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),

iv

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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Page

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.