Behavior of concrete after exposure to elevated temperatures

The occurrence of a fire in a building raises a number of important questions regarding the post-fire or residual strength and stiffness of any exposed concrete members. Loss of residual strength and stiffness due to heating has been demonstrated by a number of previous studies, most of which have utilized experiments where unstressed specimens were heated and allowed to cool prior to testing them to failure at ambient temperature. This paper describes elevated temperature tests where the specimens were loaded during heating and cooling prior to testing them to failure at ambient temperature. This paper describes the results of such testing undertaken on eighteen identical plain concrete cylinders (Ø 65mm ×150mm) which were exposed to temperatures of 100, 200, 300, 400, 500, and 600°C whilst they were subjected to 30% of maximum load during the heating and cooling phases, prior to testing at ambient temperature. The properties examined were residual compressive strength, residual secant modulus of elasticity and the effect on the stress-strain relationship of concrete. The exposure to high temperature decreased the residual concrete compressive strength, concrete modulus of elasticity and initial slope of stress-strain curves for the cylinders. It was found that the decrease in secant stiffness of concrete is more pronounced than the corresponding decrease in residual compressive strength. These results are compared with the findings from overseas studies which were conducted on specimens that were not loaded during heating. Finally, models for temperature-dependent residual compressive strength and modulus of elasticity are proposed