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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-043
Date: July 2007
Chapter 4. Evaluation
The results presented in this report represent the initial findings of the study. More detailed information will be available at the conclusion of the 96-week test period for the bench-scale tests, especially the chloride content and the degree of corrosion on the bars at the conclusion of the tests, and with the incorporation of the results of on-going ASTM G 109(29) and field tests. The results obtained to date, however, provide a useful comparison of the relative performance of the systems and of the overall performance of the epoxy-coated reinforcement.
As illustrated in figures 38a, 38b, and 39, the corrosion losses on damaged (exposed) areas on epoxy-coated reinforcement are, for the most part, higher but of a similar magnitude to the average corrosion losses exhibited by uncoated conventional reinforcing steel. As discussed earlier, in reference to the bare bar macrocell test results, the relatively higher losses on the damaged areas may result because the losses recorded for uncoated conventional steel represent values that are averaged over the full contact surface, all of which may not be corroding. The superior performance (over the 15-week test period) by the mortar-wrapped macrocell specimens containing epoxy-coated reinforcement bodes well for epoxy-coated bars in the field. The tests indicate that due to the natural variation in chloride concentration within concrete, all damaged areas on epoxy-coated reinforcement will not come in contact with high chloride contents at the same time. If uncoated steel were used in its place, however, a portion of the unprotected steel would be expected to undergo corrosion.
In terms of overall performance, the use of concrete with a lower water-cement ratio provides an advantage for both uncoated and coated reinforcement in uncracked concrete due to its role in delaying penetration of chlorides. The same advantage does not appear to be available in all cases for cracked concrete; in the current study, concrete with a lower water-cement ratio resulted in a lower corrosion rate for uncoated steel, but not for damaged epoxy-coated reinforcement.
As has been observed in other studies,(11) increasing the adhesion between the epoxy coating and the reinforcing steel does not appear to provide an advantage over conventional ECR.
In uncracked concrete, the use of corrosion inhibitors and the use of the primer coating containing calcium nitrite appears to provide added protection for damaged epoxy-coated reinforcement, and in general, the lower the water-cement ratio, the better the protection. The epoxy-coated reinforcement with the primer coating appears to be the most sensitive of the systems incorporating a corrosion inhibitor to the water-cement ratio, performing better when used in concrete with the lower water-cement ratio. The advantages of corrosion inhibitors, however, were lost to varying degrees in cracked concrete, that is, in cases in which chlorides had direct access to the reinforcing steel. To date, conventional epoxy-coated reinforcement (ECR) has performed better than the other systems in cracked concrete.
The test results for the multiple coated (MC) bars indicate that, in cases in which either both layers are penetrated or just the epoxy is penetrated, the zinc coating provides some protection to the underlying steel. This protection, however, is obtained through the sacrificial loss of zinc.
Key points, as yet unknown, but which, ideally, will be determined prior to the conclusion of this study, are the corrosion threshold of the zinc coating relative to that of exposed steel and the ability of the 50-µm (2-mil) coating to substantially delay corrosion loss of the underlying steel reinforcement.
It is useful to consider one other aspect of the corrosion of reinforcing steel when assessing the relative performance of the different systems that are under study. For conventional steel, an average total corrosion loss of 25 µm (0.001 inch) results in the production of a volume of corrosion products that is adequate to cause concrete to crack.(35) This level of corrosion will be attained in 10 to 15 years at the corrosion rates shown in figure 18. In contrast, work by Torres- Acosta and Sagües(36) and analysis by Ji et al.(21) and Gong et al.(37) demonstrates that, for typical damaged areas on epoxy-coated bars, the corrosion loss on the damaged area must be 100 times higher or 2500 µm (0.10 inch) to cause concrete to crack. This 100 to 1 ratio, coupled with the observation that corrosion rates are similar on conventional steel and exposed portions of damaged epoxy-coated reinforcement, indicates that epoxy-coated steel should provide a service life significantly longer than the desirable range of 75 to 100 years, if the service life of a structure is judged based on concrete cracking, as it typically is. In that case, all of the systems tested that incorporate epoxy-coated reinforcement will provide a service life in excess of 75 years. The appropriateness of this conclusion, however, will be further tested as this study is completed and must be tempered by concerns with the potential effects of reductions in the bond strength between the coating and the steel for bars embedded in concrete, which have not, as yet, been addressed in this study.