Peak shear strength damage model and fracture characterisation of sandstone–concrete under freeze–thaw cycles

Freeze–thaw (F–T) cycles can degrade the shear capacity of sandstone–concrete interfaces in cold-region structures. This study investigates the coupled effects of F–T cycling and interfacial roughness on interface damage and shear behaviour. Direct shear tests were performed under a constant normal stress of 0.2 MPa on interfaces with varying roughness (Types T0–T45; JRC = 0–19.43) after 0–20 F–T cycles; damage was assessed using UPV, microhardness, DIC and 3D scanning. The UPV decreased monotonically with F–T cycles by 24.8%–28.94%, indicating progressive internal deterioration. The peak shear strength decreased by 36.77%–26.29% from 0 to 20 cycles, while higher roughness mitigated strength loss, yielding 3.27%–10.49% higher peak strength than smoother interfaces at the same cycle level. The proposed strength model achieved R2 = 0.971 with MAPE = 3.78%, whereas the prediction errors increased at high JRC (maximum deviation of 8.6%), suggesting the need for recalibration for very rough interfaces. Crack-pattern observations indicate reduced macroscopic crack extension but increased microcrack initiation/branching after F–T cycling, and rougher interfaces promoted asperity interlocking and redistributed damage. The findings provide a quantitative basis for assessing the stability of the sandstone–concrete interface within the tested F–T regime and σn = 0.2 MPa.