Experimental evaluation on mechanical and thermal insulation properties of shotcrete under constant-variable temperature

One of the commonly used stabilization systems for rock tunnel is shotcrete. However, the hydration of shotcrete is usually accompanied by the development of internal defects and micro damage under high temperature. Additionally, in the actual high-temperature engineering, the temperature of the rock tunnel exhibits irregular changes. Therefore, in this study, the curing conditions of constant temperature (23 degrees C, 40 degrees C, 60 degrees C, 80 degrees C) and variable temperature (23-40-60-80 degrees C, 80-60-40-23 degrees C) are set, respectively. The mechanical properties and structural properties of polyurethane shotcrete and ordinary Portland shotcrete are introduced. It is determined that the limit temperature for inhibiting the development of sprayed concrete's compressive and splitting strength is between 60 degrees C and 80 degrees C, and the limit temperature for inhibiting the development of bond strength is about 40 degrees C. Meanwhile, the influence of high temperature on the thermal insulation performance of shotcrete is considered. The results that the self-foaming performance of polymer(polyurethane) reduces the thermal conductivity of shotcrete. Based on this phenomenon, the heat transfer model of shotcrete at room and high temperature is established to clarify the heat transfer process of shotcrete. Results will provide theoretical basis for further research of durability and heat insulation performance of protective spraying material for tunnel under high temperature.