Influences of Interface Roughness and Loading Direction on Tensile Behavior of Rock–Concrete Bimaterials Under Brazilian Test Conditions

The roughness and stress state of a rock–concrete interface significantly affect the tensile behavior and failure mechanisms. It is of great significance to investigate rock–concrete specimen with the natural rough interfaces and loading directions to improve the load-bearing capacity of the surrounding rock-lining structure. Natural rough surfaces were obtained by splitting tests, and the fractal dimension D of the rough surface was quantitatively calculated based on the cube covering method. Rock–concrete Brazilian disk specimens with different rough interfaces were prepared by a 3D engraving technique. Then, the bimaterial specimens were subjected to the Brazilian splitting test with a loading angle between 0° and 90° to investigate the mechanical response of the rock–concrete specimen. The results showed that the influence of the loading angle on the mechanical properties of the bimaterial exceeded the interfacial roughness, and the peak load increased first rapidly and then increased slowly with the threshold of θ = 60°. The peak load at θ = 0° was only approximately 40% of that at θ = 60°. The magnitude of the increase in the peak load of the rock–concrete bimaterial decreased with the loading angle. The peak load increase of the bimaterial with D = 2.142 was 69% compared with the smooth interface at 0°, while the increase was less than 4% when θ = 90°. According to DIC results, when the stress state of the rock–concrete interface is mainly tensile or shear, a rough interface can inhibit crack propagation and thus improve the tensile strength of the sample. Finally, the effects of interface roughness and loading angle on the stress state of the bimaterial were analyzed. A deviation of the classical Brazilian splitting equation was presented based on the DEM.