyrene etc. An inhibitor is usually added to the styrene to preventcross linking until the addition of a catalyst or promoter. The prin-cipal advantages of polyester for FRPs are low viscosity, fast curetime, dimensional stability chemical resistance and moderate costs.The disadvantage is high volumetric shrinkage during processing.

Vinylester Resin: They are produced by the reaction of mono-functional unsaturated acids such as methacrylic or acrylic acid withepoxy resin. They have advantages over polyester in terms of chemi-cal resistance and high temperature resistance and they are easier tohandle during processing. They are well suited for the manufactureof FRPs due to the low viscosity and short cure time, but they areat a disadvantage relative to epoxies because of the high volumetricshrinkage during curing.

Epoxy Resin: Epoxy resins are the most versatile matrices forFRPs. They have an exceptionally broad range of physical properties,mechanical capabilities and processing condition. One of the majoradvantages of epoxies for the manufacture of FRP laminates is thatthe exothermic polymerization process can be slowed by lowering thetemperature of the resin after the fibres have been infiltrated. Epoxyresins are known for their excellent strength and creep resistance,strong adhesion to fibres, chemical and solvent resistance, high glasstransition temperature and low shrinkage during cure.

Polyamide Resin: Polyamides are polymers containing cyclic amidgroup in the main macromolecular chain. The advantages are their out-standing resistance to heat, thermal degradation organic solvents andhigh energy radiation. They are slightly susceptible to attack by diluteacids and dissolved by strong mineral acids at high temperatures.

Resin Fillers and Additives: Fillers were first used to reduce thevolume of polymer used in an application and thereby reduce costswithout excessively degrading the properties. A common filler forreducing cost and shrinkage of polyester and vinyl ester resin is cal-cium carbonate. Other common fillers are Aluminum silicate, Kaolin,talc, mica and wollastronite. Numerous resin additives are availablefor enhancing the resistance of matrices and FRPs to flames, smoke,moisture, oxidation, chemical shrinkage and ultraviolet radiation.


8.2.3 Manufacturing process

Processing of FRP composites (Nanni 1993 and Bakis 1993) requiresthe application of specific temperature and pressure to the materialin order to accomplish several goods like correct fiber orientation, cor-rect fibre to resin ratio, correct fiber compaction, low void content,and correct degree of cure. Unless these goals are met by proper man-ufacturing methods, FRP composites can have property variations ofseveral orders of magnitude. Low void content, optimal resin content,and good bonding between matrix and reinforcement are desirablein FRP because they lead to better mechanical properties and bet-ter resistance to the bond between matrix and fibres. Voids in FRPsare most effectively eliminated during processing by applying pressurewhile raising the temperature of the FRP and applying a vacuum.Proper resin content or fibre volume of the FRPs is assessed by follow-ing the manufactures guidelines. There are many widely used methodsfor orienting/ curing FRP rebars (Meyer 1985 and KO 1987). Some ofthese methods are manual and automated lay-up, FRP moulding, tuberolling, filament winding, pultrusion, braiding, compression mouldingetc.

Among these methods, pultrusion technique is very much usefulfor manufacturing tubes, rods and flat and angle sections (RameshSundaram 1996). This technique is the reverse of the extrusion process.Here material is pulled rather than pushed through a die. This is aprocess wherein continuous fibre reinforced section of both solid andhollow cross section can be made. The orientation of the fibres is keptconstant during the entire process. Components produced generallyhave 70 to 75 per cent fibre content by volume and have very goodstrength and stiffness. The pultrusion process can be clarified undertwo categories, namely, pultrusion using resin bath and pultrusion byresin injection. In the first process, the fibre is drawn through a resinand then through a heated die. The die removes any excess resin andalso decides the final form of the component. The disadvantage withthis is that the resin should have a long pot life and thus cure timebecomes long. In the second process, a resin system is injected intothe reinforcement as it passes through the die. Here, resin system withshort cure times can be used, thereby, increasing the production rate.In these processes, the curing is done in adjacently located ovens andafter cooling, it is cut into the required length.


8.3 INVESTIGATIONS ON FRP WRAPS

In recent years, external application of Fibre Reinforced Polymer(FRP) wraps are used to increase the performance of reinforced con-crete structural elements, viz., beams, columns, and beam-columnjoints. FRP has been used widely to replace steel jacketing (ICJ 2004,Udhayakumar et.al. 2006) as they appear efficient and competitive(Taerwe and Matthys 1999, Swamy and Gual 1996 and Hollay andLeeming 1999). External application of FRP system provides a par-ticular solution to improve the overall performance of an RC framedstructure without the necessity of radical alteration to the originalstructure. Externally bonded FRP may be used for structures thathave undergone moderate earthquake damage. Use of FRP offers sev-eral advantages, related to high strength to weight ratio, resistance tocorrosion, fast and relatively simple application. One disadvantage ofFRP is its dependence on bond to the concrete; which is a functionof tensile capacity of the concrete and the type of surface preparationused. In view of the above, many points need to be clarified on the useof FRP for application in the retrofitting of structural elements. Hence,studies were undertaken at SERC to investigate the performance ofthe retrofitted structural elements using FRP wrap. The investiga-tion outlines the experimental investigations conducted on the RCstructural elements like beams and columns to assess the efficiency ofthe FRP wraps used for the retrofitting purposes. Glass and Carbon(GFRP/CFRP) fibre wraps were used in the present investigations.

8.3.1 Procedure for wrapping

In general, the specimen, which has to receive the FRP wrap has tobe prepared. The four corners of the specimen were first chamfered toa radius of about 15 mm. The surfaces of these corners were groundmechanically to remove any laitance. Then a two component primersystem was applied on the concrete surface and allowed to cure for 24hours. A two component epoxy coating was then applied on the primercoated surface and the FRP mat was immediately wrapped over theentire surface of the specimen. A roller was then applied gently overthe wrap so that good adhesion was achieved between the concretesurface and the FRP wrap. Another coat of the two component epoxywas applied over the fiber mat and allowed to cure for 7 days. In thecase of the specimens wrapped with two layers, the second wrap wasapplied following the same procedure as described above, after thefirst wrap was applied. The second wrap was also allowed to cure fora further period of seven days. The orientation of the fibers should be


kept parallel or perpendicular to the loading direction so as to resistthe load, in the case of single layer FRP wrap, i.e. parallel in case ofresisting the axial load and perpendicular in case of resisting bendingload. However, in the case of specimens wrapped with two layers ofFRP, the fiber orientation can be other than direction in the first layer.

8.3.2 Investigations on Retrofitted RC Beams under Flexure

In order to assess the efficiency of the CFRP/GFRP wraps underflexural loading, nine numbers of beams of size 100 × 200 × 1500 mmwere cast and tested under four point load test (Balasubramanianet.al. 2007). The longitudinal reinforcement steel consists of 2 Nos. of12mm dia HSD rebars and the shear reinforcement consists of 2 leggedvertical stirrups of 6mm φ @ 150mm c/c. Single layer and double layerCFRP/GFRP wraps were used to strengthen the RC beams. To studythe influence of the number of layers of wrap on the performance ofRC beam specimens, single and double layers of CFRP and GFRPwere wrapped on the test beams.

In general, the strength and ductility of the control RC beams wereimproved considerably when the beams were retrofitted with CFRPand GFRP. Among the two, from the performance and economy pointof view, it is recommended to use one layer of GFRP for retrofittingof RC structures.

8.3.3 Investigations on Retrofitted RC Beams under Shear

To assess the shear behaviour of the RC beams wrapped with CFRP,12 numbers of beam specimens were cast with various percentageof tension reinforcement, which includes five numbers of controlspecimen. Testing was done as four point bending. Five differentpercentages of longitudinal reinforcement (0.59, 0.92, 1.18, 1.84 and2.36%) were investigated. The shear span to depth ratio was kept con-stant at 2.0 for all the twenty four specimens (Balasubramanian et.al.2007). CFRP wrapping was employed to retrofit the beams.

It was seen that RC beams retrofitted with CFRP on the sides andbottom showed increased failure load in the case of the lower tensionreinforcement compared to the control specimens. In the case of thehigher tension reinforcement, there was no improvement in the failureload for the CFRP wrapped RC beams. It was also seen that RC beamsretrofitted with CFRP showed increased failure load in the case of thebeams that were wrapped on the top, bottom and sides than that ofthe beams wrapped on the sides and bottom only. It is found that theRC beams wrapped on top, sides and bottom showed higher ductility


compared to the control RC beams. In general, the shear strengtheningof the RC beams with CFRP wrap along the entire span was foundto be better among the various methods that were investigated. But,this increased shear strength is limited by the bond between concrete- repair material interface. The strength of the repair material has alimited role to play.

8.4 INVESTIGATIONS ON RETROFITTED RC COLUMNS

The strengthening of existing RC columns using steel or FRP jacket-ing is based on the well established fact that confinement of concretecan substantially increase its axial compressive strength (Hamid et.al.1997, Rane & Rane 2001 and Frieder et.al. 1997). The experimentalprogram at SERC consisted of testing seventeen square RC columns,having a cross section of 175 mm × 175 mm with an overall lengthof 1400 mm (Fig. 8.1). The following were the main objectives of theinvestigation, (i) to study the effectiveness of CFRP and GFRP wrapsand steel jacketing in increasing the axial compressive strength of RCcolumns, (ii) to study the effect of spacing of lateral ties in provid-ing confinement to concrete and (iii) to compare the performance ofthe steel jacketed columns with those of the FRP wrapped columns(Bharatkumar et.al.2006).

Based on the experimental results, it was seen that the stress straincurve in the post peak region clearly brings out the effect of confine-ment, as the RC column with closer lateral tie spacing showed a moreductile behaviour. The ductility index was also more for the RC col-umn provided with closer lateral tie spacing. There is also an increasein peak load, maximum strains as well as ductility index in the RCcolumns retrofitted with single layer of CFRP/GFRP wrap over con-trol RC columns for both the lateral tie spacing. Among the threeretrofitting techniques employed in the investigation, steel plate jack-eting showed an increase in the peak loads as compared to the FRPwrapping for both the lateral tie spacings studied. However, the duc-tility indices were much lower for the steel plated RC columns due tolack of sufficient confinement as in the case of the FRP retrofitted RCcolumns.

The investigation goes to prove that in situations, where retrofittingof structures is encountered, particularly when the lateral ties are notprovided as per design and in situations where the structures have tobe retrofitted to meet recent seismic design provisions, it is possible to


enhance the performance of the compression members of those struc-tures by providing them with a single layer of CFRP/GFRP wrap andsteel plate jacketing. In situations, where strength and ductility are ofparamount importance, a single layer of CFRP or GFRP can be usedfor retrofitting the RC columns.

8.4.1 Investigations on corroded RC slab

To study the behaviour of corrosion damaged RC slabs retrofitted withdifferent types of repair methods, a total of 13 Nos. of RC slabs (size2000× 2000× 60mm) reinforced with rebars having different levels ofcorrosion were proposed (Sundar Kumar et.al 2008). The first series (5specimens) consists of RC slabs with reinforcement having no corro-sion. The second (5 specimens) and third series (3 specimens) consistsof RC slabs with reinforcement having 10% and 20% weight loss dueto corrosion respectively. The slabs were provided with 7 nos. of 8 mmrebars in both directions. All the slabs were tested by applying anequivalent uniformly distributed load (Fig. 8.2). The slab was simplysupported on all the four sides, dial gauges were placed below the cen-tre of the slabs and below loading points. Dial gauges were also placedat the support to measure the uplift of the support. The first of the fiveslabs (of first series) was tested to failure and the remaining four RCslabs were gradually loaded to a deflection of 10mm. These pre-crackedfour slabs were then connected to the electrochemical corrosion cell toaccelerate corrosion (Fig. 8.3). To this end, a pond of 650mm × 650mm was constructed on the central portion of the slabs and watercontaining 3.5% NaCl by weight was stagnated in that area. The rein-forcement cage of each specimen was connected to the circuit so as toserve as the anode in the corrosion cell, whereas an external stainlesssteel plate of 500mm × 500mm immersed in the pond was used ascathode. A constant power supply of 5V was applied to accelerate thecorrosion of rebar. The corrosion levels in the slabs are being mon-itored through half-cell potential measurements. RC slabs subjectedto accelerated corrosion using impressed current were tested after 60days and 120 days (one each). The remaining two RC slabs (after 120days of corrosion) were repaired using CFRP wrapping single and dou-ble layer) over an area of 800 × 800mm at the centre. It was foundthat the repair using CFRP wrapping improved the performance ofthe corroded slabs.

For casting of second and third series of RC slabs, bare rebars wereexposed to 3.5% NaCl solution in alternate wetting and drying condi-tions in order to accelerate the corrosion in rebars at the laboratory.


In the wet cycle, the rebars were immersed in 3.5% NaCl solution for3.5days and in the drying cycle, the rebars were dried at atmospherictemperature for another 3.5 days. The cycle was continued till a spe-cific level of corrosion is achieved (10% and 20% for second and thirdseries respectively). A reduction in weight loss of 10% and 20% werenoticed after 135 and 260 days exposure, respecively.

Five RC slabs were cast using corroded rebars (10% loss in weight).Out of five slabs in the second series, one slab was tested to failureand all the remaining slabs were tested up to service load level. Oneof the slabs (second series) was repaired afterwards using single layerof CFRP. The repair methods adopted in the remaining slabs are asfollows: CFRP in the form of 100mm wide strips at 300c/c in bothdirections, GFRP bars in orthogonal direction at 300c/c, GFRP barsparallel to diagonal of the slab.

The reinforcement used for the third series were those which hadlost 20% weight due to corrosion. The slab specimens were cured for28 days before under taking up the strengthening work. The first slab(S3-1) was tested without any strengthening (Control Specimen), thesecond specimen (S3-2) was strengthened with CFRP sheets along thediagonals, and third specimen (S3-3) was strengthened with CFRPsheets in both the directions. All the three slabs were tested byapplying an equivalent uniformly distributed load. The second slabin the third series which consisted of 20% corroded reinforcement wasstrengthened with CFRP sheets of 250 mm width along the diagonalsand tested. The strength and deformation characteristics of repairedslab using CFRP was found to be better than the control slab (Fig.8.4). Based on the studies, it is possible to draw a conclusion that thecorroded slab may be restored to its normal strength conditions byCFRP wrapping techniques.

Based on the experimental investigations on corroded RC slabs, thefollowing conclusions were made:

• The slabs in which corrosion was induced by the method ofimpressed current recorded a greater decrease in the maximumload. This may be due to the fact that impressed current affectsthe strengthened concrete in cover region.

• The behaviour of the slabs in the initial stages does not differmuch with corrosion though the behaviour at later stages differsconsiderable. Hence, the failure in the slab will be sudden andcatastrophic.


• Slabs strengthened with GFRP rebars failed similar to punchingshear failure, due to the fact that the depth of slab is small.However, this method can perform better in thicker slabs.

• Strengthening of slab with CFRP strips was essentially found tobe most beneficial, economical, easy to apply at site with fewerdisturbances to the surroundings

8.4.2 Investigations on corroded RC columns

In order to study the repair of corrosion damaged RC column, columnsof size 150× 150× 700mm were cast (Sundar et.al 2009). Initially tennumbers of RC column specimens were subjected to impressed cur-rent under a constant voltage of 5V (Fig. 8.5). The UPV and reboundhammer readings were taken before the start of the accelerated cor-rosion test. The corrosion levels in the columns are being monitoredthrough half-cell potential measurements. After a period of 30 daysof accelerated test, the average half-cell potential observed was in therange of -430 to -600mV. At the end of 30 days of accelerated corrosiontest, the UPV were found to be in the range of 4.4 to 4.55km/sec whencompared to values in the range of 4.8 to 4.95 km/sec at the beginningof accelerated corrosion test. The rebound hammer values were foundto be in the range of 23-30 when compared to values in the rangeof 30-36 at the beginning of accelerated corrosion test. Crack widthsof the order of 0.08 to 0.3mm were noticed in the specimens. The 4RC columns each after 30 days and 60 days of accelerated corrosiontest were repaired using CFRP/GFRP wrapping. One RC column eachafter 30 days and 60 days of accelerated corrosion test were tested without any repair. RC columns were tested in 2500kN servo controlledUTM. Deformation and strains measurements were taken at specifiedload intervals; Control, corroded and repaired columns were tested.FRP wrapping of the columns is found to be effective in restoringstrength of the corroded column to their original capacity.

Accelerated corrosion test was continued further on some of thecolumns. The five RC columns after 120 days accelerated corrosiontest were repaired using CFRP wrapping. The two RC columns after120 days accelerated corrosion test were tested without repair. Fournumbers of corroded columns were repaired using CFRP and againsubjected to accelerated corrosion process. After a period of 60 daysof accelerated corrosion these columns were also tested in 2500kN servocontrolled UTM. Deformation and strains measurements were takenat specified load intervals.


Based on the experimental investigations on corroded RC columns,the following conclusions were made:

• The corroded RC column specimens exhibited considerablereduction in stiffness due to presence of corrosion cracks andcorrosion of main reinforcement as well as corrosion of stir-rups. Hence, the corroded column specimens showed higherdeformation for the same load compared to the control specimens.

• The ultimate load carrying capacity of corroded RC columnspecimen is about 18% lower than that of control specimens.

• CFRP wrapping enhances the axial load carrying capacity of cor-roded RC column in the range of 10 to 20%. Thus, the originalstrength of RC columns affected by corrosion can be restoredthrough CFRP wrapping.

• When corroded columns wrapped with CFRP were subjected forfurther accelerated corrosion test, there is little or no change inthe capacity of column, thereby indicating the effectiveness ofwrapping in preventing the progress of corrosion.

8.4.3 Investigations on corroded RC Beams

A total of 14 numbers of beams of dimension 100 × 200 × 1500 mmhave been cast with different levels of corrosion (Ramesh et. al.2010).Out of 14 beams casted, 4 beams are control specimens, 4 beamswith 10% corrosion, 3 beams with 25% corrosion and 3 beams with30% corrosion. The beams consisted of 3 numbers of 8mm diameterrebars as tension reinforcement (with different levels of corrosion). Twouncorroded rebars of 8 mm diameter were used as anchor bars for theshear reinforcement in the compression zone. The shear reinforcementconsists of 6mm diameter stirrups at a spacing of 150 mm; the spac-ing of the stirrups has been maintained constant for the entire span.Concrete of target strength 40 MPa has been adopted. Strengtheningof the beams with single and double layer of CFRP was carried out(Fig. 8.6). There is drop of about 43% in the load carrying capacityof the beam with rebars having corrosion of 30% weight loss. In allthe beams with varying corrosion level the maximum load carried bythe beam after strengthening is found to be more than that of con-trol specimens. It can be concluded that the loss in the load carryingcapacity of RC beams due to corrosion can be resorted back fully bystrengthening with CFRP wraps. However, there is a significant lossin the ductility of beam specimens when strengthened with CFRPwrap due to the failure of the strengthened specimens is essentially by


the rupture of the CFRP wraps which results in the sudden drop inthe load. Hence, one has to be very caution while strengthening theflexural member using FRP wrapping.

8.5 INVESTIGATIONS ON FRP REBARS

For more than 100 years, steel bars have been used as reinforcementin structural concrete members. The performance of the steel rein-forcement was not satisfactory in the case of structures exposed toaggressive environment. In such cases, deterioration of reinforced con-crete structures due to corrosion of steel will proceed more rapidlyand become critical. Recently, FRP rebars are used as reinforce-ment for concrete members in place of traditional steel rebars, oras additional reinforcement in the rehabilitation or strengthening ofexisting reinforced-concrete structures. In both cases, the non cor-rosive nature of FRPs sensibly improves the durability of concretestructures. However, FRP rebars exhibit linear behavior up to failure;this property makes the behaviour of the structures brittle. Besides,the low elastic modulus of the FRPs result in high deformability, lackof ductility, and increased crack width; as a consequence, the designcriterion for FRP reinforced-concrete structures shifts to serviceabil-ity limit state that check the structural behavioral aspects instead ofthe strength to ensure functionality and safety during the expectedlife of the structures (Teng et.al. 2002, ACI 440.1R, 2003 and Nanniet.al. 1995). For wide acceptance and implementation in construc-tion, a full characterization of the mechanical properties of FRP barsis needed. The performance of reinforced concrete structures mainlydepends on stress strain characteristics of rebars in tension and thebond strength between the rebar and concrete. Hence, tests were con-ducted at SERC to study the tension and bond characteristics of theGFRP rebars before evaluating the flexural behaviour of RC beamwith GFRP rebar.

8.5.1 Tension Test on GFRP Rebars

Tension test on GFRP rebars was more complex than steel bar. Inthe case of GFRP rebar, gripping mechanism (end anchorages) playsa major role. Possibility of premature failure (crushing of rebar) atanchorage zone of the rebar was a distinct happening in the case ofGFRP rebars, unless it was provided with effective anchorage. Whenthe diameter of rebar increases, the surface bond resistance requiredto hold the bar is also increases. This in turn leads to the bar slipping


and very less axial deformation takes place (Tighiouart et.al. 1998).Hence, anchorage and anchor alignment have a significant importance,as they may cause undesired failure modes. GFRP rebars also did notexhibit any yielding when tested under tension. The tensile strengthand stiffness of GFRP bar were dependent on several factors, such as,the ratio of the volume of the fiber to the resin matrix. Different testmethods for determining the tensile strength of the GFRP bars areavailable in the literature but not yet established by any standards-producing organizations (Kocaoz 2005, Canstro and Carino, 1998).

An anchorage system consisting of a steel pipe filled with an expan-sive cementitious grout (epoxy resin mortar) was used to provideconfinement pressure on the bar. Required length of GFRP specimenswas taken and a length of 300mm at both the ends was encapsulatedusing steel pipes for better anchoring. Plastic caps were used to closethe ends of the pipes and to keep the bar in the center of the pipe.The pipe was filled with expansive grout in this position and it needed24 hours to harden so that the specimen could be turned and the sec-ond anchor prepared (Fig. 8.7). The test was conducted on a servocontrolled universal testing machine; the hydraulic grip pressure wasapplied at both ends. The axial deformation was measured with thehelp of an extensometer (Fig. 8.8).

From the stress strain plot, it was clearly seen that the tensilestress-strain characteristics of HYSD rebars were different from thestress-strain behaviour of GFRP rebars. For HYSD rebars, the tensilestress-strain relationship can be idealized as bi-linear and inelasticwhereas the same for the GFRP rebar is linear and elastic till failure.It was found that the plain bars exhibited slippage at anchorage anddid not fracture. In the case of 10mm ribbed bar, fracture was observedwhen the applied stress was more than 650 MPa. The young’s modulusof plain GFRP rebar and ribbed GFRP rebar were 55GPa and 38GParespectively (Fig. 8.9). The 10mm and 12mm diameter GFRP barsbehaved similarly under direct tension.

8.5.2 Evaluation of Bond Strength using Beam Test

Bond tests using beams were performed in accordance with theRILEM specifications RC5-1978. Test beams consisted of two rect-angular blocks of reinforced concrete joined at the top by a steel balljoint and at the bottom by the reinforcement (GFRP or steel rebar)to be tested for bonding with the concrete (Fig. 8.10).


The test was conducted in a 1000kN UTM. The test beam restingon the two end supports was loaded by two point loads of equal mag-nitude disposed symmetrically with regard to the mid span as per therequirement. Two dial gauges of 0.001mm sensitivity were properlyclamped at either end of the rebar in such a way that both the rebarand dial gauge stem were in the same horizontal level. The load wasapplied gradually and the dial gauge readings were noted at regularintervals. The tests were continued until complete bond failure of thebars or until the bar fractured. The load slip curves relating to the twohalf beams were plotted. The average bond strengths at two levels ofslippage, namely, 0.01, 0.1mm and the maximum bond strength wereevaluated as follows The stress in the rebar was calculated using therelationship

Stress in the bar(σs) =1.25F

As

for specimens having

diameter of bars in the 10 − 16 mm range

Bond stress(τd =σsAs

πΦld

where, F is the total load corresponding to required amount of slip,As is the nominal area of the bar and ld is the bonded length of bar

The bond strength of 12 mm diameter ribbed GFRP rebars was2.6 and 2.3 times the bond strength of 12mm diameter HYSD rebarsat 0.01mm and 0.1mm slip respectively, whereas the maximum bondstrength was around 1.1 times that of HYSD rebars. The bond strengthof 12mm diameter ribbed GFRP rebars was found to be 1.6 and 1.2times the bond strength of 12mm diameter TMT rebars at 0.01mm and0.1mm respectively, whereas the maximum bond strength was almostequal. This may be due to the fact that the ribbed GFRP rebars hadrough surface in addition to the ribs. Plain GFRP rebars exhibitedvery low bond strength when compared to the ribbed GFRP, HYSDand TMT rebars. The plain GFRP rebars also failed in bond for verylow magnitude of loads.

8.5.3 Flexural Behaviour of RC Beams Reinforced GFRPRebars

From a static point of view, the position of steel rebars within the crosssection does not furnish a good contribution in terms of strength, whileits contribution is effective in terms of ductility and rigidity. Besides,


the use of steel reinforcements allows one to design the beam as under -reinforced, with a limited amount of FRP reinforcement. The behaviorof a hybrid GFRP-steel reinforced beam was recently analyzed by Newhook, 2000; the yielding of steel ensures the ductility, and the strengthof the GFRP increases the ultimate capacity after steel yielding. Aieloet al. (2002) showed that steel reinforcement in combination with FRPreinforcement is advantageous from a deformability point of view. Anadequate amount of steel reinforcement within the cross section, infact, allows for the reduction of the deformability of FRP reinforced-concrete beams under service conditions. (Nehdi et al.2005), madean effort to investigate the performance of GFRP and hybrid steel-GFRP reinforced beam column joints. (Krishnamoorthy et al.2006)studied the performance of RC slabs reinforced with a combinationof both GFRP and steel rebars and found that the load deflectioncharacteristics are similar to the HYSD rebars.

The experimental studies were carried out to evaluate the flexuralbehavior of RC beams reinforced with GFRP rebars in the con-crete cover region along with steel reinforcement as the main tensionreinforcement (Bharatkumar et.al. 2007). In all, four beams of size150 × 300 × 3000mm were cast and tested under four point bending,which included one control beam reinforced with HYSD rebar andhaving a 75mm cover. Ribbed type GFRP rebars of 10 mm and 12mm diameters were used in the investigation. The covers providedfor the steel rebars and the GFRP rebars were 75 mm and 20 mmrespectively. The experimental investigation was carried out on fourreinforced concrete beams, one reinforced with only HYSD rebars, onereinforced with only GFRP rebars, and two reinforced with a combina-tion of GFRP and HYSD rebars. The Load deflection plots of beamsreinforced with only GFRP (G1) rebar and the control beam (Control)are shown in Fig. 8.11. The Load deflection plots of beams reinforcedwith a combination of HYSD and GFRP rebars and the control beam(Control) are shown in Fig. 8.12. It was concluded that the use of steelreinforcement in combination with FRP reinforcement was advanta-geous from deformability point of view. An adequate amount of steelreinforcement within the cross section, in fact, allows for the reductionof the deformability of GFRP reinforced-concrete beams under serviceconditions. The increase of stiffness was more evident for beams rein-forced with GFRP rebars placed near the outer surface of the tensilezone and HYSD rebars placed at the inner level of the tensile zonecompared to the RC beams reinforced with only GFRP rebars. Theresults of the investigation goes to prove that in situations where larger


cover is to be provided due to aggressive environment, the GFRP barscan be successfully used in the cover concrete portion along with theconventional steel reinforcement in the reinforced concrete structures.


The use of advanced composites as external reinforcement of concreteand other structures has progressed well in the past decade in selec-tive applications where their cost disadvantage is outweighed by anumber of benefits. There are clear indications that the FRP strength-ening technique will increasingly continue to be the preferred choice formany repair and rehabilitation projects involving buildings, bridges,historic monuments and other structures. The education and trainingof engineers, construction workers, inspectors, and owners of struc-tures on the various relevant aspects of FRP technology and practicewill be crucial in the successful application of FRP materials in civilengineering construction.

8.7 ACKNOWLDEGEMENT

This lecture note is being published with the kind permission ofthe Director, CSIR-Structural Engineering Research Centre, Chennai.Authors wish to thank all the staff member of Advanced MaterialsLaboratory for their help.

8.8 REFERENCES

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4. Bakis, C. E. “FRP Reinforcements: Materials and Manufac-turing”, Fibre Reinforced Plastic Reinforcements for ConcreteEdited by A Nanni, 1993, pp 13–58.

5. Meyer, R. W., “Hand Book on Pultrusion Technology”, Chapmanand Hall Publications, New York, 1985.

6. Ko E. K., “Braiding”, Section 8 of Engineering Materials HandBook, Vol. 1, Composties, ASM International, 1987, pp 519–528


7. Ramesh Sundaram, “Fabrication Process for Composite”, Proc.of a Workshop on FRP Composites, IISc, Bangalore, 1996.

8. Special issue of The Indian Concrete Journal, Vol. 78, No. 10,October 2004

9. Udhayakumar V. , Bharatkumar B. H., Balasubramanian K. andKrishnamoorthy T. S., “Investigations on the Properties of FibreReinforced Plastic Wrap”, Proc. of 5th ASPIC, SERC, Chennai,2006, pp 577–586.

10. Taerwe L., Matthys S., “FRP for concrete construction: activitiesin Europe”, ACI Concrete International 1999;21(10): 33–6.

11. Swami R.N, Gaul R., editors, “Repair and strengthening ofconcrete members with adhesive bonded plates”, ACI SP-165.Michigan: American Concrete Institute; 1996.

12. Hollaway L.C, Leeming M.B, editors, “Strengthening of Rein-forced Concrete Structures using Externally-bonded FRP Com-posites in Structural and Civil Engineering”, Cambridge: Wood-head Publishing; 1999.

13. Krishnamoorthy et.al., “Seismic Retrofit of RC Buildings usingFRP Composites”- A report prepared by SERC, Chennai andDepartment of Civil Engineering, IIT, Madras, as a part of DSTsponsored project, July 2003.

14. Balasubramanian K., Krishnamoorthy T. S., Bharatkumar B. H.,Udhayakumar V., and Lakshmann, N., “Investigations on the RCStructural Elements Retrofitted using FRP Wraps”, Journal ofStructural Engineering, Vol. 34, No. 1, April-May 2007, pp 63–69.

15. Hamid Saadatmanesh, Mohammad R. Ehsani and Limin Jin,“Repair of Earthquake Damaged RC Columns with FRP Wraps”ACI Structural Journal, Vol. 94, No. 2, 1997, pp 206–215

16. Rene Suter and Rene Pinzelli (2001), “Confinement of ConcreteColumns with FRP Sheets”, Proceedings of the Fifth Interna-tional Conference of Fibre Reinforced Plastics for ReinforcedConcrete Structures (FRPRCS 5), pp 793–802.

17. Frieder Seible, Nigel Priestley, Gilbert A Hegemier DonatoInnamorato, “Seismic Retrofit of RC Columns with ContinuousCarbon Fibre Jackets”, Journal of Composites for Construction,Vol. 1, No. 2, 1997, pp 52–62.

18. Bharatkumar B. H., Balasubramanian K., Krishnamoorthy T.S.,and Lakshmanan, N., “Investigations on the Behaviour ofRetrofitted RC Columns under Axial Load”, Proc. Og 5th AsianSymposium on Polymers in Concrete, September 2006, Chennai,pp 611–621.


19. Teng J.G., Chen J. F., Smith S. T., Lam L., “FRP StrengthenedRC Structures”, 2002, John Wiley & Sons, Ltd.

20. ACI 440.1R-03, “Guide for the Design and Construction of Con-crete Reinforced with FRP Rebars”, ACI, Farmington, Michigan,2001.

21. Nanni A, Bakis, C. E and Boothby T. E, “Test Methods for FRP-Concrete Systems Subjected to Mechanical Loads: State of theArt Review”, Journal of Reinforced Plastics and Composites, Vol.14, 1995, pp 424–557.

22. Tighiouart B., Benmokrane B., and Gao, D., “Investigation ofbond in concrete member with fibre reinforced polymer (FRP)bars”, Construction and Building Materials 1998;12;453-462.

23. Ramesh G., Sundar Kumar S., Bharatkumar B. H., Krishnamoor-thy, T. S., “Experimental Studies on Flexural Behaviour of RCBeams”, Proc. of International Conference on Advances in Mate-rials Mechanics and Management 2010 at College of EngineeringTrivandrum, during January 2010, pp 134–141.

24. Sundar Kumar, S., Ramesh, G., Bharatkumar, B. H., and Krish-namoorthy, T. S., “Performance of FRP Strengthened ReinforcedConcrete Columns at Various Levels of Reinforcement Corrosion- an Experimental Study” International Journal of 3R; RepairRestoration and Renewal of Built Environment, Vol. 1, No. 3,July-September 2010, pp 95–101.

25. Kocaoz S., Samaranayake V. A., and Nanni, A., “Tensile char-acterization of glass FRP bars”, Composites: Part B 2005;36;127–134

26. Castro F., and Carino, J., “Tensile and Non Destructive Testingof FRP bars”, Journal of Composites for Construction 1998;17-27

27. RILEM CEB FIP. Test of the bond strength of reinforcement ofconcrete: test by bending. Recommendation RC.5, 1978:5.

28. Newhook, J. P, “Design of under-reinforced concrete T-sectionswith GFRP reinforcement”, Proc., 3rd Int. Conf. on AdvancedComposite Materials in Bridges and Structures, 2000, pp 153–160.

29. Aielo M. A, Ombres L., “Structural Performances of ConcreteBeams with Hybrid (Fiber-Reinforced Polymer-Steel) Reinforce-ments”, Journal of Composites for Construction, 2002, 6(2), pp133–140.

30. Nehdi M, Said A., “Performance of RC Frames with Hybridreinforcement under Reversed Cyclic Loading”, Materials andStructures, July 2005, 38, pp 627–637.


31. Krishnamoorthy T. S, Balasubramanian K, Bharatkumar B. H,Udhayakumar V., Lakshmanan N., “Investigations on the Flex-ural Behaviour of RC Slabs with GFRP Rebars”, SERC ProjectReport No: CCL-OLP 11141-RR-2006-2, May 2006.

32. Bharatkumar B. H., Udhayakumar V., Balasubramanian K.,Krishnamoorthy T.S, and Lakshmanan N., “Experimental Inves-tigations on Flexural Behaviour of RC Beams Reinforced WithHYSD and GFRP rebars”, Proc. of Proceedings of the Interna-tional conference on Recent developments in Structural Engineer-ing (RDSE 2007), 2007, pp 1078–1085.

Fig. 8.1 Test Set-up of RC Column in a 2500kN Servo-ControlledUTM

Fig. 8.2 Testing of RC slab in progress


Fig. 8.3 Accelerated corrosion of pre-cracked RC slabs

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30Defln. (mm)

Load

(kN

)

S3-1S3-2S3-3

Fig. 8.4 Load vs deflection plot for strengthened CFRP slab

Fig. 8.5 RC columns subjected to Impressed current technique


Fig. 8.6 Corroded rebar RC beams Strengthened with CFRP

Fig. 8.7 GFRP Test specimen

Fig. 8.8 Test set-up


Fig. 8.9 Stress Strain Pot for GFRP and HYSD Rebars

Fig. 8.10 RILEM RC-5 Bond strength Beam Details

Fig. 8.11 Comparison of Load Vs Deflection of Control beam andbeam with GFRP


Fig. 8.12 Comparison of Load Vs Deflection of Control beam andbeam with HYSD and GFRP (H1 and H2)

9 Self-Compacting Concrete as a Repair

Material

J. Annie PeterDeputy Director

CSIR-SERC, CSIR Campus Taramani, Chennai-600 113, [email protected]

9.1 INTRODUCTION

Self Compacting Concretes (SCC)s are being used more and more tobuild new structures as they have a very high degree of compactabil-ity. They facilitate the casting of densely reinforced sections that aredifficult to consolidate. Such concretes accelerate the placement rateand result in superior surface quality and durability. Self compactingconcrete can also be used for repair of damaged/deteriorated elementspresenting difficulties for placement and consolidation and is feasibleto produce high quality concrete with limited degree of surface defects,in stability and cracking. This can be accomplished by removing thedeteriorated concrete and pouring SCC into the watertight formworksassembled in-situ so that the geometry of the damaged elements canbe fully restored upon removal. This method is economical when largevolumes of repair are carried out. However, the repair can prove tobe successful only if the repair material i.e., SCC interacts well withthe parent concrete and forms a durable barrier to guard against re-initiation of problems further arising, since a dimensionally unstablerepair material is placed against a dimensionally stable substrate con-crete, as no significant drying shrinkage and creep is likely to existin the substrate concrete due to its long term exposure to the envi-ronment and the service loading. Hence, properties such as shrinkage,creep and elastic modulus are considered important for specificationof repair materials. These properties are largely related to the con-stituents of the mix. Hence, it is essential to optimize a SCC mix byconsidering both the fresh and hardened concrete properties and strikea balance between the two.




Information on mix proportioning methodology, material char-acteristics, self-compactability measuring devices are described indetail.

9.1.1 Mix Proportioning of SCC

The mix proportioning of SCC involves a complex optimisation of var-ious ingredients to achieve self compactability in the fresh state anddesired properties in the hardened stage. Guidelines for mix propor-tioning according to JSCE (Table 9.1) EFNARC and ERAMCO arepresented in the following sections.(i) European Federation of National Trade Association (EFNARC)Mix Proportioning Method

The guide lines recommended by EFNARC are also based onOkamura’s method. The difference is that instead of fixing coarseaggregates limit at 0.5, a higher amount is permitted in the case onrounded aggregate (up to 0.6). The proportion of sand in the mor-tar is varied between 40% and 50%, and water to powder ratio andsuperplasticizer dosage is determined through mortar slump flow andV-Funnel test. In this method the relative proportions of the key ingre-dients of the mix is to be computed by volume rather than by mass.For quick reference, typical ranges of proportions and quantities ofthe ingredients to obtain self compactability are also furnished. Fur-ther modifications can be made to achieve the required strength andother durability performances. The sequence of mix proportioning isgiven in Table 9.2.

• Coarse aggregates are computed as a percentage of its bulk den-sity. General ranges are between 50 and 60%. Higher proportionsof coarse aggregate are permitted for smaller sized aggregates aswell as for aggregates rounded in shape. All types of aggregatesare suitable. The normal maximum size is generally 16-20mm;however particle sizes up to 40mm or more have been used.

• Typically water contents should not exceed 200 litres/m3

• Very fine synthetic fibres may prevent flow and generally thecontent should not exceed 1 kg/m3

(ii) ERAMCO Mix Proportioning MethodThis is an extension of the EFNARC document. The major changeis in the existing test methods which have been formatted as per ENtest method standards. The typical range of constituents in SCC byweight and volume is shown in Table 9.3. The fine aggregate content

Self-Compacting Concrete as a Repair Material 161

balances the volume of the other constituents, typically 48-55% oftotal aggregate weight.

9.1.2 Selection of Ingredients

Selection of ingredients/materials plays a very significant role in mix-ture proportioning of Self Compacting Concrete (SCC). Besides thematerials used in conventional concretes, SCC mixtures have combina-tions of certain ingredients that enhance flowability significantly whileretaining their stability. This consists of combinations of admixturesand specific particle size distribution. The quality of the materialsneeds to be consistent as well. Thus a good understanding of theinfluence of the various ingredients on self compacting propertiesis essential prior to designing a SCC mixture. For SCC to be selfcompactable it should exhibit three principal characteristics such asflowability, passing ability and resistance against segregation. This pre-sentation discusses the materials that are used to make SCC and themost widely used test methods for evaluating its self compactability.

9.1.3 Materials

SCC consists of cement, aggregates, mineral admixtures, chemicaladmixtures and water. Some of the aspects to be considered duringselection of the materials are listed below:

Cement

SCC can be produced with most of Portland cements. Most of theresearch on SCC is being done using Ordinary Portland cement.Though all cements conforming to various IS standards are suitable,selection of cement should be based on their compressive strength,fineness and compatibility with other ingredients. Cements of vari-ous strengths are available. The strength of cement decides the targetstrength of concrete. Similarly, finer cements have higher reactivitywith water and hence has a bearing on the progress of hydration andthe rate of strength development. The early age strength and ultimatestrength is also higher with higher specific surface area. However, thefiner the cement the higher the water demand for SCC to achieveflowability. Care should be taken to determine the water demand fordifferent batches of cements even of the same type to achieve a pasteof normal consistency.

9.1.4 Mineral Admixtures

Mineral admixtures such as fly ash, GGBS, silica fume, metakaolinand rice husk ash are always used in developing SCCs to make up the


demand for higher powder content. They are useful in enhancing thedeformability and stability of the fresh SCC. Their large surface areagives a corresponding increase in internal forces resulting in an increasein the cohesiveness of the concrete. Further being spherical they act asball bearings to lubricate the mix giving it a greater mobility. Due toimproved packing contributed by the very small size of the particles,aggregate cement interface is stronger when mineral admixtures arepresent. The concretes will be homogeneous and dense and hence isless susceptible to segregation. This results in improved strength andimpermeability. Concretes incorporating these admixtures develop lessheat due to hydration. The main chemical effects of mineral admixtureaddition to concrete are a reduction of calcium, potassium and sodiumhydroxides due to pozzolanic reactivity. They act as nucleation sitesfor C-S-H. The main physical effect on the microstructure of hardenedconcrete is the refinement of the pore structure. Another importantfactor is the improvement in quality of the transition zone betweenthe aggregate particles and cement pastes.

Due to high surface area and high content of reactive silicon dioxidein silica fume, metakaoline and rice husk ash, they are more reactivethan fly ash and GGBS. They are usually incorporated into concreteat 5-15% by weight of total binder. Fly ash and GGBS have less ofSiO2 content, are coarser and hence less reactive. These admixturesare used in higher dosages of 30 to 50 % of the binder content.

For these admixtures to be effective, uniform distribution in con-crete is essential. Further due to their high fineness an increased waterdemand is likely. Hence these admixtures should always be used inconjunction with a superplasticiser.

Conformity of these admixtures with the respective codes should beensured. As different sources of these mineral admixtures may interactwith different efficiency, trials to establish the optimum dosages ofthese materials may often be required.

Inert fillers like limestone powder are frequently used to makeSCCs.

Chemical Admixtures

SuperplasticisersThe role of superplasticisers or High range water reducers (HRWRs) ismuch more significant in Self Compacting Concretes. To produce SCCat very low levels of water-cement ratios without needing unacceptablyhigh cement contents the use of superplasticisers is required. These arehigh molecular weight water soluble polymers. These admixtures work


on the principle of electrostatic repulsion. The superplasticisers getstrongly adsorbed onto cement surfaces with the negative charges buildup resulting in cement particle repulsion. The water thus gets releasedfrom the flocculated cement. Improved dispersion of cement grainslead to enhanced early age strength. Lignosulphonates, Melamine,and naphthalene based superplasticisers have been used successfully,either individually or in combination. Lignosulphonate based plasti-cizers may be combined with melamine superplasticisers in order toextend their workability retention. The dosage rates of the superplasti-cisers can be high in order to achieve the required workability. It shouldbe noted that there is generally a saturation dosage of superplasticisersabove which no further increase in workability will occur. This can eas-ily be determined using a marsh cone. The efflux time is measured atthe same free water cement ratio for a series of admixture dose rates.This will enable the maximum effective level of admixture additionto be identified. Compatibility between different admixtures used incombination as well as compatibility between admixtures and differentcement types must be considered when materials are selected. Flowcone tests may be useful Superplasticisers perform more effectivelywith certain cements.. Workability is greatly enhanced by delayedaddition of the superplasticiser rather than adding it with the mixingwater.

Polycarboxylated ether based superplasticisers work on the princi-ple of steric hindrance and are effective at lower dosages and hencebest suited for use in SCC.

Viscosity Enhancing Agent

Viscosity-enhancing admixtures (VEAs), also known as thickeningagents are useful in enhancing the cohesion and stability of SCCs.These admixtures can reduce the risk of separation of the heteroge-neous constituents of SCC during transport, placement, and provideadded stability to the cast concrete while in a plastic state. The incor-poration of a VEA enables the production of a stable and yet highlyflowable concrete to facilitate filling of congested reinforced elements.Viscosity enhancing agents produce higher deformability of the freshconcrete in its flowing state and poorer deformability while in a stateof rest. Addition of a viscosity agent can strongly reduce the signifi-cant influence that wrongly estimated aggregate moisture content hason fresh properties. The water content per m3 can be varied by asmuch as 10 litres while the concrete still retains the self compactingproperties.


Concrete incorporating a VEA can be sticky and viscous, especiallywhere there is a high concentration of VEA. The combined action ofVEA and SPs can result in some delay in setting, especially at highSP concentrations. The use of VEA also increases the Air-entrainingAgent (AEA) demand. The effect of VEA on mechanical propertiesshow that in some cases, slight reduction of strength up to 10% couldbe expected due to additional entrapment of air in the fluid.

Table 9.1 lists the different types of viscosity enhancing agents.These can be classified into five classes according to their mode ofaction in concrete. These classifications are as follows: Class A: Watersoluble synthetic and natural organic polymers that increase the vis-cosity of the mixing water. They include natural gums, cellulose ethers,polyethylene oxides, polyacrylamide and polyvinyl alcohol. Class B:Organic water-soluble flocculants that become adsorbed onto cementgrains and increase viscosity due to enhanced inter-particle attractionbetween cement grains. They include styrene co-polymers with car-boxyl groups, synthetic polyelectrolytes and natural gums. Class C:Emulsions of various organic materials which enhance inter-particleattraction and supply additional superfine particles in the cementpaste. They are acrylic emulsions and aqueous clay dispersions. ClassD: Water-swellable inorganic materials of high surface area whichincrease the water retaining capacity of the paste, such as bentonites,silica fume and milled asbestos. Class E: Inorganic materials of highsurface area that increase the content of fine particles in paste andthereby, the thixotropy. These materials include fly ash, hydrated lime,kaolin, various rock dusts and diatomaceous earth, etc.

The VEAs could be in liquid or powder form. The commonly avail-able VEAs include Kelcocrete, a powder based product containingdiutan gum and used in concentrations ranging from 0.05-0.20% ofthe water content, Celbex 208 (Fosroc), a cellulose based liquid admix-ture used at 0.7-1.2% dosage, Rheomac UW 450 and Glenium stream(Master Builders), a cellulose based liquid admixture used at a dosageof 0.26-1.3%, and Sikament 100 SC (Sika Qualcrete).

9.1.5 Aggregates

IS 383- lists the standard specifications for use of aggregates in con-crete. Knowledge of aggregate characteristics such as specific gravity,bulk density, particle size distribution and moisture content is essen-tial prior to proportioning of SCC mixtures. Since aggregate grading,shape and surface texture have a major role in affecting the rheologicalproperties of SCC, these characteristic may also be considered while


proportioning. The particle shape should ideally be equidimensionali.e. not elongated or flaky. Aggregates should be relatively free of flatand elongated particles. Elongated aggregates should be avoided orlimited to a maximum of 15% by weight of total aggregates. Com-pared to rough textured angular and elongated particles smooth androunded aggregates require less of cement paste to produce flowingconcretes. Crushed rock aggregates are generally preferred to smoothgravels as there is evidence that the strength of the transition zone isweakened by smooth aggregates. However smooth rounded aggregatesincreases the deformability of fresh concrete.

A maximum size of 10 to 14 mm is usually selected although aggre-gates up to 20 mm may be used. These restrictions are imposedby the need for the concrete to be able to flow though narrowspaces and though the reinforcement without segregation and blocking.aggregates should be strong and free of internal flaws or fractures.

Aggregates of high intrinsic strength are generally preferred. Gran-ites, basalt, lime stones and sandstones are being successfully used inSCC. However aggregate strength is usually not a factor for normalstrength concretes because they are several times stronger than thematrix and the transition zone.

Fine aggregate shall consist of natural sand or manufactured sandor a combination. Fine aggregates should be selected so as to reducethe water demand hence rounded particles are thus preferred tocrushed rock fines where possible. The grading curve of fine aggre-gate should however be smooth and free of gap grading to optimizewater demand. The finest fractions of fine aggregate are helpful toprevent segregation.

The silt, clay dust content of both fine and coarse aggregate shouldbe as low as possible. Presence of deleterious substances either incoarse or fine aggregate could adversely affect the workability, settingand hardening and durability characteristics of concrete. In practicelow void contents can be achieved by using smoothly graded coarseaggregates with suitable proportions of graded sand.

Materials which belong to this category normally contain coarseaggregates (up to 10mm size) to make the material more economical.Materials used in high volume repairs are due to the fact that largeraggregates (quantity and size) are contained within the mix. Conse-quently, the possibility of cracking in the repair patch is decreased.The type and quantity of coarse aggregate have an enormous effect onthe free shrinkage of repair materials.

Test Methods for Evaluation of Self Compactability


This section covers test methods and apparatus used for assessmentof self compactability of fresh SCC so that they can be placed satisfac-torily without compaction. Most of these methods enable a rapid andcost effective assessment of SCC and have been accepted universally.However, a combination of tests is to be carried out systematically toestablish the Flowing Ability, Filling Ability, Passing Ability and Seg-regation Resistance for the development of SCC mixtures as shown inTable 9.2. No single test has been proved capable of measuring all thecharacteristics. Typical maximum and minimum ranges of test valuesfor acceptance of SCC are shown in Table 9.3.

9.1.6 Tests Methods for Flowing Ability

Slump flow (including T50 time), V-funnel and Orimet can be used todetermine the flowing ability.

Slump Flow Test

The slump flow test is used to assess filling ability (free deformabilityin the absence of obstructions) of SCC. The test measures the extentof spread of concrete after lifting the cone in terms of the diameterof the spread. The test is easy to conduct both in the laboratory andsite.

The equipment consists of a traditional slump cone of 300 mmheight, 200 mm base diameter and 100 mm top diameter (Fig. 9.1).The other requirements for the test are: Base plate of minimum size of900 mm × 900 mm (which is water tight and with a smooth surface)with concentric diameter of 500 mm marked on it, a scoop preferablywith a rounded mouth not more than 100 mm wide, scale graduatedin mm and a stop watch.

The slump cone is placed centrally on the base plate and filled withconcrete up to the top. The conical mould requires approximately 5.5litres of concrete to fill. Lift the cone perpendicular to the base plateand simultaneously start the stop watch. Record the time taken forthe spreading concrete to reach a diameter of 500 mm (T50). Whenthe concrete stops flowing, measure the diameter of spread in twoperpendicular directions. The test result is the mean value of the con-crete spread rounded to the nearest 5 mm. The spread concrete is alsoassessed visually. Any occurrence of segregation is also recorded.

The higher the slump flow the greater the filling ability.


Orimet Test

The Orimet test is a practical test method for rapid assessment of fill-ing ability and uniformity of fresh SCC mixes. The orimet is a simple,rugged, durable, easily maintained and portable apparatus with goodsimulation of movement of fresh SCC during placing in site.

The apparatus consists of a vertical casting pipe of 120 mm internaldiameter fitted with an interchangeable orifice at its lower end. Aquick- release tap door is used to close the orifice. For concretes withaggregate of 20 mm maximum size the orifice diameter is normally70 mm to 80 mm. An integral tripod supports the casting pipe (Fig.9.2). A sample of approximately 7.5 litres of fresh concrete is required.A bucket having a volume of at least 10 litres to collect the concretedischarged from the Orimet and a stop watch with accuracy of 0.2 sto measure the flow time (FT) is required for the test.

The Orimet is set on firm ground and ensured that the trap dooris closed. Concrete is poured into the casting pipe. A bucket is placesunder the trap door. Open the trap door within 1 minute of filling thepipe and simultaneously start the stopwatch. Stop the stopwatch assoon as all concrete has emptied when viewed from top of the pipe.

High values of flow time indicate mixes of high viscosity. A mix oflow segregation resistance can cause coarse aggregate to settle in theorifice area, increase greatly the FT or cause a blockage and a partial/ intermittent flow.

V Funnel Test

The V funnel is used to determine the filling ability (flowability) ofSCC. It is not applicable if the maximum size of aggregate exceeds 25mm.

The equipment consists of a V-shaped funnel as illustrated in Fig.9.3. The funnel is of rectangular cross section of 490 mm × 75mm attop tapering to a bottom opening of 65 mm × 75 mm. The dischargeorifice of the funnel is equipped with a trap door. The funnel is made ofsteel or acrylic and placed vertically on a supporting stand. A sampleof fresh concrete of approximately 12 litres is required. A bucket witha capacity of 15 litres to receive the concrete under the funnel and astop watch with an accuracy of 0.1 s is also required.

The trap door of the funnel is closed and concrete filled in thefunnel. Start the stop watch on opening the tap door. The time takenfor the concrete to flow out of the funnel is recorded. A high flow time


can be associated with a low deformability due to high paste viscosity,a high interparticle friction or a blockage of flow.

9.1.7 Test methods for Passing Ability (blocking)

Passing ability determines how well a fresh SCC will flow throughconstricted spaces and between reinforcement. The aim is to assess thelevel of risk that the coarse aggregate in the mix will become wedgedbetween bars or form arches, which will partially or fully block, orobstruct flow of the fresh mix. The L- box test, J-Ring test and U-Boxgives an indication as to the filling and passing ability.

L-Box Test

The test assesses the effect of reinforcement on free flow of con-crete constrained by formwork. Blocking caused by oversized coarseaggregate o its excessive content can be detected as well as block-ing generated by moderate /severe segregation. A concrete can beregarded as possessing a degree o segregation resistance, if the par-ticles of cease aggregate are seen to be distributed on the concretesurface all the way to the end of the horizontal part.

The L-Box apparatus comprises of a vertical column section and ahorizontal trough (Fig. 9.4) The vertical column has an inside dimen-sion of 200 mm × 100 mm × 600 mm and the horizontal trough hasdimensions of 200 mm × 120 mm × 700 mm. A sample of approxi-mately 12 litres of concrete is required for the test. Concrete is allowedto flow from the vertical column once a trap door is opened. The con-crete then passes through reinforcing bars placed in the horizontaltrough immediately beyond the trap door of the apparatus. The hor-izontal trough has a length of 800 mm. The time taken for a concreteto flow a distance of 200 mm and 400 mm in the horizontal section ismeasured. The height of the concrete at both the ends of the horizon-tal trough is also measured and expressed as the blocking ratio. It isthe ratio between the height of concrete surface in the vertical columnpart (h1) and the height of the concrete surface in the trough at itsfar end (h2) after the passage through vertical reinforcing bars.

J-Ring test

The test measures the effect of reinforcing bars on the free movementof SCC. The J-Ring is used in combination with the slump cone or theOrimet test. The equipment consists of a ring placed on several rebarswith adaptable gap widths (Fig. 9.5). For 20 mm maximum size ofaggregates the gap between the rebars is 55 mm and for 10 mm sizeof aggregates the gap is 35 mm. J-Ring with slump cone requires 5.5


litres of SCC to fill the mould whereas in combination with Orimetabout 7.5 litres is required.

When used in combination with the slump cone which is placedconcentrically with the J Ring the concrete is allowed to flow throughthe bars. The final diameter of the concrete when the flow stops ismeasured. The concrete is considered self compacting when the diam-eter with and without the J- Ring do not differ by more than 50 mm.

U-Box

The test is used to measure the filling ability of SCC.The apparatus consists of a vessel which is divided by a middle

wall into two compartments (Fig. 9.6). A sliding gate is fitted at thebottom of the wall. Deformed reinforcing bars with nominal diametersof 13mm are installed at the gate with centre to centre spacing of 50mm. This creates clear spacing of 35 mm between the bars. The testrequires a volume of approximately of 20 litres. Stop watch and ameasuring scale are required when performing the test.

Initially close the partition gate in the U-box. Concrete is pouredinto the first compartment. The concrete is leveled. After the elapse of1 minute open the gate by sliding the door upwards to let the concretesample flow to the second compartment through the clearance of thereinforcement bars installed at the gate. Record the time from theopening of the gate till the completion of flow of the concrete. Theheight of concrete in the second compartment is measured.

The concrete is considered to achieve a good filling ability when thefilling height of concrete is approximately 300 mm If the filling heightis significantly less than 300 mm the concrete does not have sufficientfilling ability.

9.1.8 Fill- box Test Apparatus

The test is used to measure the filling ability of self compacting con-crete with a maximum aggregate size of 20 mm. The apparatus consistsof a transparent container with a flat and smooth surface. In the con-tainer are 35 obstacles made of PVC with a diameter of 20 mm anda centre to centre distance of 50 mm (Fig. 9.7). At the top side thereis a filling pipe (diameter 100 mm and height 500 mm) with a funnel(height 100 mm).About 45 litres of concrete is needed to perform thetest.

The container is filled with concrete through the filling pipe byadding 1.5 to 2 litres of fresh concrete into the funnel until the concrete


has just covered the first top obstacle. After the concrete has come torest measure the height at the side at which the container is filledon two places and calculate the average height (h1). Repeat this onthe opposite side (h2). The difference in height between two sides ofthe container is a measure of the filling ability. Calculate the averagefilling percentage. Average filling %F = [(h1 = h2)/2 h1]*100

9.1.9 Test Methods for Segregation Resistance

Segregation in SCC tends to show as a non-uniform distribution ofaggregates, particularly concerning coarse aggregate, which may settleat the bottom. The Orimet and V-funnel test can also detect severestatic segregation when coarse aggregate settles and prevents completeflow of the sample.

Wet Screen Stability Test Method

The test quantitatively measures SCCs resistance to segregation, bydetermining how much separation occurs between the coarse aggregateand the mortar in a sample of concrete .The test consists of taking asample of 10 litres of concrete, allowing it to stand static for a periodof 15 minutes to allow any internal segregation to occur. Next pour 2litres of the concrete on to a 5 mm sieve of 350 mm diameter, from aheight of 500 mm which stands on a sieve pan on a weigh scale. Recordthe weight of the sample. Allow 2 minutes for the concrete flow throughthe sieve. After 2 minutes, the mortar which passed through the sieveis weighed and expressed as a percentage of the weight of the originalsample on the sieve.

A concrete where less than 5% of the sample passes the sieve hasa high resistance to segregation. Such a concrete may be too viscousto be able to fill minute voids, and will almost certainly result in poorquality finished surface. A concrete where 5-15% of the sample passesthe sieve can possess optimum amount of resistance to segregation(satisfactory stability). A concrete where 15-30% of the sample passesthe sieve is likely to be susceptible to segregation (critical stability).Aconcrete where more than 30% of the sample passes the sieve is likelyto be susceptible to severe segregation (very poor stability).In addition,it is not acceptable if bleed water is detected during the test. Thisinduces a risk of settlement, washing out and modified permeability.

9.1.10 Details of typical SCC mix for repair

A SCC mix which contains more coarse aggregate content will exhibitlower drying shrinkage and creep and higher elastic modulus. Similarly,the free shrinkage of repair materials decreases when cured in water.


A typical SCC mix which can be used for repair which comprises ofordinary Portland cement of 53 grade, 10 mm maximum size gravel,a medium graded sand, superplasticizers and polypropylene fibres tocontrol shrinkage cracks is given in Table 9.4.

9.2 REFERENCES

1. Rilem Report 23, Self-compacting Concrete -State -of-the-artReport, (2000) Ed. by A. Skarendahl and O. Petersson.

2. Rilem Report 24, Workability and Rheology of fresh Concrete:Compendium of tests, (2002) Ed. by P. J. M Bartos, M. Sonebiand A. K. Tamimi.

3. EFNARC (2002) Specifications and Guidelines for Self Compact-ing Concrete.

4. Lea’s Chemistry of Cement and Concrete (1998), Ed. by PeterC.Hewlett.

Table 9.1 JSCE 2002 Manual for Mixture Proportioning of SCCRecommendations

Constituents Powder type VMA type Combination type

Coarse aggregate 0.28 to 0.35 m3/m3 0.28 to 0.36 m3/m3 0.28 to 0.35 m3/m3

Water content 155 to 175 kg/m3 - -

w/p 28-37% by mass of - -cement or 0.85 to1.15 by volume ofcement

Powder content 0.16-0.19 m3/m3 - > 0.13m3/m3

Air content (for frost 4.5% - -resistance)


Table 9.2 EFNARC (2002) Specifications and Guidelines for SelfCompacting Concrete

Constituents RangesCoarse aggregate 28-35 % by volume of the mixWater/powder 0.8-1.1 (by volume)Powder content 400-600 kg/m3 (160-240 litres/m3)Cement content 350-450 kg/m3

Air content 20%

Table 9.3 Typical range of SCC mix proportions according toERAMCO

CONSTITUENT RANGE BY MASS RANGE BY VOLUME(kg/m3) (litres/m3)

Coarse aggregate 750-1000 270-360Water Content 150-210 150-210Powder content (kg/m3) 380-600 -Cement content (kg/m3) - -Paste content - 300-380Water/Powder - 0.90-1.10

Table 9.4 List of Viscosity Enhancing Agents

Natural Polymers Inorganic Synthetic / Semi-syntheticmaterials Polymers

Natural gums Fly ash Cellulose etherderivatives

Guar gum Silica fume Hydroxy-propylmethylcellulose

Welan gum Hydrated lime Hydroxyl cellulose

Diutan gum GGBS Carboxy methylcellulose

Locust bean gum Kaoline Alginate

Agar Bentonites Polyethylene oxide

Gum Arabic Rock dust Polyacrylamide

Xanthan gum Diatomaceous earth Polyacrylate

Rhansan gum Milled asbestos Polyvinylalcohol

Welan gum Aqueous clay Styrene Co-Polymersdispersions with carboxyl groups

Plant Protein SyntheticPolyelectrolytes

Decomposed starch


Table 9.5 Typical Self Compacting Concrete Mix for Repairs

Constituent Quantity(kg/m3) Type/sourcePortland 340 OPC (53 grade)cement Flyash 160 Class F (North Chennai

Thermal Power Plant)Coarse aggregate 940 10 mm rounded gravelFine aggregate 730 River SandWater 185 PotableViscosity modifying 0.07 —AgentFibres 910g/m3 Polypropylene

Slump cone

1000

mm

1000 mm

500 mm

Fig. 9.1 Slump Flow Test

1090

980

600

60

Fig. 9.2 Orimet Test


212.5 (225) mm

65 mm

425

(450

) mm

75 mm

490 (515) mm

Fig. 9.3 Funnel Test

100

600

H1

200

Unit: mm

0-200

800

H2 1500-100

0

200

Rebars 3 Φ 12mmGap 35 mm

Fig. 9.4 L-Box Test


Fig. 9.5 J-Ring Test

45 cm

2400Pa

Concrete

14 cm

14 cm

28 cm

14 cm

4@5cm = 20cm

FillingHeight

Reinforcing Bars(D13 mm)

Sliding Door

Middle Wall

R1R2

59 cm

Fig. 9.6 U-Box Test


Placement

Guide hopper

7@350 = 350150

500300

h2

φ 16mm

6@50

= 3

00 m

m

h1

Fig. 9.7 Fill-Box Test

Fig. 9.8 Wet Screen Stability Test

10 Mechanism of Corrosion and Repair of

Corrosion Damaged Concrete Structures

J. PRABAKARScientist

CSIR-SERC, CSIR Campus, Tharamani, Chennai-600 113, India.Email: [email protected]

10.1 INTRODUCTION

During the past several decades, concrete structures had suffered fromsafety and serviceability problems due to deterioration of concrete.Generally concrete is a very durable material, the environmental fac-tors such as weathering action, chemical attack, abrasion and otherdeterioration process may change the properties of concrete with timewhen rebar is embedded into the concrete. The deterioration of Rein-forced Cement Concrete (RCC) Structures is due to the corrosion ofsteel used in concrete. Corrosion of reinforcing steel results in thebuild-up of voluminous corrosion products generates internal stresseswhich lead to cracking and spalling of the cover concrete. The param-eters which influences the corrosion process in RCC structures are thecover thickness, the quality of concrete,, environmental conditions, pHand chloride levels and presence of cracks etc. The main causes of rebarcorrosion are due to ingress of chloride ions or diffusion of CO2 gas,from atmosphere. A lowering of the pH by penetration of free chlorideions through the concrete cover to the steel, or by the carbonation ofthe concrete cover due to penetration of atmospheric carbon dioxide,can cause breakdown of the passive layer.

In general, good quality concrete provides an excellent protectionfor steel reinforcement. The steel used in concrete are remains inpassive state due to high alkalinity of concrete. The time to initiate cor-rosion is determined largely by the amount and the quality of concrete,cover thickness as well as permeability of concrete. Once de-passivationoccurs, corrosion propagation is governed by anodic, cathodic and/orelectrolytic properties of corrosion cell. The rate of chloride diffusion




is influence in concrete with water to binder ratio and the proper-ties of paste such as type of cement, mix ratio and percentage ofsupplementary cementing material, temperature and humidity.

The corrosion of steel in concrete leads to repair and rehabilitationwhich causes incredible cost. There is an increasing amount of researchbeing performed to investigate methods of corrosion prevention, or tominimize corrosion damage where it has already begun. There is anobvious need to improve the product, but inevitably there will also bea perpetual need for repair and rehabilitation.

10.1.1 Corrosion Process And Mechanism

Reinforcement corrosion is one of the most common causes for rein-forced concrete structures deterioration. Corrosion damage to thereinforcing steel results in the build-up of voluminous corrosion prod-ucts generating internal stresses and subsequent cracking and spallingof the concrete. The main causes of rebar corrosion are due to ingressof chloride ions and CO2 which destroys the natural passivity ofreinforcement located in alkaline concrete condition. In general goodquality concrete provides excellent protection for steel reinforcement.Due to high alkalinity of concrete pore fluid, steel in concrete initiallyand in most cases, for sustained long periods of time, remains in apassive state. Initiation of corrosion occurs either due to reduction inalkalinity arising from the breakdown of the passive layer by the attackof chloride ions. The time to initiate corrosion is determined largelyby the amount and the quality of concrete, cover thickness as well aspermeability of concrete. Once de-passivation occurs, corrosion prop-agation is governed by anodic, cathodic and/or electrolytic propertiesof corrosion cell (Pal et al 2002).

Chloride salts are highly soluble in water. The chloride ions dif-fuse through concrete pores. The chloride ions present in the pores ofconcrete are dissolved in water and penetrate. Then the chloride ionsattack the passive layer due to higher concentration of chloride ionsthan hydroxyl ions. The chemical reaction takes place is given below.

The passive layer is destroyed with very less drop of pH value.Chlorides act as a catalyst to corrosion when there is sufficient con-centration at the rebar surface to break down the passive layer. Theyare not consumed in the process but help to break down the passivelayer of oxide on the steel and allow the corrosion process to proceedquickly. Then the concrete reinforcement tends to corrosion and leadsto concrete deterioration as shown in Fig.10.1 (Mohammad, 2007).

Mechanism of Corrosion and Repair of Corrosion Damaged Concrete Structures 179

The process of concrete structure deterioration due to chloride cor-rosion can be divided in to two phases. They are initiation period(Ti) and propagation period (Tp) as shown in Fig. 10.2 (Tutti, 1982).During the initiation period the chloride ions penetrate in to coverconcrete and accumulate around concrete reinforcement. The initia-tion period is determined mainly by the diffusion rate of chloride ionsin concrete. Propagation period is a process in which reinforcementbegins to corrode due to chloride ions. The corrosion products accu-mulate around concrete reinforcement and cause cracking along thereinforcement due to expansion pressure of corrosion product. Thepropagation depends on oxygen in dissolved state and the moisturecontent in the concrete.

The negative chloride ions promote corrosion of steel in concreteand accelerate corrosion and the chemical reaction takes place asshown below.

Fe → Fe2+ + 2e −Fe2+ + 2Cl− → FeCl2

Fecl2 + 2OH → Fe(OH)2 + 2Cl−

2Fe(OH)2 + 1/2O2 → Fe2O3 + 2H2O

Chloride ions can enter into the concrete from de-icing salts thatare applied to the concrete surface or from seawater in marine envi-ronments. Other sources include admixtures containing chlorides,contaminated aggregates, mixing water, air born salts, salts in groundwater, and salts in chemicals that are applied to the concrete surface.If chlorides are present in sufficient quantity, they disrupt the passivefilm and subject the reinforcing steel to corrosion (Steven F Daily).

Carbonation Attack : Moisture content in concrete plays an impor-tant role for chemical process of carbonation. The relative humidityof concrete around 60 to 75% is favour for the progress of carbonation(Verbeck, 1958). The chemical reaction takes place as shown below.

CO2 + H2O → H2CO3

H2CO3 + Ca(OH)2 → CaCO3 + 2H2O

H2CO3 + CaCO3 → Ca(HCO3)2

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O


The carbon dioxide gas dissolves in the presence of moisture contentand forms dilute carbonic acid. Then the carbonic acid reacts withcalcium hydroxide to form calcium carbonates. If the concentration ofthe CO2 gas present is high enough, carbonic acid continues to formand react with the carbonates present to produce bicarbonates. Thisreaction continues as long as the bicarbonates present in the solutionand thus more CO2 is required. The reverse reaction takes place whenany of these are lost and calcium carbonate will then be precipitateduntil sufficient CO2 gas has been released to stabilize the bicarbonateremaining in solution (Hewlett, 1998, Taylor, 1997).

The pH value of pore water in the hardened concrete is generallyfrom 12.5 to 13.5 depends upon the alkali content of cement. The highalkalinity forms a thin passivating oxide layer around concrete rein-forcement and it protects from the action of water and oxygen. Dueto carbonation effect the pore fluid being neutralized, the pH dropsto value between 8 and 9. Then the passive layer around concretereinforcement is decayed and leads to concrete deterioration. The cor-rosion of steel in concrete begins by two distinct processes. One isthat the corrosion follows an electrochemical process and the otheris the physical process due to which damage to concrete occurs. Themechanism and the factors which influence the processes are discussedbelow:

10.1.2 Electrochemical Process

In its simplicity, the electrochemical process of corrosion can be con-sidered as the metallurgy I reverse. Steel is produced from the basiciron ore which is oxide in nature. Energy is added to make the oreinto steel and during the electrochemical process by corrosion, elec-trons get liberated dissipating the energy added and thereby the steelgoes back to its oxidized form.

In respect of reinforcing steel, this process can occur under two sit-uations. Immediately after production in the factory, the rods comeout is light blue colour. During transportation and storage, a thinoxide film gets formed and this acts as the passive layer. However,during handling, it is likely that the passive layer may get mechani-cally destroyed crating locally depassivated spots. Such spots in thepresence of water and oxygen create galvanic cells, forming anodic andcathodic sites and highly localized corrosion can take place. Such cor-rosion is known as localized pitting corrosion. The process follows anelectrochemical phenomenon creating a potential gradient and currentflow between the anodic and cathodic locations.


As there is a chance of corrosion even before placing the steel inconcrete, it is necessary that the reinforcing rods are well protectedduring storage. This can be achieved by keeping the rods under coveredsheds, placing them on wooden supports and providing a cement slurrycoating. Another situation is when the rod is embedded in concrete. Inthis situation, the electrochemical process progresses by forming theanodic and cathodic sites, involving chemical reactions as given below:

Fe → 2e− + Fe + + (Anode)

1/2O2 + H2O + 2e− → 2(OH−) (Cathode)

4FE(OH)2 + 2H2O + O2 → 4Fe(OH)3 (RedRust)

3Fe + 8OH− → Fe2O4 + 8e − +4H2O (BlackRust)

The electrochemical process is greatly influenced by the pH value ofconcrete and the chloride. The state of a metal can be easily assessedby measuring the electrical potential with respect to a standard elec-trode. The influence of pH and chloride content on electrode potentialcan be understood from the classic pH potential-diagram proposed byPourbiax. The diagram gives an idea on the regions of various reactionsthat can take place depending on pH, chloride content, and electrodepotential. These regions represent immunity, general and pitting cor-rosion and passivity. This diagram forms the basis of identifying thepresence of corrosion activity in a rebar embedded in concrete whiledoing half cell potential survey on a structure.

The factors which influence the electrochemical process can besummarized as follows:

• pH value

• Chloride content

• Moisture within the concrete influenced by the humidity ofenvironment or direct contact with water

• Oxygen supply which controls the rate of corrosion.

In addition to above factors, electrical resistivity of concrete alsoinfluences the electrochemical process. Very dry concrete can have ahigh resistivity of ore than 100 kilo ohm.cm. The moisture and otherchemicals can reduce the electrical resistivity, thereby increasing theconductivity. It is established that when the resistivity of concrete fallsbelow 5000 ohm.cm, the conductivity of concrete will become highand under such internal environment the rebar becomes susceptible tocorrosion.


10.1.3 Physical Process

In reinforced concrete structures, the corrosion of reinforcement isunique in the sense that the corrosion process causes extensive damageto the concrete. The physical process mainly consists of the expansiveforces caused by the volume growth of the corrosion product and oncethe stress induced by this fore exceeds the tensile strength of con-crete, cracking occurs. As further corrosion takes lace, spalling occurs.Generally presence of active corrosion process in the reinforcement ofconcrete member becomes known only when the symptom, namely,corrosion stain and / or cracking is manifested. There is always a timelag between the corrosion initiation and manifestation of the symptom.As mentioned earlier, time for corrosion initiation can be estimatedby measuring the diffusion coefficient of concrete with regard to chlo-ride ion and using this parameter in Fick’s second law of diffusion.In actual structure, measurement of corrosion rate is required. Basedon the electrochemical understanding, it is established that corrosioncurrent can be measured using a technique called Linear PolarisationResistance method. (For ore details refer L11). The corrosion currentcan be correlated to corrosion rate as:

1.0 μA/cm2 = 1.10 × 10−2mm/year

10.1.4 Approach To Investigation Of Corrosion Damaged RccStructure

The corrosion affected RCC structures can be systematically inves-tigated as per the following to assess condition of the structuresand based on that a suitable repair materials are indentified forstrengthening.

• Visual observations

• Documentations

• Measurement of geometrical parameters

• Experiments for evaluating material properties and member behav-ior Non destructive testing

• Concrete Integrity and strength Evaluation

• Electro Chemical parameters Evaluation Partially destructivestesting Load tests

• Interpretation and analysis of test results

• Formulation of repair measures

• Post repair evaluation


10.1.5 Visual Observation and Documentation

A detailed visual inspection and documentation are most importantin any field investigation. The study mainly consist of the followingactivities

Visual Inspection Documentation MeasurementTypes of cracks Both by drawing and photographs Column, beam,(width, depth, Types and pattern of cracking, slab dimensionslength pattern ) spalling, abnormal distress, VerticalRust staining discoloration, deformation alignmentSpalling of History of construction Deflections andconcrete Original quality deformations ifDampness Analysis and design methods with anyDrainage Assumption madeFoundation Types of materials usedEnvironment

10.1.6 Non Destructive Testing (NDT)

The following Non Destructive Tests are the important tests can beused for assessing the concrete integrity, strength and corrosion leveletc. The data obtained form the NDT can be considered for qualitativemeasurement and can have the confident level of about 80%.

• Rebound Hammer test

• Ultrasonic test

• Corrosion Level Measurement

• Half cell potential test

• GCOR6

• Galva Plus

• Half Cell Potential Meter

• Concrete Resistivity meter

• Permeability test

• Cover meter test

10.1.7 Partially Destructive Test (PDT)

The rebound Hammer and ultrasonic pulse velocity tests can giveindirect evidence of concrete quality and where as a more realisticassessment on concrete can be made by core sampling and testing.The PDT can give a quantitative measurement and can give the actualconcrete strength exists in the structure. The PDT method can alsohelp in assessing the following parameters.


• Evaluation of Concrete Strength

• Carbonation attack

• pH and Chloride Level

10.2 MEASUREMENT AND IDENTIFICATION OF

CORROSION LEVEL

The corrosion prone areas and locations can be identified in the struc-ture by interpreting the test data obtained with following methods.

10.2.1 Carbonation and pH Value

The common method for testing the carbonation depth of hardenedconcrete is by measuring the change in the concrete pH value (Parrott,1987). From the Fig. 10.3 it clearly shows that how quickly the pHdrops between carbonated and un-carbonated regions. The carbona-tion depth for some of the mixes are predicted using parabolic equationbased on the measured carbonation depth result. The parabolicequation is represented as

X = KtnWhere,X = Depth of Carbonation in mmK = Rate of Carbonation Depth in mm

√week

t = Time in weeksn = 0.5From the above equation the (K) value is calculated using the mea-

sured carbonation depth value (X) and age in week (t) the result hastaken. By applying the calculated value of (K) the carbonation depthvalue (X) can be identified for the age in weeks (t) applied to theequation.

10.2.2 Chloride Content

Chloride level can be determined by collecting powder samples fromthe RCC structure or from concrete core samples. The estimation ofchloride level at cover regions is most important. The chloride deter-mination can be obtained by titration method and also by RapidChloride Test Kit. The corrosive environment within concrete getsestabilished once the pH value is lowered to 10 and less or the chloridelevel reaches the threshold limit of about 0.40 to 0.60% by weight ofcement. The guide lines for identification of corrosion prone locationsbased on chloride level is given in Table-1.


10.2.3 Half Cell potential Survey

Corrosion being an electrochemical phenomenon, the electrode poten-tial of steel rebar with reference to a standard electrode undergoeschanges depending on corrosion activity. The common standard elec-trodes used are (i) Copper-Copper Sulphate Electrode (CSE) (ii)Silver-Silver Chloride Electrode (SSE) (iii) Standard Calomel Elec-trode (SCE). The measurement consists of giving an electrical con-nection to the rebar and observing the voltage difference betweenthe rebar and a reference electrode in contact with concrete surface.The test set-up for the the half cell potential is shown in Fig. 10.4.Generally, the potential values become more and more negative asthe corrosion becomes more and more active. However, less negativepotential values may also indicate the presence of corrosion activity,if the pH values of concrete are less. The general guidelines for iden-tifying the probability of corrosion based on half cell potential valuesas suggested in ASTM C 876 are given in the following Table.2.

It is important to realize that the potential of any metal in cementconcrete environment is a function of a large number of variables suchas concrete composition, pore liquid, concrete resistivity, cover thick-ness, degree of polarization, etc. Hence, no quantitative conclusion canbe drawn from it.

10.2.4 Resistivity Test

The corrosion of a specific length of reinforcement is dependent on thealgebraic summation of the electrical currents originating from thecorroding sites on the steel and flowing through the moist surround-ing concrete to non-corroding sites. Hence the electrical resistance ofconcrete plays an important role in determining the magnitude of cor-rosion at any specific location. This parameter is expressed in terms of”Resistivity” in ohm centimeter or kilo ohms centimeter. The factorswhich govern the resistivity values are:

• Constituents of concrete

• Chemical contents of concrete such as moisture, chloride level, andother ions regardless of whether or not these were introduced byformulation, atmospheric or sea water penetration.

• Type of pore structure of concrete.

Table-3 below gives the general guidelines for resistivity values indi-cating probable corrosion risk in normal concrete structures based onthe work carried out by various researchers.


For a general monitoring, a resistivity check is important becauselong-term corrosion can be anticipated in concrete structures whereaccurately measured values are below 10,000 ohm-cm. Further, if resis-tivity values fall below 5,000 ohm-cm. corrosion must be anticipatedat a much earlier period (possibly within 5 years) in the life of a struc-ture. The principle of resistivity testing in concrete is similar to thatadopted in soil testing. However, when applied in concrete, a few draw-backs should be realised. The method essentially consists of using a 4probe technique in which a known current is applied between two outerprobes and the voltage drop between the inner two elements is readoff allowing for a direct evaluation of resistance R. Using a mathemat-ical conversion factor, resistivity is calculated as r = 2 p.R.a where ’a’is the spacing of probes. The principle of four-probe resistivity test-ing is illustrated in Fig. 10.5 given below. The following drawbacksare important to note while analyzing and interpreting the resistivityvalues.

• The value obtained represents only the average evaluation overthe depth regulated by the chosen probe spacing and not that ofconcrete at steel interface.

• Skin effect of concrete with varying drying conditions.

• The instrument should have adequate ’IR’ drop compensation formeasurement.

Following Table-4 gives guidelines for a qualitative identificationof corrosion prone areas based on our experience and also based onthe work carried out by various researchers on normal concrete aftercombining the results of half cell potential and resistivity.

Table-4 Corrosion probability based on resistivity and potential

10.2.5 Corrosion Rate Measurement

In reinforcement concrete structures, determination of actual rate atwhich the reinforcement is corroding assumes larger importance. Onemethod is known as ’linear polarization resistance’ (LPR) method forthe on-site study of corrosion rates of steel in concrete (6). The funda-mental principle of Linear Polarisation is based on the experimentallyobserved assumption that for a simple model corroding system, thepolarisation curve for a few mill volts around the corrosion poten-tial obeys a quasi-linear relationship. The slope of this curve is theso-called ’Polarisation Resistance(Rp):

Rp = (Δ/Δl)ΔE → 0


From this slope, the corrosion rate can nbe determined using Stern-Geary equation

Icorr = B/Rp

Where B is a constant which is a function of the Tafel Slopes anda, c are determined from the formula given below.

B =βaβc

2.3(βa + βc)

The value of B usually lies between 13 and 52 milli volts dependingon the passive and active corrosive system. For onsite measurement,the testing system consists of a potentiate, counter electrode, referenceelectrode, and the reinforcement as a working electrode. It is necessarythat for measurements in concrete, the potential should have electronicohmic compensation (IR) drop or otherwise, the value is to be obtainedby calculation or separate experiment. This works on the principle ofLPR technique.

10.3 CORROSION PROTECTION SYSTEM

The steel corrosion in concrete can be protected with suitable meth-ods that reduce the corrosion of metals embedded in concrete, whichreduces the deterioration of concrete. The selection of methods shallbe considered and compatible to environment factors, bond, durabilityperformance and safety requirements. The following methods can befollowed to protect the steel from corrosion.

• Concrete Quality

• Cover Concrete

• Corrosion Resistance Steel

• Chemical Admixtures

• Mineral Admixtures

• Coating on Steel and Concrete Surface

• Corrosion Inhibitors

• Cathodic Protection

• Electrochemical Chloride Removal

10.4 REPAIR OF CORROSION AFFECTED STRUCTURES

Selection of materials and application methods for the repair, protec-tion, and strengthening of concrete structures is very important. It


is necessary to match the properties of the concrete being repairedas closely as possible and therefore, cementitious compositions usingsimilar proportions of ingredients are the suitable choice for repairmaterials. Types of cementitious compositions materials available forrepair of corrosion affected RCC member are as follows.

• Conventional Concrete

• Conventional mortar

• Dry Pack Mortar

• Proprietary Repair Mortar

• Ferrocement

• Fibre-reinforced Concrete

• Grouts

• Chemical Grouts

• Low Slump Dense Concrete

• Shotcrete

Apart from the cementitious materials, the improvement of prop-erties of hardened concrete by the addition of polymers is well knownand are as follows.

• Polymer Cement Concrete

• Polymer Mortar

The general repair materials being used in the construction indus-tries are as follows.

Repair operation MaterialSealing of fine cracks Epoxy resinsSealing of large cracks Portland cement mortarand joints Polymer mortar Putties

and caulksGeneral sealing of surfaces Synthetic polymers and

asphalt coatingsLocalized patching of surfaces Concretes or mortars using

portland cement Rapid-settingcements Polymer resins

Overlays and shotcreting Portland cement concrete Fibrereinforced concretesLatex modified concretePolymer concretesAsphaltic concrete


10.5 REPAIR METHODOLOGY

The repair methodology shall be chosen based on the causes ofconcrete deficiencies is essential to perform meaningful evaluationand repair. In general, any repair works undergoes the followingactivities.

• Concrete Removal

• Surface Preparation

• Repair Techniques and Material Installation

• Protective System

• Quality Control

• Performance Objectives

• Quality Control Procedures During the Repair

• Testing or Inspection Agency Qualifications

• Maintenance After Completion of Repairs

10.6 FACTORS TO BE CONSIDERED DURING REPAIR

Safety is one of the main aspects when designing a concrete repair,strengthening system. It is very much essential to understand thebasic principles of structural mechanics and have an understandingof material behaviour to evaluate and design a structural repair andstrengthening procedure. The following design care shall be takenthroughout the repair.

• Current Load Distributions

• Compatibility of Materials

• Creep and Shrinkage

• Vibration

• Water and Vapour Migration

• Safety

• Material Behaviour Characteristics

10.7 REPAIR TECHNIQUES AND METHODS

10.7.1 Small cracks

If the cracks are reasonably small (crack width = 0.75mm - 5.00mm),the technique to restore the original tensile strength of the crackedelement is by injection of epoxy with pressure .

• The external surfaces shall be cleaned


• PVC injection ports shall be placed along the surface of the cracksand are secured in place with an epoxy sealant.

• The centre to centre spacing of these ports may be approximatelyequal to the thickness of the member.

• After the sealant has cured, a low viscosity epoxy resin shall beinjected into one port at a time, beginning at the lowest part ofthe crack in case it is vertical or at one end of the crack in caseit is horizontal.

• The resin shall be injected till it is seen flowing from the oppositesides of the member at the corresponding port or from the nexthigher port on the same side of the member.

• The injection port should be closed at this stage and injectionequipment moved to the next port and so on.

The smaller the crack, higher is the pressure or more closely spacedshould be the ports so as to obtain complete penetration of the epoxymaterial throughout the depth and width of member. Larger crackswill permit larger port spacing, depending upon width of the member.This technique is appropriate for all types of structural elements suchas beams and columns. In the case of loss of bond between reinforcingbar and concrete, if the concrete adjacent to the bar has been pulverizdto a very fine powder, this powder will dam the epoxy from saturatingthe region. So it should be cleaned properly by air or water pressureprior to injection of epoxy.

10.7.2 Wider Cracks

For cracks wider than 5 mm or for regions in which the concrete hascrushed, a treatment other than injection is indicated. The followingprocedure may be adopted.

• Removal of loose material and replaced with any of the materialsi.e., expansive cement mortar, quick setting cement or gypsumcement mortar

• If found necessary, additional shear or flexural reinforcement isprovided in the region of repairs. This reinforcement could becovered by mortar to give further strength as well as protectionto the reinforcement

• In areas of very severe damage, replacement of the member orportion of member can be carried out.

• In the case of damage to walls and floor diaphragms, steel meshcould be provided on the outside of the surface and nailed or


bolted to the wall. Then it may be covered with plaster or micro-concrete .

10.7.3 Repair of Wider Cracks and Spalling in the Concrete

The repair measures generally consist of the following steps.

• Removal of damaged cover concrete in the columns and the extentof removal will depend on the damage, however, for the purposeof uniformity and quantity measurements, the concrete up to thereinforcement needs to be removed.

• After removal of cover concrete, the reinforcements shall beexposed and thoroughly cleaned both mechanically and chem-ically to remove all loose rust and other particles, using com-pressed air or water jetting.

• The exposed rods shall be given a coating of Nitozinc primer forprotecting the existing reinforcement and the coating shall beallowed to cure for the period specified by the supplier.

• After curing the primer coating, the exposed areas shall bewrapped with weld mesh of 10G × 10G with opening 100 ×100 mm to the shape of the chosen member (column/ beam).The weld mesh shall be tightly secured to the exposed concreteby using “U” nails.

• After tying the weld mesh, the exposed face shall be renderedwith a bond coat in order to provide bond between the existingold concrete and the new concrete to be poured. The area forrendering the bond coat shall be decided based on the settingtime of the bond coat since the new concrete is to be pouredwhen the bond coat is tacky before setting. Based on the settingcharacteristic of bond coat, the quantity of new concrete requiredto be poured is estimated prior to concreting.

• The replacement of cover concrete shall be either Polymer Modi-fied Mortar (PMM). The PMM is a ready to use mortar which willhave high flowing characteristics. After carrying out the worksmentioned from Sl. Nos. (i) to (v) above, the member shall beprovided with a shuttering giving adequate space of at least 50mm from the chipped faces and reinforcement. The height ofshuttering for columns shall not exceed 1.0 m for a single. Themortar is mixed with water as per the manufacture specificationsand poured into the form work In place of PMM, shorcrete maybe used for the above repair work as explained.


10.7.4 Strengthening of RCC Beams Affected Severely due toCorrosion

The strengthening methods of dis-stressed RC members shall beselected based on the functional requirement and the different methodsavailable are as follows.

• Jacketing with Conventional Concrete

• Jacketing with Micro Concreting

• Jacketing with Polymer concrete

• Jacketing with Self Compacting Concrete

• Wrapping with FRP laminates

• Steel Jacketing

The beams are to be supported by props. Remove the damagedcover concrete in the beams and the thickness of removal will dependon the extent of damage. However, for the purpose of uniformity andquantity measurements, the concrete up to the reinforcement can beremoved. After the removal of cover concrete, the loose particles are tobe removed either using compressed air or using water jetting. Applyanticorrosive coating ( Nitozinc primer) over the existing rods for pro-tecting the reinforcement from further corrosion and the coating shallbe allowed to cure for the period specified by the supplier. Apply bondcoat over the old concrete to provide bond between the existing oldconcrete and the new concrete. Shear connectors have to be provided.These shear connectors have to be fixed with an epoxy to a minimumlength of 100 mm to the old concrete and the free end has to be bentas L-shape and tie with new steel reinforcement. Alternatively, ’U’- hooks may also be provided for anchoring the new reinforcementto the beam. Provide additional steel reinforcement according to theweight loss occurred in the original bars by measuring diameter. Themain rods have to be taken into the column as per the design. Curingshall be done for a minimum period of 15 days. While jacketing thebeam, be ensure about the anchoring of steel coming from the columnmembers.

One panel of 600 × 600 × 100 mm for each day’s concreting. Fromthe panels, minimum 6 Nos. of 100×100×100 mm cubes shall be cutand tested for compressive strength at 14 days (3 Nos.) and 28 days(3 Nos.). The panels shall be prepared and cured in the same way ascarried out in the structure


10.7.5 Typical Repair/Strengthening of Columns

Remove the damaged/ loose cover concrete in the columns by means ofelectrical chipper or any other means and the thickness of removal willdepend on the extent of damage (i.e up to the sound concrete depth).However, for the purpose of uniformity and quantity measurements,the concrete up to the cover of the reinforcement can be removed. Afterthe removal of cover concrete, the loose particles are to be removedusing compressed air or water jetting. Apply bond coat over the oldconcrete to provide bond between the existing old concrete and thenew concrete. Shear connectors have to be provided as per the detailsgiven in figure enclosed. These shear connectors have to be fixed withan epoxy to a minimum length of 100 mm to the old concrete andthe free end has to be bend as L-shape and tie with weld mesh to beprovided as shown in Fig.. Jacketing of columns shall be done by usingself compacting concrete by providing suitable shuttering to a heightof 1.0 m as first lift. Give minimum one day interval for each lift forhardening the concrete. Curing shall be done for a minimum periodof 15 days.

10.7.6 Specification of Self Compacting Concrete (SCC)

Mix details Chemical AdmixturesCement - 350 kg/m3 Master BuilderSand - 950 kg/m3 Technologies (MBT)Fly Ash (Class F/C) - 150 kg/m3 Glenium - 51 (SP)Coarse aggregate Glenium - Stream (VEA)(10 mm graded) - 720 kg/m3

Water - 190 kg/m3

Super plasticizer(S.P.) - 0.45 % of (Cement +Fly Ash) Viscosity EnhancingAgent (VEA) - 0.05 % of water

10.7.7 Procedure for preparation of SCC

Initially, aggregate (10mm graded) with one third of water are to beadded to the mixer and allow to mix for 60 seconds. Then fine aggre-gates (sand), cement and fly ash are added to the mixture and allowto mix for 60 seconds and add chemical admixtures such as, S.P, VEAto the two third of water and add to the mixer to mix for another 90seconds. Now the self compacting concrete is ready for pouring.


10.8 NOTE

• Leakage of cement slurry through shutter joints should not occurand ensure perfect shuttering and in case of any gap at bottomof the shuttering, make the gap sealed.

• Slight tapping can be made on the out side of the shutteringduring pouring of self compacting concrete (SSC) to remove airvoids if any.

• Water Curing must be done immediate after 24 hours bywrapping gunny bags for a minimum period of fifteen days.

10.9 CONCLUSIONS

The reinforcement corrosion in concrete needs serious consideration bythe designers and constructors. The information discussed in this noteson corrosion of steel reinforcement in concrete shall bring awarenessand understanding of the mechanism certainly help to take appropri-ate precaution at the design and construction stage itself. The use ofproper materials and repair methods for strengthening the structureis highlighted.

10.10 REFERENCES

1. Advanced Course on ’High Performance Materials and Method-ologies for Construction and Rehabilitation of Concrete Struc-tures’ , Organized by Structural Engineering Research Centre(SERC), during January 19-21, 2000.

2. ACI manual of Concrete Practice, 2009, Part-6, ACI 506R-05 toACIITG-5.1-07.

3. Allan P. Crane, Editor “Corrosion of reinforcement in concreteconstruction”, Ellishorwood Ltd., Chichester, 1983.

4. Hewlett, Arnold, “Lea’s chemistry of cement and concrete”, pp1053 - 1087, 1998.

5. Mohammad A. El-Reedy, “Steel reinforcement concrete struc-tures”, Assessment and Repair of Corrosion, available on internet,http://hotfile.com/dl/57030679/6450a06/1420054309.zip.htmlpp,2007.

6. Steven F. “Daily Understanding Corrosion and Cathodic Protec-tion of Reinforced Concrete Structures” (http://www.estig.ipbeja.pt/pdnl/ Sub-paginas/Conservacao%20de%20edificios files/Documentos/Material%20de%20apoio/Betao/corrosao.pdf).


7. Tutti K, “Corrosion of steel in concrete”, CBI - Forskning 4.82,Cement Och Betonginstitutet, Stockholm, 1982

8. Taylor, H F W, “Cement chemistry”, 2nd Edition, ThomasTelford Publishing, London, 1997.

9. Verbeck G. J, “Field and laboratory studies of the sulfateresistance of concrete In Performance of concrete resistance ofconcrete to sulfate and other environmental conditions”, Thor-valdson symposium, University of Toronto Press, pp.113-24,1968.

10. Revision of IS 456-1999 code of Practice for Plain and ReinforcedConcrete- overview of modifications.

Table 10.1 Interpretation of Chloride and pH values for corrosionprone areas

Sl.No Test Results Interpretation

1 High pH values greater than 11.5 and No Corrosionvery low chloride content

2 High pH values and high chloride content greater Corrosion pronethan threshold values (0.4 - 0.6 5 by weight of cement)

3 Low pH values and high chloride content Corrosion prone(0.4 - 0.6 5 by weight of cement)

4 Low pH values and high chloride content Corrosion prone

Table 10.2 Corrosion risk by half cell potential

Corrosion Potential

More than 95 % More negative than - 350 mV

50 % -200 mV to -350 mV

Less than 5% More positive than -200 mV

Table 10.3 Corrosion risk from resistivity

Resistivity Corrosion probability(ohm - cm)

Greater than 20,000 Negligible

10,000 - 20,000 Low

5,000 - 10,000 High

Less than 5,000 Very high


Table 10.4 Corrosion probability based on resistivity and potential

Sl.No Test results Interpretations

1 High resistivity greater than 10,000 ohm cm and No active corrosion - relativelylow potentials - more positive than -200 mV (CSE) cathodic

2 Low resistivity below 10,000 ohm cm and Initiation of corrosion activitypotentials between -200 mV to -250 mV (CSE) - relatively anodic

3 Low resistivity about 5,000 ohm cm and potentials - Presence of corrosion activity200 mV to - anodic-350 mV (CSE)

4 Low resistivity below 5,000 ohm cm and potential High intensity of corrosion -more negative than -350 mV (CSE) fully anodic

5 Higher potential gradient and high conductivity High rate of corrosion

O2OH¯

Cl¯ Cl¯

OH¯Fe2+

H2O

O2 + H2O

Cathode Cathode Reinforcement

Passive Film

Cement Matrix

Anode2e¯

O2 + H2O

Fig. 10.1 Corrosion of steel in Concrete by Chloride Attack

Degr

ee of

Cor

rosio

n

Significant Level of Damage

Propagation Period (tp)

Design Life = ti + tp

Initiation Period ( ti)

Rate of Corrosion

ti depends on:• cover depth• w/c ratio• curing regime• cement type• environment temperature

tp depends on:• availability of O2• availability of H2O• OH - concentration

••• -

Fig. 10.2 Service Life model for design life (Tutti, 1982)


7

8

9

10

11

12

0 10 20 30 40 50Depth from Surface, mm

pH

NormalConcrete

NeutralisedConcrete

Depth at whichpassivationis lost

pH indicatedby phenolpthalein

Fig. 10.3 Change in pH with depth of carbonated concrete

ReferenceElectrode

CorrosionPotential

Cu/CuSO4Sponge

Steel Rod

Concrete

V

Fig. 10.4 Set up for half cell potential survey

V

Ia

P = 2πa V/I

Current flow Equipotential

line

Fig. 10.5 Principle of Resistivity measurement

.

11 Repair and Retrofitting of RC Structures -

Case Studies

K. Balasubramanian and V. RajendranHitech Concrete Solutions, Chennai Pvt. Ltd.,

Chennai-600 077, India.Email: [email protected]

11.1 INTRODUCTION

Construction activities account for a major component of the budgetin developing countries, including India. Cement concrete is the mostextensively used material for the construction of large infra-structuralfacilities world-wide. Significant distress or deterioration is beingobserved in Reinforced Concrete(RC) structures, such as bridges,multi-storeyed buildings, hyperboloid cooling towers and chimneys,particularly in coastal regions even well within their expected lifespan. Concrete despite its inherent deficiencies, is the most extensivelyused material for the construction of large infrastructure facilities.In the foreseeable future, there seems to be no alternative to con-crete as a construction material. Ensuring durability of concrete isone of the important issues to be addressed in evolving strategies tobring about sustainable development. Maintenance and repair of con-structed facilities is presently a growing problem globally, involvingsignificant expenditure. Strengthening, upgrading and retrofitting ofexisting structures are among the major challenges that modern civilengineering field is facing these days. The building deficiencies can bebroadly classified as Local Deficiencies and Global Deficiencies.

Local deficiencies are element deficiencies that lead to the failureof individual elements of the buildings, such as, crushing of columns,flexural and shear failure of beams etc. Unaccounted loads, inadequateconfinement, unauthorized alterations, poor quality of construction,poor detailing, lack of anchorage of reinforcement, inadequate shearreinforcement, insufficient cover, inadequate compaction and curing,etc., and environmental deterioration are reasons for local deficiencies.




Global deficiencies refer to the deficiencies of the building as awhole. Certain structural design concepts that may work adequatelyin non-seismic areas perform poorly when subjected to earthquakemotions. Examples are framed structures with strong beams and weakcolumns, or framed structures employing open ground storeys. Foreither case, a single storey sway mechanism can develop under lat-eral loading. Global deficiencies can broadly be classified as planirregularities and vertical irregularities, as per IS 1893 (Part I):2002.

This lecture notes presents two case studies, in which a corrosionaffected wharf at Chennai and a hydel power station at Srinagar havebeen rehabilitated successfully.

11.2 CASE STUDIES

11.2.1 Performance of sacrificial anodes in the rehabilitation ofcorrosion affected finger jetty

The increase in the number of structures affected by corrosion hascreated more awareness in the minds of the researchers to investigatethe various corrosion protection methodologies to be adopted duringand after the completion of construction of reinforced concrete struc-tures. Usage of different types of surface coating on rebars for thecorrosion protection has some limitations on account of many factors,like reduction in bond stress between the concrete & rebar and so on.The attempts of the various rehabilitation organizations in restoringthe corrosion affected structural elements back to their original loadcarrying capacity has proved to be a very complicated process anda short lived one. Such rehabilitation methods involve exorbitantlyhigh costs, besides causing a lot of disturbance to the occupants.Hence, recourse is being made by researchers as well as repair andrehabilitation experts to identify newer and cost effective corrosioncontrol techniques that will give long term satisfactory performance.One such method that is being widely and successfully employed allover the world is the self regulating sacrificial galvanic protection sys-tem. The case study describes in detail investigations conducted on acorrosion damaged Finger jetty, the repair methodology suggested forthe rehabilitation of the structure and executed. The repair method-ology proposed included the provision of galvanic anodes. The casestudy also describes in detail the monitoring of the repaired Fingerjetty through half cell potential and ultrasonic pulse velocity mea-surements conducted over a period of one and a half years from thetime of completion of the repair to assess the effectiveness of the repair

Repair and Retrofitting of RC Structures - Case Studies 201

methodology. The investigations have clearly demonstrated that gal-vanic anodes have proved to be an effective corrosion control techniquefor reinforced concrete structures.

11.2.2 Galvanic Anode Protection System

Many new systems and materials have been developed to delay theonset of corrosion and to increase the durability of reinforced concretestructures situated in marine environment. However, most of the sys-tems and materials that have been developed only delay the initiationtime of corrosion. Once the corrosion is initiated, the damage to thereinforced concrete structures is very extensive. Hence, the need ofthe hour is the development of corrosion control systems that will notonly be economical, but perform well over a period of time in adverseenvironmental conditions.

Galvanic corrosion protection methods were originally developed inthe 1820s. Over the years, self regulating galvanic corrosion protectionsystems have been widely used to protect underground steel structures,such as, pipelines and tanks. Self regulating galvanic protection sys-tems were first used on reinforced concrete structures around 1960.Recent technological advancements in the development of self regulat-ing galvanic anodes have led to a significant increase in their use forprotecting reinforcing steel in concrete structures.

Galvanic anodes used for galvanic protection are typically con-structed using aluminum, magnesium or zinc. For reinforced concreteapplications, zinc has become the most common sacrificial anode usedpresently. There are several reasons for the usage of zinc namely.

(a) Zinc has high corrosion efficiency i.e. higher percentage of elec-trons are discharged from the zinc as it corrodes. These electronsare available to protect the steel.

(b) As zinc corrodes, it has a relatively low rate of expansioncompared to other metals, including steel. This makes zincanodes particularly suitable for application where the anodes areembedded into the concrete structure.

(c) Zinc anodes are suitable for use in prestressed and/or post-tensioned concrete because their native potential is generally notsufficient to generate atoms or cause hydrogen embitterment ina concrete environment.

Galvanic anodes are covered with a precast mortar matrix saturatedwith lithium hydroxide (LiOH). These anodes are designed to be tied


directly to the reinforcing steel to extend the life of concrete patchrepairs. Fig. 11. 1 gives a view of a sacrificial zinc anode system thatwas used in the present study.

Because of its simplicity in installation, the galvanic anodes haveproved to be a better corrosion protection system in the case ofrepair/rehabilitation of reinforced concrete structures. They have alsoproved to be extremely successful during the maintenance of the struc-tural members the world over. However, their use in the repair andrehabilitation of corrosion damaged structures is still in its infancyin India. Fig. 11.2 shows the ease with which a sacrificial zinc anodesystem is being installed during the rehabilitation of a structure.

11.2.3 Description of Structure

The main components of the Finger jetty (Fig.11.3) situated atChennai are as follows:

• The Finger jetty was built on 95 piles arranged in four rows,intermediate rows having 22 piles in each row and eastern rowhaving 23 piles and western row having 28 piles.

• The spacing between two piles was observed as 3.330m intransverse directions and varying between 10.00m to 11.30m inlongitudinal direction, except at twin pile locations.

• The modified pile muffs, where fenders are fixed are of size 1.85mto 1.9m in longitudinal direction and 2m to 2.25m in transversedirection and extend to a height of about 3.4m up to the bottomof deck slab.

• The following are the beam sizes on the Finger jetty:

• Longitudinal beams 1000mm × 400mm (excluding deck slab)

• Transverse beams 1000mm × 750mm (excluding deck slab)

• Slab thickness 400mm with wearing coat

• Top level of the deck Varies between +4.5m to +4.15m

11.2.4 Investigations at Site

The following tests were conducted to assess the quality of concreteand extent of corrosion in the various structural elements of the Fingerjetty:

1. Ultrasonic pulse velocity test2. Half cell potential test


The following structural elements were investigated:

1. Piles2. Pile caps

The half cell potential and UPV values obtained during the inves-tigations prove that the corrosion is active and that the integrityof concrete is doubtful and that the structure requires immediaterehabilitation(Figs. 11.4 & 11.5)

11.2.5 Repair Methodology

Based on the analysis of the test results, a repair methodology wasproposed to be adopted for the piles, pile caps and beams. It wasdecided to rehabilitate the berthing wall also with reinforced concreteelement to take care of the berthing load vibrations. In view of thefact that the Finger jetty has to accommodate higher capacity vessels,the pile size was increased to the size of the pile cap so that it will actas a fender column to take care of higher berthing loads.

A proper support system was designed and placed in position beforetaking up the repair and rehabilitation. After the support system wasinstalled, the spalled/loose concrete were chipped from face of thepiles.

The heavily corroded pile liners were cut and removed from -0.20 mfrom the low tide level using under water cutting gear. All the spalled,cracked concrete and pre-applied mortars were removed by chipping toexpose the reinforcing steel. The concrete was removed about 20mmbehind the rebars. The repair sequence was so chosen that no twoadjacent piles were chipped off at a time. In fact, the sequence adoptedwas such that every 4th pile was chipped, rehabilitated before the otherpiles were taken up.

As the concrete was contaminated with chlorides, the chipped ofsurfaces of the concrete were repeatedly cleaned with potable waterusing high pressure water jet equipments during the low tide level.The exposed rebars were also cleaned with high pressure water jetand mechanical cleaning where ever required. The existing corrodedrebars were coated with zinc based protective coating.

Since the repair methodology involved provision of a micro concretejacket from the design point of view, shear connectors were providedat every 500mm c/c on the faces of piles and pile caps in a staggeredmanner. The shear connectors were anchored using polyester resin.The additional reinforcement was tied and also welded at a few places


to the shear connectors so that the connectivity to the core concreteof the structure is ensured.

The galvanic anode used in the rehabilitation of the structure wasan amphoteric zinc block embedded within a specially formulatedcementitious mortar having a pore solution pH, which is sufficientlyhigh for corrosion of the anode to occur and for passive film for-mation on the anode to be avoided as described in patent numberPCT/GB94/01224. Galvanic anode was positioned in such a way toensure all round contact with the jacketed micro concrete and wasattached to the existing/ additional reinforcement using the wire ties.Galvanic anode fixing tool was used to tighten the wire ties, so thatno free movement was possible, thus ensuring electrical continuity.Fig. 11.6 shows a view of the fixing of Galvanic anode to the pile.To check the electrical continuity between wire ties and reinforcementbar, a voltmeter was used.

11.2.6 Post repair investigation

After the Finger jetty was rehabilitated, half cell potential measure-ments were conducted on the piles and pile caps at intervals of 6months and up to a period of 2 years to check the performance ofthe repair methodology adopted, especially the provision of the selfregulating galvanic anode. Half cell potential survey was conductedusing the prefixed corrosion monitoring junction box. Care was takento ensure that the same locations before repair were again subjectedto half cell potential test to assess the efficiency of the self regulatinggalvanic anodes.

The UPV tests were conducted immediately after repair to assessthe integrity of the structural members, viz, piles, pile muff, pile capsand deck beams to assess the performance of the repair methodologyas well as the execution of the rehabilitation. Care was taken to ensurethat the same locations before repair were again subjected to UPV testto assess the efficiency of the repair methodology. The UPV tests wereconducted on the above structural members at every 6 months intervalup to a period of 2 years.

The half cell readings taken before and after completion of therehabilitation and at intervals of 6 months till the end of the 2nd yearperiod from the date of completion of the rehabilitation are listed inTable. 11.3. The half cell potential reading values show values, whichare more positive than -200 mV at the end of 2 years and as perthe recommendations of ASTM C-876, the rehabilitated structuralmembers have high probability of no corrosion. Hence, it is clearly


evident that the self regulating galvanic anode system is performingwell in the rehabilitated jetty in terms of corrosion protection.

11.3 REHABILITATION OF A HYDEL PROJECT NEAR

SRINAGAR

The Upper Sind Hydel Project (USHP)-Stage II, Kangan consists ofthree power stations with 3 x 35 MW generators. The power generatingmachinery was not able to generate power in the units II and IIIof the station almost from the inception from the early 2000. HenceVibration studies along with NDT investigations were undertaken bySERC, Chennai.

Based on the results of the investigations carried out on the rein-forced concrete columns of the USHP, the following recommendationswere made by SERC, Chennai.

• The Ultrasonic Pulse Velocity values and Rebound Hammer read-ings indicate that, in general, the integrity of concrete in the RCcolumns may be considered as satisfactory.

• The results of the tests for chloride content, sulphate content andpH levels indicate that, in general, these salts are within theirrespective permissible limits and do not indicate the presenceof any corrosive environment within the concrete at the time ofinvestigation.

• The results of the UPV tests clearly indicate that the eight con-crete pedestals supporting the stator support pads at the LGBfloor level in units II and III have undergone severe damage.Fig. 11.7 shows the typical view of RC pedestals of upper brack-ets in unit III. Considering the long term safety and to ensurethe trouble free performance of the machinery, and to keep thevibrations within the permissible limit, it is necessary that theabove eight concrete pedestals in Units II and III may be disman-tled and recast, as per the design requirements of the machineryinstalled.

• The exact extent of damage in the concrete slab diaphragm sup-porting the rotor radial thrust pads (4 numbers) in units II andIII can be assessed only after the removal of the machinery andwith closer inspection. A retrofitting methodology can be formu-lated after a closer and thorough inspection after the removal ofthe entire machinery.


The repair work consisted of dismantling the 8 eight concretepedestals supporting the stator support pads at the LGB floor levelin units II and III and then recasting them with Microconcrete aftermaking the necessary arrangement to support the machinery by meansof hydraulic jacks. Grouting was also carried out in the concrete slabdiaphragm supporting the rotor radial thrust pads (4 numbers) inunits II and III. Fig. 11.8 shows the view of pedestal supporting theupper bracket.

After the successful completion of the repair work, NDT inves-tigations were again conducted by SERC, Chennai to evaluate theefficiency of the repair methodology suggested and executed. The UPVtests were conducted on the accessible locations on the eight concretepedestals supporting the stator support pads (upper bracket). TheUPV values were found to be stable and the average values are above4.00 km/sec, which indicates that the integrity of concrete is very good.Further, no visible distress could be noticed in the concrete pedestals.

11.4 SUMMARY

In order to rehabilitate and improve the corrosion resistance of Fingerjetty, half cell potential and UPV measurements were conducted onthe various structural elements. Based on the analysis of the half cellpotential readings and UPV values, a repair methodology was designedwhich included micro concrete jacketing and provision of Galvanicanodes. The following are the conclusions drawn based on the postrepair investigations:

The UPV measurements clearly reveal that the integrity of theconcrete in the rehabilitated structural elements of the Finger jettyis good, indicating the efficiency of the micro concrete jacketing tech-nique designed and executed. The Finger jetty has not shown anydistress on account of corrosion even after a period of nearly 2 yearsas evident from the half cell potential readings taken at every 6 monthinterval. The provision of galvanic anodes i.e. the galvanic protectionsystem is performing well in the Finger jetty and from the pattern ofthe half cell potential readings observed over a period of 2 years, itmay be concluded that this may continue to perform well for a fewmore years without causing any problem. In addition to that, even ifthe corrosion were to reoccur after probably 5 years, it is required onlyto cut open the particular place to install another piece of Galvanicanode, instead of resorting to a expensive large scale rehabilitation


measure resulting in closing down of the operation of the Finger jettyduring the period of rehabilitation.

It can be concluded that the galvanic protection system using thegalvanic anodes are techno commercially viable system to be adoptedfor the rehabilitation of the corrosion damaged marine structures andthey can be a useful tool to be installed even during the construc-tion of the marine structures resulting in considerable savings to thegovernment agencies.

In the case of the Hydel Project at Srinagar, it can be seen thatproper identification of the cause of the distress through field studiesand suggestion of the appropriated repair methodology and its exe-cution will go a long way in solving many issues associated with thefunctioning of vibrating structures.

Fig. 11.1 A view of the Galvanic anode

Fig. 11.2 Typical view of installation of galvanic anode in anystructure


Fig. 11.3 A view of the corrosion affected Finger jetty beforerehabilitation

Fig. 11.4 A view of the half cell potential test in progress on thepile cap

Fig. 11.5 A view of the UPV test in progress on the pile


Fig. 11.6 View of the positioning of Galvanic anode, form work &jointing compound

Stator support pad(Upper bracket) - 8 Nos

Concrete pedestalsupporting upper bracket

Concrete slab diaphragmsupporting lower bracket

Rotor support pad(Lower bracket) - 4 Nos.

Cylindrical barrelstructure

Fig. 11.7 Typical plan view showing the details of recast RCpedestals of upper brackets in unit III


Fig. 11.8 A view of the recast pedestal supporting the upperbracket

12 Fire-Affected Concrete Structures and its

Rehabilitation

P. Srinivasan,Assistant Director

CSIR-SERC, CSIR Campus, Taramani, Chennai-600 113, India.E-mail: [email protected]

12.1 INTRODUCTION

Concrete as a versatile material has high adaptability to satisfymany aspects in civil engineering structures such as functional needs,economy, maintenance, aesthetic acceptability, and protection againstcorrosive environment and fire. When a fire has occurred, the require-ments are generally for an immediate and thorough appraisal carriedout with clear objectives. Such an appraisal must begin as soon as thebuilding can be inspected and generally before the removal of debris.The fire resistance of a concrete structure is frequently well above itsminimum requirements, and hence rehabilitation by repair will, there-fore, be preferable to demolition and rebuilding. Rehabilitation mayrequire less capital expenditure than demolition and rebuilding andmay also provide a direct saving as a result of earlier re-occupation.

The compressive strength of concrete is reduced to 25% of itsunfired value when heated to 300◦ C and 75% at 600◦C and the elasticmodulus also gets reduced in the same manner (The Concrete Society,1990). The temperature estimation based on the color change seems tobe the traditional practice for fire-damaged concrete members. Whenconcrete is heated above 300◦C, the color of concrete changes fromnormal to pink or red (300-600◦C), to whitish grey (600-900◦C) andbuff (400-1000◦ C).

The idea of making an assessment of the fire-damaged concretestructure is to arrive at the estimation of temperature, extent ofdamage to concrete and reduction in the strength of concrete and rein-forcement The stiffness damage test (SDT) has been used to study thechange in strength of concrete affected by fire (Nassif, 1995). The study




of microstructure of the fire damaged concrete samples using scanningelectron microscope and stereo microscope will give the estimationof temperature. (Wei-Ming Lin, 1996). Color image analysis has beenapplied on concrete core samples to estimate the temperature and alsothe depth of damage (Short, 2001). Optical microscopy has been usedto determine the depth of damage based on the crack density measure-ments (Georgali, 2004). The methods mentioned above are conductedin the laboratory on samples collected from the structure.

Ultrasonic pulse velocity test, which is a non-destructive testmethod, is widely practiced for the evaluation of the quality of a con-crete structure. This is a very simple test and can be carried out on astructure at a faster rate. The ultrasonic pulse velocity measurementsmade on a structure will provide a qualitative estimation of the dam-aged members with the undamaged one (Hung-Wan Chung, 1985 andAndrea Benedetti, 1998). The depth of concrete affected by fire canbe calculated using the ultrasonic pulse velocity values (Mani, 1986).The application of the ultrasonic scanning, tests on concrete core andreinforcement samples have been applied to two case studies alongwith the load test carried out after repair are discussed in this paper.

12.2 APPROACH FOR ASSESSMENT OF THE FIRE

AFECTED REINFOCREMENT OF CONCRETE

STRUCTURES

A general approach for carrying out a scientific investigation of a fireaffected reinforced concrete structure and the parameters that are tobe evaluated from these tests are given below.

Stage I : Preliminary inspection (inspection before removalof debris)Visual inspection and documentation include:

• Source of fire and its location in the building

• Locations of portions with extensive, moderate and no-damage

• Color of concrete

• Areas showing cracks, spalling of concrete and exposure ofreinforcement

• Damage of structural steel sections and their locations

• Collection of damaged samples such as steel, aluminum, glass,etc.

Fire-Affected Concrete Structures and its Rehabilitation 213

Stage II: Detailed investigation

• Estimation of temperature : Based on the collected samples suchas melted metals, broken glass pieces, color of concrete, etc.

• Duration of fire by collecting data from eyewitnesses or firefighting personnel

• List out the damage and categories, i.e., severe, fair, moderate,and no-damage.

• Insitu tests

• Ultrasonic scanning on RC members

• Rebound hammer test

• Load test if required

• Laboratory tests

On concrete core samples from affected and unaffected areas andcarry out the following

– Observe the change in color due to heat

– Observe the texture of concrete

– Conduct UPV scanning after dressing

– Determine the depth of concrete affected by fire.

– Determine the Compressive strength and Modulus of elasticityof core samplesOn Steel samples from affected and unaffected areas

– Carry out tests to determine tensile strength, modulus ofelasticity and percentage elongation

Stage III: Assessment and classification of damage

Based on the UPV values, the members may be classified as

(a) Unaffected - members with hair cracks and UPV values greaterthan 3.5 km/sec

(b) Moderately affected - members with wide cracks and UPV valuesbetween 2.5 and 3.5 km/sec

(c) Fairly affected - members with major cracks, spalling of concrete,and UPV values below 2.5 km/sec

(d) Severely affected - major cracks, spalling of concrete, exposureand debonding of Reinforcement and finally the load carryingcapacity can be calculated based on the parameters evaluatedusing the various test results.


12.3 INTERPRETATION OF INSITU AND LABORATORY

TEST RESULTS

Visual Inspection The visual inspection of the fire affected struc-ture and the status of some of the components of the structure such asaluminum, glass panes, etc. after the fire do suggest the approximatetemperature to which the structure was subjected. The temperaturecan be further confirmed by conducting ultrasonic scanning on con-crete, tension test on structural steel and reinforcing steel and testson concrete core samples.

Ultrasonic Scanning Results

Taking the UPV values for the un-affected members as the basis thevelocity values of the members affected by fire can be compared andprobable temperatures to which the portions of members were sub-jected to can also be estimated. The depth of concrete affected by firecan be calculated using the relationship between the velocity profileswith temperature (Mani, 1986)

Core Sampling and Testing

Tests on core samples give direct evidence on residual compressivestrength and temperature to which the concrete member is subjectedduring fire. The pulse velocity values of these core samples can becompared to confirm the estimated temperature and the correctnessof estimation of the depth of damaged concrete Study of core samples,their density and compressive strength bear a relation which helps toconfirm the estimated temperature.Residual Strength of Steel

To assess the residual properties of the reinforcement, samples fromdifferent locations are to be collected and tested for yield and ultimatestrength, percentage elongation and modulus of elasticity. The reduc-tion in the strength and modulus of elasticity will give an idea of thetemperature to which the member has been subjected to fire.

Based on the above test results, parameters such as probable tem-perature, depth of concrete removal, average ultrasonic pulse velocityin the core concrete, the residual strength of concrete etc., can beevaluated. Once the classification of damage has been worked out, therepair measures can be formulated.


12.4 REPAIR OF FIRE DAMAGED CONCRETE

STRUCTURES

Repair of fire-damaged concrete structures requires restoration of anyloss in strength, durability, and fire resistance of concrete and steel.Generally repair of fire affected structures shall consist of the followingtypes depending on the extent of damage.

(i) Type I for unaffected members

(a) Remove loose particles if any and clean the surface

(b) Replaster the area if required

(ii) Type II for moderately affected members

(a) Remove loose particles

(b) Clean the surface with high pressure water jet or sand blasting

(c) Inject cement grout followed by low viscosity epoxy

(d) Replaster the surface with cement mortar, if required

(iii) Type III for fairly affected members



(c) Inject cement grout followed by low viscosity epoxy

(d) Gunite with high strength gunite in layers (not exceeding 20mm) over a layer of welded mesh of 10 G × 10 G - 100 mm× 100 mm in each layer of gunite or replace the fire-affectedconcrete by polymer modified mortar or Jacketing with micro-concrete.

(iv) Type IV for severely affected members



(c) Inject cement grout followed by epoxy

(d) Provide additional reinforcement, if required

(e) Gunite with high strength gunite in layers (not exceeding 20mm) over a layer of welded mesh of 10 G × 10 G - 100 mm× 100 mm in each layer of gunite or replace the fire-affectedconcrete by polymer modified mortar or jacketing with micro-concrete.


12.5 CASE STUDIES

In the following section two case studies on fire damaged concretestructures are reported, one on RC framed structure of an industrialbuilding and the other on cooling tower.

12.5.1 Investigation of fire damaged RC framed structure of anindustrial building

The building is a reinforced concrete framed structure having columnsand beams running in perpendicular directions and is covered byR.C.C. slab. Fig. 12.1a shows a portion of the RC frame (with greasemarkings made for UPV measurements) and Fig. 12.1b shows a typicalbeam affected by fire.

Visual inspection

The visual inspection of the fire affected structure, and the statusof some of the components after the fire did suggest the approximatetemperature to which the structure was subjected. It can be seen fromthe Table. 12.1, that the temperature to which the concrete structurewas subjected can be estimated approximately between 300◦C and600◦C. The temperature was further confirmed by conducting ultra-sonic scanning, tension test on reinforcement samples and tests on coresamples.

Assessment from ultrasonic scanning and tests on coresamples

The ultrasonic scanning was carried out on 36 columns and 32 beamswith a grid spacing of 150 mm × 200 mm. A typical UPV data for abeam is shown in Fig. 12.2 A good quality concrete of M20 grade willhave a velocity of 4.0 km/sec. The lower velocity values at grid linesB and C indicate that the bottom portion of beam was affected morecompare to grid line - A i.e., top portion of beam. The temperaturewas estimated to be 300◦ C to 400◦ C and the depth of correction as40 to 50 mm.

Core samples around 20 numbers were collected on both affectedand unaffected areas. The typical core sample details are given inTable. 12.2 with the velocity values and compressive strength, andalso the depth of correction.

Assessment of residual strength of steel

The reinforcements in several locations were exposed and some of thereinforcements were in deflected condition especially in the roof slabportions. In order to assess the residual properties of the reinforcement,


samples from different locations were collected and tested mainlyfor yield and ultimate strength, percentage elongation and modulusof elasticity. Table. 12.3 shows the test results including estimationof temperature on steel samples taken from portions unaffected andaffected by fire.

After repair, the load test was conducted as per standard practice.The deflection and the recovery were found to be within allowablelimits.

12.5.2 Assessment of a fire-affected RC cooling tower

Condition assessment was made on a fire-affected cooling tower as perthe procedure mentioned above. Fig. 12.3 shows the cooling tower andFig. 12.4 shows the core sampling on the structure. The UPV data forthe shell portion is given in Fig. 12.5. It can be seen that the maximumdamage has occurred in grid lines 10to 25 whereas the portion in gridlines 1 to 5 have undergone less damage. The test results on coresamples indicate the depth of correction to be 40 to 50 mm.

12.6 CONCLUSION

A systematic investigation using visual observation and in-situ testingby ultrasonic scanning together with the tests on core samples and onreinforcement samples will adequately help to assess the condition ofa fire-affected reinforced concrete structure in a more appropriate andeconomical way. Depending upon the damage caused, the structurecan be restored.

12.7 REFERENCES

1. The Concrete Society, “Assessment and Repair of fire-damagedconcrete structures”, Technical Report 33, The Concrete SocietyLondon., 1990.

2. Nassif A. Y., et al., “A new quantitative method of assessing firedamage to concrete structures” ,Magazine of Concrete Research,47, No.172, 1990 pp 271–278.

3. Wei-Ming Lin T. D., Lin ., and Powers-Couche L. J., “Microstruc-tures of Fire-Damaged Concrete” ACI Materials Journal, V.03,No.3, 1996, pp 199–205.

4. Short N. R., Purkiss J. A., and Guise S. E., “Assessment of firedamaged concrete”, Construction and Building Materials, Vol.15,2001 pp 9–15.


5. Georgali B., Tsakiridis P. E., “Microstructure of fire-damagedconcrete. A Case Study” Cement and Concrete Composites, 2004pp 1–5.

6. Andrea Benedetti “Ultrasonic Pulse Propagation into Fire-Damaged Concrete” ACI Structural Journal, V.05,No.5, 1998 pp259–270.

7. Hung-Wan Chung and Kwok Sang Low., “Assessing fire damageof concrete by the ultrasonic pulse technique”, American Societyof Testing and Materials, 1985, pp 84–88.

8. Mani K., and Lakshmanan N., “Determining the extent of dam-age due to fire in concrete structures by ultrasonic pulse velocitymeasurements”, Indian concrete Journal, Vol.60, No.7, 1986, pp187–191.

Table 12.1 - Estimation of temperatureCriteria Material Approximate Remarksadopted temp. ◦CColor Concrete 300 - 600 Greenish grey to

pinkBehaviour of Aluminum More than Verge of meltings

material 600Degree of Steel-concrete More than Debonding of steeldamage 800 from concrete after

fire (observation)Core sample Concrete More than Pink color upto

600 fire from surfaceand whitish greyand collapsedconcrete

Table 12.2 - Tests on Core Samples (typical)Estimated Depth of UPV Estimated UPV Cube

temperature correction at location velocity of dressed compressive◦C mm km/sec km/sec core Strength

km/sec N/mm2

300 50 3.57 3.99 3.94 19.15

500 90 3.10 3.40 3.85 18.50


Table 12.3 - Test on Reinforcement Samples (typical)Status of Ultimate Yield % Young’s % Estimatdamage stress stress elongat modulus decrease in temp.

N/mm2 N/mm2 ion N/mm2x ultimate ◦105 stress

Undamaged 561.5 465.00 8 12.13 - -

Slightly 510.0 430.00 9.0 1.97 7.53 300

Severe 400.0 265.0 30.0 1.86 28.8 500

Fig. 12.1a Fire Affected RC Frame


Fig. 12.1b Damage in Beam due to Fire (Exposure ofReinforcement)

3.23 2.15 3.15 2.90 2.67 2.15 2.82 3.36 3.23 3.00 2.95 3.53 A

2.62 1.97 2.24 2.14 1.99 2.19 2.29 2.75 2.95 2.20 1.85 2.67 B

2.13 1.59 1.87 1.37 1.49 1.49 1.57 1.48 1.32 1.08 1.39 1.45 C

1 2 3 4 5 6 7 8 9 10 11 12 Grid

Note: 1.Size of beam - 400 × 400 × 4500 mm 2. Estimated Temperature - 300 to 400◦ C 3.Depth of correction - 40 to 50 mm

Fig. 12.2 Ultrasonic Pulse Velocity Values for BeamAffected by Fire

Fig. 12.3 Fire Affected RCC Cooling


Fig. 12.4 Core Sampling from the Tower

Fig. 12.5 UPV Values for the Shell of Cooling Tower

.

13 Condition Assessment of Concrete

Structures Subjected to Vibration

K. MuthumaniHead-Advanced Seismic Testing and Research Laboratory,


13.1 INTRODUCTION

The interest in the ability to monitor a structure and detect dam-age at the earliest possible stage is pervasive throughout the civil,mechanical, and aerospace engineering communities. For the purposesof this discussion, damage is defined as changes introduced into a sys-tem which adversely affects the current or future performance of thatsystem. These systems can be either natural or man-made. However,depending on the levels of exposure, these systems may not show theadverse effects of this damaging event for many years or even futuregenerations. Implicit in this definition of damage is that the concept ofdamage is not meaningful without a comparison between two differentstates of the system, one of which is assumed to represent the initial,and often undamaged, state. The need for quantitative global damagedetection methods that can be applied to complex structures has ledto the development and continued research of methods that examinechanges in the vibration characteristics of the structure. The currentstate of aging infrastructure and the economics associated with itsrepair have also been motivating factors for the development of meth-ods that can be used to detect the onset of damage or deterioration atthe earliest possible stage. Finally, technological advancements includ-ing increases in cost-effective computing memory and speed, advancesin sensors including non-contact and remotely monitored sensors andadaptation and advancements of the finite element method representtechnical developments that have contributed to recent improvementsin vibration-based damage detection. Additional factors that have con-tributed to these improvements are the adaptation and advancements




in experimental techniques such as modal testing, and development oflinear and nonlinear system identification methods.

13.2 MODAL TESTING

Experimental modal analysis is basically a procedure of experimen-tal dynamic testing, modeling and inverse computation. The primarypurpose is to develop a dynamic model for a structural system usingexperimental data. Experimental modal analysis (EMA) produces amodal model that consists of

1. Natural Frequencies2. Modal damping Ratios3. Mode shape vectors.

Once a modal model is known, standard results of modal analysiscan be used to extract an inertia matrix (Mass), a damping matrixand a stiffness matrix, which constitute a complete dynamic model forthe experimental system.

In particular EMA is useful in design, diagnosis and control ofstructural systems primarily with regard to vibration. In componentmodification, one can modify inertia, stiffness and damping param-eters in a structural system and determine the resulting effect onthe modal response (Natural frequencies, damping ratios and modeshapes) of the system. In modal response specification, one can estab-lish the best changes, from the design point of view, in systemparameters (inertia, stiffness and damping values and their degreesof freedom), in order to give a specified change in the modal response.In sub-structuring, two or more sub-system models are combined usingdynamic interfacing components, and the over-all model is determined.

Diagnosis of problems like mechanical faults, performance degrada-tion, component deterioration, impending failure etc. of a structuralsystem requires condition monitoring of the system, and analysis, eval-uation of the monitored information from time to time. Diagnosismay involve the establishment of changes, both gradual and sudden,patterns and trends in these system parameters.

The standard steps of experimental modal analysis are

• Obtain a suitable (admissible) set of test data, consisting offorcing excitations and motion responses, for various degrees offreedom of the test object.

Condition Assessment of Concrete Structures Subjected to Vibration 225

• Compute the frequency transfer functions (Frequency ResponseFunctions) of the pairs of test data using Fourier analysis. DigitalFourier analysis using Fast Fourier Transform (FFT) is the stan-dard way of accomplishing this. Either software based (computer)or hardware based instrumentation can be used.

• Curve fit analytical transfer functions to the computed trans-fer functions. determine natural frequencies, damping ratios andresidues for various modes in each transfer function.

• Compute mode shape vector.

• Compute inertia (mass) matrix, M, stiffness Matrix, K anddamping matrix, C.

13.3 STEADY STATE HARMONIC TESTS

The instrumentation for steady state harmonic tests consists of amechanical exciter, speed control unit, a vibration pick-up, a vibra-tion meter and an instrumentation tape recorder. The mechanicalexciter gives a sinusoidal force given by F = A0f

2 (sin 2πft) wheref is the operating frequency, and A is a constant depending on theeccentric moment. Using the speed control unit, the frequency is var-ied. The mechanical exciter-speed control system can be replacedusing an electro dynamic shaker-power amplifier -signal generatorsystem. In this case the existing force has constant amplitude anddoes not vary with frequency-Accelerometers, velocity pick-ups, dis-placement pick-ups, etc., is used to measure the response. The latestinstrumentation system consists of data acquisition card, computer,and associated software. When the frequencies are well separated thedamping associated with individual mode can be obtained using the

relation ξ =(

f1−f2

2fn

)where f1, f2 are frequencies corresponding to

half power points, on either side of the resonant frequency. The ampli-tudes of half power points are equal to 0.707 times the amplitude atresonance.

13.4 FREE VIBRATION TESTS

Free vibration tests are extremely useful to determine the fundamentalfrequency and associated damping in a structural system. Droppingof a weight, snapping of a tensioned wire attached to the structuralsystem, etc., can set-up free vibrations in a beam which can be mea-sured. The frequency is determined by counting the number of cyclesin a given time interval, and the damping factor determined using


the relation ξ = 12πn

loge

(x0

xn

)x is the initial amplitude and x is the

amplitude after n cycles.

13.5 AMBIENT VIBRATION TESTING

Vibration levels are measured on buildings and structures under windloading, due to traffic inducted excitation, pile driving and otherconstruction activities, and in an industrial environment. The datacollected can be used for system identification that is to determinethe overall stiffness and damping parameters. The random responsemeasured at number of salient locations simultaneously is analyzedusing FFT to obtain the dominant frequencies, and mode shapes. Theplot of amplitude of vibration against frequency then can be com-pared with standards to estimate the level of human comfort, safety tostructures and so on. In an industrial environment a pronounced levelof amplitude at a particular frequency may indicate the undesirableperformance of a machine or its foundation.

13.6 DEVICES FOR MEASUREMENT OF DYNAMIC

RESPONSE SIGNALS

A Comprehensive range of transducers and the associated signal pro-cessing equipment are available for the measurement of dynamicparameters like acceleration, velocity, displacement, strain, load andpressure.

13.6.1 Acceleration Transducers

Acceleration is the natural choice for the measurement of seismicground movement, condition monitoring of machinery vibration andhigh frequency application like blast and impact. The advantage ofacceleration transducers is that they do not require any non-vibratingstatic reference. The simplest accelerometer can be thought of as asingle degree of freedom system and the acceleration to be measuredis applied to the base of the SDOF system. The relative displacementsuffered by the spring is proportional to the absolute acceleration atthe base and some how this relative displacement is to be converted toan electrical voltage for measurement and recording. A peizo-electricalmaterial is typically used as the spring in the SDOF system and it ismounted either in a shear set up or in the compression set up (Fig.13.1). The peizo-electric crystal is characterized by its ability to pro-duce electric charge proportional to the applied stress. The applied


stress is proportional to the relative displacement of the spring mate-rial, which in turn is proportional to the base acceleration. The naturalpeizo-electric materials are quartz and Rochelle salt but the moderntransducers use the man made ceramics like barium titonate, leadZirconate-titonate and lead metaniobate. The natural frequencies ofsuch a system are very high, typically in the order of 20-50 kHz and theuseful frequency range of such accelerometers is in the range of zeroto 0.2 of their natural frequency. The peizo-electric accelerometers arevery rugged and can sustain very high shock loads in the order of thou-sands of ’g’. The distinct draw back of such accelerometers is in signalconditioning and in transmitting the signal. Being self generating pick-ups they have very little energy available and the charge generated (q)is typically in the order of pico Coulombs in a capacitance-(c) of a fewthousand pico Farads. Hence the voltage generated is v = q/c is inthe order of few milli-volts.The output impedance of the device isz = 1/(2πfc) is very large at low frequencies. Hence connection to anamplifier give rise to low frequency attenuation and possible instabil-ity at low frequencies. Towards eliminating some of the problems acharge amplifier is used as the conditioner and the typical minimumfrequency of the peizo-electric accelerometer is around 1.0 Hz

Care is required in the choice of connecting cables between thepickup and the amplifier, which is normally a co-axial cable. Standardco-axial cables suffer from tribo-electric effects, whereby spurious elec-tric charge is generated due to friction between the di-electric and theouter braid covering. The manufacturers to counteract these effectssupply special low-noise cable and care should be taken to ensure thatconnectors do not become contaminated with dirt and swarf, other-wise poor low frequency performance and noise will result. Instead ofa separate charge amplifier, peizo-electric accelerometers are availablewith built-in micro-electronic amplifier with an advantage of low out-put impedance such that conventional lengthy coaxial cables can beused to conduct the output voltage.

The attempt towards extending the range of accelerometers for lowfrequencies (fraction of a Hertz) as experienced in the case of windand ocean wave responses saw the emergence of un-bonded straingauge accelerometers where a pre-tensioned strain gauge wire is usedin the place of the piezo-electric crystal. The voltage generated isproportional to the strain change of the wire, which in turn is pro-portional to its base acceleration. However the maximum sustainableshock acceleration is in the order of 100s of g and the natural fre-quency of the system is also low. The more common type of strain


gauge accelerometers are based on the peizo resistive effect and makeuse of semi-conductive strain gauges where change in resistance isproportional to the applied stress. Unlike a metallic strain gauge, thepeizo resistor has a resistance change, which is large compared toits change in length due to applied stress. Unfortunately it tends tobe highly temperature sensitive and an elaborate temperature com-pensation effect is required. The frequency response of peizo-resistiveaccelerometers extends to zero frequency and they can be calibratedby rotation in the earth’s gravitational field.

Servo accelerometer (Force Balanced Accelerometer) is the mostprecise and costly transducer. It employs an inertial mass which isfree to move in one axis by means of a pivot or hinge. The displace-ment of the mass is sensed by some form of inductive or capacitivenon-contacting displacement transducer and the resulting signal isamplified and applied to a torque or force generator in such a senseas to tend to restore the mass to its original position. Phase shiftis normally introduced in the feed-back loop and this electricallycontrols the damping, The loop gain controls the spring constant elec-trically. The moving element and the hinge are made of quartz andhave stable mechanical properties. Such accelerometers are capable ofresolving micro-g and find application as sensing elements in complexaeronautical and marine inertial navigational systems.

13.6.2 Velocity Transducers

The velocity transducers employ the principle of emf generation by amoving flux system in a coil. They are constructed such that the mag-net is supported within the coil by means of springs (Fig. 13.1). Thearrangement is similar to an accelerometer, but unlike an accelerome-ter, which is used below its natural frequency, the velocity transduceris used above its natural frequency. Their useful frequency range is10-1000 Hz The main application of these transducers is for machinemonitoring. Their inherent ruggedness, reliability and self-generatingcharacteristics make them ideally suitable as in-built pickups on thebearing of high frequency machines. They do not require elaborateamplifiers and the simple voltage amplifiers are sufficient and thecost of the pickup and the amplifiers is very small compared to theacceleration measuring systems.

13.6.3 Displacement Transducers

The linearly variable differential transformer (LVDT) type consists ofa three winding transformer with a moveable core attached to the


input shaft(Fig. 13.1). The central primary winding is energized byan alternating current at a frequency between 2-10kHz. Since the twoouter windings are connected in opposite phase, when the core is cen-trally located, the induced emf in the secondary windings add up tozero. However if the core is displaced,, the flux linkages become unbal-anced and a net emf proportional to the displacement appears on thesecondary output. In order to provide a useable signal, the AC volt-age is to be demodulated and this is carried out in a special unit.An LVDT is often used in the actuator of the servo hydraulic sys-tem as a displacement sensor and is incorporated in the center of theactuator. Another similar transducer uses a core to create a differen-tial change in the inductance in the two halves of a centrally tappedcoil. The transducer is normally used in a bridge arrangement ener-gized at high frequency which enables the inductance unbalance tobe detected. Care is necessary to ensure that capacitance changes inconnecting cables are not large enough to affect the bridge balancesignificantly.

13.7 VIBRATION INDUCING DEVICES (SHAKERS)

Three types of vibration generators (exciters or shakers) as they arealso called are commonly used.

Mechanical exciters are used in dynamic testing of prototype struc-tures including heavy machine foundations. Two eccentric masseslocated on two shafts which are internally connected through a gearare made to rotate in the same plane at the same speed but in oppo-site directions. Their relative positions are such that the resultant oftheir centrifugal forces add up in one direction while it becomes zeroin the normal direction. The dynamic force in this kind of shaker isproportional to square of the exciting frequency.

One of the shafts is connected to the shaft of a DC motor whichis driven by a variable thyristor based speed drive. Upper frequencylimit of shaking is governed by the rotating speed of motor (usually50 Hz). Shakers of this type with a dynamic capacity of say 2t and 30Hz are indigenously available.

Electro dynamic shakers are based on the induction principleinvolving the interaction of magnetic field and electric current. Theassociated power amplifier - which drives the shaker limit the lowfrequency range to 5 Hz but it, can excite the structure at high frequen-cies giving a wide range of frequencies of operation. Large static loadscannot be sustained directly on this kind of shakers. These types of


exciters are normally expensive especially with large dynamic capabil-ities. They are used to test small sized models of prototype structuresin order to identify structural resonance which is associated with veryhigh frequencies.

Electro hydraulic shakers are, however, the most ideal ones for lowfrequency structural testing. The main element in this shaker systemis the double acting jack with an electronically controlled servo-valvefitted on it. The system is externally controlled by an electrical signalamplified by a servo amplifier which feeds the required current to theservo valve which, in turn, checks the flow of hydraulic fluid into andout of the actuator. These shakers provide very high force levels (ofthe order of even 1000t) and large displacement (upto 200 mm). Thefrequency range of the shaker is usually zero upto 100 Hz.

Both electro-dynamic and electro-hydraulic actuators can be usedto generate random signals consisting of digital data in the form ofdisplacement or acceleration time history. Normally, they are used forwave form like sine, sweep sine, and periodic pulses.

13.8 FREQUENCY ANALYSIS

Any time domain signal can be converted to frequency domain andvice-versa. Periodicity of the signal is assumed for the time durationof the acquired signal. The sine, cosine and the constant terms towhich the signal is broken down are orthogonal functions and themathematical process by which the conversion is carried out is calledas Fourier analysis. The Fourier analysis for the digitized values is thediscrete Fourier transform and the algorithm to speed up the numericalintegration is due to Cooley and Tuckey. This algorithm is easy toprogram and is also available as a firmware into the EPROM of themain processor that constitutes the core of a fast-Fourier transform(FFT) analyzer. Mathematically, Fourier transform of the time signalcan be written as,

x(f) =

∞∫−∞

x(t)e−i2πft.dt


Similarly converting from frequency domain to time domain can beachieved by an inverse FFT and this can be defined as,

x(t) =

∞∫−∞

x(f)ei2πft.df

The analog to digital conversion (ADC) process of the time signalinvolves two important considerations. One is the sampling time or thetime interval between the two consecutive pieces of the digital timehistory and the second is the minimum amplitude that can be cap-tured. The minimum amplitude of resolution depends on the numberof bits that constitute the ADC processor. For example a 14 bit pro-cessor can store a minimum voltage of 10.0V/213 . (with a 10.0 V fullscale). The sampling rate is determined by the maximum frequencyof interest and the Shannons’ theorem (or Nyquist’s frequency) statesthat the sampling time is such that

Δt =1.0

2.0fmax

The resolution in the time domain is dictated by the maximumfrequency of interest and the resolution in the frequency domain isdictated by the number points acquired. If ’N ’ number of points areacquired then the frequency resolution is

Δf =1.0

N.Δt

13.9 CASE STUDY

13.9.1 Evaluation of the Dynamic Characteristics of a TurboGenerator Supporting Structure

The turbo generator foundation for the 500 MW super thermal powerstation is one of the few structures in India supported on spring-damper assembly. The weight of the foundation is around 1500 tonnesand it is meant to support the turbo-generator machine and the pipingsystem weighing 2500 tonnes. The plan dimensions of the foundationare 33.0m * 15.0 m. The structure has five bearing points throughwhich the dynamic load of the machine is transferred to the founda-tion. There was an interruption during the casting of the foundation,which should have been done as a single pour and the machine man-ufacturer insisted on establishing the quality of the concrete and the


structure before erecting the machine. The ultra-sonic pulse velocitymeasurements had been taken and were found to be consistently morethan 4000m/sec. Dynamic characteristics of the structure were alsoevaluated in a frequency range of 5.0 to 55.0 Hz by in-situ excitationthrough contra-rotating eccentric shakers. The mechanical shakerscould attain a maximum dynamic load of 3600 kg at the maximumfrequency of 60.0 Hz and at lower frequencies, the force falls propor-tionately as the square of the frequency. This meant a very smallexcitation force at low frequencies and hence the measuring systemwas required to be extremely sensitive to pick up these small vibrationlevels.

The structure was excited by fixing the mechanical shaker at eachof the machine bearing. The resulting steady state accelerations at allthe bearing points including the excited bearing were measured andrecorded through a five channel charge amplifier and instrumentationtape recorder system. The recorded analog data was played onto a dualchannel fast Fourier transform analyzer and the amplitude componentcorresponding to the excitation frequency was synthesized and noted.The amplitude at each frequency was also normalised to a unit forceand the resulting compliance in terms of micrometer per kN (similarto flexibility in the dynamic sense) is plotted against the frequency.A typical plot showing the variation of the compliance at bearing-4 when the excitation was at bearing-1 is shown in Fig. 13.2. Thefigure also shows in dotted lines the compliance of bearing-1 when thebearing-4 was excited. The coincidence of the two curves establishesthe Maxwell’s reciprocity in the dynamic domain and also proves thevalidity of the experimental data.

The compliance curves thus generated were later on used to numeri-cally evaluate the dynamic response of the foundation after accountingfor the mass of the machine.

13.9.2 Excessive Vibrations of a Bearing in A T G Pedestal

Turbo generator foundations support high speed machinery. The speedof the rotor corresponds to the frequency of power supply which is50Hz in India. TG foundations are reinforced concrete structures withcolumns and beams. Individual beams carry bearing pedestals. Theaxis of the shaft is parallel to the longer dimension of the framedstructure. The beams that support the bearing pedestals run perpen-dicular to the longitudinal axis of the shaft, here afterwards referredto as transverse axis. Due to the rotation of the turbine shaft dynamicforces in the vertical and transverse directions are produced. These are


transmitted to the bearing pedestals through the bearings which havecertain stiffness and damping properties. The bearing - shaft inter-action itself is a complex problem. However no dynamic force in theaxial direction is envisaged or designed.

One of the 210 MW unit about to be commissioned, exhibited largeamplitude vibration in the axial direction. This was to be investigatedand corrected. The discussion with the authorities revealed that themachine has been fully checked for its balancing and is in good condi-tion. The commissioning of the equipment has been postponed for overa year because of the excessive axial vibration when the equipment wasworking at trial runs. A number of experts have suspected that theproblem is due to the local resonance of the supporting beam in thehorizontal mode. They have suggested additional mass to be addedto the beam, which was done. However the problem of axial vibrationpersisted. The quality of construction was stated to be good.

To ensure that the quality of construction particularly in theconcerned beam was acceptable, non-destructive testing using ultra-sonic pulse velocity measurements were carried out, and the resultsindicated good to very good quality concrete.

A detailed scheme of dynamic measurements was carried out on thebeam supporting the bearing pedestal. Fig. 13.3. gives the side eleva-tion and plan view of the beam. Vibration levels were measured alongvertical lines 1 to 7, and along horizontal lines a to g at intersectionpoints. Three directional sensors were used to measure the vibrationlevels. Vibrations levels were also measured at bearing levels in threedirections at locations 1 to 10 indicated in Fig. 13.3. The T.G. itselfwas used as the exciter, and vibration levels were measured at fourfrequencies namely 10.125,47.5,50, and 51.75 Hz at no load conditionand at 48.5Hz after synchoranisation at an output level of 165 MW.

The peak response at locations 1 to 10 at bearing level indicated val-ues between 50 to 80 microns in the axial direction, 25 to 30 microns inthe vertical direction and 10 to 20 microns in the transverse direction.A close study of the data clearly revels that the beam is undergoingtorsional vibrations. Since the lever arm to the added mass in an earlierexercise has been very less from the axis of rotation lying between hor-izontal lines b and c, it has not produced the desired result. An FEMmodeling was made of the beam together with the bearing housingrigidly bolted to the beam. The modulus of elasticity was chosen asto reproduce the torsional frequency corresponding to 47.5Hz

It is clearly recognized that the problem of excessive axial vibrationis due to the local resonance under torsional mode of the beam which


is lying close to the operating speed, and this has to be moved away.Stiffening of the girder is infeasible. Adding dampers also poses con-siderable problem. Added to this, the installation is frequently visitedby VIPS, and the repair measure envisaged shall not be an eye sore.Adding mass at a distance from the centre of rotation and below thebeam level is not possible due to the piping systems and other auxiliaryequipment in place. An out of the box thinking led to the suggestionthat the hood covering the equipment in the segment around the bear-ing location can be used. It was suggested that about a tonne of masscan be shaped in the form of hood and rigidly bolted to the beam.The centre of mass of the hood is at a large distance from the centreof rotation, and can significantly contribute to the mass moment ofinertia. The above thought was implemented in the FEM model andfound to be feasible. The suggestion has since been implemented, andhas avoided the problem of excessive axial amplitude at the bearinglocation.

13.9.3 Integrity Evaluation of Bridge Structures

Bridges in coastal areas are corrosion prone and the alternating cyclesof stress imposed on the bridges by the moving loads accentuate thecorrosion process. The bridges are subjected to vibration by the mov-ing loads, which are chaotic in space and time. The vibration signalsof the bridge at significant points are composed of the mixed moderesponse of the bridge. The frequency synthesis of the response signal islikely to show the frequency components at the first few flexural modesof the bridge and also at its torsional mode. The continuous monitor-ing of the averaged response signal over a period is likely to show thedecrease in natural frequencies of the bridge due to degradation in thesectional properties of the bridge.

13.9.4 Integrity Evaluation of Pile Foundations through StressWave Propagation Method

The wave propagation is the mechanism by which a transiently excitedpulse travels through an elastic medium. A steady state vibration canalso be characterized as standing wave pattern with the superpositionof the incoming and outgoing waves. The stress waves can be classi-fied as uniform and dispersive waves. The wave velocity of a uniformwave is a material property and independent of the frequency of exci-tation whereas a dispersive wave has wave velocity dependent on thefrequency of excitation. For example the axial stress wave travellingthrough a prismatic rod is of uniform type whereas the flexural or


shear wave travelling through the same rod is of dispersive type. Forexample a square shaped flexural pulse generated through a lateralimpact on a rod will have its time base elongated as the wave travelsas the high frequency component travels fast. However if the impactis an axial one, the shape of the square pulse is retained and there willbe amplitude decay.

If a small impact is given to a rod and the response is sampledat a high rate (in terms of micro seconds) the observed response willbe as in fig. with the reflected wave arriving at the impacted pointfor every 2l/c time interval. (’c’ is the wave velocity of the axially

propagating wave and is equal to√

E/ρ. In the case of a deformity inthe pile due to necking or enlargement at a depth of ’a’ from the pilehead, the propagating wave has a momentum and energy imbalanceat the suddenly changing cross section and to preserve the originalenergy and momentum a reflection takes place. The total wave energyis forked and is transformed as reflected and and refracted forms. Thisprinciple is made use of in the geotechnical application, towards non-destructive testing of pile foundations. The magnitude of the reflectedwave from the pile deformity is proportional to the reduction in thearea and its length. (Fig. 13.4)

13.10 REFERENCES

1. Bendat, J., Piersol, S., Random Data: Analysis and MeasurementProcedures, John Wiley NY, 1986, USA.

2. Gatti, P., Ferrari V., Applied Structural and Mechanical Vibra-tions Theory, Methods and Measuring Instrumentations, E & FNSpon, 1999, London.

3. Norton M. P, Fundamentals of Noise and Vibration Analysis forEngineers.

4. Lyon R. H, DeJong R. G, “Design of a High Level Diagnostic Sys-tem”, Jl. of Vibration, Acoustics, Stress and Reliability in Design,1984.

5. Stewart, R. M, “Application of Signal Processing Techniques toMachinery Health Monitoring”, Chapter-23 Noise and Vibrationedited by R. G. White and J. G Walker, 1982, Ellis Horwood.

6. Cooley, J. W., Tuckey, J. W., “An algorithm for machine cal-culation of Complex Fourier Series”, Jl. of Mathematics ofComputaion, Vol-19, 1965.

7. Bloch, H. P, Geitnet F. K., Machinery Failure Analysis andTrouble-shooting, Gulf Publishing, Houston, USA, 1986.


8. Tavner P. J, Gayden B. G, Ward D. M, “Monitoring of Generatorsand Large Motors”, IEE Proc., B 133(3), 1986.

9. Collacott, R. A., Mechanical Fault Diagnosis, Chapman and Hall,1977, London.

10. Collacott, R. A., Vibration Monitoring and Diagnosis, GeorgeGodwin ltd, London, 1979.

11. B & K Application Notes 14-227, Notes on the use of VibrationMeasurement for Machine Condition Monitoring

12. Srinivasulu, P., Lakshmanan, N., Muthumani, K., Gopalakrish-nan, N., In-situ evaluation of the Dynamic Characteristics of a500 MW Turbo-Generator Foundation, SERC Project - 454, 1992.

13. IEEE-344, Guide for the Seismic Qualification of Class-I elec-trical equipment for nuclear power plant generating station, TheInstitution of Electrical and Electronic Engineers (IEEE), NY,USA, 1971.

14. USNRC, Standard Review plan 3-7-2, Seismic System Analysis,USA.

15. Srinivasulu P., Muthumani K., Gopalakrishnan N., SathishkumarS., Seismic Qualification Tests on Control Valves, SERC Report,Project - CNP- 478, 1998.


Piezo-electric effect

Seismic Mass

Outer casing

Magnet as massElectric coil

Outer casing

ExcitationOutput

PrimarySecondary

Core

Fig. 13.1 Construction of Transducers

10 20 30 40 500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Excitation:# 1 -Response: # 4Excitation:# 4 -Response: # 1

Frequency (Hz)

Fig. 13.2 Forced Variation of Response in TestedFoundation


a

b

c

d

e

f

g

4 3 2 1 5 6 7

25cm

35cm

40cm

60cm

45cm

45cm

+

+

+

+

+

+

+

+

+

+

26.5cm 30cm 55cm 65cm 65cm

Side Elevation

Generator End

ControlRoom

BearingIV

55cm 30cm 26.5cm

7

6

10

9

8

5

3

4

2

1

Fig. 13.3 Layout of Measurement Points for VibrationLevels on the Transverse Beam of Bearing


lDisplacement

Reflected pulsesfrom tip

timeTransient response of a pile without defects

t = 2 l/c t = 2 l/c

a)

laDisplacement

Reflected pulsesfrom tip

Reflected pulsesfrom constriction

timeTransient response of a pile with defect

t = 2 l/c

t = 2 a/c

t = 2 l/c

b)

Fig. 13.4 Integrity Monitoring of Piles through Stress wave

.

14 Application of Fiber Optic Sensors for

Performance Assessment of Concrete

Structures

K.RavisankarScientist - G,


14.1 INTRODUCTION

Structural monitoring is used as a diagnostic tool for detecting or infer-ring the presence of defects/damages and for scheduling maintenanceoperations. The information obtained from monitoring is generallyused to plan and design maintenance activities, increase the safety,verify hypotheses, reduce uncertainty, and to widen the knowledgeconcerning the structure being monitored. Structural monitoring hasfound applications in two types of structures in particular: innovativenew structures and problematic ageing structures. In the case of newlybuilt constructions, it has become common practice to instrumentthose that present innovative aspects in terms of the types of materi-als used, structural design or geometry. Old structures with problemshave benefited from structural monitoring to extend their useful lifespan safely, making full use of the available structural reserves. Moni-toring is linked with safety. Unusual structural behaviours are detectedin monitored structures at an early stage; therefore, the risk of sud-den collapse is minimized and human lives, nature and goods arepreserved. Early detection of structural malfunction allows for anin-time refurbishment intervention that involves limited maintenancecosts. Well-maintained structures are more durable and an increase indurability decreases the direct economic losses (repair, maintenance,reconstruction) and also helps to avoid losses for users that may suf-fer due to a structural malfunction. New materials, new constructiontechnologies and new structural systems are increasingly being used




and it is necessary to increase knowledge about their on-site perfor-mance, to control the design, to verify performance, and to create andcalibrate numerical models.

The service phase is the most important period in the life of astructure. During this phase, construction materials are subjected todegradation by ageing. Concrete cracks and creeps, and steel oxidizesand may crack due to fatigue loading. The degradation of materials iscaused by mechanical (loads higher than theoretically assumed) andphysico-chemical factors (corrosion of steel, penetration of slats andchlorides in concrete, freezing of concrete, etc.). As a consequence ofmaterial degradation, the capacity, durability and safety of a structuredecreases. Monitoring during service provides information on struc-tural behaviour under predicted loads, and also registers the effectsof unpredicted overloading. Data obtained by monitoring is usefulfor damage detection, evaluation of safety and determination of theresidual capacity of structures.

Structures have different life periods: construction, testing, service,repair and refurbishment, and so on. During each of these periods,monitoring can be performed with an appropriate schedule of mea-surements. The schedule of measurements depends on the expectedrate of change of the monitoring parameters, but it also depends onsafety issues. Structures that may collapse shortly after a malfunctionoccurs must be monitored continuously, with maximum frequency ofmeasurements. However, the common structures are designed in sucha manner that collapse occurs only after a significant malfunction thatdevelops over a long period. Therefore, in order to decrease the costof monitoring, the measurements can be performed less frequently,depending on the expected structural behaviour.

It is desirable to determine the safety of the critical structures bythe Non-Destructive Testing (NDT) for evaluation of their strengthand integrity. Fiber optic sensors are attractive sensing devices as anNDT tool, given their small size, light weight, and dielectric glass con-struction that renders them immune to electrical noise and electromagnetic interference. Optical fibers offer the possibility to be embed-ded within cement or concrete without affecting their performanceand used as sensitive, but rugged, transducers of mechanical pertur-bations. Fiber optic sensors have the capability to be embedded priorto curing, into the concrete structural elements for non-destructiveevaluation of structural integrity and the measurement of the internalstate of stress. Sensors can also be surface mounted on concrete or steelmembers. There are challenging problems and issues while applying

Application of Fiber Optic Sensors for Performance Assessment of Concrete Structures 243

fiber optic sensing technology for monitoring of concrete structures.Systematic studies on the new sensors have been carried out in thelaboratory to solve the issues/problems. In this lecture, the potentialof fiber optic sensors for performance monitoring of concrete struc-tures has been highlighted. Details of the laboratory studies carriedout in solving some of the technical challenges and issues for imple-menting the fiber optic sensing technology to the field problems arealso covered.

14.1.1 Fiber Optic Sensors

Fiber optic sensors are fabricated using high strength silica, whichpossesses an inherent immunity to corrosion and Electro-MagneticInterference (Eric Udd 1995). The properties of optical fibers allowinnovative approaches for the design of optical sensors. Due to this rea-son, a number of fiber optic sensor types have been developed. Fiberoptic sensors can be classified under different categories. Localized,distributed and multiplexed sensors are based on sensing methods.Intensity, interferometric, polarimetric and spectrometric sensors arebased on transduction mechanism. Extrinsic Fabry-Perot Interfero-metric (EFPI) sensors and Fiber Bragg Grating (FBG) sensors bothare being used for long-term/performance monitoring of concretestructures.

14.1.2 EFPI Fiber Optic Sensors

EFPI sensors, which are of interferometric type, reported to be goodfor strain sensing in civil engineering applications (Ravisankar, K2002). In EFPI type sensor, a cavity comprising of two mirrors (reflec-tion) which are parallel to each other and perpendicular to the axis ofthe optical fiber form the localized sensing region. Here the referenceand sensing optical fiber are one and the same up to the first mirror,which constitutes the start of the sensing region. Fabry-Perot cavity isformed between the air - glass interface of two fiber end faces alignedin a hollow core fiber. Changes in the separation between the twofiber end faces, known as change in cavity length, cause interferomet-ric fringe variations. The interference pattern generated is sinusoidalin shape and directly related to the intensity of the applied strain. Theperiod of the wave form constitutes a fringe and by proper calibration,the magnitude of the strain can be determined.


14.1.3 Fiber Bragg Grating (FBG) sensors

Fiber Bragg Grating sensors are the most promising optical fibersensors based on the state-of-the-art technologies (Raymond, M. Mea-sures, 2001). FBG fiber optic sensors have initially begun to be usedextensively in the telecommunication industry for dense wavelengthdivision de-multiplexing, laser stabilization and erbium amplifier gainflattening at 1550nm wavelength range. In addition, the characteristicsthat an FBG reflects a specific wavelength that shifts slightly depend-ing on the strain applied are ideal for mechanical sensing. Hill andcoworkers first observed fiber photosensitivity in germanium-dopedsilica fiber in 1978 (Kathy K., 2006). Since then and entire class ofin-fiber components, called the Fiber Bragg Grating, have been intro-duced. Fiber Bragg Gratings are periodic structures that are imprinteddirectly into the core of glass optical fiber by powerful ultraviolet radi-ation. Such structure consists of a periodically varying refractive indexover typically several millimeters of the fiber core. The specific charac-teristic of FBG for sensing applications is that their periodicity causesthem to act as wavelength sensitive reflectors. During imprinting pro-cess, the intensity of the ultraviolet illumination is made to occur ina periodic fashion along the fiber core. At a sufficiently high powerlevel, local defects are created with in the core, which then give rise toa periodic change in the local refractive index. This change in refrac-tive index created is permanent and sensitive to a number of physicalparameters, such as pressure, temperature, strain and vibration. Thusby monitoring the resultant changes in reflected wavelength FBG canbe used for sensing applications to measure various physical quantities.

14.2 LABORATORY INVESTIGATIONS

There are challenging problems and issues while applying fiber opticsensing technology for performance monitoring of concrete structures.Systematic studies on the new sensors have been carried out in thelaboratory to solve the following issues/problems:

(i) Safe embedment of fiber optic sensors in concrete structures

(ii) Elimination of errors due to temperature induced apparent strain

(iii) Assessment of performance of the sensors under static and cyclicloading

(iv) Assessment of the long term stability of the fiber optic sensors

(v) Instrumentation for measurement of interfacial strains in FRPstrengthened concrete elements


14.3 INSTALLATION OF FIBER OPTIC SENSOR

Installation of fiber optic sensor in concrete structures is a challengingtask and it is preferable to go for embedment type sensors. Embeddingbare fiber optic sensor in concrete structures is not advisable becauseof their fragility. The sensor may get damaged during concreting orcompacting time, and hence they must be properly protected. Anotherimportant aspect of sensor embedding is the ingress/egress of the sen-sor lead to/from the host structure. The optical lead wires, which arevery fragile, also need to be protected from damage at ingress/egresslocations. One method of safeguarding the sensor is by providing aprotective layer called encapsulation between the optical fiber and thesurrounding concrete. The properties of this encapsulation can have amajor influence on the life and functionality of the sensor. The encap-sulation should be compatible to the surrounding concrete material toensure complete strain transfer. Hence selecting a suitable material asencapsulation is very important.

14.3.1 Sensor Protection Systems for EFPI Fiber Optic Sensor

A method of protection system using a pair of epoxy sheets has beendeveloped (Kesavan K., 2004). Here, one 50mm long EFPI fiber opticstrain sensor was packaged using two cast epoxy sheets of 100 × 10 ×2mm size. A groove was cut in one of the epoxy sheets and a 50mmlong fiber optic strain sensor was bonded using epoxy cement. Thenanother epoxy sheet was placed over and sealed using liquid epoxy.Another method of encapsulation using rod assembly has been devel-oped to embed the EFPI fiber optic sensor in concrete (Kesavan, K.,2010). In this technique, one 10mm long EFPI fiber optic sensor wasbonded to a steel rod of 5mm diameter and 60mm long with weldedend flanges. The sensor with the signal carrier was suitably protectedagainst damages (Figs. 14.1 & 14.2). In this method, the steel rod iscovered in such a way that the strain transfer takes place only throughthe end flanges

14.3.2 Sensor Protection Systems for FBG Fiber Optic Sensor

Stainless steel housing based package was designed (Biswas.P., 2010)and two samples of packaged FBG sensors was prepared as shown inFig. 14.3. The length between two flanges, flange diameter and flangethickness are of 70 mm, 12 mm and 5 mm respectively. The innerdiameter of the tube is 3.5mm with a wall thickness of 0.5mm.


14.3.3 Performance Evaluation of Packaged Sensor

Experiments were carried out to assess the level of strain transferthrough the packaged EFPI fiber optic sensor and FBG fiber opticsensor by embedding these packaged fiber optic sensors inside thetwo concrete cylinders separately (Fig. 14.4). After sufficient cur-ing, the cylinders were additionally instrumented with four surfacemounting electrical resistance strain gages to compare the response ofthe embedded sensor. The instrumented cylinders were tested undercompressive loading and the responses of the embedded fiber opticsensor, and electrical resistance strain gages were recorded. The strainresponse obtained from embedded fiber optic sensor was comparedwith the average of the four conventional electrical resistance straingage responses (Figs. 14.5 & 14.6). To check the reliability, experimentswere repeated on an another specimen and agreement was found tobe good.

When structures are subjected to high stresses due to over loading,accidents and natural calamities like earthquake etc., the embeddedsensors should withstand this high stresses and record the responseof the structure accurately. In order to study the performance of thepackaged fiber optic sensors under such loading conditions, experimen-tal investigation was carried out. For this study, packaged EFPI fiberoptic sensors was embedded inside the concrete cylinder of size 150mm diameters and 300 mm long during casting of the cylinder andthe cylinder was tested under compressive load using an UTM. Theresponse from embedded fiber optic sensor was recorded up to the fail-ure of the cylinder. From the observations, it is found that embeddedfiber optic sensor response and the strain gage response is close upto elastic limit (with in 1% variation), it is also found that embeddedfiber sensors continued to work without any damage or degradationeven after attaining a strain range of around 2000 με .

Experiments were carried out to study the performance of the pack-aged EFPI fiber optic sensor under flexural loading. For this study,two RCC beams (150 × 200 × 1500mm size) were cast and packagedEFPI fiber optic sensor was embedded in concrete at 30mm below thetop surface of each beam. Electrical resistance strain gages were alsobonded on the surface of each beam to compare the strain response offiber optic sensors (Fig. 14.7).

The instrumented beams were loaded by applying four-point bend-ing load. Load was applied in steps up to the failure of the beam andresponses from all the sensors were recorded. The strain responses from


fiber optic sensor and electrical resistance strain gages were found tobe good.

Bridges and other critical civil engineering structures operate ina dynamic environment subjected to repeated cyclic loading. Theintegrity of structures under such load conditions can not be pre-dicted from their responses under static loads. Predicting fatigue lifeof structures subjected to repeated load cycles during their service isan important issue. The life of structural components is significantlyinfluenced by fatigue. Reliable performance of packaged fiber optic sen-sors under cyclic/fatigue load is to be ascertained while using themfor health monitoring (Parivallal, S., 2004).

Experiments were carried out to study the performance of pack-aged fiber optic sensors (both EFPI and FBG) under fatigue load.For this study, concrete cylinders of size 150mm dia and 300mm longwere embedded with packaged fiber optic sensors (both EFPI andFBG) were prepared. The cylinders were instrumented with surfacemounting electrical resistance strain gages on the surface after cur-ing. A sinusoidal loading, ranging from a minimum of 8.49MPa toa maximum of 14.1MPa, at a frequency of 10Hz was applied to theinstrumented concrete cylinders using servo-controlled UTM. The per-formance of the embedded fiber optic sensors was evaluated up to 2million cycles of loading. The fiber optic sensor measurements wereconsistent with the load amplitudes during fatigue test. The responsesfrom the embedded packaged FBG sensors and surface mountedelectrical resistance strain gages were found to be matching well.Fig. 14.8 shows the responses from strain gages and packaged FBG sen-sor around 2 million cycles. Experiments were also carried out to assessthe performance of packaged fiber optic sensor under high-stress, low-cycle loading. For this study, the instrumented concrete cylinders weresubjected to high-stress, low-cycle loading using an UTM. Eight cyclesof loading-unloading were applied to the instrumented cylinders. Ineach cycle, the minimum stress was kept constant at 2.83MPa andthe maximum stress was varied from 14.15MPa to 53.79MPa. Stressversus strain for each of the cylinder was plotted. During the test, amaximum of around 1500 με was measured from the embedded sensorand the sensor was found to be working well even after attaining thehigh strain range.

14.3.4 Temperature Studies

In cases while making strain measurements at variable temperatureenvironment, the indicated strain is equal to the sum of stress-induced


strain in the test specimen and the temperature induced apparentstrain of the sensor bonded to the test specimen. With the thermal out-put expressed in strain units, correction for this effect can be made bysimply subtracting (algebraically) the apparent strain from indicatedstrain. To study the performance of EFPI and FBG sensor for themeasurement of thermal strain and temperature, experimental studieswere carried out for EFPI and FBG sensors independently.

14.3.5 Apparent Strain Calibration for EFPI Fiber Optic Sensor

In order to correct the temperature effects, temperature calibrationwas carried out for EFPI fiber optic sensors from laboratory experi-ments on two structural materials, namely, steel and concrete, usingcommercially available EFPI fiber optic strain sensors. A steel speci-men of size 300 × 20× 3mm was prepared and two fiber optic strainsensors, one temperature compensated for steel and the other withoutany temperature compensation, were bonded adjacent to each other.A temperature sensor (electrical resistance type) was also bonded(adjacent to fiber optic strain sensors) using suitable adhesive to mea-sure the surface temperature of the specimen. The instrumented testspecimen was placed inside a temperature controlled oven and the tem-perature was raised in steps from ambient temperature to a maximumof 80◦ C. The temperature of the test specimen was allowed to stabilizeat each stage, before measurements were carried out. Strain from fiberoptic strain sensor and temperature from temperature sensor wererecorded for each temperature setting. While conducting temperaturecalibration studies for concrete, a temperature controlled water bathwas used instead of a temperature controlled oven to eliminate the dry-ing shrinkage effect. Also the concrete specimen was soaked in waterfor sufficient period to obtain saturated condition. A concrete cylin-der of 150mm diameter and 300mm long was chosen as test specimenfor conducting temperature calibration study. The concrete cylinderwas instrumented with two surface mounted fiber optic sensors, onetemperature compensated for steel and the other without any temper-ature compensation. A temperature sensor (electrical resistance type)was also bonded (adjacent to fiber optic strain sensors) using suit-able adhesive to measure the surface temperature of the specimen.Fig. 14.9 shows the temperature Vs strain plots, from which appro-priate temperature correction coefficients can be obtained

The average value of slope of the above plots gives the apparentstrain per degree Celsius for the particular sensor bonded to the partic-ular structural material. From the experiments using non temperature


compensated fiber optic strain sensors, it is seen that the apparentstrain per degree Celsius is very close to the thermal expansion coeffi-cient of the host materials used in the experiments. Hence using a noncompensated EFPI fiber optic strain sensor in a test specimen, one candirectly measure the thermal expansion coefficient of any material.

14.4 APPARENT STRAIN CALIBRATION FOR FBG FIBER

OPTIC SENSOR

For this experiment, a dual FBG Sensor, each with grating length of15 mm and one FBG sensor was bonded with the adhesive on thesurface of a mild steel specimen and the second FBG sensor was keptfree ended on the surface of specimen for sensing temperature alone.Conventional resistance based temperature sensor was also fixed tomeasure temperature. To study the behaviour of FBG fiber optic sen-sor under temperature, the instrumented specimen was placed inside aoven and temperature initialization was done at ambient temperatureof 26.4◦ C and recorded the initial values of FBG sensors and straingage based temperature sensor. Then the temperature was increasedup to 65◦ C at 5◦C interval, corresponding wavelength shifts in bothFBG sensors & strain values from temperature sensor (resistancebased) were recorded.

The Bragg wavelength shifts in both the FBGs are same due tochange in temperature, while additional effect of strain results in largerwavelength shifts for the FBG which is bonded. Wavelength shift dueto temperature is subtracted from total shift of the first FBG to getthe thermal strain alone due to temperature.

14.5 STUDIES ON LONG-TERM STABILITY ASSESSMENT

OF FIBER OPTIC STRAIN SENSORS

Long -term stability assessment of EFPI fiber optic sensors, subjectedto a sustained loading was carried out. For this study, two specialself straining frame (Fig. 14.10) was designed and fabricated. Two7mm diameter high strength prestressing wires were instrumentedwith EFPI fiber optic sensors and a temperature sensor. The instru-mented prestressing wires were tensioned by means of a hydraulicjack. After locking the prestressing force on the instrumented wiressuitably, the strains from the two fiber optic sensors were measured.The measurements from the fiber optic sensor and temperature sensorwere carried out periodically. The measured strain data for a duration


of 400days was corrected for temperature effect and strain vs. timewas plotted (Fig. 14.11). The strain output is almost constant duringthis period, indicating that fiber optic strain sensors are stable andsuitable for long-term monitoring of structures.

14.6 INSTRUMENTATION FOR MEASUREMENT OF

INTERFACIAL STRAINS IN FRP STRENGTHENED

CONCRETE ELEMENTS

Reinforced concrete structures strengthened with Fiber ReinforcedPlastics (FRP) have been widely accepted since they have the promi-nent characteristics that the structures strengthened with conven-tional materials cannot compare with. FRP composites exhibit highstrength to weight ratio, corrosion resistance and convenient to usein repair/strengthening applications. Some methods that have beenadopted for repair of concrete structures with FRP include wrappingof the cracked members, adhesion of FRP plates/sheets to the tensionface of the members, etc. A common cause of failure in such strength-ened members is associated with debonding of FRP substrate fromthe concrete in an abrupt manner. This may be due to stress concen-tration at the fiber cutoff point and existing of transverse cracks alongthe member span. In order to understand the mechanism of debondingand also for evaluating the long-term performance of strengthened con-crete structures, it is essential to embed strain sensors at the interfacebetween the damaged concrete and the FRP fabric.

The requirement for any embedded sensor for monitoring differ-ential strain in FRP strengthened concrete structures is that thesensor should not be detrimental to the operational requirement of thestrengthened structure. Due to the compatibility with FRP materials,fiber optic sensor is a good choice for embedding at the interface of theFRP strengthened concrete structures. Surface preparation, bondingtechnique, thickness of adhesive layer, compatible protective coating,embedment length of the sensor etc are some of the issues in fiber opticsensor instrumentation for FRP strengthened concrete structures.

Experimental investigations were carried out for understanding theissues in placing FBG sensor at the interface of concrete and FRPand to measure the interfacial strain. The experimental program con-sists of testing small concrete prisms connected with Carbon FiberReinforced Polymer composite (CFRP) and subjected to axial loadconditions. The specimens consist of two concrete prisms with dimen-sions of 100×100×250 mm. Two prisms were connected through two


CFRP sheets strips 200×50 mm wide externally bonded to the oppo-site sides of the concrete surface by a wet lay-up process. Steel barsof 20 mm diameter were inserted in the cast exiting 15 cm from oneend of each prism in the way to apply the pull from hydraulic machineduring testing. At first the concrete surface was cleaned with an ironbrush and then the surface was coated with a layer of primer andair cured for 24 hours. Once the surface was ready, the FBG sensorswere bonded on to the concrete surface. Polymide coated FBG sen-sors were used for the instrumentation since polymide is compatiblefor both concrete and the FRP. Two different types of FBG sensors;single and dual gages were bonded at the interface. The advantage ofusing dual gage is: the initiation and propagation of debonding of thewrap is identified whereas with single gage the strain at a particularpoint only can be measured with out any idea of mode of debonding.On the left side of face 1, a dual gage with two gratings of size 3mmspaced by 20mm was bonded. On the right side of the face 1, a singlegage of 25mm long was bonded. Similarly on the left side of face 2,a 25mm long grating was bonded and on the right side of face 2 adual gage was bonded. The instrumentation scheme is as shown inFig. 14.12. The FBG sensors were placed very near to the face of theconcrete prisms. The FRP sheets were bonded to the concrete prismsusing epoxy adhesive. There is no contact between the two concreteprisms except through the FRP sheets. In the middle of the speci-mens, where the two concrete prisms are in contact, a paper surface isinserted to create a no bond area. Specimens were prepared in labora-tory condition of ambient humidity and temperature. After bondingthe sensors properly, a coat of saturant was applied to the concretesurface over the wrapping area. Over the saturant, the CFRP fibermat was placed and subjected to pressure by gentle rolling. The sec-ond coat of saturant is then applied over the fiber mat gently. Thenit was allowed to cure for 24 hrs. Four conventional strain gages werebonded to the outer surface of the CFRP at locations exactly abovethe FBG sensors bonded at the interface of concrete and FRP sheetsto compare with the strains measured by the embedded FBG sensors.The specimen was tested in the Universal Testing Machine (UTM), aload controlled machine under tension. All the FBG’s were connectedto the FBG interrogator and the strain gages were connected to thestrain gage data logger and all the gages were initialized at zero load.Axial tensile load was applied to the specimen by pulling the two barsfixed in the machine. The strain response from all the sensors was


measured continuously. The load was applied gradually until there iscomplete failure of the specimen.

The specimen was loaded up to failure and the response from thesensors were recorded continuously. The specimen failed at a load of15.4 kN by complete debonding of the FRP fabric from concrete. Thefiber optic sensor embedded at the interface of concrete and FRP hadregistered higher level of strain than the strains measured by conven-tional electrical resistance strain gages on the surface of the FRP. Thedual FBG sensor in face 2 at the interface had linear response upto 4kN and the behaviour changed to non linear indicating the initiationof debonding at that location. On further loading, the response fromthe FBG sensor near the edge of the prism (FBG Face 2 RS Dual 2)increases as there was debonding of the FRP from concrete. After thefirst FBG sensor near the edge (FBG Face 2 RS Dual 2) reaches peakvalue, the response from the second FBG sensor on the same fiberstarts increasing (FBG Face 2 RS Dual 1). This shows the propaga-tion of debonding of the FRP fabric from the location of the first FBGsensor to the next one in the same fiber. When the load was furtherincreased, the strain sensed by the second FBG increases at higherrate and there was complete separation of the FRP from the concreteat side 2 for a load of 15.4kN showing a sudden drop. Hence with mul-tiple FBG sensors the initiation and propagation of debonding can bevery well monitored in the FRP strengthened concrete structures. It isalso seen that debonding was not detected directly by the externallybonded strain gages. Since these strain gages were bonded to the outerface of the FRP, they stop sensing the strain after debonding, as thefabric gets detached from the concrete surface.

14.7 SUMMARY

Fiber optic sensors are a practical and real sensing technology alterna-tive to conventional NDT techniques. Among the primary benefits forusing fiber optic sensors are their immunity to electro magnetic noisecoupled with their small size that allows for direct embedment into theconcrete and composite materials. Technology on sensors, interroga-tion instruments, installation methods etc are improving, but need tocontinue to improve for widespread applications. Concrete construc-tion would benefit greatly from in-situ structural monitoring usingfiber optic sensors that could detect a decrease in performance orimminent failure. In this lecture, the potential of fiber optic sensorsfor integrated sensing and monitoring of concrete structures has been


brought out. Details of the laboratory studies carried out in solvingsome of the technical challenges and issues for implementing the fiberoptic sensing technology to the field problems are covered.

14.8 REFERENCES

1. Eric Udd (1995), ’Fiber Optic Smart Structures’, John Wiley &Sons, Inc., New York.

2. Raymond, M. Measures. (2001). ’Structural Monitoring withFiber Optic Technology ’, Academic Press, California.

3. Ravisanakar, K., et.al (2002), “Experimental Studies on FiberOptic Sensors for Smart Structure Applications”, SERC ResearchReport, SERC, EML-RR-2001-3, 2002.

4. Parivallal, S., Ravisankar, K., Kesavan, K., Sreeshylam, P. andSridhar, S, (2004), “Performance evaluation of fiber optic sensorsunder fatigue loading”, SERC Research Report , SERC, EML-RR-2004 - 3, May 2004

5. Kesavan.K, Ravisankar.K, Parivallal.P and Narayanan.T (2004).’A Technique for Embedding EFPI Fibre Optic Strain Sensors inConcrete’. Experimental Techniques, pp31-33.

6. Kathy K. (2006). Optoelectronic Applications: Fiberoptic Sens-ing - Fiber sensors lay groundwork for structural health monitor-ing. Laser Focus World, 42 (2), 63-67.

7. Kesavan.K, Ravisankar.K, Parivallal.S, Sreeshylam.P and Srid-har.S (2010), ’Experimental studies on fiber optic sensors embed-ded in concrete’, Measurement, vol. 43, pp 157-163.

8. Biswas.P, Bandyopadhyay.S, Kesavan.K, Parivallal.S, Arun Sun-daram.B, Ravisankar.K, Dasgupta.K (2010) ’Investigation onpackages of fibre Bragg grating for use as embeddable strain sen-sor in concrete structure’. Sensors and Actuators, A: Physical,Vol.157, Issue 1, Jan.2010, pp77-83.


Fig. 14.1 Cast epoxy sheet encapsulated EFPI fiber opticsensor

Fig. 14.2 Details of steel rod packaged EFPI fiber opticsensor

Fig. 14.3 Details of packaged FBG fiber optic sensor


Packaged FBG Sensor Packaged EFPI Sensor

Fig. 14.4 During embedding packaged fiber optic sensorsinside the concrete cylinders

0

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Fiber optic sensor encapsulated with epoxy sheetsAverage of four electrical resistance strain gages

Fig. 14.5 Comparison of Strain response-epoxyencapsulated EFPI fiber optic sensor vs electrical resistance

strain gage

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-500-450-400-350-300-250-200-150-100-500Micro strain

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N

Packaged FBG sensorStrain Gage (Average)

Fig. 14.6 Comparison of packaged FBG fiber optic sensorvs electrical resistance strain gage


Epoxy encapsulated fiber optic sensor

Fig. 14.7 Instrumentation details of RCC beam

-70

-60

-50

-40

-30

-20

-10

0

10

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Time in Seconds

Mic

ro st

rain

Packaged FBG SensorStrain Gage (Average)

Fig. 14.8 Typical response from embedded packaged FBGfiber optic sensor and Strain gage during fatigue test


0

50

100

150

200

250

300

350

400

450

30 40 50 60 70 80 90

Temperature in o C

Mic

rost

rain

Fiber optic sensor temperature compensated for steelFiber optic sensor without any temperature compensation

Slope = 8.1 µε/°C

Slope = 0.8 µε/°C

Fig. 14.9 Temperature calibration curves for apparentstrain correction- concrete specimen

Self straining frame

Instrumented Prestressed wire

Fig. 14.10 Experimental set-up for long-term stabilityassessment of fiber optic sensor


0

500

1000

1500

2000

2500

3000

0 50 100 150 200 250 300 350 400 450

No. of Days

Mic

rost

rain

Specimen 1 (Stress=418 MPa)Specimen 2 (Stress=360 MPa)

Fig. 14.11 Plot of strain vs. time(days) for long-termperformance assessment fiber optic sensor

Face 1

LS LS

RS RS

FBG Interrogator

FBG Interrogator

Dual Gage 3mmgrating spaced at 20mm

Single Gage with 25mm grating

Dual Gage 3mmgrating spaced at 20mm

Single Gage with 25mm grating

FBG face 2 RS Dual 2

FBG face 2 RS Dual 1

Face 2

Fig. 14.12 Instrumentation scheme for interfacial strainmeasurement in concrete elements

15 Evaluation of Residual Pre-stress in

Concrete Structures

S. Parivallal and K. KesavanAssistant Director

CSIR-SERC, CSIR Campus, Tharamani, Chennai-600 113, India.Email: [email protected], [email protected]

15.1 INTRODUCTION

Most of the critical civil engineering structures, in particular bridges,are constructed using reinforced / prestressed concrete as structuralmaterial. These structures undergo distress with time due to environ-mental and other unfavorable operating conditions. It is well knownthat the time dependant phenomenon such as creep and shrinkageof concrete also reduces prestressing force over time. Thousands ofconcrete bridges presently in operation worldwide are in need ofrehabilitation through major works of repairs. In the future, the reha-bilitation of existing structures will constitute an exceptionally largefield of operation that will extend for many years. Timely retrofittingmeasures help to reduce damages and improve service life. In orderto assess the safety and serviceability and to take a decision aboutthe possible repair measures to rehabilitate the distressed concretestructures, it is necessary to reliably estimate the existing level ofstress.

Assessing the existing stress of prestressed concrete structures inservice is fairly a difficult task and the researcher is often facedwith lack of actual design/construction information and environmen-tal service conditions. It is first necessary to generate scientificallyand systematically the required data relating to the existing level ofprestress, in order to take a decision about the residual strength andpossible repair measures to rehabilitate the distressed prestressed con-crete members. Determination of in-situ stress in the concrete surfaceis one way to assess the prestress available in the prestressing steel.




15.2 CONCRETE CORE TREPANNING TECHNIQUE

(SERC,1998)

Concrete core trepanning technique has been developed for assessingthe existing stress in prestressed concrete structures in-service. Thistechnique is based on the measurement of strain release due to localelastic stress relief, caused by core drilling and creation of normalstress-free boundaries.

In this technique, a strain gage is fixed at the center of theintended core aligned in the direction of maximum stress (for uniaxialstress condition). On drilling the annular hole around core, the straingage measures the complete elastic strain relief due to core drilling.Arrangement of strain gage in the core is shown in Fig. 15.1. An annu-lar hole of 50mm dia. is formed by diamond core drilling and the strainrelease is recorded till the cutting depth reaches to the required depth.Special instrumentation procedures, water proofing of gages and leadwire connections are developed to minimize errors during measure-ments. This technique has the advantage of measuring the full strainrelease and the data reduction is also simpler. The released strain isof the opposite polarity to the in-situ stress. After a sign change, thestrain is multiplied by the elastic modulus of concrete to determinethe in-situ stress. The core samples taken from the measured locationscan be used, to determine the elastic modulus of concrete.

15.3 LABORATORY STUDIES USING CORE TREPANNING

TECHNIQUE (KESAVAN,2000)

Laboratory studies were carried out to formulate proper procedure tomeasure and assess the reliability of the concrete core trepanning tech-nique for the determination of existing stress in prestressed concretestructures.

Experiments were carried out to assess the depth of the cuttingrequired to get maximum strain release in core trepanning technique.For this purpose, two reinforced concrete beams (150 × 100 × 1500mm) were cast. On each beam at 10 locations, 30 mm size linearstrain gages were bonded (five each at top and bottom) along thelongitudinal direction, with distance between gages being around 150mm. A special test set-up was designed and fabricated to apply axialcompression to the beam, by means of a hydraulic jack (Fig. 15.2).A core of 50 mm diameter was formed by diamond core drilling, till

Evaluation of Residual Pre-stress in Concrete Structures 261

the depth equals to diameter of the hole. For every 10 mm depth ofcutting, the released strains were noted.

From these studies, it is observed that for 50 mm diameter coredrilling using 30 mm gage size, the maximum release occurs at a cut-ting depth of 20 to 30 mm and there is no need to cut deeper, nor it isrequired to remove the core (Fig. 15.3). Also it was observed from thestudies conducted on beams that the average of released strain due tocore cutting are around 80-90% of the existing strain.

15.4 EXISTING STRESS MEASUREMENT IN

PRESTENSIONED PSC BEAM - EXAMPLE

(PARIVALLAL, 2001)

In order to carry out further reliability studies on core trepanningtechnique, a seven year old pretensioned concrete beam (T-section)was chosen. Instrumentation details of the beam are given in Fig. 15.4.The beam was prestressed with 18 number of 5mm dia. high tensilesteel wires with an initial prestressing force of 360kN. Seven sectionswere identified for instrumentation and measurement.

The easiest way to calculate the existing prestress is by finding thestress at the neutral axis of the beam, where all the bending stressesdue to prestress as well as gravity loads vanish. The calculated neutralaxis of the T-beam in consideration is found to fall very close to thetop flange and hence it was not possible to cut a core at the neutralaxis and hence to be interpolated by cutting at least two cores in thesame cross section. One core at top of the flange (normal to the topsurface) and two cores below the neutral axis on either side of thebeam (Fig. 15.4) were cut out at every section and from the releasedstrain values, the strain at the neutral axis was calculated.

Fig. 15.5 shows the released strain for a typical core of a seven yearold concrete beam. It is seen that the released strains at web left andweb right are identical, which show the reliability of the measurementsand absence of significant lateral bending. From the measurement ofstrain at top and bottom, the strain released at the neutral axis posi-tion is calculated. The existing prestressing force at various sectionsis evaluated using the appropriate material properties. The averageprestress calculated is 283.8kN, which is in good agreement with theapplied prestress, after taking into account the losses due to shrinkage,creep etc.


15.5 CASE STUDIES

Developed concrete core trepanning technique was used in assessingthe existing level of stress / prestress in various prestressed con-crete structures. Case studies of assessment of residual prestress inprestressed concrete structures using core trepanning technique arepresented here.

15.5.1 Determination of Existing Level of Prestress in BridgeGirders(SERC, 1997)

A two-lane fly-over bridge was investigated to assess the existing con-dition of the bridge (Fig. 15.6). The bridge consists of 9 suspendedspans each 30.48m length, supported on cantilever box type ham-mer heads monolithic with the solid piers. The cross section of thebridge shows two single cell rectangular boxes, the top slab of which ismonolithically connected together. There are 8 diaphragms, all spacedequally. The box girders are prestressed with internal tendons origi-nally and are subsequently strengthened with external tendons also.Two spans of the fly-over bridge, which are highly deteriorated, wereinvestigated for assessing their conditions. Three locations on eachgirder were instrumented for determining the existing level of prestress(Fig. 15.6). Concrete core trepanning technique was applied to deter-mine the existing level of pre-compression in the prestressed concretegirders.

15.5.2 Existing Stress Determination in Vierendeel Girder ofthe Roof Truss System(SERC, 1998)

An experimental investigation was carried out by SERC, to assess thesafety and serviceability of the roof system of a Workshop building( Fig. 15.7). The scope of the project includes assessing the existinglevel of prestress in the Vierendeel girders of the roof truss systemfrom the knowledge of existing stress levels in the bottom chords ofthe Vierendeel girder. Concrete core trepanning technique was used forthe measurement of existing stresses. In all, nineteen locations (fourlocations on the top surface and the remaining in the centroidal lineof the sides) of the bottom chord were instrumented on 10 differenttrusses (out of total 36 trusses). From this investigation, it was possibleto estimate the level of prestress in the bottom chords of the girders.


15.5.3 Existing Stress Measurement in Ribs of ConcreteHorizontal Silo (SERC, 1999)

The concrete silo structure measures 276m30m in plan. The structureshas a parabolic profile arch ring made up of seven corrugated pro-filed precast concrete elements of varying lengths, that are connectedtogether by in-situ concrete beams running longitudinally along thelength of the building. The arch rings are designed as two hingedarches. Each precast element has a trapezoidal profile with ridge andvalley portions as shown in Fig. 15.8. In order to balance the stressesof the composite structure if necessary, prestressing cables have beenprovided longitudinally and along the profile of the arch rings. In all,fifteen locations (seven locations on the side surface and eight loca-tions on the ridge) were selected in nine different ribs and instrumentedalong the rib axis. From the investigation, the existing stresses werecomputed.

15.5.4 Determination of Existing Level of Prestress in PSCGirders of the Iron Ore Berth(SERC, 2001)

This is a 37 years old iron ore berth structure consisting of an approachdeck having a length of about 143m of steel gratings supported by RCbeam on either sides. These beams rest on the RC pile cap supportedby two RC piles. The iron ore berth deck has seven spans (vary-ing from 16.44m to 17.69m). The width of the berth is about 22m.The structural system for this consists of 20nos. of post tensionedbeams arranged side by side @ 1m c/c. The group of girders are alsoprestressed laterally through diaphragms, after laying the deck con-crete(Fig. 15.9). The entire deck including PSC girders is supportedover pier cap formed over prestressed concrete crip that rests on theballast bed found over the sea bed available at -22m approximatelyfrom MSL. In order to obtain the prestress in the identified PSC gird-ers , the position of the neutral axis was determined from the geometryof the girder in order to avoid the bending stress contribution. Thetrepanning technique was carried out at three selected spans. Theinstrumented locations in PSC girder is as shown in Fig. 15.9. Thestress in the beam was calculated from the measured strain.

15.5.5 Assessment of Residual Prestress in a PrestressedConcrete Bridge at Srisailam(SERC, 2002)

The bridge is a balanced cantilever prestressed bridge comprising tenspans and supported on cylindrical piers with the end spans on abut-ments. The overall length of the bridge is 530.36m and the span length


is 48.77m. The piers are hammerhead type with articulations support-ing the suspended spans. Each of the piers was constructed integralwith prestressed cantilever girders on either side of the pier head,extending for 9.14m length from centre of pier to serve as hammerheads. The gaps between the cantilever arms having a span of 30.48mwere bridged with suspended girders resting on the cantilever ends.Based on the request made by the sponsor, an experimental investi-gation to assess the loss of prestress was carried out on the hammerhead supported by pier P7, which is highly deteriorated. The hammerhead is a cast - in - situ multi - cell box section, consisting of five gird-ers with top and bottom flanges to form an integral box section (Fig.15.10). Due to inacceability, only the outer surfaces of the extremegirders of the hammer head were available for instrumentation. Fourlocations on each of these extreme girders of the hammer head wereselected for instrumentation. Out of these four locations, two were onthe cantilever portion on pier P7 projecting towards Hyderabad sideand the other two on Srisailam side. In all, eight locations were instru-mented at the centroidal axis of the hammer head (Fig. 15.10). Theresidual prestress forces were obtained from the investigation.

15.5.6 Determination of Existing Level of Prestress in PSCGirders of the Approach Jetty to Intake Well(SERC,2005)

An experimental investigation was carried out to assess the conditionand formulating recommendations for remedial measures of approachjetty and intake structure. This approach jetty and intake well wereconstructed around 30 years back. The approach jetty consists of pre-stressed concrete girders with cast-in-situ deck slab. It has 26 spansof 15.24m consisting of precast prestressed concrete girders supported1.22m diameter piles with capping beams. The first eleven spans aresupported on single pier and the remaining on two piers. The widthof the jetty is 3.66m. Fig. 15.11 shows the typical cross section ofjetty structure and instrumented locations. Concrete core trepanningtechnique was used for the measurement of existing stresses. The exist-ing stress measurement was carried out on outer girders of the threespans (worst affected, moderately affected and unaffected) only. Fromthe measurements, the prestressing force available was calculated, andbased on this, the safe load carrying capacity of the jetty structure wasevaluated.


15.5.7 Experimental investigations on super-structure of theMahatma Gandhi Bridge at Patna(SERC, 2009)

An experimental investigation was carried out to assess the residualprestressing force of the Mahatma Gandhi Bridge at Patna. The bridgeis a balanced cantilever bridge having 59m span on each side of the pieras shown in Fig. 15.12. In order to evaluate the residual prestress, twodifferent pier spans, namely, span P23 at upstream side and span P26at down stream side were identified for the investigation. Out of thetwo selected, span P23 of U/S side is older and distressed comparedto the span P26 of D/S side. In each span, both cantilever girderswere instrumented at the inner surface of the box girder. In each arm,three sections were identified and at each section four locations wereinstrumented at two locations on the centre of gravity of the crosssection and the other two are at the top and bottom of the web. Fromthe measured strains at CG of the section, the residual prestressingforce in each girder is obtained.

Investigations of railway bridge girder near Villupuram(SERC,2010)

Studies were carried out to measure the existing level of stress due tothe self weight, prestress and super imposed dead loads (wearing coat,parapet, ballast, permanent way, etc.) in the identified span of thebridge. The prestressed concrete bridge is located between Villupuramand Mayavaram section (Fig. 15.13). Existing stress measurement wasdone by concrete core trepanning technique at the selected locations(Fig. 15.13). Twelve locations were identified for measuring the exist-ing stresses by concrete core trepanning technique. These locationsinclude mid span, quarter span and near support. From this study,existing stress condition of the girder was evaluated for its strengthevaluation.

15.6 SUMMARY

For assessing the existing stresses on distressed prestressed concretestructures concrete core trepanning technique can be used. Laboratorystudies were conducted to evaluate the reliability of the concrete coretrepanning technique. Case studies of assessment of residual prestressin prestressed concrete structures using core trepanning technique arepresented here. The details of the case studies will be presented in thelecture. Using this concrete core trepanning technique, it is possibleto estimate the probable value of existing prestress with a high degree


of reliability in prestressed concrete members. This will go a long wayfor the designer to design suitable rehabilitation measures.

15.7 REFERENCES

1. SERC Report, “Experimental Techniques for Existing StressDetermination in Prestressed Concrete Structures”, 1998.

2. Kesavan K., Parivallal S., Ravisankar K., Narayanan T., andNarayanan R.,“Non-Destructive Evaluation of Existing stressin Prestressed Concrete Members”, Proceeding of the NationalSeminar NDE-2000, pp 39–45., 2000

3. Parivallal S., Kesavan K., Ravisankar K., Narayanan T., andNarayanan R., “Assessment of Existing Prestress in PrestressedConcrete Structures” Proceeding of the National Seminar onTrends in prestressed Concrete, 2001 pp 271–279.

4. SERC Consultancy Report, “Determination of Existing PrestressLevel in Girders of Old Fly Over Bridge of Visakhapatnam Porttrust”., 1997

5. SERC Consultancy Report, “Existing Stress Measurement ofPrestressed Concrete Vierendeel Girders, Chennai Port Trust”.,1998

6. SERC Consultancy Report, “Report on the investigation to assessthe condition of horizontal silo and suphala plant structures inRCF factory at Chembur”., 1999

7. SERC Consultancy Report, “Report on the safety audit of pre-stressed concrete members of the iron ore berth of Visakhapatnamport trust”., 2001

8. SERC Consultancy Report, “Assessment of residual prestress ina prestressed concrete bridge at Srisailam”., 2002

9. SERC Consultancy Report, “Determination of Existing Level ofPrestress in PSC Girders of the Approach Jetty to Intake Well”,2005

10. SERC Consultancy Report, “Residual prestress evaluation in theMahatma Gandhi bridge at Patna”, 2008

11. SERC Consultancy Report, “Structural Assessment of a Pre-stressed Concrete Girder in Railway Bridge No. 493 in Servicebetween Villupuram and Mayavaram”., 2010


50mm φ Concrete core


50 mm φ50 mm


30mm Strain gage

50mm φ

1 1

Plan Section 1-1

Fig. 15.1 Concrete core trepanning technique

Test floor

500

PedestalJac

Load cell

Core locations

Pedestal

500 6 7 8 9 10

100

RCC Beam 1500 mm long.

Elevation

1 2 3 4 5

150 1000

1 2 3 4 5

Plan All dimensions in mm

Fig. 15.2 Experimental setup for existing strainmeasurement in axially loaded RCC beam


0

10

20

30

40

50

0 20 40 60 80 100 120 140 160 180MICROSTRAIN

DEP

TH (m

m)

Location 1Location 2Location 3Location 4Location 5

Fig. 15. 3 Released strain in axially compressed RCCbeam

View 1-1

402

254 50

162

3

7 11

2

162

Flange

Web

Left

Rig

ht

Neutral Axis

Elevation

Instrumented Section

1 2 3 4 5 6 7

Sect

ion

5 Se

ctio

n

2

Sect

ion

6

Sect

ion

7

Sect

ion

4

Sect

ion

3 1

1 800 300 730 1 170 150 360 950 740 5200

5mm Prestressing wire, 3×6 nos

All dimensions in mm

Fig. 15.4 Instrumentation Details of the PretensionedPrestressed Concrete Beam


10

20

30

40

50

0 50 100 150 200 Micro strain

Location 2 Top

Location 2 Bottom left

Location 2 Bottom right

Fig. 15.5 Trepanning Technique Applied to PrestressedConcrete Beam

L1

Section 1-1

L1 L2

N O T E: A LL D IM E N SIO N S A R E IN m m L1 – L3 IN DIC A T ES INS T R U M E N T E D LO C AT IO N S

L3

29718 1

1

Plan L1-L3 Indicates instrumented locations

Centre Line of the Bridge

2743 mm

1499 mm

7772 mm

Fig. 15.6 Details of fly over bridge girder


Elevation

Plan

A

A

Cross section A-A

Fig. 15.7 Vierendeel girder truss roof system of aworkshop building

30000

18500

3000

G.L

Elevation

Ridge Side 400

1500 1500

430

170

170

Cross section at A(showing instrumented locations)

Top view

2,64,000 6000 6000

30000

A

All dimensions are in mm

Fig. 15.8 Typical horizontal silo


Precast prestressed beams

Rail

22000 3800

Fender wall Galvanised ladder

• All dimensions are mm

• B1 To B5, B3T & B3B Indicates location of Instrumentation

B1 B2 B3 B4 B5

B3T

B3B

Elevation of typical PSC girder

Fig. 15.9 Instrumentation details of typical PSC girder ofIron Ore Berth

Plan

6858 5893 P7HD1P7HD2 P7SD1 P7SD2

P7HU1 P7HU2 P7SU2

Upstream

Downstream

P7SU1 Srisailam side Hyderabad side

4953

Pier P7

A

A

Section A-A

165

1981

292 1372

922 170

P7SU 3

P7SU 1 P7S D1

Fig. 15.10 Instrumentation details of prestressed hammerhead


Fig. 15.11a A View during investigation on ApproachJetty to Intake Well

b) Typical cross section of approach jetty at mid span

Instrumented location

Instrumented location

Fig. 15.11b Determination of Existing Level of Prestressin PSC Girders of the Approach Jetty to Intake Well


Fig. 15.12 Experimental investigations on super structureof the Mahatma Gandhi Bridge at Patna

19700

17700

VILLUPURAM END MAYAVARAM END

MWTCQVMWTC MVWTC

QVMWBC MWBC MVWBC

250

500600

350

5700

500

CLWeb

CL Slab Thk.

C ore C utting Location (Q uarte r S pan - V M E nd)

VMWTC VMETC

VMWBC VMEBC

Fig. 15.13 Locations of Existing Stress MeasurementUsing Trepanning Technique

.

16 Risk Informed Inspection Planning for RC

Structures

K. Balaji Rao and M. B. Anoop,Scientists, Risk and Reliability of Structures Group,

CSIR-SERC, CSIR Campus, Taramani, Chennai-600 113, India.e-mail: [email protected]; [email protected]

16.1 INTRODUCTION

The problem of condition assessment of existing structures based onfield investigation data is gaining importance as many infrastructuralfacilities are becoming aged. Typically, an engineer is called upon toaddress issues regarding the condition assessment, re-qualification/lifeextension of existing structure, remaining life assessment with respectto its future usage (see Fig. 16.1). A common feature of any assessmentproblem is the observation, observer and the inference. While, gener-ally, physical, statistical and modelling uncertainties are addressed inengineering problems, there are certain characteristics unique to theassessment problems (Fig. 16.2):

1. The available structure is only one and better defined than a struc-ture construed at the design stage (but yet to be constructed). Also,the environment in which the structure located is better definedthan normally assumed at the design stage. However, it is possiblethat the uncertainties in defining the live loads may still be prevail-ing unless more structure/site specific live load surveys have beencarried out.

2. While the uncertainties arising out of environment/mechanicalloading and structural system properties are small, the maximumuncertainty arises out of the judgments made by the experts regard-ing the state of health or the condition of the structure based onthe in-service inspection data.

Some advanced but more subtle differences in uncertainty mod-elling stems from the following observations:




1. Classical physics represents that striving to learn about the naturein which we essentially seek to draw conclusions about objectiveprocesses from observations and so ignore the consideration of theinfluences which every observation has on the object to be observed.Conversely quantum mechanics makes possible the treatment ofatomic processes by partially foregoing their space-time descriptionand objectification.

2. The concept of statistical ensemble may not hold good since we aredealing with a single structure/system. The non-determinism of thesystem and about the loading is minimal as discussed earlier.

The lecture presents the details of research carried out at CSIR-SERC in this area (see Fig. 16.3) and also covers some practicalapplications related to the assessment of remaining life of reinforcedconcrete structures subjected to chloride induced corrosion of rein-forcement. Specifically, the application of Brunswikian theory forcondition assessment and the use of quantum statistical probability(QSP) distributions for handling uncertainties are presented.

16.2 BRUNSWIKIAN THEORY FOR CONDITION

ASSESSMENT

An important aspect in remaining life estimation is the interpretationof the data from field inspections and making expert judgement aboutcondition state of the structural member. Subjective and inaccuratecondition assessment has been identified as the most critical technicalbarrier to highway bridge management (Aktan et al., 1996). Therefore,due consideration needs to be given to the quality of the data and theexpert interpreting the data. Human judgement plays an importantrole in the condition assessment and decision making. A promising the-ory is the Brunswikian theory, the application of which is researchedupon by various investigators (viz., Gigerenzer et al., 1991; Adelmanet al., 2003). For instance, using the concepts of Brunswikian theory,Gigerenzer et al. (1991) proposed probabilistic mental models (PMM)for modelling the human mental process in making decisions. Thesalient details of Brunswikian theory for corrosion damage assessmentas presented by Balaji Rao et al. (2004) are given below.

16.2.1 Brunswikian theory

Brunswik (1952) pointed out that one’s knowledge of a distal ’initialfocal variable’ is mediated by more proximal ’cues’ (or information)that one has about it. The lens model proposed by Brunswik (1952)

Risk Informed Inspection Planning for RC Structures 277

conceptually represented the situation wherein one individual has tomake a judgement about the true state of the distal variable usingmultiple pieces of information. A simple lens model is shown in Fig.16.4. The lens model provides the means for measuring certain char-acteristics descriptive of judgment behavior. This can be achieved byconstructing statistical models of expert judgments using regressiontechniques. The most commonly used model is the generalised lin-ear model. In the generalised linear model, the actual criterion valueand the judged criterion value are modelled as linear combinationsof the cue values. Using the generalised linear model, the correlationbetween the judgement and the actual environment (represented asthe achievement of the expert, ra) can be determined (Balaji Rao etal., 2004). The achievement index, ra, can be regarded as a measureof the accuracy of the judgements made by the expert.

Based on the assumption that people are good judges of thereliability of their knowledge, Gigerenzer et al. (1991) proposed prob-abilistic mental models (PMM) for cognitive processes in judgement.Two important aspects of PMM are that probabilistic inference ispart of the cognitive process and that uncertainty is part of theoutcome. Using the PMM models, the over- or under-confidencelimits associated with an expert for the different confidence levelscan be determined based on the judgements made on a number ofbaseline cases. This treatment would enable to characterise the think-ing process with respect to various confidence levels. The over- orunder-confidence takes into account the relative bias of the expert.

Brehmer and Hagafors (1981) expanded the Brunwikian lens modelto a multilevel lens model to study the use of experts in complexjudgement making. Such a multilevel lens model is used in the presentstudy to model the condition state assessment of reinforced concrete(RC) bridge girders. The procedure for condition assessment usingBrunswikian theory is given below.

16.2.2 Condition assessment using Brunswikian theory

The distal stimuli of the multilevel lens model, used in conditionassessment of RC bridge girders, is the corrosion of reinforcement,which gives rise to the proximal stimuli to the observer/instrumentin the form of appearance and corrosion current/potential. Theinformation on proximal stimuli (such as rust stains, amount ofcracking and spalling, corrosion current density) are recorded by theobserver/instrument (cues). These cues, together with corrosion stateof reinforcement are the distal stimuli for the expert, who is making


a decision regarding the condition state. The information recorded bythe observer/instrument (cues) are corrected for the evaluation abil-ity/human error (in the case of human observer) and for the detectioncapability and correctness of detection (in the case of instrument).The corrected data is the proximal stimuli for the expert who makesa judgement regarding the distal stimuli, namely, the corrosion stateof the reinforcement.

By integrating the information required for condition assessment,and supplying the same along with cues, the aim would be to rationallycapture the thinking process of an expert in arriving at the judgementregarding the condition state. It is known that the mental process canbest be described in the probabilistic basis. A number of experts areasked to make judgement regarding the condition state independentlyusing the same set of cues. The expert is asked to identify the conditionstate(s) in which he believes the member is in, and to attach confi-dence level(s) for his judgement from a confidence scale. Consistentwith probabilistic mental thinking, the experts would judge the proba-ble condition states of corrosion affected RC bridge girders, along withrespective confidence levels. The judgements of all the experts are com-bined on the basis of achievement index for each expert. Thus, insteadof classifying judges as experts or non-experts, they are considered asrational to different degrees (Reid, 1999).

16.2.3 Remaining life estimation

The proposed multi-layer Brunswikian lens model is integrated withMarkov chain (MC) model for remaining life assessment of corrosion-affected reinforced concrete structural members. In this study, thedegradation in the resistance of the RC flexural member due tochloride-induced corrosion of reinforcement is modelled by calculat-ing the ’capacity ratio’, ν(t), of the member at time t as the ratio ofthe load carrying capacity of the member at any time t to the requiredcapacity for the structural member according to relevant design stan-dards. ν(t) is considered as the measure of corrosion damage to thestructural member at time t. In this case, the state space is the corro-sion damage state of the member and the index space is the time. Thestochastic evolution of the system, modelled by homogeneous MC canbe completely described by the Transition Probability Matrix (TPM),P. By computing the values of ν(t) for two consecutive years, the1-step TPM, P, can be computed. The n-step TPM, P n, can be com-puted for determining the corrosion damage state of the structural


member at the end of n years. The corrosion damage state probabil-ities at any time can be determined from the n-step TPM for thattime period, using the methodology given by Balaji Rao and AppaRao (2004). By comparing the capacity ratio at any time with a tar-get value, the service life of the structure with respect to safety canbe determined.

16.3 EXAMPLE

The remaining service life of a reinforced concrete T-beam for a bridgeis estimated using the proposed methodology. The random variablesconsidered along with their statistical properties are given in Table 1.The probabilitycapacity ratio ≤ 0.5 with age of the structural memberbefore inspection is shown in Fig. 16.5. From Fig. 16. 5, it is noted thatat 19 years of age, the probability of capacity ratio being less than orequal to 0.5 becomes 0.01. An inspection is carried out at this time.The information (cues) obtained from the inspection (see Table 2) arepassed on to five experts, who have been asked to make judgementsregarding the corrosion damage state and to assign confidence levelsfor their judgements. Using these values, the corrosion damage stateprobabilities are determined (see Table 3), and the state vector for thecorrosion damage state combining the judgements of all the experts isobtained. The probabilitycapacity ratio ≤ 0.5 with age of the struc-tural member including the effect of inspection is shown in Fig. 16.5.It can be noted from Fig. 16.5 that the probabilitycapacity ratio ≤0.5 = 0.01 when the structural member is 22 years of age, and hence,the remaining life of the structural member can be considered to be 3years from the time of inspection against the limit state of probabil-itycapacity ratio ≤ 0.5 = 0.01. Thus, by carrying out an inspection,the engineer has now the option to postpone the repair activities upto a period of three years for the problem considered. This type ofinformation can be generated using the proposed methodology, whichwill be useful for making decisions regarding repair.

16.4 PROCEDURE FOR INCLUDING IN-SERVICE

INSPECTION RESULTS

16.4.1 Effect of In-Service Inspection (Balaji Rao et. al, 2004)

An inspection was carried out at t = t1 and the remaining pipe wallthickness was determined. Let d be the original wall thickness, and dl

be the loss in wall thickness over a period of time t1.


Assuming a uniform loss of thickness, rate of loss = d1

t1= r

P(Detection of loss of a given thickness) = PoD(d1).Typical PoD curves are shown in Fig. 16.6. The probability of non-

detection, according to draft NUREG-1661, Chapter 6 is given by

PND = ε +1

2(1 − ε) erfc

[υln

(A/A∗

)]whereA = area of the crackA∗ = area of the crack at 50% PND

ε = best possible PND for very large cracksυ = slope of the PND curve.

The values for the parameters of the PoD curves for different levelsof inspection performance are given in Table 16.4. Vibration fatigueand thermal fatigue are two degradation mechanisms due to whichcracks can develop suddenly between two ISI intervals. In such cases,it is prudent to use monitoring.

16.4.2 Modification to Markov Chain

Modification to the original Markov Chain is as follows:

a. Based on ISI write down initial state vector using PoDb. Get modified gradation rate and introduce as correction factor

for the rate predicted earlier. Use this modified equation in thecomputation of TPM.

c. The virtual time would start from the time of ISI.

16.5 CORROSION INITIATION IN REINFORCED

CONCRETE BRIDGE GIRDERS USING BAYESIAN

TECHNIQUE

Development of reliability-based service life models require that themodels can incorporate the information generated during in-serviceinspection; that is, the models/model parameters can be updatedbased on in-service inspection data. Use of Bayesian methods forincorporation of information obtained during in-service inspection incondition assessment and thus in realistic service life estimation ofexisting structures is well established (viz., Mori & Ellingwood 1994a,b). However, in most of the above investigations, conjugate distribu-tions are used in decision making. While the use conjugate distributionhelps in making the problem more mathematically tractable, it may


not be possible to include the greater degree of engineering judgmentin decision making regarding expected service life.

A methodology for the assessment of time of corrosion initiationin reinforced concrete bridge girders using Bayesian techniques is pro-posed (Balaji Rao et al., 2003). The methodology will be useful forrealistic service life assessment based on data from field inspection.Attempt has been made to show how engineering judgment can beused in formulating the likelihood function used in Bayesian decisionmaking. The form of likelihood function is generally not known. Deter-mination of the form requires engineering and statistical judgment orbackground. The form of likelihood function should be so chosen thatit will in-crease the likelihood of observations made based on dataobtained from field investigations. Likelihood functions were formu-lated for two different cases, which will arise in practice: i) in morenumber of cases the chloride concentration obtained from field inves-tigation is less than the mean chloride concentration estimated earlierby the designer, and, ii) in more number of cases the chloride con-centration obtained from field investigation is more than the meanchloride concentration estimated earlier by the designer. Effectivenessof the proposed methodology was demonstrated by applying it to thechloride concentration data obtained from field investigations on Gim-systraumen Bridge, Norway (Fluge, 2001). From the measured chlorideprofiles at the end of 11 years, surface chloride concentration and dif-fusion coefficient values for 236 locations were determined and werereported in Fluge (2001). It is noted that out of the 236 observations,in 163 cases, the chloride concentration at the level of reinforcementdetermined based on field investigations exceeds the mean predicted bythe designer, i.e., in more number of cases, the chloride concentrationfrom field investigations is more than the mean chloride concentra-tion estimated by the designer (case ii). From the three values ofprobability of corrosion initiation obtained, namely, 0.805 (based onthe prediction at the design stage), 0.912 (based on the point esti-mate -computed using relative frequency approach- from informationobtained from field investigations), 0.960 (based on updated chlorideconcentration using the proposed methodology), it is noted that thevalue obtained using the proposed methodology corroborates with theengineering decision taken to repair the bridge girder at the end of11 years (Fluge, 2001). This also suggests that the forms of the priordistribution and the likelihood function used in this investigation areappropriate. Thus, the prediction made using the Bayes techniques is


more realistic, and the use of proposed methodology helps in makingbetter decisions (Fig. 16.7).

16.6 MAINTENANCE SCHEDULING FOR

CORROSION-AFFECTED RC STRUCTURAL

ELEMENTS

The design of structure should take into account the possible degra-dation that may occur during its service life, thus facilitating thescheduling of maintenance activities (which can be optimised) andavoiding costly repairs/replacements. A methodology for maintenancescheduling, based on estimation of the reliability of corrosion affectedreinforced concrete structural members taking into consideration thetime and degree of repairs, is proposed (Balaji Rao et al., 2002). Themethodology uses the concepts of virtual aging, failure rate and time-variant reliability analysis. Due to the repair, a part of the degradedresistance of the member is restored. The amount of restoration ofresistance depends upon the degree of repair, z, defined as the ratio ofrestored resistance to the degraded resistance. The concept of virtualresistance ratio is used to take into account the effect of repair on theresistance of the member (see Fig. 16.8). The virtual resistance ratioat any time is considered as a random variable to take into accountthe stochasticity in the material properties, cross sectional dimensionsand level of degradation. Since the virtual resistance ratio is boundedbetween zero and one, a truncated distribution is used for representingthe variations in this quantity. The reliabilities of a 6m span simplysupported beam subjected to chloride-induced corrosion of reinforce-ment was determined using the proposed methodology. The beam hasbeen designed according to IS 456-2000 for moderate exposure con-ditions. However, in practice the beam was found to be exposed tosevere exposure conditions. The reliabilities of the beam against dif-ferent damage levels (see Balaji Rao et al. (2002) for definitions ofdamage levels) at different time intervals are computed. The reliabil-ities of the beam against damage state 3 (corresponding to 25% lossin area) are shown in Fig. 16.9. The methodology is general and canbe used to estimate the reliability against any specified damage level.Knowing the required reliability levels against specified damage, it ispossible to select the optimal time and degree of repair.


16.7 QSP DISTRIBUTIONS FOR HANDLING

UNCERTAINTIES (BALAJI RAO, 2007)

While application of MC for stochastic modelling is well accepted inengineering for systems which are described using classical statisti-cal mechanics, its usefulness in modelling systems at various scales isstill an issue receiving recent attention. In Balaji Rao (2007), thisissue is addressed by defining a metric and through identificationof isometries associated with space-time symmetries and the use ofthese concepts for reversible systems. As mentioned earlier, in thecase of assessment problems (see Fig. 16.1), the concept of statisticalensemble may not hold good since we are dealing with a single struc-ture/system. Also, the non-determinism of the system and about theloading is minimal. To address the problem of non-existence of ensem-ble in a real world, Wallace (2001) suggested ’Quantum Interpretationof Statistical Probability (QISP)’, which is explained below.

16.7.1 Description of states at equilibrium in classicalstatistical mechanics (CSM)

1. The possible states of a classical statistical system are given by thepoints in some phase P .

2. At any given time t, the specific system under consideration has adeterminate state given by a specific point in P - though this pointis assumed not to be exactly known.

3. At time t, the probability that this determinate state is in a givenregion of P is given by some probability distribution over P .

4. The time - evolution of the system is deterministic (given by Hamil-ton’s equations) and so knowing the probability distribution at onetime tell us what it is at all other times.

5. A system is said to be at equilibrium when the probabilitydistribution does not vary in time.

On conceptual side, there is a problem of defining the probabilitydistribution over phase space, though interpreted in a relative fre-quency terms. Because, the observed system is only one!. With theabove problem of non-existence of ensemble in a real world, Wal-lace (2001) suggests ’Quantum Interpretation of Statistical Probability(QISP)’.


16.7.2 Quantum Interpretation of Statistical Probability(QISP)

’Ignorance’ probability in the sense of a probability distribution overa space of many possible states of a system, one of which is actual,has to be looked at critically in statistical mechanics. As such, the useof ’probability’ density operator in statistical mechanics needs furtherexamination. When a density operator, is used to describe a statisti-cal system, it is to be understood as the determinate-though highlynon-pure-’entanglement’ density operator which describes that specificsystem (Fig. 16.10). The map is of from

p (ρ) →∫

Dρ p(ρ)ρ

where p(ρ) is the given probability distribution over entanglement den-sity operators ρ and the map in (3) is many-to-one. While map (3)is for a realistic quantum systems, to get a feel for ρ the form for anisolated quantum system is presented below.

ρ =∑

i

p(i) |i > < i|

where p(i) is a given probability distribution p(i) over some (notnecessarily orthogonal) states |i >.

The following six reasons for proposing the above conjecture weregiven in Wallace (2001). Out of these six, the first three reasons areconceptual and the other three are more dynamical and probably moreimportant.

1. In classical statistical mechanics, the main problem is under deter-mination of probability distribution by the statistical facts. Thisproblem would be automatically solved in QISP.

2. It would make the concept of ’ensemble’ rather less problematic.By defining the density operator to be describing the system (singlesystem under consideration) totally avoids the confusion of ensem-ble of classical statistical mechanics (which is more of a theoreticalabstraction than a reality). In particular, concepts like entropy aredefined, in CSM, to apply to an ensemble rather than an individualsystem, as in QISP. In quantum mechanics, if QISP holds, then itmakes sense to describe a single system as being in a macrostate(i.e., described by an entanglement density operator), and we shouldbe able to assign macrostate properties such as entropy to that


single system. This may make at more coherent to describe aunique system as having ascertain probability distribution. Thisredescription of single systems has relevance for the reduction ofthermodynamics to statistical mechanics.

3. If QISP holds, then the (highly problematic) probabilities of statis-tical mechanics are to a large extent removed from consideration,to be replaced with the probability intrinsic to quantum mechanics.However, this problem needs more research.

4. QISP allows us to construct ’transcendental’ account ofequilibrium-that is, a justification of the equilibrium state indepen-dent of any causal story as to how systems get into equilibrium inthe first place- for quantum mechanics which in some way is similarto classical statistical mechanics. In the case of classical statisticalmechanics the system equilibrium is decided in such way that thepossible realizations of microstates are combined in such way thatit is consistent with observed or to be modeled macrostate. Since weare considering equilibrium system behaviour we are talking aboutsteady state modeling. The invariant quantity, assuming no dissi-pation, is energy. Hence, the candidate distributions proposed formicrostates should be based on conservation of energy or shouldhave energy as time invariant quantity. The microcanonical distri-bution hypothesized should satisfy the law of conservation of energy(it may be quickly recalled that the microcanonical distributionmay be Boltzmann’s distribution or equipartition distribution). Inquantum mechanics also the concept of transcendental equilibriumis some what similar, except that in addition to above points (1)-(3), wherein we have density operator defined on states of quantumsystem (mostly entangled) are definite states of the system. Hence,some kind of eigen value analysis seems to help define the densityoperators on states of system. But all the studies from decoherencesuggests that (in the absence of dissipation) the only density oper-ators which are invariant under decoherence are projections (andsums of projections) onto eigenspaces of the conserved quantities.For a system with energy as the only conserved quantity, thoseinvariant density operators are microcanonical operators and theirsums.

5. One of the important concepts, generally invoked in classical sta-tistical mechanics, for describing the system in equilibrium isthe concept of stationarity and much stronger property beingergodicity. Ergodicity is generally assumed to have mathemati-cal simplicity/tractability and in engineering due to limitations


imposed by experimentation (assuming that the process can be wellapproximated by a stationary process). The assumption of ergodic-ity is not required or it is natural to a quantum mechanical systemsince we neither have ensembles nor we have pdf evolving in time orconstant defined over state space. We are handling a single system(dynamical) which is in equilibrium with environment (taken careof by decoherence of pure states of system).

6. If the plausibility of observation (4), dealing with equilibriumbehaviour, is accepted, then the microcanonical density operator(interpreted as an entanglement density operator) is the only stateof the system (at given energy) which is a valid equilibrium state-allother states evolve towards that state, so any probability distribu-tion over any other states will not be an equilibrium distributionat all. In other words, QISP holds at equilibrium, because thedynamics of the system force it upon us.

Demonstration of the use of QSP distributions in determining the ele-ments of Transition Probability Matrix and the effect of considerationof QSP as against the classical statistical probability distributions willbe presented during the lecture.

16.8 SUMMARY

The studies at CSIR-SERC on handling uncertainties in conditionassessment of structures, with emphasis on application of Brunswikiantheory for handling human judgemental aspects and the use of quan-tum statistical probability distributions for handling uncertainties, arepresented. The emphasis has been on the use of Markov chain for mod-elling the response of systems at various scales. It is to be mentionedthat, at present, both classical statistical and quantum mechanics areto be applied depending upon the scales of phenomenon being mod-elled. The concept of quantum interpretation of statistical probabilityseems to play a major role in future developments in experimentalmechanics.

16.9 REFERENCES

1. Adelman, L., Miller, S.L., Henderson, D.V. and Scholles, M. (2003),“Using Brunswikian theory and a longitudinal design to study howhierarchical teams adapt to increasing levels of time pressure”, ActoPsychologica, 112, 181-206.


2. Aktan, A.E., Farhey, D.N., Brown, D.L., Dalal, V., Helmicki, A.J.,Hunt, V. and Shelley, S.J. (1996). “Condition assessment for bridgemanagement”. Journal of Infrastructure Systems, ASCE, 2(3), 108-117

3. Balaji Rao, K. and Appa Rao, T.V.S.R. (2004), “Stochastic mod-elling of crackwidth in reinforced concrete beams subjected tofatigue loading”, Engineering Structures, 26(5), 665-673.

4. Balaji Rao, K., Anoop, M.B., Lakshmanan, N., Gopalakrishnan, S.and Appa Rao, T.V.S.R. (2004), “Risk-based remaining life assess-ment of corrosion affected reinforced concrete structural members”,Journal of Structural Engineering, 31(1), 51-64.

5. Balaji Rao, K., Anoop, M. B. and Appa Rao, T. V. S. R. (2002),“Reliability analysis of stochastic degrading and maintained sys-tems”, Proceedings of 6th International Conference on ProbabilisticSafety Assessment and Management (PSAM6), San Juan, PuertoRico, USA, June 23-28, 2002.

6. Balaji Rao, K., Satish, B., Anoop, M. B., Gopalakrishnan, S. andAppa Rao, T. V. S. R. (2003), “Application of Bayesian techniquefor corrosion state assessment of reinforced concrete bridge girders”,in Safety and Reliability, Ed. T. Bedford, P.H.A.J.M. van Gelder,Proceedings of ESREL 2003, 15-18 June, 2003, Maastricht, TheNetherlands, A. A. Balkema Publishers, pp 73–80.

7. Balaji Rao, K., Anoop, M. B., Lakshmanan, N., Gopika Vinod,Saraf, R. K. and Kushwaha, H. S., “A methodology for riskinformed in-service inspection for safety related systems - Finalreport”, Report No. SS-GAP01241-RR-04-3, March 2004.

8. Balaji Rao, K. (2007), “Markov-Chain modelling for reliabilityestimation of engineering systems at different scales - some consider-ations”, Proceedings of International Conference on Civil Engineer-ing in the New Millennium: Opportunities and Challenges, Bengalengineering and science university, Kolkata, 11-14 January 2007,(in CD-ROM). (also available at http://arxiv.org/abs/0708.1566)

9. Brehmer, B. and Hagafors, R. (1986), “Use of experts in complexjudgment decision making: A paradigm for the study of staff work”,Organizational Behaviour and Human Decision Processes, 38, pp181–195.

10. Brunswik, E. (1952), The conceptual framework of psychology,University of Chicago.

11. Enright, M.P. and Frangopol, D.M. (1998), “Probabilistic analysisof resistance degradation of reinforced concrete bridge beams undercorrosion”, Engineering Structures, 20(11), pp 960–971.


12. Fluge, F. (2001), “Marine chlorides: A probabilistic approach toderive provisions for EN 206-1”, In Service life design of concretestructures - from theory to standardization: 63-83. 3rd DuranetWorkshop, Troms, Norway.

13. Gigerenzer, G., Hoffrage, U. and Kleinbolting, H. (1991), “Prob-abilistic mental models: A Brunswikian theory of confidence”,Psychological Review, 98(4), pp 506–528.

14. Mori, Y. and Ellingwood, R. (1994a), “Maintaining reliabilityof concrete structures. I: Role of inspection/repair”, Journal ofStructural Engineering (ASCE), 120(3), pp 824–845.

15. Mori, Y. and Ellingwood, R. (1994b), “Maintaining reliability ofconcrete structures. II: Optimum inspection/repair”, Journal ofStructural Engineering (ASCE), 120(3), pp 846–862.

16. Reid, S.G. (1999), “Perception and communication of risk and theimportance of dependability”, Structural Safety, 21(4), pp 373–384.

17. Wallace, D. (2001), “Implications of quantum theory in the foun-dations of statistical Mechanics”, http://philsci-archive.pitt.edu

18. Wong, F. S. and Yao, J. T. P. (2001), “Health monitoring andstructural reliability as a value chain”, Computer-Aided Civil andInfrastructure Engineering, 16(1), pp 71–78.

Table 16.1 - Random variables considered in the exampleproblem

Variable Mean COV*

Diffusion coefficient, D 5 × 10−8cm2/s 0.10Surface chloride concentration, cs 0.30 % by wt. of concrete 0.10Critical chloride concentration, ccr 0.125 % by wt. of concrete 0.05Cover thickness, d 40 mm 0.05Rate of corrosion, rcorr 0.58 mm/year 0.30Compressive strength of concrete, fck 30 MPa 0.18Yield strength of steel, fy 415 Mpa 0.12

Table 16.2 - Data from inspection for the example problem

From Visual Rust stains Highly noticeable rust stainsInspection Cracking Several longitudinal cracks;

some cracks in stirrup directionSpalling Clearly noticeable spalling

From Field Icorr (3LP) = 6.0 A/cm2; Ecorr = -450 mVMeasurements Cover depth = 38 mm;

Remaining diameter of reinforcement = 32.0 mm


Table 16.3 - Corrosion damage state probabilities based onexperts’judgement

Corrosion Damage state probabilities

Damage state Expert 1 Expert 2 Expert 3 Expert 4 Expert 5 Combined

1 0 0 0 0 0 02 0 0 0 0 0 03 0.25 0.357 0.563 0.30 0.58 0.4074 0.75 0.643 0.437 0.70 0.42 0.5935 0 0 0 0 0 0

Table 16.4 - Parameters of PoD curves

Inspection Performance a*(% of a/t) ε νLevel 1 40 0.1 1.6Level 2 15 0.2 1.6Level 3 5 0.05 1.6

Assessment problem

Assessment problem

-Conceptual design

-Analysis

-Design

- Construction

- Maintenance

- Disposal

- Experimental studies

- Field performance evaluation

-Safety auditing of existing structures

Assessment problem

Assessment problem

-Conceptual design

-Analysis

-

-

-

- Experimental studies

- Field performance evaluation

- Safety auditing of existing structures

Fig. 16.1 The real engineering problem

BUT THE SYSTEM IS ONLY ONE!

ASSESSMENT PROBLEM

ClassicalMechanics

deterministic

Classical Statistical Mechanics

here also influence ofobservation on the objectto be observed is ignored

Bayesian decision theory

How many observations are necessaryto construct informative posterior

non-determinism

Fig. 16.2 Assessment Problem


Condition Assessment –Markov Chain approach

Corrosion Initiation Reliability Analysis

Bayesian updation based on inspection data

Corrosion PropagationTime-varying Reliability Analysis

Effect of Repairs (concept of virtual aging)

Life Prediction & Remaining Life Assessment

Human Judgemental model +Reliability analysis model

Probabilistic Models

Judgemental Models

Development of Learning

Models

–

Fig. 16.3 Studies at CSIR-SERC on condition assessment

X1

X2

X3

X4

rE,1rE,2

rE,3

rE,4

rs,1rs,2

rs,3

rs,4

Achievement, ra

EcologicalValidity, r E,i

Cue Utilization, rs,i

Cues Xj

YSYE

Fig. 16.4 Schematic Representation of Brunswik LensModel

Before inspectionAfter inspection

0.6 m

2.6 m1.E-02

0.6m

2.6m19cm

6.9cm 0.4m

8 Nos 35.8mm Φ1.E-06

Pr{c

apac

ity ra

tio <

0.5}

1.E-10

1.E-14

1.E-18

1.E-220 5 10 15 20

age (years)

Before inspectionAfter Inspection

Fig. 16.5 Probability of the structural member being in astate requiring immediate repair action according to CEB


. . .

Oustanding Very good Marginal

State of System

1.0

0.0

S1 S2 Sk

0 0

PoD

(d1)

d1/d 1.0

Fig. 16.6 Schematic representation of typical PoD curves

Prediction at design stage

Point estimate

Proposed methodology

Probability of corrosion initiation0.5 0.6 0.7 0.8 0.9 1.0

Prediction at design stage

Proposed methodology

Point estimate

Fig. 16.7 Probability of corrosion initiation at the end of11 years for the Gimsystraumen bridge girder

1.0

V1[x(t1)]

V2[x(t2)]

V1=(1-z1)*{1-V1[x(t 1)]}

V2=(1-z2)*{1-V2[x(t 2)]}

t1 t2t1* t2

*

t

x(t)

V1[x(t)] = x[t-(t 1-t1*)]

V2[x(t)] = x[t-(t 2-t2*)]

V0[x(t)] = x(t)

0.0

V1[x(t1)]

V2[x(t2)]

V1=(1-z1)*{1-V1[x(t 1)]}

V2=(1-z2)*{1-V2[x(t 2)]}

t1 t2t1* t2

*

t

V1[x(t)] = x[t-(t 1-t1*)]

V2[x(t)] = x[t-(t 2-t2*)]

V0[x(t)] = x(t)

Fig. 16.8 Renewal process with perfect/imperfect repair


z = 0.95

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60age (years)

relia

bilit

y

Truncated normal

Truncated log -normal

Damage state 3 : 25% loss in area of reinforcement- degree of repair

z = 0.90

300 mm

500 mm

3, 25 mm Φ

8 mm Φ, 2L, 150 mm c/c

Simply supported beamclear span = 6.0 mWidth of supporting walls = 200 mmLoad from the slab =

22.5 kN /mfck = 25 N/mm 2

fy = 415 N/mm 2

Clear cover = 30 mmEffective depth =

449.50 mm

Truncated normalTruncated log-normal

z -

300 mm

500 mm

3, 25 mm Φ

8 mm Φ, 2L, 150 mm c/c

2

2

Fig. 16.9 Reliabilities for the beam against damage state 3

By defining the density operator to be describing the system- avoids confusion of ensemble- concepts like entropy can be applied to this system

which is considered to be in macro state of equilibrium- ensures reduction of thermodynamics to CSM

Stationarity (S) and Ergodicity (E) invoked in CSM are automatically satisfied in QS since we are handling a single system which is in equilibrium with environment (taken care of by de -coherence of pure states)• System is one + Environment is varying =>System is

changing => Automatic satisfaction of S & E??

Fig. 16.10 Quantum interpretation of statisticalprobability

17 Distress in Prestressed Concrete Members

and their Rehabilitation

K. RamanjaneyuluDeputy Director


17.1 INTRODUCTION

Concrete is one of the most versatile and widely used of all construc-tion materials in India. If properly prepared and placed in position, ithas adequate durability under normal conditions of exposure. In theearly years of concrete construction, it was thought that the concretestructures would last forever, without any maintenance. However, anumber of structures built during the last 50 years, have suffered dura-bility problems resulting with different degrees of deterioration, witheven a few cases of total collapse. This has triggered off the necessityfor developments for distress assessment and evolving necessary repairmethodology of these structures. The deterioration has been partic-ularly noticed in structures located in the regions of severe exposureconditions such as coastal areas.

Many prestressed concrete (PSC) bridges constructed in India dur-ing the last 3 decades and located in the coastal areas have shownsigns of distress in the form of development of cracks in the gird-ers, potholes in the deck, malfunctioning of bearings, and corrosionof HTS wires/stands. In prestressed concrete bridges, the corrosion ofprestressing cables can lead to substantial loss in prestress and ulti-mately sudden collapse of spans as seen in Mandovi bridge in Goa in1986. Considerable distress was also noticed in Thane - Creek bridgein Maharashtra, Sharavati bridge in karnataka constructed in 1970,Zuari Bridge in Goa constructed in 1983, Narmada bridge in Gujaratconstructed in 1977, to name a few.

One must keep in mind that any compound produced from a nat-urally available stable material will try to revert back to the original




constituent material from which it is made of, with the passage oftime. For example, we know cement is manufactured from the natu-rally available lime-stone. The man made compound concrete containalkaline calcium hydroxide which combines with atmospheric carbon-dioxide to revert back again into a stable calcium carbonate (i.e.),lime-stone. This process is called Carbonation. Similarly, the steel ismade from more stable Iron-oxide. In the presence of atmosphericoxygen and water, the steel reverts back again into stable iron-oxide.This process is called Corrosion. The three C’s i.e., Carbonation, Cor-rosion and Construction practices are the main mechanisms that causeretrogression to concrete structures.

Evaluation of damage is essential in selecting a suitable repairmethod. To evaluate the damage, it is necessary to determine theextent, cause of damage and whether or not the cause is still active.Selection of a repair material must be based on the evaluation ofthe damage, characteristics of repair material, and local conditions.If detailed evaluation of a damaged structure reveals that the orig-inal construction was of poor quality concrete, a lasting repair canprobably be achieved with a high quality concrete or other patchingmaterial. If a high quality concrete has deteriorated, a lasting repaircan only be achieved by protecting it from the exposure conditionsthrough the use of high quality impermeable material. Repair materi-als must be compatible with the concrete that is being repaired. Theyshould respond in same way to changes in temperature and loads asthat of original structure and they should blend well with structureand its appearance.

This lecture presentation deals with some of the more commonretrogression mechanisms of PSC structures. It also covers some ofthe common repair techniques used to retrofit the retrogressed pre-stressed concrete structures. In the end, some of the case studies arealso presented.

17.2 DISTRESS CAUSING MECHANISMS

Deterioration of PSC structures may arise from a number of inde-pendent causes. The source of these causes may be grouped intothree general categories, viz., design and construction deficiencies,environmental effects and changes in use.

Designs that do not follow specifications and good con-struction practices can lead to structural deterioration. Insufficient

Distress in Prestressed Concrete Members and their Rehabilitation 295

concrete cover for reinforcements, inadequate spacing between ten-dons, grouping of tendons, incomplete grouting of tendons, impropercompaction of concrete, bad drainage system, improper joints and baddetailing are some of the common design and construction deficiencies.

Environmental effects include material quality, environmentalaggression (chloride), freeze-thaw deterioration, alkali-silica reaction,support movement, carbonation, shrinkage and thermal strains. Thereis also a possibility of some or many of these effects may be actingsimultaneously aggravating the condition of state of deterioration ordistress These actions are acting continuously to the structures andtheir effects are cumulative with time.

Changes in use are a significant factor affecting the deterioration. Itmay include increase in traffic volume, increase in maximum permittedvehicle size or increase in the number and frequency of large sizedvehicles on the bridge. Wear and fatigue are two other mechanismsthat directly lead to deterioration of the PSC bridges.

17.3 ENVIRONMENT

The environmental factors may be classified as (1) Natural and (2)Manmade. The natural factors include variations in ambient temper-ature and relative humidity of the air, presence of chlorides, sulphatesetc. Manmade factors include resultant of pollutants such as carbondioxide, sulphur dioxide etc. from effluents let out in the surroundingenvironment by nearby industries. The environmental factors influencethe quality of concrete as well as reinforcements to a greater extent inIndia, being a hot weather country. High temperature and alternatinghigh and low humidity have the greatest adverse affect on the qual-ity and integrity of concrete. The durability of a concrete structurewill be determined by the rate at which the concrete deteriorates asa result of chemical reaction. The most important chemical reactionsare acid attack, sulphate attack, alkali attack, effect of carbonationand chloride penetration.

17.3.1 Alkali Silica Reaction (ASR)

While choosing the aggregates, the major factor to be considered isalkali-aggregate reaction which may lead to destructive expansion ofconcrete. This relates to action between the reactive silica presentin the aggregate and the alkalis added in concrete including thosepresent in mixing water and cement. Due to this reaction, a swellinggel is generated. This gel causes expansion and cracking of concrete


in both micro and the macro level. Among the various geological for-mations relevant to aggregates in India, perhaps quartzite, basalt aremost vulnerable minerals in this context. Destructive alkali-aggregatereaction can take place only in the presence of moisture. Thus struc-tures exposed to high humidity and contact with water/moisture aremore vulnerable.

The degree of expansion due to alkali-aggregate reaction is alsodependent on temperature. As such, structures in hot countries likeIndia are more vulnerable. The following preventive methods areappropriate:

• Choosing non-reactive aggregates

• Limiting alkali content in cement to 0.6% expressed as Na2Oequivalent

• Neutralising any higher value of alkali by using blended cement

• Preventing contact between concrete and any source of moisture.

17.3.2 Sulphate Attack

Sulphate attack is characterised by the chemical reaction of sul-phate ions with the aluminate component and sulphate, calcium andhydroxyl ions of hardened cement. The reaction between these sub-stances, if enough water is present, causes expansion of the concrete,leading to cracking with an irregular pattern. Concrete may to someextent be protected against sulphate attack either by choosing thetype of cement that is impervious to sulphate attack or by ensuring asufficient degree of impermeability using suitable admixtures.

17.3.3 Corrosion

In prestressed concrete structures, the untensioned reinforcement andprestressing steel are normally protected against corrosion by passi-vation due to the alkalinity of the surrounding concrete (pH values ofconcrete is generally greater than 12.5). Corrosion would not occur aslong as there are no breaks in the passive layer over the steel. Thelayers may be broken by two mechanisms. One involves carbonation,the other chloride ions.

(a) CarbonationThe ambient air contains 0.03% carbondioxide. Rapid industrial-isation and pollution due to automobiles increases the CO2 levelin atmosphere. The combustion of 1 Kg of petrol or diesel pro-duces about 3.1 Kg of CO2 increasing the CO2 level in ambientair. CO2 combines with the soluble calcium to form an insoluble


calcium carbonate, a process known as carbonation. Carbonationis very rapid on the surface, but diminishes rapidly with depth.One may expect a depth of carbonation of 1 mm in 3 months, 10mm in 10 years and 20 mm in 30 years. In the carbonated con-crete the alkalinity diminishes appreciably and below a pH of 9.5,the reinforcement is no longer protected against corrosion. Therate of carbonation is affected by variations in (1) strength, (2)density, (3) aggregate size and distribution, (4) moisture content,(5) cement content, (6) humidity of the air, and (7) CO2 contentof the air.Carbonation can work its way in from the surface of the con-crete to the reinforcing steel, reducing the pH of the concretesurrounding the steel and allow corrosion to start. The interfacebetween carbonated and non-carbonated concrete is abrupt butfairly uniform. Consequently, corrosion due to carbonation is gen-erally characterised by a widespread surface rusting, even thoughit may occur in patches of different intensity, reflecting local vari-ations in steel and concrete characteristics. Fortunately, for soundconcrete with a low water-cement ratio, carbonation is seldom aconcern.

(b) Chloride PenetrationMost corrosion problems are related to chloride - either in areaswhere deicing salts are used or in marine areas. When the con-centration of chloride gets higher than a threshold level, corrosionstarts. The presence of chloride ions in concrete, can cause depas-sivation of steel even if the associated pore solution is highlyalkaline. The surface of the steel, therefore, becomes activatedlocally forming an anode, while the rest of the passive surfaceserves as the cathode. Since the latter is much larger, the dis-solution of the ions in the anode is highly localised and a pit isformed which is most dangerous corrosion for prestressing steel.This localised pitting corrosion, with non-expansive corrosionproducts, can quite possibly develop without visible signs on thesurface of the concrete.Effective controls should be endorsed on the total permissiblechloride content in concrete. This should be limited to 0.1% ofthe weight of cement for prestressed concrete and 0.15% for rein-forced concrete. Chloride may be present in cement, aggregates,water and /or admixtures. Frequent onsite checks of materialscan effectively control the chloride content in concrete.


(c) Inadequate Grouting of Cable DuctsInadequate or delayed grouting of prestressed cable ducts havealso contributed to corrosion of steel in quite a few cases. Properspecification for materials, equipment and workmanship as perstandard practices for grouting should be adhered in prestressedconcrete structures.

17.3.4 Hydrogen Embrittlement

When steel is pickled (dissolved) in acids, the hydrogen atoms evolvedat the surface of the steel penetrates into the latter with the resultsteel becomes so brittle that it fractures on being subjected to tensilestress. The brittleness is greater according to the level of hydrogen isabsorbed, i.e., according as the acid acts upon the iron, for a greaterlength of time. Even quite small amounts of hydrogen are, however,sufficient to cause considerable deterioration of the tensile strength andductility of the wire. Prestressing steel must therefore, on no account,be exposed to the action of acids. Acids occurring in crude oil, humicacid, hydrochloric acid, sulphuric acid, phosphoric acid, hydro cynicacid, gases like sulphur dioxide etc., are highly dangerous. One bridgein Brazil, the prestressing cables, each comprising 182 heat-treatedwires fractured within a few days after being tensioned. The causewas traced to a grouting compound with high sulphur content. Inanother case, slight traces of sulphur in a lorry on which prestressingwire was being conveyed were enough to cause fracture of the wirewhen it was subsequently tensioned.

17.3.5 Stress Corrosion

Stress corrosion occurs when steel is subjected to tensile stress and atthe same time exposed to corrosive environment. As a result of thiscorrosion, sudden brittle fracture of material occurs. The followingthree conditions must exist: (1) Stress corrosion susceptibility of thesteel, (2) Action of a corroding agent (chemical, moisture and electricpotential difference), (3) The presence of tensile stresses in the steel.If one of these three conditions is not satisfied, no stress corrosion willoccur.

17.4 CORROSION PROTECTION

The basic ways of providing corrosion protection for steel in concreteare by changing the environmental exposure conditions, the electro-chemical nature of the exposed surface of the steel. Some of the waysof preventing/delaying the corrosion damage are:


(i) Use post-tensioning

(ii) Use air-entraining agent

(iii) Use pozzolonas

(iv) Provide adequate cover

(v) Maintain low water-cement ratio

(vi) Consolidate the concrete thoroughly

(vii) Provide adequate curing

(viii) Include Provisions for repairing cracks.

FHWA in USA, research demonstrated the benefits of epoxy coat-ing of reinforcement, along with a silane sealer for the concrete and useof silica fume pozzolonic admixture for the concrete. The finer poz-zolona improves the imperviousness of resulting concrete. Comparativefineness of cementitious materials is given in Table 17.1

17.5 MANIFESTATION OF DISTRESS IN PSC

STRUCTURES

Manifestation of distress in PSC structures is through:(i) Cracking, (ii) scaling, (iii) delamination, (iv) spalling, (v) leach-

ing, (vi) rust stains, (vii) deformations, (viii) hollow or dead sound,and (ix) excessive deflections/ movements.

DELAMINATION

Delaminations occur when layers of concrete separate from bridgedecks or beams at or near the level of the outer most layer of reinforc-ing steel. Such areas give off a hollow sound when tapped with a rodor hammer. The major cause of delaminations is the expansion result-ing from corrosion of reinforcing steel. It occurs with either repeatedchloride deicer applications or continued exposure to a marine environ-ment. Inadequate cover over reinforcing steel will reduce the initiationtime of corrosion. Vehicular exhaust and emission on bridges may alsocause delaminations. When sufficient moisture and oxygen are presentwith a chloride ion content above 0.77 kg /m3, corrosion of reinforce-ment will occur in most bridge deck concrete. At the beginning stage ofdelamination, the repair can be carried out with epoxy resin injectionat the delaminated section. If delamination is in an advanced stage,delaminated concrete has to be removed and special repair proceduresare to be followed.

The horizontal cracking (delamination planes) is not visible untilspalling occurs. In the absence of structural cracking, the chloride ion


should diffuse through the porous medium (concrete) according toFick’s law. Some of the other sources that contribute to the formationof horizontal cracking (delamination planes) are:

1. Poisson’s effects due to high prestress levels.2. Transverse shrinkage and restraint due to end block3. Temperature effects4. Torsion due to differential camber5. Temperature rises due to heat of hydration, etc.

17.6 SPALLING

A spalling is defined as the depression resulting when a fragment ofsurface concrete gets detached from a larger mass by any impact, byaction of weather, by pressure, or by expansions within the largermass. The major cause of spalls is the same as that for delaminations.Spalling that occurs at joints may be caused by corrosion of steel atthe expansion joints, or from impact of traffic. Usually the area ofactive steel corrosion and chloride contaminated concrete is consider-ably larger than the area of spalled or delaminated concrete. If onlythe area of spalled or delaminated concrete is removed and repaired,a continuing repair program may be required. However, if the chlo-ride contaminant is removed and repaired and the deck is properlywater proofed to avoid further chloride contamination, either througha bonded topping or overlay, a more durable repair will be obtained.

17.7 SCALING

Scaling of concrete surface is defined as local flaking or peeling awayof the near surface portion of concrete. Scaling may be classified aslight if there is only loss of surface mortar with no exposure of courseaggregate, medium or severe if there is loss of mortar with increasingexposure of coarse aggregate and very severe if there is loss of coarseaggregate with the mortar.

The most generally accepted explanation of scaling involves thegeneration of internal pressure during freezing of solution containedin saturated voids. Scaling also occurs when concrete is subjected toalternate wetting and drying or to concentrated solutions of chloridedeicers. Although the extent of scaling may be easily determined, mea-surement of the chloride ion content of the concrete is advisable to


evaluate future spalling potential. Impervious and high strength sur-face coating of less than 6mm thick have been used, when scaling isin its early stages1.

17.8 CRACKING

Cracking that potentially endanger the structural adequacy of themember should be immediately considered for repair. Many cracksdo not require detailed repair procedure. If the cracks are active, i.e.,the crack width is increasing due to continuing over loads or due tostructural settlement, complete replacement of the member or detailedrepair procedures are necessary. It may be necessary to measure orestimate crack widths. This can be done with measuring microscopesor feeler gauges. If necessary, the extent of cracking can be evaluated bypulse velocity by impact echo techniques. Epoxy resins generally canbe used to repair cracks. It may be convenient to widen the cracks andthen fill them with latex mortar. Cracks in prestressed members shouldnot be repaired without consultation with an engineer to determinethe reason for the cracks.

17.9 EVALUATION OF DAMAGE

Before designing repair scheme in damaged prestressed concrete struc-ture, assessment of the extent of damage is needed. In particular, in thecases where concrete is damaged extensively and/or some prestressingwires / strands are severed, the stress levels remaining in non-severedtendons are difficult to determine. If the extent of damage or strandstress levels can be determined inexpensively and reliably, repairs toa girder can then be designed to restore its original strength. Timelyinspection and rectification will improve the service life of the structureconsiderably.

Strength evaluation of existing structure is necessary due to thefollowing reasons:

• To decide whether the damaged structure can be replaced orrepaired. During the process of rectification, need arises toidentify which components have to be rehabilitated or replaced.

• Selection of rectification measures depend on the type and extentof damage or deterioration. Strength evaluation will provide aclear idea for economical selection of rectification measures atoptimized cost.


• When a calculation has shown that the structure is not capa-ble of meeting the present standards, due to change in loadingregulations, loading models in the codes or strength models

• When inspection has revealed loss of section/capacity such thatthe strength may have fallen below the level needed for meetingthe load Criteria.

• When there is reason to believe that the boundary conditions,load distribution or section resistance are different from thatassumed in the analysis model.

• To measure directly the stress spectra and to evaluate possiblefracture or estimate remaining service life.

Residual strength assessment is essential for deciding and design-ing different rectification techniques. With different Non-DestructiveTesting (NDT) methods, localized behavior of the deteriorated struc-ture can be predicted. Different analytical/experimental approachesare developed and reported in recent past for global strength assess-ment of existing structures. Adaptation of any particular method ormethods mainly depends on the extent of damage that the struc-ture had undergone, cost of assessment, rectification and life periodextension.

17.9.1 Nondestructive Methods for Condition Assessment ofExisting Bridge

Diagnostic process is the first stage of strength evaluation of any struc-ture, which include techniques to identify the critical parts or elementsof the structure, identify the causes of distress, monitor the structuralperformance, warn against failure, and provide statistical data for thedevelopment of design and evaluation criteria. Before conducting anassessment on existing bridge, different data are required which shallsuitably be incorporated to analytical or experimental data for overallstrength evaluation procedure. The data required are collected fromnondestructive tests. The data are,

• Actual state of structure which include evaluation of stresses,strains, deflections, cracks and any permanent deformation ofexisting bridge

• Accumulated damage, corrosion of steel reinforcement, changesin material properties, loss of geometrical section etc.,

• Loss of prestress in case of prestressed girders due to timedependent effects,


• Extreme load events like earth quake and disasters due to cycloneetc.,

Recent advances in nondestructive testing methods are quite sat-isfactory for evaluation of material strength of damaged structures.Different diagnostic procedures and available non-destructive tests aresummarised by Nowak2. In any method, defects are detected by obser-vation of changing response of the interrogating medium. Interrogatingmedium in non-destructive test include electromagnetic waves, stresswave, electrical resistance/potential, magnetism and charged particles,etc.

17.9.2 Visual Inspection

Visual inspection by an experienced engineer often provides a goodoverview of the condition of the structure. It allows identificationof presence of cracks, delaminations, spalling, corrosion or surfacedeposits. Visual inspection may reveal severe damage to exposedstrands, such as nicks, severed wires, kinks, extensive yielding. Gen-erally, it is assumed that if cracks around a strand do not close afterimpact, the strand has lost a significant portion of its prestress force.Indeed, it is quite possible that a strand and the surrounding concretecould exhibit none of these physical attributes, but still be signifi-cantly damaged. For instance, with spalling of large areas of concrete,it is possible that undamaged strands could lose some of their preten-sioning force through shortening of the stressed strand, which may bereflected by camber of the damaged girder.

If the damage is severe, visual inspection is insufficient. Differenttechniques adopted for non-destructive test are summarised below.IRC SP 40- 19933 gives different non- destructive tests to be conductedbefore the overall assessment test for damage detection.

Besides visual inspection of concrete surface the surface, of mainreinforcement can also be inspected for any corrosion using Endoscopy,by drilling holes at regular intervals carefully following cable profile .However care is required not to affect the cable itself during drillingand on completion they have to be filled with epoxy modified mortar.

17.9.3 Tests on Concrete

Different non-destructive tests on concrete are summarized by Malletin state of the art review on repair of concrete bridges10.


Depth of concrete cover is measured by cover-meter. Wide rangeof cover-meters are available which measures concrete cover to anaccuracy of 5mm.

Hardness of concrete and strength can be measured by reboundhammer. The main limitation of this instrument is that it relates thestrength only to a very limited depth, and is governed by surfacetexture and carbonation. (Nowak-2)

Quality of concrete is measured by sonic and ultrasonic pulse veloc-ity methods. These methods are based on measurement of travel timeof acoustic waves.

Flaws in concrete are measured by impact echo method. Thismethod was developed by Carino and Sansalone4 in 1990s. Themechanical impact generates a short duration stress pulse, whichtravel as p and s waves. These are reflected by discontinuities. Later,Bungley5 developed spectral peak plotting for finding flaws in concrete.In 1993, Krause, Wiggenhauser and Wilsch presented an advancedpulse echo method for ultrasonic testing of concrete.

Different methods are established for determination of surface per-meability of concrete structures. In that initial surface absorption test(ISAT) is the method standardised in BS 18816.

Chemical methods are used to evaluate depth of carbonation andchloride ion content. The IRC -SP 403, has given phenolphthalein testfor detection of carbonation of concrete. Bungley carried out moredetailed analyses from the microscopic examination of the sectiontaken from a small drilled core sample.

The chloride content is measured in laboratory by Mohr’s methodusing potassium chloride as indicator in a neutral medium or byvolumetric titration method in acidic medium (IRC SP 40-1993).

Concrete delamination due to corrosion of reinforcement in bridgedecks is detected by Thermography7, which measures the difference insurface temperature of sound and unsound concrete, to detect areasof delaminations due to corrosion.

Radar techniques are used to detect voids, position and continu-ity of reinforcements, ducts, delaminations or other anomalies. Themethod was first used by Pocock and Hartley in 1990. Flohrer andBrenhardt8 described the application of radar technique to detect thelocation of prestressing tendons.

For the measurement of existing stress in concrete, slot-cuttingmethod was developed by Abdunner9. In this method, a 4mm wide


slot was cut in 10mm increments to a depth of 80mm. After each incre-ment, a special jack was inserted into the segmental slot and pressurerequired for restoring the former strain distribution was noted.

Gifford and partners described precision coring a strain gauged areaof concrete to estimate the principal stresses present. Elastic constantsare obtained by in-situ jacking test in the core-hole.

17.9.4 Tests on Steel

Corrosion of steel in concrete is the main reason for strength loss withage of reinforced/prestressed concrete bridge girders. Corrosion is anelectrochemical process. The probability of corrosion is proportional tothe corrosion current which is controlled by the resistivity of the con-crete. Different methods for corrosion detection of reinforced concretebridge girders are reported and summarised by Mallett10.

Measurement of total resistance of a wire can be a preliminarymethod of estimation of corrosion of cables as the cross sectional areaof wire reduces with corrosion, thus increasing the electrical resistanceof the wire.

Radiography is used to give picture showing the position and sizeof bars. This is based on the principle that loss of energy of gammarays passing through a heterogeneous medium is greater in zones ofhigher density material. The main disadvantage of this method is thatit will not show the extent of corrosion.

Ultrasonic methods are used to detect distress or fracture in pre-stressing tendon if length of the tendons are small.

17.10 IN-SITU STRESS DETERMINATION TECHNIQUES

IN PRESTRESSED CONCRETE GIRDERS AND

BRIDGE DECKS

The ability of any prestressed concrete structure to support all presentand anticipated loads depends on the amount and distribution ofresidual prestress. Condition assessment of the existing structures alsorequires determination of prestress.

The present day trend is that during construction, the bridge isinstrumented with sensors to determine the loss of prestress. This willprovide a correct index to the health of a bridge from its inception, i.e.from the construction stage onwards. Presently instrumentation suchas concrete strain gages, vibrating wire gages, etc. are being used forinstrumenting critical structures such as Nuclear Power Plants. The


current developments in Fibre Optic sensors hold promise for reli-able measurements at reasonable cost in future. This has also beenextended to some selected bridge applications. Such instrumentedstructures can be classified as Intelligent structures. If the prestressedconcrete girders are instrumented during construction, they are calledpriori instrumented girders. Most of the old bridges are not instru-mented during construction, the girders of such bridges are known aspriori uninstrumented girders.

The following methods are available for determining the residualprestress in priori uninstrumented prestressed concrete girders.

(a) Steel stress relief hole method

(b) Concrete stress relief core method

(c) Decompression moment method

(d) Special methods

17.10.1 Steel Stress Relief Hole Method

Steel stress relief hole method is an experimental method of determin-ing prestress in a prestressed concrete member by drilling a relativelysmall hole either in prestressed or non-prestressed steel existing in thebeam. The stress relief is caused by this drilling. The hole diameterand depth are equal. The hole diameter can be anywhere from 0.8mm to 3.0 mm (standard size 1.6mm) depending upon the diameterof prestressed wires or non-prestressed reinforcement bars. As the holeis drilled into a stress field, the stress field around the hole is affectedand the radial stress at the edge of the hole experiences a total stressrelief. This stress relief is measured using electrical resistance foil straingages fixed on the wire prior to drilling, which are aligned radially tothe hole.

17.10.2 Concrete Stress Relief Core Method

Concrete stress relief core method is an experimental method of deter-mining the precompression in a prestressed concrete member by takingout a concrete core and measuring the stress relief caused by the holeformed by removing the core with electrical strain gages pasted earlier.

17.10.3 Decompression Moment Method

In decompression moment method, residual prestressing force in amember is determined by carefully observing the reopening of a flexu-ral crack in the member during flexural load test. After the first crackhad developed, the beam will be unloaded as a result of which crack


may get closed. Load is slowly reapplied, and the reopening of thecrack on the bottom face is carefully monitored. at the instant of crackopening, the stress at the bottom fiber is zero. Since the beam sectionproperties, weight and the applied loads are known, the residual pre-stress existing in the member can be calculated by the well-knownflexural formula.

17.10.4 Special Methods

In the special methods, the existing prestress in a girder is measuredby nullifying the strain release caused by a free boundary with externalpressure. Strain sensors are affixed at a location where the prestressis to be determined. When a slot or hole is made at that location afree boundary occurs and the resulting strain release is measured bythe prefixed sensors. Uniform pressure is then applied by means ofjack or any other device along the free boundary to such an extentthat the strain release is nullified. This pressure gives the residualprecompression in the member at that location.

17.11 REHABILITATION TECHNIQUES -CASE STUD

17.11.1 Retrofitting of a Typical PSC Girder Bridge usingExternal prestressing

Structural Engineering Research Centre (SERC), Chennai, had carriedout condition assessment of a distressed prestressed concrete girderbridge in which heavy prestress loss has been observed. External pre-stressing was suggested for retrofitting of the bridge to carry the ratedloading. It was decided to measure strain, deflection responses dur-ing external prestressing with a view to know the state of stressesin the structural elements and to ensure that the state of stresses iswell within the permissible limits so that the whole operation of theexternal prestressing could be carried out without any distress beingcaused to the concrete due to increase in stresses. It was also sug-gested to carrying out load testing of the retrofitted span with a viewto check the rating of the bridge and to verify/ensure safety of thebridge during vehicular movement. The details of the instrumentationand measurements carried out for strain and deflection responses ofthe retrofitted span during external prestressing and load testing arepresented in the following sections.

17.11.2 Instrumentation for Strain Measurement

Linear precision foil strain gages, 90mm long with 120 ohm resis-tance, with preattached lead wires (1m long), were used for strain


measurements. Strain gages were bonded using compatible adhesiveand standard procedure was followed for strain gage installation. Theleads from the strain gages were connected to the strain measuringequipment (data logger) by using shielded, low resistant instrumen-tation cables. All the strain gages were connected to the instrumentwith quarter bridge-3 lead wire configuration and the stability of thestrain gages were monitored and checked before the actual tests.

For installing electrical resistance strain gages on the bridge deck,pits of 300 × 300mm size to the required depth (to remove the bitu-minous overlay upto the top surface of the deck) were made at theidentified locations. The gages installed on the top of the deck andat bottom flange of girders were used to estimate the extreme fibrestresses developed due to external prestressing operations. The straingages were installed before commencement of external prestressingoperations. It may be noted that these gages would give the stresschanges due to prestressing operations alone.

17.11.3 Instrumentation for Deflection Measurement

A precision theodolite/total station (Fig. 17.1) was used for mea-surement of deflections during prestressing operations as well asduring the load testing. The theodolite station was kept on the topdeck of the span adjacent to the instrumented span, where externalprestressing/load test was carried out.

Using the proposed scheme of instrumentation, deflections of theindividual girders, as well as the deflections at the mid-span of the deckduring external prestressing operations/load testing were measured.Five theodolite targets were installed at mid span on the top surface ofthe deck (after removing bituminous overlay). Further, theodolite tar-gets at one-quarter and three-quarters of the span were also installedalong central line of the deck. For installing theodolite targets on thebridge deck, pits of 200 × 200mm size, to the required depth (tillit touched the top surface of the deck) were made at the identifiedtheodolite target locations. For installing reflection target sheet, a steelstand at each location was fixed in each pit, using plaster of paris. Fig.17.2. shows the arrangement of seven theodolite targets in the span.

17.11.4 Sequence of External Prestressing Operation

The external prestressing was carried out by M/s FPCCL, Mumbai,for all the four PSC girders in each span. Each girder was stressedusing two Nos. of 8T13 prestressing cables, with a total prestressingforce of 1000 kN (2×500 kN). The stressing pressure for each cable


(for 500 kN) has been worked out to be 160 kg/cm2. The elongationfor the above prestressing force has been worked out to be 126mm. Toaccount for anchorage slip of about 8mm, each 8T13 cable was stressedupto 165-170 kg/cm2. The stressing operation was carried out usingtwo stressing jacks. The stressing sequence is shown schematically inFig. 17.3.

External prestressing was applied as per the sequence specifiedabove. The interior girder ’G2’ was stressed first. Out of the twocables used for prestressing for each PSC girder, one cable was stressedfrom one side and the second cable was stressed from the other side.Tensioning of each cable was done in seven stages. The tensioningpressures (in kg/cm2) at different stages were: 50, 75, 100, 120, 140,160 and 165-170 kg/cm2 respectively. Strain gage measurements wererecorded at each stage of stressing. Deflection measurements were alsotaken at seven theodolite target locations of span using high precisiontheodolite/total station at different stages of prestressing, as in thecase of strain measurements. The deflections which occurred duringthe different stages of external prestressing compared reasonably wellwith the theoretical deflections computed. Then the stressing of sec-ond interior girder ”G3” was taken up followed by outer girder “G4”.Stressing of outer girder ’G1’ was done after the stressing of G4 onthe same day. The method adopted for stressing, stages of stressingand method of measurement of strains and deflections for girders G3,G4 and G1 were the same as those adopted for girder G2. The detailsof progressive (cumulative) deflection of girders G1, G2, G3 and G4after completion of external prestressing of each girder are arrived.Deflection of bridge, at mid span, at girder locations, during exter-nal prestressing is shown in Fig. 17.4. It has been observed that thedeflections of PSC girders and strain values measured during externalprestressing operations compared reasonably well with the theoreticalvalues computed. The external prestressing operation was completedsuccessfully.

17.11.5 Load Testing and Measurement of Response

After the completion of the external prestressing of the four girders ineach span, load testing of span was taken up. A TATA 2515C (Cum-mins) vehicle was used for load testing of the bridge span. The grossunladen weight of the vehicle, front axle weight, rear axle weights ofunladen vehicle were determined using a weigh bridge. Gross LadenWeight (GLW) of the test vehicle was computed as 25.0t (W) to sim-ulate bending moment at mid span due to Class B loading as per


IRC: 6 [2000]. The test load was applied in stages of 0.5W (12.5t),0.75W(18.75t), 0.9W(22.5t), and 1.0W(25.0t), where “W” is the GrossLaden Weight of the test vehicle. For each stage of load application,test vehicle was placed on the bridge deck so as to induce maximummoment. The additional weights were loaded/added on to the testvehicle by placing pre-weighed sand bags, each weighing 40 Kg, onthe test vehicle. Fig. 17.5 shows the test vehicle at centre of spanduring particular stage of loading. During all stages of load applica-tion, deflections were measured at selected theodolite target locationsand strains were measured using electrical resistance strain gages. Foreach stage of load application, the loaded test vehicle was brought tothe intended/marked position (at centre of span) and deflections andstrains were recorded instantaneously and after a period of five min-utes. The test vehicle was then taken off the bridge and instantaneousrecovery of deflections and strains were recorded. Further, recovery ofdeflections and strains 5 minutes after the removal of the load werealso recorded. Maximum deflection due to Gross Laden Weight (W= 25.0t) was found to be 3.1mm which is less than maximum per-mitted deflection of 28.65mm (1/1500 of span = 28.65mm). It wasalso noticed that the recovery of deflections after the removal of loadswas within the stipulated values given in IRC: 6. The strain valuesrecorded during the various stages of loading were also found to bewell within limits and on lower side compared to the strains inducedduring external prestressing.


In the past and during the present, the final acceptance of pouredconcrete is by strength measured through a test sample (cube) thatmay or may not represent the quality of the in- place concrete. Thissituation has to change with emphasis on design for durability. Deci-sions on durability require detailed testing to assess durability basedcharacteristics of concrete, technical knowledge and judgement. Reg-ular inspection, proper maintenance and timely repair / restorationwill go a long way in reducing damage to concrete structures. Withthe present advances in sensor technology and automation, continuousmonitoring of structures leading to intelligent structures will, in thenear future, ensure timely warning for changes in state of structure.


17.13 REFERENCES

1. ACI Committee 546, “Guide for repair of concrete bridge super-structure”, ACI Manual of Concrete Practice, Part2 , 1995.

2. Nowak A., “Diagnostic Procedures for Bridges”, Proceedings ofthe NATO Advanced Research Workshop on Bridge Evaluation,Repair and Rehabilitation, Maryland, USA, 1990 pp 73–84.

3. IRC SP- 40,“Guidelines on techniques for strengthening andrehabilitation of Bridges”, New Delhi 1993.

4. Carino N. J., and Sansalone M., “Flaw detection in concreteusing the impact-echo method”, proc. of NATO advanced work-shop on Bridge evaluation, repair and rehabilitation, Baltimore,Maryland, USA, 1990 pp. 101–118.

5. Bungley J. H., “Testing concrete in structures: A guide to equip-ment for testing concrete in structures”, CIRIA Technical Note143, 87., 1992

6. BS:1881, “Testing concrete: Part207-Near to surface strengthtests; Part 208:Initial Surface Absorption”, 1992

7. Clemina G. G., and Mckeel W. T.jr., “Detection of delamina-tion in bridge decks with infrared thermography”, Transportationresearch record, No. 664, Vol.1, pp. 180–182

8. Flohrer, and Brenhardt, B., “Detection of prestressed steel ten-dons behind reinforcing bars, detection of voids in concretestructures - a suitable application for radar systems”, Proc. 2ndInt. Conf. On bridge management, pp. 18–21 Apr, 1993.

9. Abdunur C., Duchene J. L., “Structural assessment of bridge withtransversal cracks”. First International Conference on BridgeManagement held at University of Surrey Guildford, 1990,pp.489–500.

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12. Thomas B., Ramakrishna Rao M.N., Khare P. S., “ConditionAssessment of a Prestressed Concrete Bridge Deck under Dis-tress”, Proc. of the Int. Seminar on Failures, Rehabilitation andRetrofitting of Bridges and Aqueducts, Nov. 1994, Bombay, Vol.1, pp 255–259.

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Seminar on Failures, Rehabilitation and Retrofitting of Bridgesand Aqueducts, Nov. 1994, Bombay, Vol. 1, pp 97–127.

Table 17.1 Comparative fineness of cement materials

Sl. No. Material Fineness

1 Silica fume 20000 m2/kg(0.20 to 0.1 micron)

2 Fly ash 400 to 700 m2/kg (5 to 3 micron)

3 Blast furnace slag 350 to 600 m2/kg (6 to 3.5 micron)

4 Ordinary portland cement 300 to 400 m2/kg (7 to 3 micron)

Fig. 17.1 A View of Theodolite Set Up

Fig. 17.2 View of Theodilite Targets for the Measurementof Deflections


Fig. 17.3 Stressing Sequence During External Prestressing

Fig. 17.4 Deflected Profile of the Bridge at Mid-spanduring Prestressing


Fig. 17.5 Test Vehicle at the Centre of Span

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Recent Developments in Condition Assessment, Repair Materials and Repair - Retrofitting Technique

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