Abstract: In order to explore the interaction mechanism between soil and structure, indoor direct shear tests under three median sizes of particles d₅₀ (1.21，4.56，8.91 mm), three joint roughness coefficients (0.4, 9.5, 16.7) and three shear rates (1, 5, 10 mm/min) were designed. The stress changes and volume variables in the shear process were monitored, and the effects of roughness on the shear characteristics of sand-concrete interface with different particle sizes under different shear rates were studied. The results show that, when the median size of particle is 1.21 mm, the shear strength of sand-concrete interface first increases and then decreases with the increase of JRC. When the median size is 8.91 mm, the interfacial shear strength increases with the increase of JRC. The internal friction angle of sand-concrete interface increased with the increase of JRC. With the increase of particle size, the internal friction angle of sand-concrete interface first decreases and then increases. At different shear rates, the final dilatancy increased with the increase of particle size. When the shear rate was 5 min/mm, the final shear expansion of sand-concrete interface was the smallest. The shear strength of sand-concrete interface increased with the increase of JRC after reaching a critical particle size.

Key words: direct shear test, sand-concrete interface, roughness, particle size, shear properties