Microstructural design of concrete reinforcing bars for improved corrosion performance

Corrosion of steel in concrete is a complex phenomenon. The corrosion performance of reinforcing steel embedded in concrete is dependent on many variables. These variables include differences in materials constituents, microstructure, processing procedures, concrete mixture proportions, structural design, construction methods, loadings, cracks, and environmental conditions. Reinforced concrete structures are often expected to have relatively long service lives under severe loading and exposure conditions. Often, the primary governing factor affecting the service life of the structure is the corrosion performance of the steel reinforcement. For many systems, concrete is often the only protective cover that prevents the ingress of aggressive ions and eventual corrosion of the reinforcing steel. Significant advances in state of the practice have been exhibited with the use of high-performance concrete materials. Due to the high cost associated with deteriorating reinforced concrete structures, however, other approaches need to be investigated to ensure long-term serviceability of these structures. A complementary approach to improving the quality and corrosion performance of reinforced concrete structures is to utilize reinforcement that has been microstructurally designed to resist corrosion. This approach can increase the redundancy of the protective system, decrease corrosion activity, and increase the service life of reinforced concrete structures. To properly evaluate the corrosion performance and to better predict the service life of reinforced concrete structures, it is necessary to evaluate the characteristics of the bulk concrete, the steel-concrete interface, the steel mill scale, the passive film, and the steel microstructure. This research program investigated the performance of microstructurally designed reinforcing steel for improved corrosion resistance when embedded in concrete and exposed to accelerated chloride environments. Reinforced concrete specimens containing ASTM A 615 and controlled rolled dual-phase ferritic martensitic (DFM) reinforcing steels were embedded in concrete and subjected to chloride solutions. Samples were then evaluated for mass loss and macrocell current flow for a period of approximately 1 year. The results from the macrocell and mass loss testing indicate that the controlled rolled DFM reinforcing steel exhibited less mass loss due to corrosion than the ASTM A 615 reinforcing steel.