Abstract_project Asli 2

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CHAPTER-1

INTRODUCTION

To keep a high level of structural safety, durability and performance of the

infras tructure in each country, an eff icient sys tem for early and regular

s tructural assessment is urgently required. The quali ty assurance during

and af ter the cons truct ion o f new s tructu res and af ter recons t ruction

processes and the characterisation of material properties and damage as a

function of time and environmental influences is more and more becoming

a serious concern.

Non-destructive testing (NDT) methods have a large potential to be part of

such a system. NDT methods in general are widely used in several industry

branches. Aircrafts , nuclear facilities, chemical plants, electronic devices

and other safety cri t ical ins tal lat ions are tes ted regularly with fas t and

reliable test ing technolog ies . A varie ty o f advanced NDT methods are

available for metallic or composite materials.

In r ecen t yea rs , inn ov at iv e NDT metho ds , which can b e u sed for the

assessment o f exis ting s tructu res , have become avai lab le fo r concrete

structures, but are still not established for regular inspections. Therefore,

the object ive of th is project is to s tudy the applicabil i ty, performance,

availability, complexity and restrictions of NDT.

The purpose of establishing standard procedures for nondestructive testing

(NDT) o f con crete s truc tu res i s to q ua li fy and q uant ify the mater ia l

propert ies of in-si tu concrete without in trus ively examining the material

properties. There are many techniques that are currently being research for

the NDT of materials today.1.1 Background of the problem

Concrete structures as many other engineering structures are subjected to deterioration that

affect their integrity, stability and safety. Faced with the importance of the damages noted

on the structures, the current choices are directed towards the repair of the existing

structures rather than towards the demolition and construction of new ones. But before any

repair work being done, it is common practice to determine the causes of the deterioration

so that successful repair can be done. Many repair work fail because the exact causes ofthe deterioration was not adequately identified.

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This identification process comprises many methods including non-destructive testing

methods. Non-Destructive Testing is usually undertaken as part of the detailed

investigation to complement the other methods. Sometimes, the conclusions of the

investigation are based essentially on these tests.

First developed for steel, it has not been easy to transfer the NDT technology to the

inspection of concrete (Carino, 1994)[2].

Because of the characteristics of reinforced

concrete the non destructive testing (NDT) of concrete structures is more complex than the

NDT of metallic materials (Rhazi, 2001)[13]

Since the spread of their application in civil engineering, one of their main disadvantages

lies in the processing and interpretation of the data, which is often not trivial (Colombo

and Forde, 2003)[3]

. In order for the NDT to better achieve its role in structural

assessment there must have agreed standards and guidelines on how to do the survey in

the field and interpret the data obtained Unfortunately until now the choice of the best-

fitted technique for a specific case is not simple, the relevance of the measurement process

not guaranteed, and the question of how to cope with measurement results and how to

finally assess the structural properties remains unanswered (Rilem, 2004)[10]

1.2 Statement of the problem

The application of non-destructive testing to concrete structures is sometimes

disappointing. There are many NDT techniques, each based on different theoretical

principles, and producing as a result different sets of information regarding the physical

properties of the structure. These properties, such as velocities, electrical resistance and so

on, have to be interpreted in terms of the fabric of the structure and its engineering

properties.

The interpretation of the data is the most challenging task of the engineer assessing the

structure. The recommendations made based on the interpreted result can be very

significant. Decision on whether a structure is adequate or not, the standard and

specifications are respected or not, and the exact causes of the deterioration, depends on

the outcome of the datas interpretation. It is neither desirable that they lead to the

condemnation of a structure safe or economically repairable building, nor it is admissible

that they provide a false sense of confidence in an otherwise unsafe structure.

Therefore it is vital to study the reliability in of the NDT results of concrete structures.

How NDT results are interpreted? What are the factors affecting these interpretations?

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1.3 Objectives of the study

The objectives of the study are to:

Investigate the reliability of Non-Destructive Testing (NDT) results of concrete

structures,

Determine the factors affecting the interpretation of (NDT) results of concrete

structures.

1.4 Scope of the study

The present work focuses on the study of the reliability of non-destructive testing results

of concrete structures. It will be accomplished by comparing the tests done in laboratory

& the tests conducted on normal hardened concretes o f A double stor ied bu il ding

having two halls v iz. , Hall No.1 and Hall no.2 in Community Hall Kang

Ma i, Di st ri ct : Hosh ia rp ur ; The study will be restricted to the compressive strength of

concrete.

1.5 Limitations of the study

This study will investigate neither human being role in the reliability of NDT nor will it

focus on how to improve the reliability of the NDT testing equipments.

It will be based on the assumptions that the testing equipments are adequate and the testing

operation done with respect to the procedure from the planning of the testing to the

recording of the data.

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CHAPTER-2

LITRATURE REVIEW

The purpose of this project work is to provide an introduction to commonly used NDTmethods for concrete. Emphasis is placed on the principles underlying the various

methods so as to understand their advantages and inherent limitations. Additional

information on the application of these methods is available in IS: 13311; Part-I & II[7-8]

2.1 Historical Background

Some of the first methods to evaluate the in-place strength of concrete were adaptations of

the Brinell hardness test for metals, which involves pushing a high-strength steel ball into

the test piece under a given force and measuring the area of the indentation. In the metals

test, the load is applied by a hydraulic loading system. Modifications were required to use

this type of test on a concrete structure. In1934, Professor K. Gaede in Germany reported

on the use of a spring-driven impactor to supply the force to drive a steel ball into the

concrete (Malhotra, 1976)[6]

. A nonlinear, empirical relationship was obtained between

cube compressive strength and indentation diameter. In 1936, J.P. Williams in Englandreported on a spring-loaded, pistol-shaped device in which a 4-mm ball was attached to a

plunger (Malhotra, 1976)[6]

. The spring was compressed by turning a screw, a trigger

released the compressed spring, and the plunger was propelled toward the concrete. The

diameter of the indentation produced by the ball was measured with a magnifying glass

and scale. In 1938, a landmark paper by D.G. Skramtajev, of the Central Institute for

Industrial Building Research in Moscow, summarized 14 different techniques for

estimating the in-place strength of concrete, 10 of which were developed in the Soviet

Union (D.G. Skramtajev, 1938)[4]

.

2.2 Recent Developments:

In 1995, B. Haselwander reported, safety, serviceability & due proportion in the three

goals of building construction[1]

. The safety must not be neglected & NDT can make a

significant contribution to evaluating building safety. To achieve the second goal i.e.

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serviceability NDT can play an important role in quality assurance. The third goal i.e. due

proportion or beauty does not have NDT does not have same significance.

In 2003, Michael P. Schullar, reported that Recent advances in nondestructive technology

have led to mainstream use of several methods for evaluating the strength of the structure

[11]. Nondestructive approaches such as Rebound Hardness, stress wave transmission,

impact echo, surface penetrating radar, tomography imaging & infrared thermograph are

useful for qualitative condition survey as well as identification of internal features such as

voids or areas of distress .In-situ test methods are also available for determination of

engineering properties.Standardised methods exists for many of the evaluation approaches

& efforts are ongoing for further improve the quality of NDT testing.

In 2003, J. Mat conducted Nondestructive assessment of the actual compressive strength

of concrete, through an experimental program , involving destructive and nondestructive

applied to different concrete mixes[10]

.He established relationships for pulse velocity,

rebound hammer ,pullout test, probe penetration . The results shows good behavior for

some methods, like pulse velocity, rebound hammer. The relationships for various

methods were compared in terms of dimensionless sensitivity, for different strength levels.

The results showed decreasing sensitivity with increasing strength.

In 2004, P. Turgut, in his Research developed the correlation between concrete strength

and UPV values[12]

, the most appropriate curve is found and shown depending on the

correlation between the curve obtained from existing reinforced structures and the curve

obtained from studies on laboratory originated specimens. Depending on these curves the

best fit formula is found as:

Sn=0.3161e1.03v

n

With the formula Sn = 0.3161e1.03V

n, obtained with the correlation of earlier researches'

findings and this study's findings, approximate value of compressive strength in any point

of concrete can be practically found with ignoring the mixture ratio of concrete through

using only longitudinal velocity variable (Vn).

In 2007, Shibli R. M. Khan, established correlation between UPV tests and destructive

tests, and presented them in the form of regression equations that display standard errors

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between 3.2 to 6.7 Mpa[15]

. The proposed relationships can be used for concrete

strength estimation that is normally required in building or structural assessment,

especially with the present trend of constructing modern structures with high performance

concrete.

In 2009, Ehsan Moshtagh and Ali Massumi reported Seismic assessment can be achieved

by NDT tests[3]

. He proposed two methods viz. forced vibration method and ambient

vibration method for seismic assessment of existing buildings, which is the first step

towards rehabilitation of constructions.

In 2009, Jeffrey Wouters in his paper Applications of Impact-Echo for Flaw Detection,

reported that .Impact-echo testing has been highly successful in locating and evaluating

numerous types of flaws in concrete and masonry structures [9]. He suggested that a

thorough understanding of the actual testing parameters and variables will result in more

accurate and successful flaw detection.

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CHAPTER-3

TESTS AND METHODOLGY

The quality of new concrete structures is dependent on many factors such

a s t yp e o f c em en t, t yp e o f a gg re ga te s, w at er c em en t r at io , c ur in g,

environmental condit ions etc. Besides th is, the control exercised during

cons t ruct ion a lso contr ibutes a lot to achieve the des i red qual ity . The

present system of checking slump and testing cubes, to assess the strength

of concrete, in structure under construction, are not sufficient as the actual

s t rength o f the s t ructu re depend on many o ther facto rs such as p roper

compaction, effective curing also.

Considering the above requirements, need of testing of hardened concrete

in new s tructures as well as o ld s tructures , is there to assess the actual

condition of structures. Non-Destructive Testing (NDT) techniques can be

used effectively for investigation and evaluating the actual condition of the

s tructures . These techniques are relat ively quick, easy to use, and cheap

and give a general indication of the required property of the concrete. Thisapproach will enable us to find suspected zones, thereby reducing the time

and cost of examining a large mass of concrete. The choice of a particular

NDT method depends upon the property of concrete to be observed such as

strength, corrosion, crack monitoring etc.

The subsequent testing of structure will largely depend upon the result of

preliminary testing done with the appropriate NDT technique.

The NDT being fast, easy to use at s ite and relatively less expensive can be

used for

(i) Testing any number of points and locations

(ii) Assessing the structure for various distressed conditions

(iii) Assessing damage due to fire, chemical attack, impact, age etc.

( iv )Detec ting c rack s, v oids , f ractures , h on eyco mb s and weak

locations

(v) Assessing the actual condition of reinforcement

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Many of NDT methods used for concrete tes t ing have their orig in to the

test ing o f more homogeneous , meta l lic sys tem. These methods have a

sound scientific basis , but heterogeneity of concrete makes interpretation

of results somewhat d iff icult. There could be many parameters such as

materials , mix, workmanship and environment, which influence the result

of measurements.

Moreover the test measures some other property of concrete (e.g. hardness)

yet the r es ul ts a re interpreted to ass es s the d if fe rent p ro perty o f the

concrete e.g. (strength). Thus, interpretation of the result is very important

and a d i ff icu l t job where general iza t ion i s not possib le . Even though

operators can carry out the test but interpretation of results must be left to

e xp er ts h av in g e xp er ie nc e a nd k no wl ed ge o f a pp li ca ti on o f s uc h

nondestructive tests .

Var ie ty o f NDT metho ds h av e b een d ev elop ed and a re ava il ab le for

invest igat ion and evaluation of d ifferent parameters related to s trength ,

durability and overall quality of concrete. Each method has some strength

and some weakness. Therefore prudent approach would be to use more than

one method in combination so that the s trength of one compensates the

weakness o f the o ther . The various NDT methods fo r test ing concre te

bridges are listed below A. For strength estimation of concrete

(i) Rebound hammer test

(ii) Ultrasonic Pulse Velocity Tester

(iii) Combined use of Ultrasonic Pulse Velocity tester and

rebound hammer test

(iv) Pull off test

(v) Pull out test

(vi) Break off test

B. For assessment of corrosion condition of reinforcement and to determine

reinforcement diameter and cover

(i) Half cell potentiometer

(ii) Resistively meter test

(iii) Test for carbonation of concrete

(iv) Test for chloride content of concrete

(v) Profometer

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(vi) Micro covermeter

C. For detection of cracks/voids/ delaminating etc.

(i) Infrared thermo graphic technique

(ii) Acoustic Emission techniques

(iii) Short Pulse Radar methods

(iv) Stress wave propagation methods

pulse echo method

impact echo method

response method

3.1 NDE Methods in Practice

Visual inspection: The f ir st s tage in the eva lu at io n o f a con crete

s tructure is to s tudy the condition of concrete, to note any defects in the

concrete, to note the presence of cracking and the cracking type (crack

width, depth, spacing, density), the presence of rust marks on the surface,

the presence of voids and the presence of apparently poorly compacted

areas etc. Visual assessment determines whether or not to proceed with

detailed investigation.

The Surface hardness method: This is based on the principle that the

strength of concrete is proportional to its surface hardness. The calibration

chart is val id for a part icular type of cement, aggregates used, mois ture

content, and the age of the specimen.

The penetration technique: This is bas ical ly a hardness tes t , which

p ro vides a q uick means o f d etermining the r elat iv e s tr en gth o f the

concrete. The results of the tes t are influenced by surface smoothness of

concrete and the type and hardness o f the aggregate used . Again, the

calibration chart is valid for a particular type of cement, aggregates used,

moisture content, and age of the specimen. The test may cause damage to

the specimen which needs to be repaired.

The pull-out test: A pullout test involves casting the enlarged end of a

s tee l rod af ter se t t ing o f concre te , to be tes ted and then measur ing the

force required to pull it out. The test measures the direct shear strength of

concrete. This in turn is correlated with the compressive s trength; thus a

measurement of the in-place compress ive s trength is made. The tes t may

cause damage to the specimen which needs to be repaired.

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Ultra-sonic pulse velocity test: This test invo lves measur ing the

velocity o f sound th rough concre te fo r s treng th determinat ion . S ince ,

concrete is a multi-phase material, speed of sound in concrete depends on

t he r el at iv e c on ce nt ra ti on o f i ts c on st it ue nt m at er ia ls , d eg re e o f

compacting, moisture content, and the amount of discontinuities present.

This technique is applied for measurements of composition (e.g. monitor

the mixing materials during construction, to estimate the depth of damage

caused by fire), s trength estimation, homogeneity, elastic modulus and age,

& to check presence of defects , crack depth and th ickness measurement.

General ly, h igh pulse velocity read ings in concrete are ind icat ive o f

concrete of good quality. The drawback is that this test requires large and

expensive transducers. In addition, ultrasonic waves cannot be induced at

right angles to the surface; hence, they cannot detect transverse cracks.

3.2 Introduction to NDE Methods

Concrete technologists practice NDE methods for

(a) Concrete strength determination

(b) Concrete damage detection

3.2(a) Strength determination by NDE methods:Strength determination of concrete is important because its elastic behavior

& service behavior can be predicted from its strength characteristics. The

conventional NDE methods typically measure certain properties of concrete

f rom which an es t imate o f i t s s t reng th and o ther character is t ics can be

made. Hence, they do not directly give the absolute values of strength.

i) The rebound hammer test: The Sch midt r eb ou nd h ammer i s

b as ic al ly a s ur fa ce h ar dn es s t es t w it h l it tl e a pp ar en t t he or et ic al

relationship between the s trength of concrete and the rebound number of

the hammer. Rebound hammers test the surface hardness of concrete, which

cannot be converted directly to compressive strength. The method basically

measures the modulus o f e las t ic i ty o f the near surface concrete . The

principle is based on the absorption of part of the stored elastic energy of

the s pr in g throu gh p last ic d eformation o f the rock s ur face and the

mechan ical waves p ropagat ing th rough the s tone whi le the remaining

elas t ic energy causes the ac tual rebound of the hammer. The d istance

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travelled by the mass, expressed as a percentage of the initial extension of

the spring, is called the Rebound number. There is a considerable amount

of scatter in rebound numbers because of the heterogeneous nature of near

surface properties (principally due to near-surface aggregate particles).

There a re s ev eral f ac to rs o th er than con crete s tr en gth tha t inf lu en ce

rebound hammer tes t results , including surface smoothness and f in ish ,

moisture content, coarse aggregate type, and the presence of carbonation.

Although rebound hammers can be used to estimate concrete strength, the

rebound numbers mus t be corre la ted wi th the compressive s t rength o f

molded specimens or cores taken from the structure.

Rebound Hammer (Schmidt Hammer)

FIG.3.1 REBOUND HAMMER

This is a s imple, handy tool , which can be used to provide a convenient

and rapid indication of the compressive strength of concrete. It consists of

a spring controlled mass that slides on a plunger within a tubular housing.

The schemat ic d iagram showing various parts o f a rebound hammer i s

given as Fig.3.1

The rebound hammer method could be used for

Assessing the likely compressive strength of concrete with the help

of su itable co-re la t ions between rebound index and compress ive

strength.

Assessing the uniformity of concrete

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Assessing the quality of concrete in relation to standard

requirements.

Ass es sing the q ua li ty o f o ne e lement o f con crete in r el at io n to

another.

Thi s metho d can b e u sed with g reater con fidence for d if fe rent ia ting

between the questionable and acceptable parts of the test is classified as a

hardness tes t and is based on the principle that the rebound of an elas t ic

mas s d ep en ds o n the h ardn es s o f the s ur face aga in st which the mas s

impinges . The energy absorbed by the concrete is related to i ts s trength .

Despite its apparent simplicity, the rebound hammer test involves complex

problems of impact and the associated stress-wave propagation.

There is no unique relation between hardness and strength of concrete but

experimental data relat ionships can be obtained from a given concrete.

However, this relationship is dependent upon factors affecting the concrete

surface such as degree of saturat ion, carbonation, temperature, surface

preparat ion and location, and type of surface f in ish . The result is also

affected by type of aggregate, mix proportions, hammer type, and hammer

inclination. Areas exhibiting honeycombing, scaling, rough texture, or high

porosity must be avoided. Concrete must be approximately of the same age,

moisture conditions and same degree of carbonation (note that carbonated

surfaces yield higher rebound values) . I t is clear then that the rebound

number reflects only the surface of concrete. The results obtained are only

representative of the outer concrete layer with a thickness of 3050 mm.

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Principle:

The method is based on the principle that the rebound of an elas t ic mass

depends on the hardness of the surface against which mass strikes. When

the p lu ng er o f r eb ou nd h ammer i s p ress ed aga in st the s ur face o f the

concrete , the sp ring control led mass rebounds and the ex ten t o f such

rebound depends upon the surface hardness o f concrete . The surface

h ar dn es s a nd t he re fo re t he r eb ou nd a re t ak en t o b e r el at ed t o t he

compressive strength of the concrete. The rebound value is read off along a

graduated scale and is designated as the rebound number or rebound index.

The compressive strength may be read directly from the graph provided on

the body of the hammer.

The impact energy required for rebound hammer for different applications

is given below

Table 3.1 Impact Energy of Rebound Hammers

PRECAUTIONS DURING TESTING

Before commencement o f a test , the rebound hammer shou ld be tested

against the test anvil, to get reliable results . The testing anvil should be of

s te el h av in g B ri ne ll h ar dn es s n um be r o f a bo ut 5 00 0 N /m m2 . The

supplier/manufacturer of the rebound hammer should indicate the range of

readings on the anvil suitable for different types of rebound hammer.

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For taking a measurement, the hammer should be held at right angles to the

surface of the s tructure. The tes t thus can be conducted horizontal ly on

v er ti ca l s ur face and v er ti ca lly u pwards o r d ownwards o n h or izon ta l

surfaces

(Fig. 3.2 shows various positions of Rebound Hammer)

If the

situation so demands, the hammer can be held at intermediate angles also,

b ut in each cas e, the r eb ou nd n umber w il l b e d if fe rent for the s ame

concrete.

The following precautions should be observed during testing

The surface should be smooth, clean and dry

The loosely adher ing scale shou ld be rubbed off wi th a g r inding

wheel or stone, before testing

The tes t should not be conducted on rough surfaces result ing fromincomplete compaction, loss of grout, spalled or tooled surfaces.

The po in t o f impact shou ld be a t leas t 20mm away f rom edge o r

shape discontinuity.

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ii) Ultrasonic Pulse Velocity Tester

Ultrasonic instrument is a handy, battery operated and portable instrument

used for assessing elastic properties or concrete quality. The apparatus for

ultrasonic pulse velocity measurement consists of the following (Fig.3.3) (a) Electrical pulse generator

(b) Transducer one pair

(c) Amplifier

(d) Electronic timing device

FIG. 3.3 USPV TESTER

Objective:

The ultrasonic pulse velocity method could be used to establish:

(a) The homogeneity of the concrete

(b) The presence of cracks, voids and other imperfections

(c) Change in the structure of the concrete which may occur with time

(d) The quali ty of concrete in relat ion to s tandard requirement (e) The

quality of one element of concrete in relation to another

(f) The values of dynamic elastic modulus of the concrete

Principle

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The method is based on the p r incip le that the veloci ty o f an u l t rason ic

pulse through any material depends upon the density, modulus of elasticity

and Po issons ra tio o f the mater ia l . Comparat ively h igher velocity i s

obtained when concrete quali ty is good in terms of densi ty , uniformity,

homogeneity etc. The ultrasonic pulse is generated by an electro acoustical

transducer. When the pulse is induced into the concrete from a transducer,

it undergoes multiple reflections at the boundaries of the different material

phases within the concrete. A complex system of stress waves is developed

which includes longitudinal (compression), shear (transverse) and surface

( Re yl ei gh ) w av es . T he r ec ei vi ng t ra ns du ce r d et ec ts t he o ns et o f

long itudinal waves which is the fastest . The veloci ty o f the pulses i s

almost independent of the geometry of the material through which they

pass and depends only on its elastic properties. Pulse velocity method is a

convenient technique fo r invest igat ing s t ructu ral concre te. For good

quali ty concrete pulse velocity wil l be higher and for poor quali ty i t wil l

be less. If there is a crack, void or flaw inside the concrete which comes in

the way of transmission of the pulses, the pulse strength is attenuated and

it passed around the discontinuity, thereby making the path length longer.

Consequently , lower velocit ies are obtained. The actual pulse velocity

obta ined depends p r imari ly upon the mater ia ls and mix p ropor t ions o f concrete. Density and modulus of elasticity of aggregate also significantly

affects the pulse velocity. Any suitable type of transducer operating within

the frequency range of 20 KHz to 150 KHz may be used. Piezoelectric and

magneto-s tr icture types of t ransducers may be used and the lat ter being

more suitable for the lower part of the frequency range.

The e lec t ronic t iming device shou ld be capab le o f measur ing the t ime

interval elapsing between the onset of a pulse generated at the transmitting

transducer and onset of i ts arr ival at receiving transducer. Two forms of

the electronic t iming apparatus are poss ible, one of which use a cathode

ray tube on which the leading edge of the pulse is displayed in relation to

the sui tab le t ime scale , the o ther uses an in terval t imer wi th a d i rec t

reading digital display. If both the forms of timing apparatus are available,

the interpretation of results becomes more reliable.

The ultrasonic pulse velocity has been used on concrete for more than 60

years . Powers in 1938 and Ober t in 1939 were the f i rs t to develop and

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extensively use the resonance frequency method. Since then, u l trasonic

techniques have been used for the measurements of the various properties

of concrete . Also , many in ternat ional commit tees , speci ficat ions and

standards adopted the ultrasonic pulse velocity methods for evaluation of

concrete. The principle of the tes t is that the velocity of sound in a solid

material, V, is a function of the square root of the rat io of i ts modulus of

elasticity, E, to its density, d, as given by the following equation:

Where, g is the gravity acceleration. As noted in the previous equation, the

velocity is dependent on the modulus of elasticity of concrete. Monitoring

modulus of elasticity for concrete through results of pulse velocity is not

normal ly recommended because concrete does not fu lf i ll the physical

req ui rements for the v al id ity o f the equ at io n (2) n ormally u sed for

calculations for homogenous, isotropic and elastic materials

Where V is the wave velocity, is the density, is Poisson's ratio and Ed

is the dynamic modulus of elasticity. On the other hand, it has been shown

that the strength of concrete and its modulus of elasticity are related.

The method s tarts with the determination of the t ime required for a pulse

of v ibrat ions at an ul trasonic frequency to travel through concrete. Once

the velocity is determined, an idea about quality, uniformity, condition and

strength of the concrete tes ted can be at tained. In the tes t , the t ime the

pulses take to travel through concrete is recorded. Then, the velocity is

calculated as:

V = L/ T

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Where V=pulse velocity, L=travel length in meters and T=effective time in

seconds, which is the measured time minus the zero time correction.

From the l i terature review, i t can be concluded that the ul trasonic pulse

velocity results can be used to:

(a) Check the uniformity of concrete,

(b) Detect cracking and voids inside concrete,

(c) Control the quali ty of concrete and concrete products by comparing

results to a similarly made concrete,

(d) Detect condition and deterioration of concrete,

(e) Detect the depth of a surface crack and

(f) Determine the strength if previous data is available.

Factors influencing pulse velocity measurement

The pulse velocity depends on the propert ies of the concrete under tes t .

Various factors which can influence pulse velocity and its correlation with

various physical properties of concrete are as under:

Moisture Content:

The mois tu re content has chemical and physical ef fec ts on the pu lse

velocity . These effects are important to es tablish the correlat ion for the

es t imation of concrete s trength . There may be s ignif icant d ifference in

pulse velocity between a properly cured s tandard cube and a s tructural

element made from the same concrete. This difference is due to the effect

of different curing conditions and presence of free water in the voids. It is

impor tan t tha t these ef fec ts are careful ly cons idered when es timat ing

strength.

Temperature of Concrete:

No significant changes in pulse velocity, in strength or elastic properties

occur due to variations of the concrete temperature between 5 C and 30

C . Cor rect io ns to p ul se v eloc ity measu rements s ho uld b e mad e for

temperatures outside this range, as given in table 3.2 below:

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Path Length:

The path length ( the dis tance between two transducers) should be long

enough not to be s ignif icantly influenced by the heterogeneous nature of

the concrete. I t is recommended that the minimum path length should be

100mm for concrete with 20mm or less nominal maximum size of aggregate

and 150mm for concrete with 20mm and 40mm nominal maximum size of

aggregate. The pulse velocity is not general ly influenced by changes in

path leng th, a l though the e lec t ron ic t iming appara tus may ind icate a

tendency for slight reduction in velocity with increased path length. This is

because the higher frequency components of the pulse are attenuated more

than the lower frequency components and the shapes of the onset of the

pulses becomes more rounded wi th increased d is tance t ravelled. This

app aren t r ed uc tion in v eloc ity i s u su al ly s mall and wel l w ithin the

tolerance of time measurement accuracy.

Effect of Reinforcing Bars:

The pulse velocity in reinforced concrete in v icini ty of rebars is usually

higher than in plain concrete of the same composit ion because the pulse

velocity in s teel is almost twice to that in p lain concrete. The apparent

increase depends upon the p rox imity o f measurement to rebars, the i r

numbers, diameter and their orientation. Whenever possible, measurement

should be made in such a way that s teel does not l ie in or closed to the

direct path between the transducers. If the same is not possible, necessary

corrections needs to be applied. The correction factors for this purpose are

enumerated in different codes.

Shape and Size of Specimen:

The velocity of pulses of vibrations is independent of the size and shape of

specimen, unless its least lateral dimension is less than a certain minimum

value. Below this value, the pulse velocity may be reduced appreciably.

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The extent of this reduction depends mainly on the ratio of the wavelength

of the pulse vibrations to the least lateral dimension of the specimen but it

is insignificant if the ratio is less than unity. Table given below shows the

re la t ionsh ip between the pulse velocity in the concrete , the t ransducer

frequency and the minimum permissible lateral dimension of the specimen.

Table: 3.3 Effect of specimen dimension on pulse transmission.

The use of the ul trasonic pulse velocity tes ter is in troduced as a tool to

monitor basic initial cracking of concrete structures and hence to introduce

a threshold limit for possible failure of the structures. Experiments using

ultrasonic pulse velocity tes ter have been carr ied out , under laboratory

condit ions , on various concrete specimens loaded in compression up to

fai lure. Special p lots , showing the relat ion between the velocity through

concrete and the stress during loading, have been introduced. Also, stress

s tr ai n m ea su re me nt s h av e b ee n c ar ri ed o ut i n o rd er t o o bt ai n t he

corresponding s trains . Results showed that severe cracking occurred at a

stress level of about 85% of the rupture load. The average velocity at this

critical limit was about 94% of the initial velocity and the corresponding

strain was in the range of 0 .0015 to 0 .0021. The sum of the crack widths

has been es t imated us ing special relat ions and measurements . This value

that corresponds to the 94% relative velocity was between 5.2 and 6.8 mm.

Procedure of using Ultrasonic Pulse Velocity Tester

The equipment should be calibrated before starting the observation and at

the end of test to ensure accuracy of the measurement and performance of

t he e qu ip me nt . I t i s d on e b y m ea su ri ng t ra ns it t im e o n a s ta nd ar d

calibration rod supplied along with the equipment. A platform/staging of

sui tab le height should be erected to have an access to the measur ing

locations . The location of measurement should be marked and numbered

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with chalk o r s imi lar th ing p r io r to ac tual measurement (p re decided

locations).

Mounting of Transducers

The direct ion in which the maximum energy is propagated is normally at

right angles to the face of the transmitting transducer, it is also possible to

detect pulses which have t raveled th rough the concrete in some o ther

direction. The receiving transducer detects the arrival of component of the

pulse which ar rives earl ies t . This i s genera l ly the lead ing edge o f the

longitudinal vibration. It is possible, therefore, to make measurements of

pulse velocity by placing the two transducers in the fol lowing manners

(Fig.3.4)

Fig. 3.4 Various Methods of USPV Testing

(

a)

Direct Transmission (on opposite faces)

This arrangement is the most preferred arrangement in which transducers

are kept directly opposite to each other on opposite faces of the concrete.

The t rans fer of e ne rgy be tw ee n t ra nsduc er s i s ma xi mum in t his

arrangement. The accuracy of velocity determination is governed by the

accuracy of the path length measurement. Utmost care should be taken for

accurate measurement of the same. The coolant used should be spread as

thinly as poss ible to avoid any end effects result ing from the different

velocities of pulse in coolant and concrete.(b) Semi-direct Transmission :

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Thi s a rr angem ent i s use d w he n i t is not possi ble to have dir ec t

t rans miss io n (may b e d ue to l imited acces s) . I t i s l es s s en si tive as

compared to direct transmission arrangement. There may be some reduction

i n t he a cc ur ac y o f p at h l en gt h m ea su re me nt , s ti ll i t i s f ou nd t o b e

suff ic ien tly accurate . This ar rangement i s o therwise s imi lar to d irect

transmission.

(c) Indirect or Surface Transmission :

Indirect transmission should be used when only one face of the concrete is

access ible (when other two arrangements are not poss ible). I t is the leas t

s en si tive o ut o f the three a rran gements. For a g iv en p ath l en gth, the

receiving transducer get signal of only about 2% or 3% of amplitude that

produced by direct transmission. Furthermore, this arrangement gives pulse

v eloc ity measu rements which a re u su al ly inf lu en ced b y the s ur face

concrete which is o f ten having d if ferent compos i tion f rom that below

sur fa ce c onc re te . The re for e, the t es t r esul ts may not b e c or re ct

r ep re se nt at iv e o f w ho le m as s o f c on cr et e. T he i nd ir ec t v el oc it y i s

invariably lower than the direct velocity on the same concrete element.

This difference may vary from 5% to 20% depending on the quality of the

concrete . Wherever p ract icable , s i te measurements shou ld be made to

determine th is d ifference. There should be adequate acoustical coupling b etween con crete and the face o f each t rans du cer to ens ure tha t the

ultrasonic pulses generated at the transmitting transducer should be able to

pass into the concrete and detected by the receiving t ransducer wi th

minimum losses. I t i s impor tan t to ensure that the layer o f smooth ing

medium should be as thin as possible. Coolant like petroleum jelly, grease,

soft soap and kaolin/glycerol paste are used as a coupling medium between

transducer and concrete. Special t ransducers have been developed which

impart or pick up the pulse through integral probes having 6mm diameter

tips. A receiving transducer with a hemispherical tip has been found to be

very successful. Other transducer configurations have also been developed

t o d ea l w it h sp ec ia l c ir cu ms ta nc es . I t s ho ul d b e n ot ed t ha t a z er o

adjustment will almost certainly be required when special transducers are

used. Most of the concrete surfaces are sufficiently smooth. Uneven or

rough surfaces shou ld be smoothened using carborundum s tone before

placing of transducers. Alternatively, a smoothing medium such as quick

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setting epoxy resin or plaster can also be used, but good adhesion between

concrete surface and smoothing medium has to be ensured so that the pulse

is propagated with minimum losses into the concrete. Transducers are then

pressed against the concrete surface and held manually. It is important that

on ly a very th in layer o f coup l ing medium separa tes the su rface o f the

con crete f ro m i ts con tact in g t rans du cer. The d is tance b etween the

measuring points should be accurately measured. Repeated readings of the

transit time should be observed until a minimum value is obtained.

Once the u l trasonic pulse impinges on the surface o f the mater ia l , the

maximum energy is propagated at right angle to the face of the transmitting

transducers and bes t results are, therefore, obtained when the receiving

transducer is placed on the opposite face of the concrete member known as

Direc t T rans miss io n. The p ul se v eloc ity can b e measu red b y Direc t

T ra ns mi ss io n, S em i- di re ct T ra ns mi ss io n a nd I nd ir ec t o r Su rf ac e

Transmiss ion . Normal ly, Direct Transmiss ion is p referred being more

rel iable and s tand ardized. (Variou s cod es g iv e cor re la tion b etween

concrete quality and pulse velocity for Direct Transmission only). The size

of aggregates influences the pulse velocity measurement. The minimum

path length should be 100mm for concrete in which the nominal maximum

size of aggregate is 20mm or less and 150mm for aggregate s ize between20mm and 40mm. Reinforcement , i f p resent , should be avoided dur ing

pulse velocity measurements, because the pulse velocity in the reinforcing

bars is usually higher than in plain concrete. This is because the pulse

velocity in steel is 1.9 times of that in concrete. In certain conditions, the

firs t pulse to arr ive at the receiving transducer travels part ly in concrete

and partly in steel. The apparent increase in pulse velocity depends upon

the proximity of the measurements to the reinforcing bars, the diameter and

numbe r of ba rs and the ir ori enta tion w it h r espe ct t o t he pat h of

propagation. It is reported that the influence of reinforcement is generally

small i f the bar runs in the direct ion r ight angle to the pulse path for bar

d iameter less than12 mm. But if percentage of s teel is quite h igh or the

axis of the bars are parallel to direction of propagation, then the correction

factor has to be applied to the measured values.

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The ze ro t ime cor re cti on i s e qua l t o the tr ave l t ime be tw ee n t he

t ransmit ting and receiving t ransducers when they are p ressed f i rmly

together.

Determination of pulse velocity

A pulse o f long itudinal v ib rat ion i s p roduced by an e lec t ro acoust ica l

t ransducer , which is held in con tact wi th one surface o f the concre te

member u nd er t es t. After t ravers in g a k no wn p ath l en gth (L) in the

concrete, the pulse of vibration is converted into an electrical signal by a

second electro-acoustical t ransducer and electronic t iming circuit enable

the transit time (T) of the pulse to be measured. The pulse velocity (V) is

given by V = L / T

Where, V = Pulse velocity, L = Path length, T = Time taken by the pulse to

traverse the path length

Fig.3.5 testing of a beam by USPV Tester

The ultrasonic pulse velocity results can be used:

To check the uniformity of concrete,

To detect cracking and voids inside concrete,

To control the quality of concrete and concrete products by

comparing results to a similarly made concrete,

To detect the condition and deterioration of concrete,

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To detect the depth of a surface crack, and,

To determine the strength if previous data are available.

i i i) Combined use of Rebound hammer and Ultrasonic Pulse

Velocity Method

In v iew o f the r el at iv e l imitat io ns o f e ithe r o f the two metho ds for

predicting the strength of concrete, both ultrasonic pulse velocity (UPV)

and reb ou nd h ammer metho ds a re s omet imes u sed in combina tion to

a ll ev ia te t he e rr or s a ri si ng o ut o f i nf lu en ce o f m at er ia ls , m ix a nd

environmental parameters on the respective measurements. Relat ionship

between UPV, rebound hammer and compressive strength of concrete are

ava il ab le b as ed o n l ab oratory t es t s pecimen. Bet te r accuracy o n the

es timat ion o f concrete s t rength i s achieved by use o f such combined

m et ho ds . H ow ev er , t hi s a pp ro ac h a ls o h as t he l im it at io n t ha t t he

established correlations are valid only for materials and mix having same

propor t ion as used in the t r ia ls. The int r ins ic d if ference between the

lab oratory t es t s pecimen and in- si tu con crete ( e. g. s ur face t ex tu re ,

moisture content, presence of reinforcement, etc.) also affect the accuracy

of test results .

Combination of UPV and rebound hammer methods can be used for the

ass es smen t o f the q ua li ty and l ik ely compres sive s tr en gth o f in- si tu

concrete. Assessment of l ikely compress ive s trength of concrete is made

from the rebound indices and th is is taken to be indicat ive of the entire

mass on ly when the overa l l qual ity o f concre te judged by the UPV is

go od . W he n t he q ua li ty a ss es se d i s me di um , t he e st im at io n o f

compress ive s trength by rebound indices is extended to the entire mass

o nl y o n t he b as is o f o th er c ol la te ra l m ea su re me nt e .g . s tr en gt h o f

con trol led cub e s pecimen, cemen t con tent o f h ardened con crete b y

chemical analysis or concrete core testing. When the quality of concrete is

poor , no assessment of the s trength of concrete is made from rebound

indices.

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iii) Acoustic emission technique: This technique utilizes the elastic

waves generated by plastic deformations, moving dislocations, etc. for the

analysis and detection of structural defects .

However, there can be mult ip le travel paths available from the source to

the s en so rs . A ls o, e lect ri ca l inter fe rence o r o th er mechanica l n oi ses

hampers the quality of the emission signals.

iv) Impact echo test: In this technique, a stress pulse is introduced at

the surface o f the s tructu re , and as the pulse p ropagates th rough the

structure, it is reflected by cracks and dislocations.

Through the analys is of the reflected waves , the locations of the defects

can b e est imated . The main d rawb ack o f thi s t echn iq ue i s tha t i t i s

insensitive to small s ized cracks

3.2(b) Damage detection by NDE methods:

Global techniques : These techniques rely on global structural response for

damage iden ti f ica tion . Thei r main d rawback is tha t s ince they re ly on

global response, they are not sensi t ive to localized damages . Thus , i t is

poss ible that some damages which may be present at various locations

remain un-noticed.

Local techniques: These techniques employ localized s tructural analys is ,

for damage detection. Their main drawback is that accessories like probes

and fixtures are required to be physically carried around the test s tructure

for data recording. Thus, it no longer remains autonomous application of

the technique. These techniques are often applied at few selected locations,

by the instincts/experience of the engineer coupled with visual inspection.

Hence, randomness creeps into the resulting data.

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CHAPTER-4

RESULTS AND DISCUSSION

4.1 CALIBRATION TESTSPROCEDURE:

The procedure that was fo llowed dur ing exper iments cons isted o f the

following steps:

1. Various concrete mixes were used to prepare standard cubes of 150-mm

side length.

2. Concrete cubes of unknown history made under site conditions were also

brought from various sites for testing.

3 . All cubes were immersed under water for a minimum period of 24 hr .

before te sting.

4. Just before testing, the cubes were rubbed with a clean dry cloth in order

to obtain a saturated surface dry sample.

5 . Once drying was complete, each of the two opposite faces of the cube

was prepared for the rebound hammer test as described in the

specifications.6. The cubes were positioned in the testing machine and a slight load was

applied. The rebound number was obtained by taking three measurements

on each of the four faces of the cube. The rebound hammer was horizontal

in all measurements.

7 . Once the rebound hammer tes t was complete, each of the two surfaces

was p repared fo r the u l trasonic pulse velocity test as descr ibed in the

specif icat ions . Care was taken so that there was no effect of the notches

p roduced by the hammer . The t ime was measured on each o f the two

opposing surfaces and the average was recorded.

8. Once nondestructive testing on each cube was completed, the cube was

loaded to failure and the maximum load was recorded.

9. Results were plotted as shown in Figures.

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4.2 REBOUND HAMMER TEST

The operation of rebound hammer is shown in the fig.4.1. When the plunger of rebound

hammer is pressed against the surface of concrete, a spring controlled mass with a constant

energy is made to hit concrete surface to rebound back. The extent of rebound, which is a

measure of surface hardness, is measured on a graduated scale. This measured value isdesignated as Rebound Number (rebound index). A concrete with low strength and low

stiffness will absorb more energy to yield in a lower rebound value.

FIG. 4.1 OPERATION OF REBOUND HAMMER

PREPARATION OF SPECIMEN:

6 cubes were cast, targeting at different mean strengths. Further, the cubes

were cured for d ifferent number of days to ensure availabil i ty of a wide

range of compressive s trength at tained by these cubes . Size of each cube

was 150150150 mm.

TESTING OF SPECIMEN:

1 0 reading s ( rebo un d n umbers ) were o btained for each cub e, a t

different locations on the surface of the specimen.

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The cube was divided into grid blocks of equal spacing and 10 points were

marked at equal intervals for taking the Rebound Hammer test.

a) The cubes were then given a load of 7 N/mm^2 (as specified by the IS

CODE 13311) in the Compress ion Test ing Machine and the Rebound

Values were obtained.

b) The Pred ic ted Compressive S t rength as p red icted by the Rebound

Hammer was ca lcu la ted f rom the char t g iven on Rebound Hammer

FIG. 4.2 CHART FOR CALCULATING. PREDICTED

COMPRESSIVE STRENGTH FROM REBOUND

NUMBER GIVEN ON REBO UND HAMMER

c) Average of rebound numbers and standard deviations were calculated using

Equations 1 and 2 respectively as:

Where fa is the average of rebound numbers, fi is the rebound number,

And n is the total impact number and S is the standard deviation.

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TEST IS CONSIDERED RELIABLE IF THE STANDARD DEVIATION OF TEN

READINGS IS NOT MORE THAN THE FOLLOWING:

REBOUND VALUE 15 30 45

STANDARD DEVIATION 2.5 3.0 3.5

d) The cubes were then loaded up to their ultimate stress and the Breaking

Load was obtained.

Table 4.1 Rebound Number & Comparative Hardness

Fig.4.3 Components of a Rebound Hammer used in the Project

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Fig.4.4 Rebound Hammer Testing of a Specimen

The fol lowing tables l is ts the Rebound numbers (rebound index), Mean

Rebound Value, Standard Deviation, the Dead Load on the specimen at the

time of testing, the Breaking Load, the Predicted Compressive Strength as

predicted by the Rebound Hammer and the actual Compressive Strength as

obtained by the Compression Testing Machine.

SAMPLE NO. 1

S.No. R.No.

1 19

2 25

3 23

4 22

5 23

6 22

7 22

8 22

9 23

10 22

Mean 22.3

S.D 1.49

Dead Load

= 150 KN

Breaking load = 247 KN

f(ck) N/mm^2 14.2 N/mm^2

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(Predicted)

f(ck) N/mm^2

(Actual)

11.0 N/mm^2

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Table No. 4.2 a

SAMPLE NO. 2

S.No. R.No.

1 192 20

3 19

4 20

5 19

6 20

7 19

8 20

9 19

10 22

Mean 19.7

S.D 0.94

Dead Load

= 150 KN

Breaking load = 311.5 KN

f(ck) N/mm^2

(Predicted)

13.2

N/mm^2

f(ck) N/mm^2

(Actual)

13.8

N/mm^2

Table No. 4.2 b

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SAMPLE NO. 3

S.No. R.No.

1 24

2 25

3 264 26

5 26

6 25

7 25

8 24

9 25

10 25

Mean 25.1

S.D 0.73

Dead Load

= 150 KN

Breaking load = 346.5 KN

f(ck) N/mm^2

(Predicted)

18.8 N/mm^2

f(ck) N/mm^2(Actual)

15.3 N/mm^2

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Table No. 4.2 c

SAMPLE NO. 4

S.No. R.No.

1 42

2 42

3 41

4 42

5 42

6 42

7 43

8 43

9 42

10 42

Mean 42.2

S.D 0.63

Table 4.2(d)

Dead Load

= 150 KN


f(ck) N/mm^2

(Predicted)

42.6 N/mm^2

f(ck) N/mm^2

(Actual)

36.88 N/mm^2

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SAMPLE NO. 5

S. No. R. No.

1 36

2 37

3 37

4 39

5 40

6 40

7 41

8 40

9 40

10 41

Mean 39.1

S.D 1.79

Dead Load

= 150 KN


f(ck) N/mm^2

(Predicted)

36.2 N/mm^2

f(ck) N/mm^2

(Actual)

31.5 N/mm^2

Table No. 4.2 e

SAMPLE NO. 5

S.No. R.No.

1 38

2 38

3 37

4 37

5 38

6 38

7 378 37

9 38

10 38

Mean 37.6

S.D 0516

Dead Load

= 150 KN


f(ck) N/mm^2

(Predicted)

39.7

N/mm^2

f(ck) N/mm^2

(Actual)

33.8

N/mm^2

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Table No. 4.2 f

The following graph is obtained between the Predicted Compressive Strength by the

Rebound Hammer and the Actual Compressive Strength:

Fig. 4.5 Calibration Graph for Rebound Hammer with its Equation

4.3 Ultrasonic Pulse Velocity Test

PREPARATION OF SPECIMEN

9 cubes were cast , ta rgeting a t d if ferent mean s t rengths. Fur ther , the

cubes were cured for different number of days to ensure availability of a

wide range of compressive strength attained by these cubes. Size of each

cube was 150150150 mm.

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TESTING OF SPECIMEN:

3 readings of Ultrasonic Pulse Velocity (USPV) were obtained for each

cube.

The cubes were then g iven a load o f 7 N/mm 2 (as speci f ied by the IS

CODE 13311) in the Compress ion Test ing Machine and the USPV were

obtained.

The cubes were then loaded up to their ultimate stress and the Breaking

Load was obtained.

The fol lowing table l is ts the USPV in each specimen with their mean

velocity, the Dead Load, the Breaking Load and the actual Compressive

Strength as obtained by the Compression Testing Machine.

Fig.4.6 Zeroing of the Transducers

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Fig.4.7 USPV Tester used in the Project

Fig. 4.8 Compression Testing of a Specimen

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OBSERVATIONS

S.

No.

V1 V2 V3 V Breakin

g

f (ck)

N/mm2

m/sec m/sec m/sec m/sec IR

LOAD

LOAD (Actual)

1 2825 2916 2913 2884.6 150 562.5 25

2 3350 3585 3218 3384 150 669.8 29.77

3 3625 3632 3218 3491 150 720 32

4 4219 4213 4007 4146 150 841.5 37.4

5 4411 4444 4117 4324 150 875.2 38.9

6 4625 4525 4417 4522 150 893.2 39.7

Table 4.3 USPV Testing Results

The following graph is obtained between the Compressive Strength and

the Ultrasonic Pulse Velocity:

Fig. 4.9 Graph obtained for USPV Testing

Thi s gr aph ca n now a ls o be use d t o appr oxi ma te ly pr edic t the

Compressive Strength of Concrete.

Although it gives fairly approximate results but it should be verified with

some other tests like the Rebound Hammer test.

The quality of concrete in terms of uniformity can be assessed using the

guidelines given in table below:

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S . N o. P ul se V el oc it y b y C ro ss P ro bi ng

in

m/s

Concrete Quality

Grading

1 Above 4500 Excellent

2 Btween3500-4500 Good3 Between3000-3500 Medium

4 Below 3000 Doubtful

Table 4.5 USPV Criterion for Concrete Quality Grading

4.4 Study of Effect of Reinforcement on the Rebound Values and Pulse

Velocities

To Study the ef fec t o f re in forcement on the Rebound Values and the

Ultrasonic Pulse Velocities:

a) Two Beams were cas t of the fol lowing dimensions:

Length = 1.8 m Breadth = .2 m Depth = .25 m

b) Grade of Concrete Used: M20 and M25

c) The po in ts where the re in forcements ex is ted were known so the

testing was done in two stages :

By avoiding the impact of reinforcements or by trying to minimize its impact.

By undertaking the effect of reinforcements or by maximizing its Impact.

A comparative analys is is then made to know the effect of

Reinforcement on the tests

OBSERVATIONS

Grade of concrete used: M 20

Rebound No. Ultra Sonic Pulse Velocity

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S.

No.

WithoutReinforcement

WithReinforcement


WithReinforcement

1st end 29 30 2861 3155

Quarter Length 28 29 2941 3053

Mid Span 30 31 2991 3075

Length 28 29 2800 2908

2

nd

Length 29 29 2925 3224Table No.4.6 (a) Testing of Beam (M20) for effect of Reinforcement

Grade of concrete used: M 25

Rebound No. Ultra Sonic Pulse Velocity

S.

No.


With

Reinforcement


With

Reinforcement

1st end 36 36 3161 3688

Quarter Length 36 37 3141 3374

Mid Span 35 37 3191 3488

Length 39 41 3322 3778

2 nd Length 39 39 3257 3722

Table No.4.6 (b) Testing of Beam (M25) for effect of Reinforcement

The maximum variation obtained for Rebound Hammer is 3.6%, where as

in case of Ultrasonic Pulse Velocity the maximum variat ion is 16.1%.

Therefore variations are well within tolerable limits .

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4.5 TESTING OF HALL NO. 1 AND HALL NO. 2

Tests were conducted on some of the Columns, Beams and Slabs of Hall

No . 1 and H al l No. 2 f or the as se ssme nt of th ei r qua lit y. Theobservations, results and discussions have been tabulated below:

Fig4.10 community hall at Village: Kang Mal, District: Hoshiarpur

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4.5 (a) TESTING OF COLUMNS

HALL NO.1

COLUMN NO.1

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

BOTTOM

28

29

29

28.67 3313 22.5

Testing was done

over the plaster.

Medium qualityconcrete

MIDDLE

13

14

14

13.67 Over

Range

13.4

Void between

pl aste r & c olumn

face ind icated b y a

peculiar sound when

struck softly by iron

rod

TOP

15

15

15

15.33 Over

Range

14.1

Void between

pl aste r & c olumn

face ind icated b y a

peculiar sound when

struck softly by iron

rod

TABLE NO.4.7

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HALL NO.1

COLUMN NO.2

Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

BOTTOM

32

33

33

32.67 3754 31.3

Good Quality

concrete

MIDDLE

30

3230

30.67 3531 30.1

Good Quality

concrete

TOP

31

31

30

30.67 3255 30.1

Good Quality

concrete

TABLE NO.4.8

HALL NO.1

COLUMN NO.3Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

BOTTOM

36

36

36

36 3744 33.9

Good Quality

concrete

MIDDLE

34

35

35

34.67 3825 33

Good Quality

concrete

TOP

33

34

35

34 3614 32.8

Good Quality

concrete

TABLE NO.4.9

In hal l no.2 , columns are made of brick masonry so th is test i s not

applicable here

4.5 (b) TESTING OF BEAMS

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HALL NO.1

BEAM NO.1

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

1ST

SUPPORT

40

40

37

39 4468 39.9

Good Quality

concrete

MID

SPAN

32

32

34

32.67 3455 29.9

Medium Quality

concrete

2ND

SUPPORT

34

34

35

34.33 3480 30.8

Medium Quality

concrete

TABLE NO.4.10

HALL NO.1

BEAM NO.2

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

1ST

SUPPORT

33

35

32

33.33 3655 30.5

Good Quality

concrete

MID

SPAN

34

35

36

35 3845 31.6

Good Quality

concrete

2ND

SUPPORT

34

37

34

35 3440 31.6

Good Quality

concrete

TABLE NO.4.11

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HALL NO.1

BEAM NO.3Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

1ST

SUPPORT

43

39

36

39.33 4505 35.5

Good Quality

concrete

MID

SPAN

38

45

37

40 4533 36.1

Excell ent Quality

concrete. Proper

compact io n may b e

the reason

2ND

SUPPORT

45

49

45

46.33 4861 41.2

Excellent Quality

concrete. It is the

junction of three beams

& a column so heavy

reinforcement &

compaction is indicated

TABLE NO.4.12

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HALL NO.2

BEAM NO.1

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

1ST

SUPPORT

26

28

27

27 2620 20.3

Doubtful Quality

Requires Attention

MID

SPAN

25

27

27

26.33 2729 20

Doubtful Quality

Requires Attention

2ND

SUPPORT

29

28

24

27 2645 20.3

Doubtful Quality

Requires Attention

TABLE NO.4.13

HALL NO.2

BEAM NO.2

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

1ST

SUPPORT

31

31

34

32 2620 29.8

Medium Quality

concrete

MID

SPAN

33

32

32

32.33 2729 29.8

Medium Quality

concrete

2ND

SUPPORT

35

34

37

35.33 2645 31.1

Good Quality

concrete

TABLE NO.4.14

HALL NO.2

BEAM NO.3

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Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

1ST

SUPPORT

28

28

29

28.33 1955 23.8

USPV is low, there is

separat ion o f p las ter

from the beam or

internal voids &

Cracks .Requires

Attention

MID

SPAN

30

32

30

30.67 3233 25.1

Medium Quality

concrete

2ND

SUPPORT

29

33

31

31 3534 25.3

Medium Quality

concrete

TABLE NO.4.15

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4.5 (C) TESTING OF SLABS

Hall No.1

Observat ions were taken on the top roof s lab i . e . 2 nd f loor roof s lab

because the G.F. & F.F. do not have exposed surface due to application

of tiles. The 2 n d floor roof slab has been plastered to protect it from rain,

sun & others extreme conditions & to give a smooth finish .

SLAB BETWEEN A-B

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

EDGES

38

39

39

38.67 4257 34.3

Good quality concrete

MID

SPAN

ALONG

EDGES

36

35

35

35.67 3966 32.7

Good Quality

concrete

CENTRE

OF SLAB

34

34

34

34 3850 31.8

Good Quality

concrete

TABLE NO.4.16

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HALL NO.1

SLAB BETWEEN D-E

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

EDGES

33

30

28

30.33 4122 26.2

H igh USPV may be

due to heavy tors ion

s te el . O ve ra ll g oo d

quality concrete

MID

SPAN

ALONG

EDGES

32

31

31

31.33

3890 27.2

Good Quality

concrete

CENTRE

OF SLAB

29

30

30

29.67 2855 25.0

Little low may be due

to improper shuttering

o f s labs & imp ro per

compaction

TABLE NO.4.17

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HALL NO.1

SLAB BETWEEN E-F

Rebound

Value

Mean USPV

m/s

Quality

fc k

N/mm2

Remarks

EDGES

30

33

28

29.67 4005 26.4

Good quality concrete

MID

SPAN

ALONG

EDGES

28

28

27

27.33 3825 25.6

Good Quality

concrete

CENTRE

OF SLAB

30

29

27

28.67 3988 26

Good Quality

concrete

TABLE NO.4.18

HALL NO.2 (On 1 s t floor slab)

SLAB BETWEEN 3-4

Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

EDGES

47

45

44

45.33 5710 47.9

Excellent quality

concrete

MID

SPAN

ALONG

EDGES

49

48

44

47 5764 49.3

Excellent Quality

concrete

CENTRE

OF SLAB

53

51

48

50.67 6229 52.6

Excellent Quality

concrete

TABLE NO.4.19

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HALL NO.2

SLAB BETWEEN 4-5Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

EDGES

47

42

44

44.33 5612 47.8

Excellent quality

concrete

MID

SPAN

ALONG

EDGES

58

47

58

51 5562 52.4

Excellent Quality

concrete

CENTRE

OF SLAB

38

36

40

38 4079 40.5

Excellent Quality

concrete

TABLE NO.4.20

HALL NO.2

SLAB BETWEEN 5-6Rebound

Value

Mean USPV

m/s

Quality

fck

N/mm2

Remarks

EDGES

45

46

50

47 5846 51.9

Excellent quality

concrete

MID

SPAN

ALONG

EDGES

45

45

47

45.67 5760 49.1

Excellent Quality

concrete

CENTRE

OF SLAB

42

39

33

38 4832 39.5

Excellent Quality

concrete

TABLE NO.4.21

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Fig No. 4.11 Rebound Hammer Testing of a Column in Hall No.1

Fig No. 4.12 Rebound Hammer Testing of a Slab in Hall No.1

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Fig No. 4.13 USPV Testing of a Column in Hall No.1

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4.6 Interpretation of Results

From the above discussion following interpretation can be made:

4.6.1 IN CASE OF REBOUND HAMMER:

1. After obtaining the correlat ion between compressive s trength and

rebound number, the strength of structure can be assessed.

2. In general, the rebound number increases as the strength increases.

3. Rebound Number is affected by a number of parameters i .e. type

o f cemen t, typ e o f agg rega te , s ur face con di tion and moisture

content of the concrete, curing and age of concrete, carbonation of

concrete surface etc.

4. The reb ou nd ind ex i s ind icat iv e o f compres sive s tr en gth o f

con crete u p to a l imited d ep th f ro m the s ur face . The interna l

cracks, flaws etc. or heterogeneity across the cross section will not

be indicated by rebound numbers.

5. The estimation of strength of concrete by rebound hammer method

can no t b e h eld to b e v ery accurate and p ro bable accuracy o f

prediction of concrete strength in a structure is 25 percent.6. If the relationship between rebound index and compressive strength

can be found by tests on core samples obtained from the structure

or standard specimens made with the same concrete materials and

mix proportion, then the accuracy of results and confidence thereon

gets greatly increased.

7. The Rebound hammers showed erratic result when the compressive

s t reng th was below 15 N/mm2 . Abo ve 1 5 N/mm2 the predicted

compres sive s tr en gth v ar ied a lmos t l in ea rly w ith the actua l

compressive strength.

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4.6.2 IN CASE OF ULTRASONIC PULSE VELOCITY

TESTER:

1. The u lt rason ic pulse velocity o f concrete can be re la ted to i t s

density and modulus of elasticity.

2. It depends upon the materials and mix proportions used in making

concrete as well as the method of p lacing, compacting and curing

of concrete.

3. If the concrete is not compacted thoroughly and having

segregation, cracks or f laws, the pulse velocity wil l be lower as

compare to good concrete, al though the same materials and mix

proportions ar e used.4. The actual value o f the pu lse veloci ty in concre te depends on a

number of parameters, so the criterion for assessing the quality of

concrete on the basis o f pulse veloci ty i s valid to the genera l

extent.

5. The assessment of quality becomes more meaningful and reliable,

when tests are conducted on different parts of the structure, which

ha ve be en bui lt a t t he sa me t ime w it h si mil ar mat er ia ls,

construction practices and supervision and subsequently compared.

6 . The qual ity o f concrete i s usual ly speci f ied in terms of s treng th

and i t i s the re fo re , s omet imes h elpful to u se u lt raso nic p ul se

velocity measurements to give an estimate of strength.

7 . The relationship between ultrasonic pulse velocity and s trength is

affected by a number of factors including age, curing condit ions,

moisture condition, mix proportions, type of aggregate and type of

cement.

8. The assessment of compressive strength of concrete from ultrasonic

p ul se v eloc ity v alues i s n ot accurate b ecau se the cor re la tion

between u l trasonic pulse veloci ty and compressive s t rength o f

con crete i s n ot v ery c lear . Becau se the re a re l arge n umber o f

p arameter s inv olved, which inf lu en ce the p ul se v eloc ity and

compressive strength of concrete to different extents. However, if

detai ls of material and mix proport ions adopted in the part icular

57


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s tructure are available, then es t imate of concrete s trength can be

mad e b y estab li sh in g s ui table cor re la tion b etween the p ul se

velocity and the compressive strength of concrete specimens made

with such materia l and mix p ropor t ions, under environmental

conditions similar to that in the structure.

9 . The es timated s trength may vary from the actual s trength by 20

percent.

10.T he c or re la ti on o bt ai ne d f or a p ar ti cu la r g ra de m ay n ot b e

appl icab le fo r concrete o f another g rade o r made wi th d if ferent

types of material.

11. At some places over plaster in rounded columns USPV gave

no results or indicated that the velocity was out of range. In such a

p lace the rebound value was a lso very low. This p lace gave a

unique sound on striking softly with a hard material like iron which

clear ly indicated a void between the concrete o f p i llar and i t s

plastering.

4.6.3 GENERAL TRENDS IN COLUMNS AND BEAMS:

A general trend was obtained in the columns . The trend was such that

towards the base of the column the tests always showed a higher quality

of concrete i .e. , higher compressive strength. The compressive strength

goes on decreasing as we go up towards the roof

Fig No. 4.14 Variation of Strength with increase in Height of Column

A graph has been plot ted with increas ing height agains t the predicted

compressive s trength . I t is evident from the graph that the compress ive

strength goes on decreasing with increase in height of column.

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The reason for this variation is better compaction at the base. Since all

the weight of the column acts at the base higher compaction is achieved

and a lso better compact ion faci l it ies are avai lab le near the base and

process compaction becomes difficult as we go up.

No such regular trend was observed for beams or slabs.

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CHAPTER-5

CONCLUSION

Considerable engineering judgment i s needed to p roperly evaluate a

measurement. Mis interpretat ion is poss ible when poor contact is made.

For example, in some cases i t may not be poss ible to identify severely

corroded reinforcing bar in poor quality concrete. However, it is possible

to identify poor quality concrete which could be the cause of reinforcing

bar problems. The poor quali ty concrete al lows the ingress of moisture

and oxygen to the reinforcing bars, and hence corrosion occurs. Presently

the s ys tem i s l imited to p en et ra tion d ep th s o f 3 00 mm. Res earch i s

ongoing to develop a system that can penetrate to a depth of 3000mm or

more.

W he n v ar ia ti on i n p ro pe rt ie s o f c on cr et e a ff ec t t he t es t r es ul ts ,

(especial ly in opposite d irect ions) , the use of one method alone would

not be sufficient to study and evaluate the required property. Therefore,

the use of more than one method yields more reliable results .

For example, the increase in mois ture content of concrete increases the

ultrasonic pulse velocity but decreases the rebound number.

Hence, us ing both methods together wil l reduce the errors produced by

using one method alone to evaluate concrete. Attempts have been done to

corre la te rebound number and u lt rason ic pulse velocity to concretes trength. Unfortunately , the equation requires previous knowledge of

concrete constituents in order to obtain reliable and predictable results .

The Schmidt hammer provides an inexpensive, s imple and quick method

of obtaining an indicat ion of concrete s trength , but accuracy of 15 to

20 per cent is poss ible only for specimens cas t cured and tes ted under

cond it ions fo r which calib ra t ion curves have been es tabl ished . The

results are affected by factors such as smoothness of surface, s ize and

shape of specimen, mois ture condit ion of the concrete, type of cement

and coarse aggregate, and extent of carbonation of surface.The pulse velocity method is an ideal too l fo r es tab lishing whether

concrete is uniform. It can be used on both existing structures and those

under construction. Usually, i f large differences in pulse velocity are

found within a structure for no apparent reason, there is strong reason to

presume that defect ive or deteriorated concrete is present . Fair ly good

correlation can be obtained between cube compressive strength and pulse

velocity. These relations enable the strength of structural concrete to be

predicted within 20 per cent, provided the types of aggregate and mix

proportions are constant.

In summary, u l trasonic pulse velocity tes ts have a great potential for

concrete control , part icularly for es tablishing uniformity and detect ing

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cracks or defects . I ts use for predict ing s trength is much more l imited,

owing to the large number of variables affect ing the relat ion between

strength and pulse velocity.

The d ev ia tion b etween actua l r es ul ts and p redicted res ul ts may b e

attributed to the fact that samples from existing structures are cores andthe crushing compress ive cube s trength was obtained by us ing various

corrections introduced in the specifications. Also, measurements were not

accurate and representative when compared to the cubes used to construct

the plots. The use of the combined methods produces results that lie close

to the true values when compared with other methods. The method can be

extended to tes t exis t ing s tructures by taking direct measurements on

concrete elements.

Unlike other work, the research ended with two simple charts that require

no previous knowledge of the consti tuents of the tes ted concrete. The

method presented is s imple, quick, rel iable, and covers wide ranges of

con crete s tr en gths . The metho d can b e eas ily app li ed to con crete

specimens as well as existing concrete structures. The final results were

compared with previous ones from literature and also with actual results

obtained from samples extracted from existing structures.

5.1 Recommendations

Since the advantages of the nondestructive testing are its rapidity and non destructivity,

priority should be given to develop correlation relationship by laboratory specimens. For

future work, emphasis should be put on establishing correlation relationship onspecimens cured and vibrated in different conditions. Different mixes targeting the same

strength should also be considered.

As the age of the specimens has a major influence on the nondestructive parameters,

relationships should be developed to relate reading on samples of different ages.

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REFERENCES

1. B. Haselvader , Non Destructive Testing in Civil Engineering from

Contractors point of view in International symposium in Non Destructive

Concrete in Civil engineering 26-28.08.1995 ,pp(1-3)

2. Carino N.J. (1994), Non Destructive testing of concrete , History &

Challenges ,(pp13-17)

3. Colombo ,S. Forde (2003), Alternative method of AE Data processing (pp17-

21)

4. Estimating in situ strength of concrete D.G. Skramtajev of central

institute of Industrial Building Research, Mascow, 1938, (pp 37-69)

5. Ehsan Mohtagh and Ali Massumi Seismic Assessment of R.C. buildings by

estimation of effective parameters on seismic behavior using non destructive

testing in structural design of tall buildings Vol.5-Dec.2009,(pp 789-793)

6. Handbook on Non Destructive Testing of Concrete (second edition),1976

by V.M. Malhotra and N.J. Carino (pp119-126)

7. IS: 13311, (Part-1)-1992, Non Destructive testing of concrete methods of

test, Ultra Sonic Pulse Velocity.

8. IS:13311, ( Part-2)-1992, Non Destructive testing of concrete methods of

test Rebound Hammer

9. Jeffery Wouter Application of Impact Echo for flaw detection Civil

Engineering Department, Faculty of Engineering, Hashemite University,

Zarqa 13115, Jordan (pp 39-43)

10. J. Mat , Non Destructive testing for assessment of compressive strength of

High Compressive Concrete in civil engineering journal vol.15,issue sept.-

oct. 2003,vol.15,(pp 452-459)

11.Micheal P. Schullar PE, Non-destructive testing and demerge assessment

of structures in Progress of Structural engineerin

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