Evolution mechanism of interfacial mechanical properties of fiber concrete-granite composite under high-temperature variable-temperature multi-field coupling

• 55°C is the optimal HVT curing temperature, with 4.43% higher peak strength than 70°C • JRC amplifies thermal stress, with each 10-unit JRC increase boosting strength by 5.88%-6.65% • AR glass fibers reduce harmful pores, increasing interfacial toughness by 23.7% • Drucker-Prager criterion (R²>0.99) precisely captures composite failure behavior In high-geothermal tunnel engineering, the mechanical behavior at the fiber-reinforced shotcrete-surrounding rock interface is vital for support stability. This study focuses on the interfacial mechanical behavior of alkali-resistant glass fiber concrete-granite composites (ARGFS-GCSs) under high-temperature variable-temperature (HVT) conditions. Composites with different Joint Roughness Coefficients (JRC = 1.99, 12.99, 19.9) were prepared and cured under HVT with starting temperatures of 40°C, 55°C, and 70°C. Triaxial compression tests (confining pressures of 3-10 MPa), combined with nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), analyzed mechanical property evolution, failure characteristics, and micro-mechanisms. Results show peak strength first increases then decreases with temperature, with 55°C as the optimal curing temperature; strength at 70°C is significantly reduced compared to 55°C. JRC exacerbates temperature sensitivity by amplifying thermal stress, and elastic modulus is positively correlated with JRC. Failure modes: tensile-shear failure dominates at 40°C, while bending-shear oblique cracks occur at 55°C and 70°C; high JRC enhances interfacial coordinated deformation capacity. Microscopic analysis reveals HVT accelerates cement hydration but increases harmful pores, while the synergistic effect of alkali-resistant glass fibers and hydration products improves interfacial toughness. The Drucker-Prager criterion precisely captures failure behaviors under combined HVT and JRC influence, verifying interfacial cohesion as the key factor controlling peak strength. This study provides important references for anti-deformation design and durability optimization of fiber shotcrete linings in high-geothermal tunnels.