Residual Properties of TMT Steel Bars after Exposure to Elevated Temperatures

Abstract

Many reinforced concrete (RC) structures use thermomechanically treated (TMT) bars as reinforcement and are designed to withstand fire loads. Though the properties of mild steel at elevated temperatures have been extensively studied, there is little information available about how TMT bars are affected. An attempt is made here to study the effect of elevated temperature exposure on the metallographic characteristics and mechanical properties of TMT bars. Fe500 high-strength deformed TMT bars were exposed to peak temperatures up to 950°C for durations up to 240 min. The samples were then allowed to cool to room temperature in air before a microstructure examination and other tests (tensile behavior and hardness) were carried out. A temperature of approximately 500°C was found to be a “threshold” for the TMT bars, beyond which substantial damage was found to occur. It was found that in addition to temperature, the soaking period could also bring about 6–8% additional loss in strength. Exposure to elevated temperatures also showed irreversible changes in the microstructure, with hard martensite getting converted to softer ferrite–pearlite, and these changes also explain the changes in mechanical properties. Using peak temperature and soaking period as independent variables, the empirical relations for changes in the mechanical properties are also developed.