Influence of repeated heating on physical-mechanical properties and damage evolution of granite

Abstract Granite is commonly seen in tunnels, historical buildings, or energy generation/storage facilities, where repeated heating-cooling cycles can occur during regular operation or fire accidents. In this study, physical and mechanical properties as well as the cracking behaviour of Eibenstock granite during single and repeated heating-cooling cycles are described. Generally, repeated thermal cycling has only a small influence on properties and the cracking behaviour of granite. Thin section observations show that the main minerals of granite do not show big changes up to 1000 °C. P-wave velocity and open porosity measurements reveal that thermal induced microcracks mainly develop before quartz transition temperature (573 °C), and after this temperature these earlier-formed microcracks will be widened rather than newly formed. A hardening behaviour of granite after heat treatments can be observed in the temperature range of 25 °C–400 °C due to the evaporation of water. At higher temperatures, further developed or extended microcracks will lead to a reduction of both, uniaxial compression strength (UCS) and stiffness. Back-calculated parameters show that the thermal induced cracking might reduce the interlocking between the crystals by weakening or removing the cohesion rather than the friction angle. Tensile strength shows a continuous decrease with the development of thermal induced cracks, and no obvious difference can be observed between single and repeated heating-cooling scenarios. Additional testing on samples with different dwelling time shows that thermal time has a minor influence on the thermal cracking behaviour of granite. The slight difference at 1000 °C in terms of P-wave velocity and open porosity between single and repeated thermal cycles is mainly caused by repeated shrinkage and widening of cracks or closing and reopening of some microcracks. All in all, repeated thermal cycling has a negligible influence on physical and mechanical properties. The same holds for the cracking behaviour of granite.