The objective of this project was to evaluate thermal zinc diffusion (TZD) coatings for improved corrosion resistance for concrete reinforcing bar as compared with commercial coatings currently in use. Corrosion performance was determined, using standard ASTM test methods, in good quality concrete [lower water-to-cement ratio (w/c) and higher cover] cracked in flexure with periodic loading to abrade the coatings, in order to capture the two predominant failure mechanisms for coated steels. Both straight and bent bars were evaluated, as well as another set of U-bend specimens for stress corrosion. Steels evaluated were black steel, hot-dipped galvanized (HDG) steel, epoxy-coated steel (ECR), 2304 Stainless Steel (SS), low-chromium ASTM 1035 steel, and the TZD-coated steel. An epoxy coating was manually applied to the TZD bars to evaluate a coated version of the product (TZE). The performance of the corrosion-resistant reinforcing bars was significantly better than the control black bars. The overall relative corrosion resistance ranking in the cracked beam test was:

Black Steel Bar < HDG < A1035 Low-Chromium < TZE ≈ TZD < ECR, 2304 SS

The results indicate that TZD reinforcing steel could improve the corrosion performance of steel in concrete. For the U-bend specimens the HDG performed better than TZD, which may be due to higher w/c of concrete and lower cover that let chloride in too fast preventing the TZD specimen to adequately form a protective passive coating. The HDG coating was thicker and had a chromate treatment, which helped it to passivate. The epoxy-coated TZD bars (TZE) had lower corrosion currents. The damage on the bars was comparable to the TZD in the beam test but did help in the U bend tests. The coating was not optimized (brushed versus fusion bonded, not formulated for concrete use), which implies that performance could improve with a commercially applied suitable coating. Based on the results of this study, further work with departments of transportation is recommended to initiate trial testing in the field. The cracked beam method looks promising as a test method to evaluate corrosion-resistant reinforcing bars, high-performance concrete, surface treatments, and a combination of these. It might be useful to develop this flexural cracked beam test into an AASHTO provisional test method specification.

 The final report is available.