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Publication Number: FHWA-HRT-04-090
Date: JUNE 2004

Long-Term Performance of Epoxy-Coated Reinforcing Steel in Heavy Salt-Contaminated Concrete

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FOREWORD

This report describes results obtained from a long-term natural weathering exposure testing of the remaining 31 post-Southern Exposure (SE) test slabs that contained epoxy-coated reinforcing bar (ECR), black bars, and stainless steel bars and were not autopsied during the 1993–1998 Federal Highway Administration (FHWA) research project. The test slabs had been exposed to the very aggressive SE test, which involved alternating wetting with 15 weight percent NaCl solution and air drying cycles for 96 weeks. The test results confirmed that the black bars produced the highest mean macrocell current density (least corrosion resistant) among various combinations of test variables regardless of slab configuration and the stainless steel bars exhibited negligible mean macrocell current density. The performance of straight top-mat ECRs varied from 7 to 40 percent of the highest black bar case, as measured by macrocell current density, depending on the size of the initial coating damage and the type of bar in the bottom mat. ECR used in the top mat alone reduced the corrosion susceptibility to at least 50 percent of the black bar case, even when it contained coating damage and was connected to the black bar bottom mat. If straight ECRs in the top mat were connected to ECRs in the bottom mat, the mean macrocell current density was no greater than two percent of the highest black bar case and approached the corrosion resistant level of stainless steel reinforcement. Autopsy of ECR slabs exhibiting negligible macrocell current density revealed excellent condition with no sign of corrosion. However, ECRs exhibiting high macrocell current densities showed coating deterioration and exhibited numerous hairline cracks and/or blisters in conjunction with reduced adhesion, coating disbondment (permanent loss of adhesion) and underlying steel corrosion. The test results of present and the earlier FHWA studies indicate that adhesion appeared to be a poor indicator of long-term performance of the coated bars in chloride contaminated concrete and it is concluded that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion.

This report will be of interest to materials and bridge engineers, reinforcing-concrete corrosion specialists, reinforcing bar manufactures, producers of organic coatings, and manufacturers of stainless steel.

T. Paul Teng, P.E.
Director, Office of Infrastructure
  Research and Development

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for its content or use thereof. This report does not constitute a standard, specification, or regulation.

The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official policy of the U.S. Department of Transportation.

The U.S. Government does not endorse products or manufacturers. Trade and manufacturers' names may appear in this report only because they are considered essential to the object of the document.

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

 

1. Report No

FHWA-HRT-04-090

2. Government Accession No.

N/A

3. Recipient's Catalog No.

N/A

4. Title and Subtitle

Long-Term Performance of Epoxy-Coated Reinforcing Steel in Heavy Salt-Contaminated Concrete

5. Report Date

June 2004

6. Performing Organization Code

N/A

7. Authors(s)

Seung-Kyoung Lee, Paul D. Krauss

8. Performing Organization Report No.

N/A

9. Performing Organization Name and Address

WISS, Janney, Elstner Associates, Inc.
330 Pfingsten Road
Northbrook, IL 60062-2095

10. Work Unit No. (TRAIS)

N/A

11. Contract or Grant No.

DTFH61-93-C-00027

12. Sponsoring Agency Name and Address

 Office of Infrastructure R&D
Federal Highway Administration
6300 Georgetown Pike
McLean VA 22101-2296

13. Type of Report and Period Covered

Final Report
September 1998 to December 2002

14. Sponsoring Agency Code
15. Supplementary Notes

Contracting Officer's Technical Representative (COTR): Yash Paul Virmani, HRDI-10

Acknowledgements: Leo Zegler, John Drakeford, Gregory Hedien, Steve Harris, Steve Zimmerman

16. Abstract

This report describes long-term natural weathering exposure testing of the remaining 31 post-Southern Exposure (SE) test slabs that were not autopsied during the 1993–1998 Federal Highway Administration (FHWA) research project. The samples were exposed from September 1998 to December 2002 at an outdoor test yard in Northbrook, IL. The 1993–1998 research program involved testing more than 52 different bar materials and, consequently, 12 different bar types were selected for long-term durability tests in concrete exposed to the very aggressive SE test, which involved alternating wetting with 15 weight percent NaCl solution and drying cycles for 96 weeks. Periodic macrocell corrosion current between top and bottom mats and short-circuit potential data were collected during the exposure test program. Upon termination of the test program, autopsy and subsequent laboratory analysis was performed on the test slabs.

The test results confirmed that the black bars produced the highest mean macrocell current density (least corrosion resistant) among various combinations of test variables regardless of slab configuration, and that the stainless steel bars exhibited negligible mean macrocell current density. In general, bent epoxy-coated reinforcing bar (ECR) in the top mat, coupled with black bars in the bottom mat, performed the worst among all ECR cases. The straight top-mat ECRs' macrocell current density varied from 7 to 40 percent of the highest black bar case, depending on the size of initial coating damage and type of bar in the bottom mat. ECR used in the top mat alone reduced the corrosion susceptibility to at least 50 percent of the black bar case, even when it contained coating damage and was connected to the black bar bottom mat. In contrast, if straight ECRs in the top mat were connected to ECRs in the bottom mat, the mean macrocell current density was no greater than 2 percent of the highest black bar case even when rebar coatings had defects, and approach the corrosion resistant level of stainless steel reinforcement. Such improved corrosion resistance can be attributed to (1) reduction in cathodic area; (2) higher electrical resistance; and (3) reduced cathodic reaction.

Whenever an ECR slab with negligible macrocell current density was autopsied, the appearance of the extracted ECR and concrete/bar interface was excellent with no sign of corrosion. However, when ECRs specimens with high macrocell current densities were autopsied, they revealed coating deterioration due to corrosion and exhibited numerous hairline cracks and/or blisters in conjunction with reduced adhesion, coating disbondment (permanent adhesion loss), and underlying steel corrosion. No consistent trend was found between the level of macrocell current density and the extent of coating adhesion loss. The present test results and the earlier FHWA studies indicate that adhesion appeared to be a poor indicator of long-term performance of the coated bars in chloride contaminated concrete; it is concluded that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion.

17. Key Words

concrete, corrosion, durability, electrochemical impedance spectroscopy, black bar, stainless steel, epoxy-coated reinforcing steel, mat-to-mat resistance, macrocell current, corrosion rate, chloride

18. Distribution Statement

No restrictions. This document is available to the Public through the National Technical Information Service; Springfield, VA 22161

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

132

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized (art. 5/94)

SI* (Modern Metric) Conversion Factors


TABLE OF CONTENTS

EXECUTIVE SUMMARY

CHAPTER 1. INTRODUCTION AND PROJECT HISTORY

CHAPTER 2. EXPERIMENTAL METHOD

Test Specimens
Data Collection
Autopsy
Chloride Analysis

CHAPTER 3. RESULTS AND DISCUSSION

Short-Circuit Potential and Macrocell Current Density
Statistical Analysis of Test Data
Autopsy Results
Chloride Analysis

CHAPTER 4. CONCLUSIONS

REFERENCES

APPENDIX A. PHOTOGRAPHS OF TEST SLABS AT END OF OUTDOOR EXPOSURE AND AUTOPSIED BARS


LIST OF FIGURES

Figure 1. Configuration of test slab
Figure 2. Estimated chloride accumulation at 25.4 mm (1.0 inch) depth with time in the concrete test slab
Figure 3. Average macrocell current data of eight reinforcing bar materials for 96-week SE tests
Figure 4. Average mat-to-mat AC resistance data of eight reinforcing bar materials for 96-week SE tests
Figure 5. Short-circuit potential change with time (straight top (black and ECR)-black bottom-uncracked concrete) during outdoor exposure
Figure 6. Macrocell current density change with time (straight top (black and ECR)-black bottom-uncracked concrete) during outdoor exposure
Figure 7. Short-circuit potential change with time (straight top (black and ECR)-black bottom-cracked concrete) during outdoor exposure
Figure 8. Macrocell current density change with time (straight top (black and ECR)-black bottom-cracked concrete) during outdoor exposure
Figure 9. Short-circuit potential change with time (bent top (black and ECR)-black bottom-uncracked concrete) during outdoor exposure
Figure 10. Macrocell current density change with time (bent top (black and ECR)-black bottom-uncracked concrete) during outdoor exposure
Figure 11. Short-circuit potential change with time (stainless steel and ECR in both mats-uncracked concrete)
Figure 12. Macrocell current density change with time (stainless steel and ECR in both mats-uncracked concrete)
Figure 13. Mean short-circuit potential change with time (uncracked vs. precracked concrete)
Figure 14. Mean macrocell current density change with time (uncracked vs. precracked concrete)
Figure 15. Mean short-circuit potential change with time (straight vs. bent ECRs in uncracked concrete)
Figure 16. Mean macrocell current density change with time (straight vs. bent ECRs in uncracked concrete)
Figure 17. Mean short-circuit potential data classified by bar type (from table 9)
Figure 18. Mean macrocell current density data classified by bar type (from table 9)
Figure 19. Relative ratio of macrocell current density per slab configuration
Figure 20. Relationship between macrocell current density versus initial artificial coating defects
Figure 21. Short-circuit potential data classified by coating type
Figure 22(a). Macrocell current density data classified by coating type (linear scale)
Figure 22(b). Macrocell current density data classified by coating type (logarithmic scale)
Figure 23. Ninety five percent confidence intervals for short-circuit potential data
Figure 24. Ninety five percent confidence intervals for macrocell current density data
Figure 25. Ninety five percent confidence intervals for AC resistance data
Figure 26. Ninety five percent confidence intervals for impedance modulus data
Figure 27. Cutting a test slab with a gas-powered saw
Figure 28. Extraction of embedded reinforcing bars
Figure 29. Typical condition of ECR with good corrosion resistance (slab #7—top right bar)
Figure 30. Typical condition of ECR with poor performance (slab #30—top right bar)
Figure 31. Typical condition of a bent ECR with good performance (slab #1—top left bar)
Figure 32. Typical condition of a bent ECR with poor performance
Figure 33. Closeup views of ECRs exhibiting various conditions: (a) an intact ECR; (b) an ECR containing hairline coating cracks; (c) an ECR containing coating blisters and hairline coating cracks; and (d) a delaminated ECR revealing severely corroded substrate
Figure 34. Typical condition of black bars in the top mat
Figure 35. Typical condition of black bent bars in the top mat (slab #23—top right bar)
Figure 36. Corroded bottom mat (slab #19)
Figure 37. Photograph of autopsied bars extracted from slab #18 (ECR top-black bar bottom)
Figure 38. Photograph of autopsied bars extracted from slab #10 (ECRs in both mats)
Figure 39. Ninety five percent confidence intervals for number of final defects
Figure 40. Ninety five percent confidence intervals for ECR rating data
Figure 41. Ninety five percent confidence intervals for knife adhesion data
Figure 42. Ninety five percent confidence intervals for extent of disbondment data
Figure 43. Relationship between water-soluble versus acid-soluble chloride data
Figure 44. Ninety five percent confidence intervals for water-soluble chloride data at top bar depth
Figure 45. Slab #1 front, rear, and top views with specifications
Figure 46. Slab #1 extracted rebars condition
Figure 47. Slab #1 after autopsy
Figure 48. Slab #2 front, rear, and top views with specifications
Figure 49. Slab #2 extracted rebars condition
Figure 50. Slab #2 after autopsy
Figure 51. Slab #3 front, rear, and top views with specifications
Figure 52. Slab #3 extracted rebars condition
Figure 53. Slab #3 after autopsy
Figure 54. Slab #4 front, rear, and top views with specifications
Figure 55. Slab #4 extracted rebars condition
Figure 56. Slab #4 after autopsy
Figure 57. Slab #5 front, rear, and top views with specifications
Figure 58. Slab #5 extracted rebars condition
Figure 59. Slab #5 after autopsy
Figure 60. Slab #6 front, rear, and top views with specifications
Figure 61. Slab #6 extracted rebars condition
Figure 62. Slab #6 after autopsy
Figure 63. Slab #7 front, rear, and top views with specifications
Figure 64. Slab #7 extracted rebars condition
Figure 65. Slab #7 after autopsy
Figure 66. Slab #8 front, rear, and top views with specifications
Figure 67. Slab #8 extracted rebars condition
Figure 68. Slab #8 after autopsy
Figure 69. Slab #9 front, rear, and top views with specifications
Figure 70. Slab #9 extracted rebars condition
Figure 71. Slab #9 after autopsy
Figure 72. Slab #10 front, rear, and top views with specifications
Figure 73. Slab #10 extracted rebars condition
Figure 74. Slab #10 after autopsy
Figure 75. Slab #11 front, rear, and top views with specifications
Figure 76. Slab #11 extracted rebars condition
Figure 77. Slab #11 after autopsy
Figure 78. Slab #12 front, rear, and top views with specifications
Figure 79. Slab #12 extracted rebars condition
Figure 80. Slab #12 after autopsy
Figure 81. Slab #13 front, rear, and top views with specifications
Figure 82. Slab #13 extracted rebars condition
Figure 83. Slab #13 after autopsy
Figure 84. Slab #14 front, rear, and top views with specifications
Figure 85. Slab #14 extracted rebars condition
Figure 86. Slab #14 after autopsy
Figure 87. Slab #15 front, rear, and top views with specifications
Figure 88. Slab #15 extracted rebars condition
Figure 89. Slab #15 after autopsy
Figure 90. Slab #16 front, rear, and top views with specifications
Figure 91. Slab #16 extracted rebars condition
Figure 92. Slab #16 after autopsy
Figure 93. Slab #17 front, rear, and top views with specifications
Figure 94. Slab #17 extracted rebars condition
Figure 95. Slab #17 after autopsy
Figure 96. Slab #18 front, rear, and top views with specifications
Figure 97. Slab #18 extracted rebars condition
Figure 98. Slab #18 after autopsy
Figure 99. Slab #19 front, rear, and top views with specifications
Figure 100. Slab #19 extracted rebars condition
Figure 101. Slab #19 after autopsy
Figure 102. Slab #20 front, rear, and top views with specifications
Figure 103. Slab #20 extracted rebars condition
Figure 104. Slab #20 after autopsy
Figure 105. Slab #21 front, rear, and top views with specifications
Figure 106. Slab #21 extracted rebars condition
Figure 107. Slab #21 after autopsy
Figure 108. Slab #22 front, rear, and top views with specifications
Figure 109. Slab #22 extracted rebars condition
Figure 110. Slab #22 after autopsy
Figure 111. Slab #23 front, rear, and top views with specifications
Figure 112. Slab #23 extracted rebars condition
Figure 113. Slab #23 after autopsy
Figure 114. Slab #24 front, rear, and top views with specifications
Figure 115. Slab #24 extracted rebars condition
Figure 116. Slab #24 after autopsy
Figure 117. Slab #25 front, rear, and top views with specifications
Figure 118. Slab #25 extracted rebars condition
Figure 119. Slab #25 after autopsy
Figure 120. Slab #26 front, rear, and top views with specifications
Figure 121. Slab #26 extracted rebars condition
Figure 122. Slab #26 after autopsy
Figure 123. Slab #27 front, rear, and top views with specifications
Figure 124. Slab #27 extracted rebars condition
Figure 125. Slab #27 after autopsy
Figure 126. Slab #28 front, rear, and top views with specifications
Figure 127. Slab #28 extracted rebars condition
Figure 128. Slab #28 after autopsy
Figure 129. Slab #29 front, rear, and top views with specifications
Figure 130. Slab #29 extracted rebars condition
Figure 131. Slab #29 after autopsy
Figure 132. Slab #30 front, rear, and top views with specifications
Figure 133. Slab #30 extracted rebars condition
Figure 134. Slab #30 after autopsy
Figure 135. Slab #31 front, rear, and top views with specifications
Figure 136. Slab #31 extracted rebars condition
Figure 137. Slab #31 after autopsy

LIST OF TABLES

Table 1. Mix properties of concrete used
Table 2. Acid-soluble chloride concentrations (percent by weight of sample)
Table 3. Summary of corrosion performance of different reinforcing bar materials under 96-week SE testing[4]
Table 4. Summary of in-concrete test results per coating type[4]
Table 5. Summary of test slabs exposed to outdoor weathering
Table 6. Electrochemical and chloride data for test slabs containing ECR in top mat only
Table 7. Electrochemical and chloride data for test slabs containing ECR in both mats
Table 8. Electrochemical and chloride data for test slabs containing black and stainless steel bars
Table 9. Electrochemical test data classified by bar type
Table 10. Electrochemical test data classified by coating type
Table 11. Characterization of autopsied ECRs tested in the top mat
Table 12. Characterization of autopsied ECRs tested in both mats

 


EXECUTIVE SUMMARY

In May 1993, the Federal Highway Administration (FHWA) began a 5-year research project, Corrosion Resistant Reinforcing for Concrete Components. The objective of the study was to develop cost-effective new breeds of organic, inorganic, ceramic, and metallic coatings, as well as metallic alloys that could be utilized on or as reinforcement for embedment in portland cement concrete and ensure a corrosion-free design life of 75 to 100 years when exposed to adverse environments. The 1993–1998 research program involved testing more than 52 different organic, inorganic, ceramic, and metallic coatings on steel bars, as well as solid metallic bars. Specifically, these included epoxy-coated, other polymer-coated, ceramic-coated, galvanized-clad, epoxy-coated galvanized-clad, stainless steel-clad, nickel-clad, copper-clad, corrosion resistance alloy-clad, inorganic silicate-clad, solid corrosion-resistance alloy steel, solid aluminum-bronze, solid stainless steels, and solid titanium reinforcing bars. Consequently, 12 different bar types were selected for long-term durability tests in concrete exposed to the very aggressive Southern Exposure (SE) testing, which involved alternating wetting with 15 weight percent NaCl solution and drying cycles for 96 weeks. About 150 test slabs were fabricated for the selected 12 different bar types.

After the conclusion of the 96-week SE testing in 1998, 31 post-SE test slabs that were not autopsied were then exposed to a long-term natural weathering at an outdoor test yard in Northbrook, IL, from September 1998 to December 2002. Periodic macrocell corrosion current between two mats and short-circuit potential of top mat bars (while they were connected to the bottom mat bars) data were collected during the exposure test program. When the test program ended after about 7 years, autopsy and subsequent laboratory analysis was performed with the test slabs, and the results are reported here. The tests include chlorides in the concrete, condition evaluation at bar/concrete interface, and visual examination of extracted bars.

Macrocell current density was a good indicator of corrosion performance of the various reinforcements. The black bars produced the highest mean macrocell current density (least corrosion resistant) among various combinations of test variables regardless of slab configuration. The stainless steel bars exhibited negligible mean macrocell current density. Whenever an epoxy-coated reinforcing bar (ECR) slab with negligible macrocell current density was autopsied, the appearance of the extracted ECR and concrete/bar interface was excellent with no sign of corrosion, and the coating looked new with a glossy texture. However, when ECRs slabs with a high macrocell current density were autopsied, they revealed coating deterioration due to corrosion and exhibited numerous hairline cracks and/or blisters in conjunction with reduction in adhesion, coating disbondment (permanent loss of adhesion), and underlying steel corrosion. Generally, the number of final coating defects on the autopsied ECRs increased from their initial values determined before embedment in concrete. There was no consistent trend found between the level of macrocell current density and the extent of coating adhesion loss. The earlier FHWA studies investigated the adhesion of the coatings using accelerated solution immersion tests and cathodic disbonding tests. Based on the review of the test results, the adhesion, as tested by solution immersion and cathodic disbonding tests, appeared to be a poor indicator of long-term performance of the coated bars in chloride contaminated concrete after 96-week SE. These findings led researchers to conclude that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion.

In general, bent ECRs in the top mat coupled with black bars in the bottom mat performed the worst among all ECR cases. For straight top-mat ECRs, the mean macrocell current density was influenced by the size of initial coating damage and type of bar in the bottom mat. Their performance varied from 7 to 40 percent of the highest black bar case as measured by macrocell current density. However, if straight ECRs in the top mat were connected to ECRs in the bottom mat, the mean macrocell current density was no greater than 2 percent of the highest black bar case, regardless of the initial coating defect size. Both mat ECR systems behaved comparable to stainless steel bars. According to impedance modulus, alternating current (AC) resistance, macrocell current density data, and autopsy results, the excellent performance of test slabs containing ECRs in both mats should be attributed to the facts that electrical resistance was very high between the two mats, and the ECRs in the bottom mat suppressed the corrosion activity by minimizing the area for the cathodic reaction (oxygen reduction).

The 2-year saltwater ponding with alternate wetting, heating, and drying, followed by 5-year outdoor weathering, confirmed that use of ECRs in the top mat only (uncoated bottom mat) reduced the corrosion susceptibility to at least 50 percent of the black bar case, even when the coating has damage. Hence, ECR used in the top mat alone would not provide optimum corrosion protection. If ECRs are used in both mats in uncracked concrete, corrosion resistance increases dramatically, even when the rebar coatings have defects. Such improved corrosion resistance can be attributed to a (1) reduction in cathodic area; (2) higher electrical resistance; and (3) reduced cathodic reaction.

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