ACCELERATED QUANTIFICATION OF CRITICAL PARAMETERS
FOR PREDICTING THE SERVICE LIFE AND LIFE CYCLE COSTS OF
CHLORIDE-LADEN REINFORCED CONCRETE STRUCTURES
A Thesis
by
RADHAKRISHNA PILLAI GOPALAKRISHNAN
Submitted to the Office of Graduate Studies of Texas A&M University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
August 2003
Major Subject: Civil Engineering
ACCELERATED QUANTIFICATION OF CRITICAL PARAMETERS
FOR PREDICTING THE SERVICE LIFE AND LIFE CYCLE COSTS OF
CHLORIDE-LADEN REINFORCED CONCRETE STRUCTURES
A Thesis
by
RADHAKRISHNA PILLAI GOPALAKRISHNAN
Submitted to the Office of Graduate Studies of Texas A&M University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Approved as to style and content by:
David Trejo
(Chair of Committee)
Richard B. Griffin (Member)
Joseph M. Bracci (Member)
Paul N. Roschke (Head of Department)
August 2003
Major Subject: Civil Engineering
iii
ABSTRACT
Accelerated Quantification of Critical Parameters for Predicting
the Service Life and Life Cycle Costs of Chloride-Laden Reinforced Concrete Structures.
(August 2003)
Radhakrishna Pillai Gopalakrishnan, B.E., University of Allahabad, Allahabad, India
Chair of Advisory Committee: Dr. David Trejo
The use of corrosion resistant steels (instead of conventional carbon steels) and/or
high performance concrete can increase the overall service life and can reduce the life
cycle cost (LCC) of reinforced concrete (RC) structures exposed to chloride
environments. At present, no accelerated standardized test procedures are available to
directly evaluate critical parameters affecting the service life of RC systems and current
test methods can take years or decades to indirectly evaluate these critical parameters for
high performance construction materials. This prevents the engineers, designers, and
owners from using new high performance materials, especially, the corrosion resistant
steel reinforcement.
This thesis evaluates the Accelerated Chloride Threshold (ACT) test procedure
developed to determine the critical chloride threshold value of uncoated steel
reinforcement embedded in cementitious materials. Using the ACT test procedure, the
critical chloride threshold values of the ASTM A706, ASTM A615, microcomposite,
SS304, and SS316LN reinforcement types were determined to be 0.2 kg/m3 (0.3 lb/yd3),
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0.5 kg/m3 (0.9 lb/yd3), 4.6 kg/m3 (7.7 lb/yd3), 5.0 kg/m3 (8.5 lb/yd3), and 10.8 kg/m3
(18.1 lb/yd3), respectively. Using these values, the time to corrosion initiation of
chloride-laden RC systems can be determined.
The Accelerated Cracking and Spalling Threshold (ACST) test procedure has
been developed to determine the amount of steel corrosion required to cause cracking and
spalling of concrete cover. From preliminary experimental data, the critical cracking and
spalling threshold thickness for a 19 mm (0.75 inch) concrete cover with 0.45, 0.55, and
0.65 water-cement ratios has been determined to be 20.64, 16.85, and 37.46 mils,
respectively. Preliminary results indicate that for a cover depth of 19 mm (0.75 inch) the
critical cracking and spalling threshold value (mils) is equal to
2] 1[ 2.4 (12.5 / ) 11.6 ( / )10 w c w c −− + × − × and can be used to determine the time of corrosion
propagation in chloride-laden RC systems.
A parametric study with different steel reinforcement, water-cement ratios, and
chloride exposure conditions indicated that the use of corrosion resistant steels will
increase the overall service life and can reduce the LCC of RC structures exposed to
severe chloride environments.
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TABLE OF CONTENTS
Page
1 THE INTRODUCTION................................................................................................1
1.1 BACKGROUND..................................................................................................1 1.1.1 Chloride-induced corrosion in concrete: Causes.........................................1 1.1.2 Chloride-induced corrosion in concrete: Remedies ....................................2
1.1.2.1 High performance cementitious materials................................................2 1.1.2.2 High performance steel reinforcement .....................................................3
1.1.3 Critical parameters for service life prediction and life cycle cost analysis .4 1.2 PROBLEM STATEMENT AND RESEARCH OBJECTIVES...........................5 1.3 THESIS ORGANIZATION .................................................................................6
2 BASICS OF ELECTROCHEMICAL CORROSION...................................................9
2.1 INTRODUCTION................................................................................................9 2.2 FORMS OF CORROSION ................................................................................10
2.2.1 General corrosion ......................................................................................10 2.2.2 Localized corrosion ...................................................................................11
2.3 MECHANISMS OF CORROSION ...................................................................11 2.4 THERMODYNAMICS OF CORROSION........................................................14
2.4.1 Electrochemical potential of corrosion reactions ......................................14 2.4.1.1 Activity and Gibbs free energy ..............................................................15 2.4.1.2 The fundamental work-energy relationships..........................................16
2.5 KINETICS OF CORROSION............................................................................20 2.5.1 Corrosion rate ............................................................................................20
2.5.1.1 Average corrosion rate ...........................................................................21 2.5.1.2 Instantaneous corrosion rate...................................................................22
2.6 PROTECTIVE SURFACE BARRIERS ............................................................23
3 MECHANISMS OF CHLORIDE-INDUCED CORROSION IN CONCRETE ........25
3.1 CHLORIDE PENETRATION IN UNCRACKED CONCRETE.......................26 3.1.1 Diffusion of chloride ions in concrete.......................................................27
3.1.1.1 Effect of water-binder ratio ....................................................................28 3.1.1.2 Effect of cement type and supplementary cementitious materials .........29 3.1.1.3 Effect of aggregates................................................................................31 3.1.1.4 Effect of compaction and consolidation.................................................33 3.1.1.5 Effect of initial curing conditions...........................................................34 3.1.1.6 Effect of environmental conditions ........................................................35 3.1.1.7 Effect of chloride exposure conditions and time of exposure ................37
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Page
3.1.2 Mathematical models for diffusion based chloride transport in concrete .39 3.2 CHLORIDE PENETRATION IN CRACKED CONCRETE ............................43 3.3 INITIATION OF CHLORIDE-INDUCED CORROSION................................45
3.3.1.1 The passive film in concrete...................................................................46 3.3.2 Chloride-induced breakdown of passive film ...........................................48 3.3.3 The corrosion reactions after the breakdown of the protective layers ......50 3.3.4 Critical chloride threshold value ...............................................................52 3.3.5 Factors influencing the critical chloride threshold value ..........................58
3.3.5.1 Steel characteristics ................................................................................58 3.3.5.2 Cementitious material and interfacial transition zone characteristics ....60
3.4 PROPAGATION OF CHLORIDE-INDUCED CORROSION .........................61 3.5 CORROSION-INDUCED CRACKING OR SPALLING OF CONCRETE
COVER ..............................................................................................................64 3.5.1 Critical amount of corrosion products resulting in cracking and spalling 67 3.5.2 Cracking and spalling threshold thickness ................................................70
4 SERVICE LIFE AND LIFE CYCLE COST OF RC STRUCTURES EXPOSED TO CHLORIDE ENVIRONMENTS..........................................................................73
4.1 SERVICE LIFE OF RC STRUCTURES............................................................73 4.1.1 Definitions and influencing factors ...........................................................73 4.1.2 Various time phases and prediction of service life ...................................75 4.1.3 The chloride-induced corrosion initiation phase.......................................76 4.1.4 The chloride-induced corrosion propagation phase ..................................79 4.1.5 The repair and rehabilitation phase ...........................................................82 4.1.6 Methodology for predicting service life of RC structures exposed to
chloride environments ...............................................................................83 4.2 LIFE CYCLE COST OF RC STRUCTURES ....................................................85
4.2.1 Definition and factors contributing to the life-cycle cost..........................85 4.2.2 Life cycle cost analysis .............................................................................86
5 CURRENT TEST METHODS TO PREDICT SERVICE-LIFE OF RC STRUCTURES EXPOSED TO CHLORIDE ENVIRONMENTS ............................94
5.1 ACCELERATED METHODS FOR CHLORIDE PENETRATION.................94 5.1.1 Chloride penetration by cyclic wet-dry exposure .....................................94 5.1.2 Electrically accelerated chloride penetration ............................................95
5.2 CORROSION RATE MEASUREMENT BY MASS LOSS TESTS .................97 5.3 ELECTROCHEMICAL METHODS FOR CORROSION MONITORING......98
5.3.1 Half-cell potential measurements..............................................................99 5.3.2 Polarization resistance measurement techniques ....................................101
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Page
5.4 CHEMICAL METHODS FOR CHLORIDE CONTENT ANALYSIS ...........107
6 RESEARCH SIGNIFICANCE .................................................................................110
7 EXPERIMENTAL PROGRAM AND PRELIMINARY TESTS.............................112
7.1 RESEARCH OBJECTIVES.............................................................................112 7.2 THE ACCELERATED CHLORIDE THRESHOLD (ACT) TEST.................112
7.2.1 The ACT test methodology .....................................................................113 7.2.1.1 The general test methodology ..............................................................113 7.2.1.2 The ACT test layout .............................................................................114 7.2.1.3 The accelerated chloride transport system ...........................................116 7.2.1.4 The corrosion initiation detection system ............................................116 7.2.1.5 Quantification of the critical chloride concentration............................118
7.2.2 Evaluation and engineering refinement of the ACT test.........................118 7.2.2.1 Type of potential gradient (voltage) source and electrical timer..........118 7.2.2.2 Steel potential variations due to applied potential gradient .................121 7.2.2.3 Chloride migration rate and pH variations due to applied potential
gradient.................................................................................................126 7.2.2.4 Time to formation of a stable passive film...........................................130 7.2.2.5 Time for attaining a stabilized polarization resistance.........................131 7.2.2.6 Reference electrode, Haber-Lugin probe and Haber-Lugin probe
electrolyte .............................................................................................133 7.2.2.7 Voltage source - distribution box assembly .........................................135 7.2.2.8 Definition of parameters for electrochemical testing ...........................136 7.2.2.9 Mortar dust collection and modified chloride analysis method ...........137
7.2.3 Materials and experimental design: ACT tests .......................................141 7.3 THE ACCELERATED CRACKING AND SPALLING THRESHOLD (CST)
TEST ................................................................................................................146 7.3.1 The general test methodology .................................................................146 7.3.2 The ACST test layout, and procedure .....................................................146 7.3.3 Materials and experimental design: ACST tests .....................................152
8 RESULTS AND DISCUSSIONS .............................................................................156
8.1 CRITICAL CHLORIDE THRESHOLD VALUES .........................................156 8.1.1 ASTM A706 type reinforcement.............................................................158 8.1.2 ASTM A615 type reinforcement.............................................................162 8.1.3 Microcomposite steel reinforcement .......................................................168 8.1.4 Stainless steel 304 reinforcement ............................................................174 8.1.5 Stainless steel 316LN reinforcement.......................................................178 8.1.6 Summary of critical chloride threshold values........................................182
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8.2 THE DURATION OF THE CORROSION INITIATION PHASE..................184 8.3 CRITICAL CRACKING AND SPALLING THRESHOLD THICKNESS.....205 8.4 THE DURATION OF CORROSION PROPAGATION PHASE ....................209 8.5 OVERALL SERVICE LIFE AND LIFE CYCLE COST COMPARISON......211
8.5.1 Overall service life ..................................................................................211 8.5.2 Life cycle cost comparison......................................................................212
9 CONCLUSIONS AND FUTURE RECOMMENDATIONS ...................................219
9.1 RESEARCH CONCLUSIONS ........................................................................219 9.2 RECOMMENDATIONS FOR FUTURE RESEARCH...................................221
REFERENCES................................................................................................................223 APPENDIX A..................................................................................................................241 APPENDIX B ..................................................................................................................257 APPENDIX C ..................................................................................................................266 APPENDIX D ..................................................................................................................279 APPENDIX E ..................................................................................................................281 APPENDIX F ..................................................................................................................283 APPENDIX G ..................................................................................................................296 APPENDIX H ..................................................................................................................298 APPENDIX I ..................................................................................................................300 VITA................................................................................................................................302
1
1 THE INTRODUCTION1
1.1 BACKGROUND
Premature deterioration of reinforced concrete (RC) structures resulting from
exposure to aggressive environments is a serious challenge facing civil engineers,
designers, contractors, and owners. Highway bridges, marine structures, and parking
garages are typical examples of structures facing premature deterioration. The two main
causes of structural damage to RC structures are degradation of the cementitious material
and corrosion of the embedded steel reinforcement. Corrosion of steel reinforcement in
bridge structures has been recognized as the largest overall maintenance cost in the
United States infrastructure. The annual direct cost of corrosion for highway bridges is
estimated to be $8.3 billion (Koch et al. 2001). Life cycle cost analyses estimate that the
indirect cost to the user due to traffic delays and lost productivity is more than 10 times
the direct cost of corrosion (Koch et al. 2001). New technologies, which are yet to be
utilized, may help to reduce this huge economic loss.
1.1.1 Chloride-induced corrosion in concrete: Causes
The most common cause of initiation and propagation of steel reinforcement
corrosion in RC structures is the presence of chlorides, mostly from seawater and deicing
salts (e.g., sodium chloride, calcium chloride, and magnesium chloride). Another
potential source of chlorides are admixtures containing chlorides. Verbeck (1975)
described chloride ions as "a specific and unique destroyer".
This document follows the style and format of ACI Materials Journal.
2
1.1.2 Chloride-induced corrosion in concrete: Remedies
Two of the several strategies to improve the resistance of RC structures against
chloride-induced corrosion are using high performance cementitious materials, and high
performance steel reinforcement. High performance cementitious materials slow the
transport rate of chloride ions towards the steel reinforcement, thereby delaying the onset
of corrosion. High performance steel reinforcement resist corrosion activity by requiring
higher concentrations for activation of corrosion, thereby extending the service life of the
structure.
1.1.2.1 High performance cementitious materials
The use of high performance cementitious materials can improve the resistance of
RC structures against chloride-induced corrosion and the resulting premature structural
failure in two different ways:
• by retarding the rate of chloride ingress in concrete, and • by retarding the rate of corrosion of reinforcement. Retarding the chloride ingress rate in concrete can increase the time required to
attain sufficient chloride concentrations at the reinforcement level to initiate corrosion.
This retardation in chloride transport rate can be achieved by densifying the
microstructure of concrete. Dense microstructures can be achieved by various methods.
Some of these methods include using dense, durable concrete with supplementary
cementitious materials (e.g., fly ash, slag, silica fume, and metakaolin) (Dehghanian and
Arjemandi 1997, Thomas, and Bamforth 1999, Thomas and Matthews 2003). It has been
well documented that chloride ions can penetrate faster through cracked concrete than
3
through uncracked concrete (Wang et al. 1997). Hence, keeping the concrete free of
cracks can delay the chloride ingress rate.
The lower the corrosion reaction rate, the higher will be the time to corrode a
specific amount of steel. Corrosion inhibitors can effectively retard the corrosion
reaction rate by altering the chemical mechanisms in concrete (Trepanier et al. 2001,
Saricimen et al. 2002, Al-Amoudi et al. 2003). The use of corrosion inhibitors can be
effective in reducing the corrosion reaction rate even in concrete with high chloride
contamination levels. Also, some corrosion inhibitors have been reported to retard the
chloride ingress rate resulting in a delayed initiation of corrosion (Kondratova et al.
2003).
Formation of corrosion products cause expansive stresses on the concrete cover.
When these expansive stresses exceed the tensile strength capacity of the concrete,
cracking and spalling of concrete cover occurs. Balabanic et al. (1996) and many others
have reported that a reduction in water-cement ratio will result in an increased effective
tensile strength capacity of concrete cover. It is well documented that increased cover
depth will not only result in longer time requirement for chloride ions to reach the
embedded reinforcement to start corrosion but will also require more corrosion products
to cause cracking and spalling of the concrete cover (Balabanic et al. 1996).
1.1.2.2 High performance steel reinforcement
The steel industry is manufacturing various types of reinforcing steels, each with
unique corrosion and strength performance characteristics. It is well documented that the
high performance steel reinforcement (i.e., steel with improved corrosion resistance) can
4
significantly improve the corrosion resistance of RC structural elements (Trejo et al.
2000). The initial material cost may be higher for corrosion resistant steels when
compared to conventional carbon steels. But, because of improved resistance to chloride-
induced corrosion, some high performance reinforcing steels may be more cost effective,
based on average life cycle costs.
Moreover, for the same repair method, the repair frequency will be less for
corrosion resistant steels than that for conventional carbon steels. This is again attributed
towards the faster corrosion of conventional carbon steels when compared with corrosion
resistant steels. Knudsen et al. (1998) reported that for discount rates below 7% (often
used by bridge designers while selecting rehabilitation strategies), repairs using stainless
steel are more economical than that using conventional carbon steel or cathodic
protection.
Thus, the service life can be increased and life cycle cost (LCC) may be reduced
if high performance cementitious materials as well as high performance steel
reinforcement are used in RC structures exposed to corrosive environments. Key
material parameters to determine the service life and LCC are needed to assist engineers
in selecting optimal strategies for selecting materials.
1.1.3 Critical parameters for service life prediction and life cycle cost analysis
Corrosion of steel reinforcement causes the concrete surface to crack and spall,
resulting in reduced service life times. Three key parameters, the minimum chloride
concentration required to initiate corrosion of steel reinforcement, the amount of steel
reinforcement corrosion required to trigger surface cracking and spalling of the concrete
5
cover, and environmental exposure conditions are needed to predict the service life of RC
structures. Available standard test methods for determining the corrosion characteristics
of steel reinforcement embedded in concrete do not specifically evaluate these key
parameters and can take years or decades to complete, making these methods
uneconomical and impractical. Standardized short-term test methods to evaluate the
corrosion characteristics of steel reinforcement embedded in cementitious materials are
not yet available. This lack of reliable quantitative data makes decision makers hesitant
towards using the new durable corrosion resistant steel reinforcement. Thus there is an
urgent need to develop short-term test methods that provide quantitative data on the
critical chloride threshold value for steel reinforcement embedded in cementitious
materials and the amount of corrosion required to crack or spall the concrete cover,
especially when the steel industry is producing various types of reinforcing steels, each
having unique corrosion performance characteristics. This quantitative data, required to
predict the service life and life cycle cost will assist designers in making better decisions
in selecting cost effective construction materials during the design stage for RC
structures.
1.2 PROBLEM STATEMENT AND RESEARCH OBJECTIVES
The purpose of this research program is to study, using accelerated test
procedures, the influence of steel reinforcement types, water-cement ratios, and cover
depths on the overall serviceability and life-cycle cost of RC structural systems exposed
to chloride environments.
6
Various objectives of this study are:
• to evaluate and perform engineering refinement of the Accelerated Chloride Threshold (ACT) test methodology originally developed by Trejo and Miller (2002),
• to quantitatively determine the critical chloride threshold values of different uncoated steel reinforcement types embedded in a standard cementitious material using the ACT test methodology,
• to develop an Accelerated Cracking and Spalling Threshold (ACST) test method,
• to quantitatively determine the critical cracking and spalling threshold thickness of concrete cover using the ACST test methodology, and
• to study the effect of the quality of both the steel reinforcement and concrete cover on the service life and life cycle cost of RC structures exposed chloride environments.
Recommendations on selecting durable construction materials for reduced life
cycle cost of RC structures will be presented.
1.3 THESIS ORGANIZATION
This thesis includes 9 sections and several subsections. Section 1 introduces the
background to the magnitude of the problems associated with corrosion-induced
deterioration of RC structures exposed to chloride environments. An introduction on
how this premature deterioration and the resulting economic loss can be curbed or
controlled is provided. The urgent need for developing standardized short-term test
methods for efficient, reliable, and quantitative determination of critical parameters for
predicting service life of RC structures exposed to chloride environments is emphasized.
Section 2 is comprised of a brief review of basic principles and mechanisms of
electrochemical corrosion of metals in aqueous solution environment. Thermodynamic
and kinetic principles are discussed.
7
Section 3 provides a comprehensive review of the principles and mechanisms of
chloride-induced corrosion of steel reinforcement embedded in concrete. Mechanisms
such as diffusion based transport of chloride ions in concrete, and the formation and
breakdown of protective layers on the embedded steel reinforcement are presented. A
review of critical chloride threshold values for different steel reinforcement types,
cracking and spalling threshold thickness of for various concrete design parameters and
other issues is provided.
Section 4 presents mathematical models for predicting the service life of RC
structures exposed to chloride environments. A brief review of life cycle cost analysis
models is also provided.
Section 5 presents a discussion on different electrical, electrochemical and
chemical test methodologies available for determining critical service life parameters of
RC structures exposed chloride environments.
Section 6 emphasizes the significance and necessity for the development of short-
term test methodologies required to determine the critical chloride threshold level of steel
reinforcement and critical cracking and spalling threshold thickness for concrete cover.
The quantitative information on these parameters can be used for the prediction of service
life and life cycle cost of RC structures exposed to chloride environments.
Section 7 presents the experimental program followed in this thesis for
determining the critical chloride threshold values and cracking and spalling threshold
thickness values of uncoated steel reinforcement embedded in cementitious materials.
This section also includes a description and evaluation of the new accelerated test
8
methods used in the experimental program to evaluate the corrosion performance of steel
in cementitious materials.
Section 8 provides a detailed discussion on the results of the testing program.
These results include the critical chloride threshold values and cracking and spalling
threshold thickness values obtained from the experimental programs explained in section
7. Finally a parametric study on the service life and life cycle costs of RC structures with
different construction materials are provided.
Section 9 provides conclusions and recommendations for future research.
110
6 RESEARCH SIGNIFICANCE
Owners, designers, material producers, and contractors are considering the
potential use of building materials that minimize corrosion of the reinforcement,
maximizes service-life, and optimizes life-cycle costs. Both mineral and chemical
admixtures in the concrete can delay the onset of corrosion and have proven to be an
effective approach in minimizing the impact of corrosion (Maslehuddin et al. 1987,
Thomas and Matthews 1993, and Ozyildirim 1994). In addition, several reinforcing
steels have been developed to resist corrosion when embedded in concrete and exposed to
chlorides and other aggressive chemicals. The implementation of these corrosion
resistant steel reinforcement products has been relatively limited due to lack of specific
quantitative data on the corrosion performance and lack of information on the cost
justification and benefits of these products. In addition, realistic corrosion testing in
cementitious materials often takes several years to evaluate, thereby slowing the
implementation of these products. Therefore, simple, short-term procedures are needed
to evaluate the performance of the steel reinforcement embedded in cementitious
materials.
To economically justify the use of materials that enhance the corrosion resistance
of RC structures, life-cycle cost comparisons are needed. To perform life-cycle cost
analyses, the service-life must be estimated. To evaluate the service-life and life-cycle
costs of RC structures susceptible to corrosion, quantitative measures of key material
characteristics and parameters must be known. For chloride-induced corrosion, key
material characteristics include the transport rate of the chloride ions (i.e., diffusivity,
111
sorptivity, etc.) in the cementitious material, the critical chloride threshold level of the
steel reinforcement in the cementitious material, the corrosion rate of the steel
reinforcement, and the critical cracking and spalling threshold thickness for the concrete
cover.
Critical chloride threshold values for conventional steel reinforcement types have
been reported throughout the literature, but no standardized short-term method for
evaluating this parameter is currently available. There is only limited information
available on critical cracking and spalling threshold thickness for the concrete cover. No
standardized short-term method for evaluating this parameter is currently available.
Hence, development of standardized short-term test methodologies are necessary for the
determination of:
• the critical chloride threshold of steel reinforcement embedded in concrete, and
• the critical cracking and spalling threshold thickness for the concrete cover.
Quantification of these parameters can be used to better predict the service life
and life cycle costs of RC structures exposed to chloride environments. This information
will assist owners, designers, and contractors to reliably implement the use of newer,
more durable construction materials that can increase the service life and long term cost
effectiveness of RC structures.
219
9 CONCLUSIONS AND FUTURE RECOMMENDATIONS
9.1 RESEARCH CONCLUSIONS
Using new, accelerated test procedures, this thesis evaluated the critical chloride
threshold value of 5 reinforcing steel types and the amount of corrosion products
required to crack and spall concrete covers. The results from the investigations indicate
that the critical chloride threshold value and the critical cracking and spalling threshold
thickness can be determined for various steel reinforcing types and concrete covers over
relatively short test durations. But, because the test methods are new, some
modifications to these methods are also recommended.
The ACT test procedure developed by Trejo and Miller (2002) has been
evaluated and engineering refinements have been made to quantitatively determine the
critical chloride threshold values of uncoated steel reinforcement embedded in
cementitious materials. The following conclusions are drawn from the ACT test
program and related studies.
• The critical chloride threshold value of ASTM A706 reinforcement is lower than that of ASTM A615 steel reinforcement. Hence, the ASTM A706 steel reinforcement can corrode at earlier times than the ASTM A615 steel reinforcement if exposed to similar chloride exposure conditions.
• The microcomposite, SS304, and SS316LN reinforcement types have higher critical chloride threshold values than both the ASTM A615 and ASTM A706 steel reinforcement types. The SS304 steel reinforcement exhibits slightly higher critical chloride threshold value than the microcomposite steel reinforcement.
• The SS316LN reinforcement exhibits the highest critical chloride threshold value and thereby the best corrosion resistance characteristics than the ASTM A706, ASTM A615, microcomposite, and SS304 reinforcement types.
220
• The complete removal of the mill scale and surface finishing from the ASTM A615 or SS316LN steel reinforcement types did not increase the critical chloride threshold value.
• The complete removal of the mill scale and surface finish of the ASTM A706, microcomposite and SS304 steel reinforcement types did increase the critical chloride threshold value.
• The time to corrosion initiation increases as the diffusion coefficient, the water-cement ratio, and the rate of chloride buildup at the concrete surface decreases, and as the critical chloride threshold value of the embedded steel reinforcement increases.
• In general, the longest time to corrosion initiation was exhibited by SS316LN reinforcement followed by SS304, microcomposite, ASTM A615, and ASTM A706.
The accelerated cracking and spalling threshold (ACST) test has been developed
to determine the cracking and spalling threshold thickness for concrete covers with
different design parameters. The following conclusions are made from the preliminary
results from the ACST test program and related studies.
• Larger amount of corrosion products are required to crack concrete with a 0.65 water-cement ratio than that required by the concrete with a 0.45 water-cement ratio.
• Concrete with 0.55 water cement ratio required less corrosion products to cause cracking than that required by concretes with both 0.45 and 0.65 water-cement ratios.
• The corrosion-induced cracking of concrete cover depends not only on the concrete strength but also on the interconnectivity of pores in concrete, which increases with decreasing water-cement ratios.
• There exists a relationship between water-cement ratio and the critical cracking and spalling threshold thickness.
The service life and life cycle cost analysis of RC structures using different steel
reinforcement types indicate that, for the assumptions in the analysis, in low chloride
exposure conditions and low water-cement ratios the decks with the conventional steels
are more cost effective, especially at higher discount rates, than the decks with the
corrosion resistant steels. As the severity of chloride exposure increases, the use of
221
corrosion resistant steels tend to be more cost effective than the use of conventional
steels.
9.2 RECOMMENDATIONS FOR FUTURE RESEARCH
For improvement of the ACT test procedure, the following recommendations are
made:
• more accurate measurement of the ohmic drop between the reference electrode and the steel surface during the polarization resistance measurements should be implemented.
• modify the geometry of the ACT specimen by reducing the height of mortar column below the working electrode level to minimize the use of cementitious material.
• use shielded wires for making all the electrical connections (to reduce noise during the electrochemical measurements.
• implement the use of a Faraday cage or other systems, if needed, to minimize the noise during the electrochemical measurements.
• investigate the effect of lateral distance between the anode and circular edge of the steel sample on the induced overvoltage during the application of external potential gradient.
• Ensure that the top of the embedded steel sample (WE) is exactly at the level of the embedded anode mesh disk in the ACT specimen. More research is needed to further optimize the relative position of the steel specimen surface with reference to the anode mesh disk.
To obtain more information on the critical chloride threshold value of various
steel reinforcement in various environmental conditions, the following
recommendations are made:
• perform the ACT testing and determine critical chloride threshold values of steel reinforcement embedded in mortar with different supplementary cementitious materials and mixture proportions.
• perform more research to investigate the effect of mill scale on the chloride-induced corrosion characteristics of steel reinforcement.
The time to corrosion initiation values calculated using Life-365 (2000) software
and the SRC method show significant differences as the range of time to corrosion
222
initiation values increases. A simple mathematical equation is needed to better simulate
the chloride buildup rate at the concrete surface and to predict the time of corrosion
initiation.
To validate the preliminary ACST test procedure, the following issues need to
be addressed:
• perform more ACST tests with different water-cement ratios and cover depths.
• perform more ACST tests with concretes with different supplementary cementitious materials and mixture proportions.
• perform ACST tests with different steel reinforcement types because the volume of corrosion products may vary from one type of steel reinforcement to another.
• perform ACST tests with steel reinforcements with different diameters.