Significant advances in the state-of-the-practice have been exhibited with the use of high performance concrete materials. But, due to the high cost associated with the deteriorating reinforced concrete structures, other approaches should 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 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 or the passive film, and the steel microstructure. This research program investigated the potential benefits of microstructurally designing a reinforcing steel to improve the corrosion performance when embedded in concrete and exposed to accelerated chloride environments. Reinforced concrete specimens containing ASTM A615 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 macro-cell current flow for a period of approximately one year. The results indicate that the controlled rolled DFM reinforcing steel exhibited less mass loss than the ASTM A615 reinforcing steel

Dual-Phase Steels

The mechanical properties of steel are very dependent on the microstructure morphology. This, in turn, is dependent on the carbon content, alloying elements, finishing temperature, and cooling method. Conventional reinforcing steel generally consists of high carbon contents (up to 0.4%) and is generally normalized (air-cooled) after the rolling process, resulting in a pearlitic and ferritic microstructure.
Pearlite is composed of alternate lamellae of ferrite and carbide. The mechanical properties of ferrite-pearlite microstructures are a function of carbon content. Both the yield and ultimate strengths increase with carbon content, whereas the reduction in area (an indicator of ductility) decreases with carbon content. The increase in strength and decrease in ductility results from the increase in pearlite volume . The percent volume of pearlite in many steels is directly proportional to the carbon content. The ferrite is a relatively soft, ductile phase. Increasing the volume percent of ferrite will improve the ductility characteristics of the steel while reducing the strength. Pearlite is a strong, brittle load-carrying phase. Increasing the volume fraction of the pearlite phase will result in lower ductility and higher strength. But, because the pearlite generates micro-galvanic cells, this microstructure is not ideal for aggressive environments.
Like pearlite, lathe martensite is also a strong, load-carrying phase. Austenite is the parent phase that transforms to martensite upon cooling. As austenite is rapidly cooled, iron atoms are displaced by carbon atoms, resulting in a distorted lattice structure. This makes the movements of dislocations very difficult which results in the high strength of the martensite phase. Martensitic steels can obtain increased strengths by increasing the rolling temperature within the two-phase region. This process increases the martensite volume of the steel. Like pearlite, increasing martensite volumes increase the strength of the steel.

The DFM samples were produced in a commercial steel plant. Ingots were initially cast and bloomed. Samples from these ingots were delivered to the laboratory so the research team could evaluate processing techniques in order to develop a heat treatment schedule for the steel mill. The objective was to use a controlled rolled heat treatment process utilized in existing mills that could produce 60 ksi reinforcing steel.

D. Trejo, P.J.M. Monteiro, B. Gerwick, and G. Thomas)Microstructural Design of Concrete Reinforcing Bars for Improved Corrosion Performance , ACI Journal Jan-Feb., V.97, 78, 2000.

Trejo, D., Monteiro P.J.M. and Thomas, G. "Dual-Phase Ferritic Martensitic Steel for Concrete Reinforcement ", International Offshore and Polar Engineering Conference, Netherlands (1995).

Trejo, D., Monteiro P.J.M. , Thomas, G. and X. Wang "Mechanical Properties and Corrosion Susceptibility of Dual Phase Steel in Concrete" , CCR journal Vo 24, 1245 (1994).