Electrochemical Behavior of Carbon and Thermo-Treated Steel Rebars Embedded in Concrete Exposed to Chlorides

One of the most used materials to reinforce concrete structures is the carbon steel rebar, however raw materials to produce carbon steel according to ASTM-A-615 standards are have become more expensive, so most of the production of this type of steel reinforcement is made from ferrous metal waste material; but, their physical and mechanical properties are barely sufficient for the requirements that the applications demand and the fact of using raw materials of a known stoichiometric composition to improve them can result in an increase of the cost production, so the implementation of heat treatment has been the most feasible alternative to achieve the required quality checks at reasonable prices.            

There is a great quantity of electrochemical data related to the mild steel rod, but the heat treatment produces a martensite outer layer, which may cause electrochemical differences. For this reason the main objective of this work was compare the electrochemical behavior between two different types of reinforcement, such as carbon steel and thermo-treated steel by QTB (Quenching Temperature Bar) process. The research was conducted in an accelerated exposure environment through immersion-drying cycles, using reinforced concrete specimens fabricated with water/cement ratios of 0.45 and 0.65.

These specimens were monitored through electrochemical tests, such as the half-cell potential (HCP) and the linear polarization resistance (LPR). Finally the determination of the critical chloride concentration was performed with a potentiometric titration. A critical chloride concentration of 0.45% and 0.73% for total chlorides by weight of cement were obtained for thermo-treated steel and carbon steel, respectively; where the critical chloride concentration is parameter which defines the activation for a reinforced concrete structure. In this case the activation time was different, but the electrochemical results obtained in all the exposure period for both of the two reinforcing steels were similar. The results were used for develop two explications about of the activation time differences between the two reinforcing bars.