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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°C, 40°C, 60°C, 80°C) and variable temperature (23–40–60–80°C, 80–60–40–23°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°C and 80°C, and the limit temperature for inhibiting the development of bond strength is about 40°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. [ABSTRACT FROM AUTHOR]

The procedure of numerical identification of thermophysical properties of concrete during its hardening is presented. Heat of cement hydration, thermal conductivity and specific heat are determined for purpose of modelling temperature evolution in massive concrete elements. The developed method is based on point temperature measurements in a cylindrical mould and the numerical solution of the inverse heat transfer problem by means of direct search optimization algorithm. The determined thermal characteristics of concrete are not constant and depend on the maturity of concrete. The procedure was verified on set of concrete mixes designed with Portland cement CEM I 42.5R and Portland-slag cement CEM II/B-S 32.5 N. Calcareous fly ash was also used for partial replacement of cement in the mixtures. The obtained results have been compared with experimentally measured temperature in concrete and a fair agreement has been found. [ABSTRACT FROM AUTHOR]

• Highly thermally conductive binder was prepared (HTCB). • Study on grouts with the HTCB binder for geothermal applications was conducted. • Thermal and mechanical properties and durability of the grouts was assessed. • The obtained grouts possess good properties for geothermal applications. • Thermal conductivity of the obtained grouts is 24% higher than typical grouts. This paper presents the results of a laboratory study on grouts with a novel binder characterized by high thermal conductivity, designed for low-temperature geothermal applications. The highly thermally conductive binder was prepared by milling expanded graphite together with Portland clinker to obtain a homogenous mixture containing exfoliated graphite. Cement grouts with the obtained binder were prepared to determine their thermal and mechanical properties and durability in both dried and water-saturated state. Thermal properties were determined by thermal conductivity and thermal diffusivity measurements. Compressive and flexural strength tests were carried out to evaluate the mechanical properties of grouts. The durability of grouts was determined by evaluating the impact of wet-dry and heating–cooling cycles on their mechanical strength and thermal conductivity. Furthermore, the quality of the interface between grouts and HDPE pipe was determined by bond strength, hydraulic conductivity and thermal efficiency tests. The results showed that milling expanded graphite together with Portland clinker is an effective way to obtain a binder that consists uniformly dispersed graphite particles, providing high thermal conductivity. The use of the binder in grouts preparation allows to obtain good properties in terms of their use in low-temperature geothermal applications. Especially, this binder improves the thermal efficiency of the grouts. The value of thermal conductivity of grouts based on HTCB is about 24% higher than that of typical grouts with sand. Furthermore, the grouts are characterized by good mechanical properties and durability. The binder also enables to achieve a good quality of the interface between the grout and HDPE pipe. [ABSTRACT FROM AUTHOR]