Experimental Investigation of the Compressive Strength of Normal-strength Air-entrained Concrete at High Temperatures

Abstract

Over the past decades, the mechanical properties of concrete during fire have been extensively investigated; however, not many studies were aimed at the properties of air-entrained concrete (AEC), and even fewer were aimed at the properties of AEC during fire, i.e. at high temperatures. The lack of studies is unfortunate as the higher porosity of concrete caused by the air entrainment (AE) could decrease the pore vapour pressure, which could in turn decrease the amount and intensity of micro-cracks and spalling of concrete, which is the main cause of concrete structure failures at high temperatures. This study investigates how the AE affects the heat-induced concrete spalling and the compressive strength of normal-strength concrete at temperatures ranging from 20 °C to 800 °C. When performing the experiments, heat treatments were first conducted on reference, i.e. non-air-entrained, and air-entrained specimens. Immediately after the heat treatments, compression tests were performed on the hot specimens in order to obtain the stress-strain diagrams and the compressive strengths of the investigated materials at high temperatures. The results obtained by the experiments suggest that the AE reduces the risk of massive concrete spalling when concrete is exposed to a high rate of temperature increase. The results also show that the AE reduces the compressive strength of concrete when the concrete is subjected to high temperatures for a long time period. This indicates that when using an air-entraining agent, a higher strength reduction should be assumed at high temperatures. It does, however, suggest that AE is very beneficial in the environments where a high rate of temperature increase can be expected.