Effects of heating/cooling on recovery of strength and carbonation resistance of fire-damaged concrete

This study investigated the effects of elevated temperature and cooling method on the compressive strength and carbonation depth of concrete specimens which were re-cured for different durations. The concrete specimens were heated to different temperatures up to 750°C and were then cooled in air or by spraying with water. After cooling to room temperature, the specimens were post-fire cured in a controlled environment for a total duration of 90 d. Compressive strength and accelerated carbonation tests were conducted to measure the changes in the compressive strength and carbonation depth of the concrete specimens. The experimental results indicate that the compressive strength of air-cooled concrete exposed to temperatures of 150, 300 and 450°C reached respectively 101·6%, 97·2% and 81·9% of the original values. However, after exposure to temperatures of 600°C and 750°C, the compressive strength of the air-cooled concrete decreased to 40·4% and 26·2% of the original values respectively. The compressive strength values of water-cooled concrete heated to 150, 300, 450, 600 and 750°C gradually changed to respectively 102·7%, 96·7%, 80·2%, 46·9% and 28·7% of the original values. The carbonation resistance of concrete was not recovered for concretes exposed to temperatures above 450°C and the carbonation resistance of the water-cooled concrete was lower than that of air-cooled concrete. After exposure to a temperature of 600°C, similar variations were observed in concretes with water-to-cement ratios of 0·55 and 0·35.