Effects of temperature gradient and curing time on shear properties of concrete-rock interfaces in geothermal tunnels: experimental investigations

Understanding the evolution of concrete-rock interface properties under gradient thermal conditions is critically significant for ensuring the durability of support structures in high geothermal tunnels, where extreme thermal gradients threaten structural integrity. This study aims to investigate the shear behavior and failure mechanisms at this interface under simulated one-sided heating conditions (50 °C, 95 °C) representing tunnel environments. Our innovative methodology employs a custom experimental system, integrating direct shear tests at 3-day and 28-day curing ages with multi-scale characterization (SEM, XRD, CT) to link microstructure to performance. The main conclusions are that curing age dictates temperature effects; at 3 days, moderate heat (50 °C) enhances density/P-wave velocity via accelerated hydration, while 95 °C causes degradation. By 28 days, both temperatures reduce these properties. SEM/XRD/CT identify high-temperature-induced porosity, cracks, and disordered hydration products. Shear strength exhibits four-stage behavior, increasing with normal stress but critically degrading under high temperature/long curing. Two failure modes emerge: Type I (bonding surface failure) and Type II (mixed failure in adjacent concrete). The transition between modes depends on temperature and curing age. Mechanistically, thermal gradients cause uneven hydration and severe drying shrinkage, concentrating stress, initiating micro-cracks, and weakening the interface. Moderate curing temperatures enhance early performance, but strong gradients and high temperatures drastically impair long-term shear strength and structural resilience. The study establishes a novel temperature-dependent failure criterion, providing a theoretical basis for optimizing concrete in geothermal tunnels. Limitations include the simulation of one-dimensional heating and omission of cyclic thermal effects.