Thermal damage evolution of granite under slow and high-speed heating conditions

A continuum based numerical approach is used to investigate the effects of slow and high-speed heating, including the ISO 834 fire curve, on thermal cracking of granite. During slow heating, thermal stress variations are mainly controlled by the heterogeneous thermal expansion of mineral grains. As a result, the stress concentrations are more isolated and dispersedly distributed throughout the sample. High-speed heating produces large thermal gradients especially at the outer surface of the samples, inducing big thermal strain increments and subsequent high compressive stresses. Larger cracks are formed due to a wider range of concentrated tensile stresses inside the sample during high-speed heating. At low temperatures, slow heating has a stronger influcence on micro-cracking. Due to larger cracks and newly induced micro-cracks, high-speed heating has an increasingly important impact on crack pattern in granite at high temperatures. Although shear failures show a significant increase at 800 °C, tensile cracks are still the majority of total failures in both, slow and high-speed heating scenarios. The final crack pattern in granite under fire conditions is a result of different cracking behaviors caused by various heating rates. This consequently affects strength and final failure pattern of granite.