Thermo-mechanical behaviour of concrete under biaxial compression loading at elevated temperatures

Understanding the behaviour of concrete at elevated temperatures is essential for designing fire-resistant structures, especially under different loading and restraint conditions. Research on concrete behaviour at higher temperatures under biaxial loading conditions is limited. Further, effect of different biaxial loading ratios on concrete behaviour is not investigated yet. To bridge these gaps, this study proposes a unique testing regime for the assessment of thermo-mechanical behaviour of concrete under various biaxial load ratios. First, the biaxial mechanical loads are applied on the concrete specimen with a central hole. In next step, the thermal loading is applied on the hole surface while the specimen is under sustained loading conditions. This approach simulates the realistic tension and compression zones as well as the state of stress near the hole surface under the applied loading. This testing method has several advantages including the ability to generate higher stresses near the hole surface than the loading applied and also study the effect of compressive and tensile zones on the thermal and spalling behaviour of concrete. Test results showed increased cracking and greater expansion in the minor axis of concrete specimen at lower load ratios. Further, thermo-mechanical analysis in ABAQUS revealed that samples experienced tensile stresses greater than the tensile strength of concrete between 2 and 3 minutes when heated to a thermal condition simulating that of the Hydrocarbon temperature-time curve and between 9 and 14 minutes under the Standard temperature-time curve; thus, demonstrating that heating rate and biaxial loading ratio can significantly affect the behaviour of concrete during a fire.