U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-04-090
The black bars produced the highest mean macrocell current density among various combinations of test variables regardless of slab configuration, i.e., presence of crack and bar shape (bent vs. straight). The highest mean macrocell current density (1.3 A/cm2 (8.4 A/in.2)) was obtained from the black bent bar coupled with black bottom bars in uncracked concrete. In contrast, the stainless steel bars exhibited negligible mean macrocell current density, which was only 1 percent of the highest black bar case. Autopsy revealed that the corrosion morphology of black and stainless steel bars was consistent with the macrocell current density data. Macrocell current density was a good performance indicator of test slabs.
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. When they were coupled with black bars in the bottom mat, the size of the coating defect became a critical factor for controlling macrocell current density. In the case of straight top mat ECRs containing 0.004 percent of initial coating defect coupled with black bottom bars, the mean macrocell current density was 7 to 21 percent of the highest black bar case, depending on whether the slab was precracked. If straight top mat ECRs containing 0.5 percent coating damage were connected to the black bottom bars, the current values increased to more than 40 percent of the black bar value, regardless of precracks in the concrete.
However, if straight ECRs in the top mat were connected to ECRs in the bottom mat in uncracked concrete, the mean macrocell current density decreased to no greater than 2 percent of the highest black bar case, regardless of the initial coating defect size. They behaved comparable to stainless steel bars.
For bent ECRs, even ones containing 0.004 percent coating damage produced noticeable macrocell current density when they were connected to black bottom bars, such that mean macrocell current density increased to 38 and 49 percent of the highest black bar case, regardless of initial coating defect size. In general, bent ECRs coupled with black bottom bars performed the worst of the ECR cases.
Mean macrocell current density varied significantly by coating type; this could be related to coating quality. However, when ECRs were used in both mats, such variation disappeared, and all bars behaved very well with very low mean macrocell current density.
The SCP data obtained during this test program exhibited large data scatter with time, and no consistent trends developed that could lead to compare the corrosion performance of different bar types and shapes.
Whenever an ECR slab with negligible macrocell current density was autopsied, the appearance of the ECR and concrete/bar interface was excellent with no sign of corrosion, and the coating looked new with a glossy texture. However, when severely corroded ECRs recognized by high macrocell current densities were autopsied, they revealed coating deterioration due to corrosion and exhibited numerous hairline cracks and/or blisters in conjunction with extensive coating disbondment and underlying steel corrosion. When test slabs exhibited severe damage, electrochemical test data collected from those slabs did not provide meaningful results.
Final defects were classified as bare area, mashed area (mechanical damage), coating crack, and holiday. Generally, the number of final coating defects on the autopsied ECRs increased from their initial values determined before embedment in concrete. This phenomenon was particularly pronounced for the poorly performing bars due to development of coating cracks, which was the most frequent form of coating deterioration. Accumulation of multiple rust layers beneath disbonded coating is also a common corrosion morphology observed on severely corroded ECRs.
Reduced adhesion was usually initiated at the initial coating defects. It was much more pronounced on the top mat ECRs, irrespective of whether they were removed from ECR top-black bottom slabs or the ECR top-ECR bottom slabs, compared to those extracted from the bottom mat. The ECRs removed from the bottom mat also exhibited the lowest number of final defects and the strongest knife adhesion. No consistent trend was found between the level of macrocell current density and the extent of adhesion loss. Earlier FHWA studies investigated the coatings' adhesion using solution immersion tests and cathodic disbonding tests.[2,4] 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 concrete. These findings suggest that there is no direct relationship between loss of adhesion and the effectiveness of ECR to mitigate corrosion.
According to impedance modulus, AC resistance, macrocell current density data, and autopsy results, the excellent performance of test slabs containing ECRs in both mats comparable to stainless steel bars may be attributed to the facts that electrical resistance was very high between the two ECR mats, and the ECRs in the bottom mat suppress the corrosion activity at the top mat ECR by minimizing the size of the available cathode.
This 7-year laboratory and outdoor exposure study confirmed that using ECRs in the top mat of simulated bridge decks can reduce the corrosion susceptibility to at least 50 percent of the black bar case, even when ECR with poor coating condition is used. If ECRs are used in both mats in uncracked concrete, this corrosion resistance can increase dramatically, and can approach the corrosion-resistant level of stainless steel reinforcement. This conclusion is valid even when they contain coating defects.