Experimental Study on Shear Characteristics of Fiber-Reinforced Shotcrete-Rock Interface Under High-and-Variable Temperature.

Stability of fiber-reinforced shotcrete (FRS)-rock interface under high-and-variable temperature conditions during high geothermal tunnel construction is related to the safety of tunnel support. The rough rock specimen preparation method and the high-and-variable temperature conservation model were proposed based on the tunnel environment. In the present work, direct shear tests were conducted on composite specimens of FRS and rock, which were subjected to high-and-variable temperature curing (with initial curing temperatures of 46, 57, and 71 °C) and different levels of normal stress (0.5, 1.0, and 2.0 MPa). The characteristics of roughness surface changes before and after shear were analyzed based on industrial type optical surface scanning. The acoustic emission (AE) of the shear process of the combined test and the microscopic characteristics of the concrete under high-and-variable temperature curing conditions were investigated. The main findings are as follows. (1) The curing temperature, normal stress, and roughness exert significant influences on both the shear strength and failure mode of the interface between rock and FRS. Shear strength was reduced by 36% and shear displacement by 30.7% at an initial curing temperature of 71 °C compared to 46 °C at an interface height of 0.35 mm and a normal stress of 1 MPa. The interface protrusion height of 0.35 mm and the initial curing temperature of 46 °C increased the normal stress from 0.5 to 2.0 MPa, the shear strength increased by 65%, and the shear displacement increased by 28%. The shear strength increased by 28%, shear displacement decreased by 39%, and the number of AE events at the peak shear stress increased by a factor of 11 for specimens with a normal stress of 0.5 MPa, an initial curing temperature of 46 °C, and a bulge height of 3 mm relative to 0.35 mm. (2) The post-peak evolution of shear stress can be categorized into four types: fast drop, fast drop followed by rebound, fast then slow drop, fast then slow drop with fluctuations. (3) Shear failures at the interface and on the rough rock surface primarily occur on the concrete side, and the influence of the initial curing temperature on the failure mode weakens with increasing roughness and normal stress. (4) The inclusion of alkali-resistant glass fiber (AR-GF) mitigates the detrimental effects of high temperature on concrete and concurrently enhances the shear strength of the interface between rock and FRS. These research findings can provide valuable insights and support for the investigation, design, and construction of FRS support systems in high-temperature underground engineering projects. Highlights: The rough rock specimen preparation method based on 3D scanning and 3D engraving and the high-and-variable temperature curing model were proposed. Shear mechanics and acoustic emission characteristics of the shotcrete-rock interface after high-and-variable temperature curing were investigated. The rough surface analysis method based on industrial surface scanning was proposed to study the characteristics of rock rough surface changes. The effect of initial curing temperature on the shear strength of the shotcrete-rock interface decreases with increasing normal stress and roughness. [ABSTRACT FROM AUTHOR]