Strength Deterioration Mechanism of Interface between Soil–Rock Mixture and Concrete with Different Degrees of Roughness under Freeze–Thaw Cycles

The hydrothermal field status of soil–rock mixture–concrete interface changes significantly under freeze–thaw cycles, which causes pore structure changes, interface debonding, and cracking; therefore, the shear strength deteriorates and the safe operation of buildings in cold regions is affected. Indoor direct shear and nuclear magnetic resonance (NMR) tests and numerical simulations were conducted to explore the strength deterioration mechanism at the interface between the soil–rock mixture and concrete with different roughness degrees under freeze–thaw cycles in this study. The results suggest that freeze–thaw cycles have little effect on the rotation and displacement of particles, and roughness has a considerable effect. There are primarily three types of pores in the interface layer: (1) micropores; (2) mesopores; and (3) macropores. By increasing the number of freeze–thaw cycles, the T2 spectral peaks of the pore structures shifted to the upright side and the total T2 spectral area increases. The increase in the roughness degrees tends to increase the proportion of mesopores and enhances the interfacial strength. The results could benefit research on soil–rock and concrete interface systems and provide effective guidance for the construction, operation, and maintenance of pile foundation engineering in cold regions.