Abstract: To reveal the degradation mechanism of the mechanical properties of grout-treated joints in high-temperature environments, high-temperature treatments were applied to sandstone joint samples grouted with different water-cement ratios, followed by direct shear tests to analyze the effect of high temperature on the shear performance of grout-treated joints. The results indicate that an increase in grout strength is associated with higher peak shear stress and smaller displacement. In contrast, high-temperature treatment yields less noticeable post-peak softening, delayed failure, and increased plastic deformation. Both increased grout strength and roughness help improve shear strength. However, with increasing temperature, the cohesion of the rock-grout interface and the compressive strength of the rock face decrease non-linearly, indicating that the grout reinforcement effect weakens with higher temperatures. High-temperature-induced debonding and softening at the rock-grout interface are the two main mechanisms responsible for shear strength degradation. Based on the three-dimensional morphological parameters of Grasselli, a shear strength criterion for grout-treated joints considering the high-temperature effect is proposed. Comparison with experimental data and literature shows that this criterion can accurately predict the peak shear strength of grout-treated joints after high-temperature exposure, providing a theoretical basis for the stability evaluation of rock masses in high-temperature environments or after fire exposure.

Key words: high temperature, sandstone, grouted structural surface, strength model, rock-grout interface, shear