Influence of substrate properties on fracture behavior of rock-concrete bi-material central interface cracked Brazilian disks at different loading angles

Rock-concrete (R-C) bi-material structures are crucial for safety in tunnel construction and similar projects. The load-bearing properties and fracture patterns of structures made of concrete combined with rocks of varying properties exhibit notable differences. This study conducts experiments and numerical simulations on R-C central interface crack Brazilian disk specimens, investigating the effects of rock type, interface crack length, effective modulus ratio, and tensile strength ratio on fracture behavior under different loading angles. Quasi-static compression tests were carried out on sandstone-concrete (S-C) and granite-concrete (G-C) specimens at varying angles, revealing significant differences in fracture patterns and load-bearing capacity. The fracture patterns of S-C show interface extension cracks, tensile wing cracks, and vertical tensile cracks, while, G-C specimens exhibit additional single-wing crack patterns; Peak loads increase with loading angle, with G-C specimens initially having lower peak loads than S-C specimens at angles from 0° to 15°, but surpassing them at higher angles. Then, discrete element models of R-C were constructed using PFC2D and validated with experiments. The influence of interface crack length on the fracture patterns of S-C and G-C specimens was initially investigated using numerical methods. Subsequently, fracture propagation simulations for R-C samples with varying effective modulus and tensile strength ratios were performed. The simulation results show that smaller interface cracks lead to more secondary cracks in concrete, resulting in a heightened degree of fragmentation. Increasing effective modulus and tensile strength ratios result in various crack patterns: interface propagation crack, wing crack in rock and concrete, wing crack only in rock or concrete, vertical tensile crack in rock and concrete, and vertical crack only in rock or concrete; The peak load shows an increasing and then decreasing trend with increasing effective modulus ratio, while with the increase of tensile strength ratio exhibits an increasing and then steadily changing pattern.