Thermal Degradation and Residual Strength Assessment of Reinforced Concrete Members Exposed to Elevated Temperatures

Abstract

This research involves the degradation of RC members upon exposure to fire and studying their residual strength behavior. The temperature was elevated upto 1050°C with focus on its cooling regimes using air cooling and water quenching. Experimental evaluation was conducted using compressive strength, flexural strength, and Ultrasonic Pulse Velocity (UPV) measurements. This was accompanied by microstructural analysis through Scanning Electron Microscopy (SEM-EDS). The results indicate that concrete retains most of its mechanical integrity up to 200-250°C, followed by progressive degradation beyond 400°C, with critical strength loss occurring in the range of 600-800°C. Water-cooled specimens exhibited relatively improved residual performance at higher temperatures. At the same time, air-cooled specimens showed greater deterioration due to prolonged thermal exposure. UPV measurements demonstrated a strong correlation with internal damage and residual strength. This confirms their effectiveness as a non-destructive evaluation technique. Microstructural observations revealed increased porosity, Interfacial Transition Zone (ITZ) cracking, and decomposition of hydration products at elevated temperatures, which directly contributed to strength reduction. Using the experimental data, a regression-based predictive model was developed to correlate residual compressive strength with temperature and UPV. The model demonstrated strong agreement with experimental results, achieving a prediction accuracy exceeding 90%. The findings provide a comprehensive understanding of the thermo-mechanical behavior of RC members under fire exposure. Overall, it highlights the potential of combining non-destructive testing with predictive modeling for reliable post-fire structural assessment.