Physico-mechanical characterization of Higher Himalayan granite under the thermal treatments of different heating–cooling conditions

Although point load strength is considered as a best proxy for uniaxial compressive strength and also incorporated in the routinely used rock mass rating (RMR) system, the effects of temperature treatments on the point load strength has not gained ample attention over the years. Accordingly, in this investigation, two different cooling techniques (i.e. water- and air-cooling methods) has been used in order to study the influence of different heating–cooling treatments on the physical properties, microstructural characteristics and point load strength of Himalayan granite collected from Sangla valley, Himachal Pradesh. The temperatures for heat treatment were targeted at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C. As a response to thermal treatments, increase in effective porosity, decrease in density and increase in damage coefficient occurs which causes exponential decrease in point load strength. It decreases as high as 74% and 81% under air-cooling and water cooling, respectively, after heating of about 600 °C with reference to thermally untreated specimens. The microstructural study reveals that the grain boundary is quite intact, and the thermal-induced cracks are less pronounced up to 200 °C in both the thermal treatments. However, the increase in crack density due to thermal stresses and thermal shocks induce additional micro-cracks like intra-, inter- and trans-granular cracks, at and beyond 300 °C onwards and their coalescence with each other at higher temperatures (i.e. ≥ 500 °C) under both the thermal treatments contribute towards the variation in point load strength of thermally treated granites.