Study on the fracture performance for rock-concrete interface in the high geothermal tunnel environment

This study investigated the effects of the hot-dry geothermal tunnel environments on the interface fracture properties of rock-concrete composite materials. The interface fracture energies and fracture toughnesses in mode I and II of the rock-concrete specimens were experimentally investigated by the wedge splitting test. The interface fracture energies were calculated based on the P-CMOD curves, and the crack propagation length was obtained via the digital image correlation (DIC) technology. The initial fracture toughnesses and unstable fracture toughnesses of the rock-concrete interface were calculated by the finite element method. Before the finite element models of the rock-concrete composite specimens were established, a genetic algorithm was adopted to determine the cohesive force distribution at the rock-concrete interface. The correlation between the fracture behavior and the mode ratio was also investigated in this paper. The results show that the hooked-end steel fibers can markedly enhance the interface fracture energy which reached up to 161 N/m at 28 days. The opening mode fracture is the main destruction mechanism during the crack propagation. The hot-dry geothermal tunnel environment can always reduce the interface fracture toughness, and this reduction is pronounced in a higher temperature curing environment. The sliding mode fracture is more prone to occur in a higher temperature curing environment. Furthermore, the introduction of hooked-end steel fibers can significantly improve fracture toughnesses, and this promotion is even more remarkable at a later curing age. The group with the addition of hooked-end steel fibers at the ages of 3 days and 28 days were 0.059 MPa·m1/2 and 0.085 MPa·m1/2, and 0.138 MPa·m1/2 and 0.186 MPa·m1/2 for the initial and unstable fracture toughnesses, respectively.