Experimental Study on Shear Mechanical Properties of Heterogeneous Concrete Surfaces Under Freeze–Thaw Cycling

This study experimentally investigated the shear mechanical properties of heterogeneous concrete surfaces during freeze–thaw cycles. Artificial concrete joint specimens with identical morphologies were subjected to direct shear tests under varying freeze–thaw cycles (0, 5, 10, 20, and 30 cycles) and normal stresses (2 to 4 MPa), and the changes in the porosity of the specimens were observed. The results demonstrated that an increase in the number of freeze–thaw cycles resulted in a continuous decrease in the peak shear strength, pre-peak shear stiffness, and residual shear strength of the structural surface, with reductions of 11.5–44.4%, 15.7–31.7%, and 14.5–38.5%, respectively; the increase in porosity exhibited a pattern of rapid growth initially, followed by a slower rate as the number of freeze–thaw cycles increased. The enhancement of normal stress can, to a certain extent, suppress freeze–thaw damage; however, its strengthening effect weakens as the number of freeze–thaw cycles increases. Based on the experimental data, the degradation models of peak shear strength, pre-peak shear stiffness, and residual shear strength considering the influence of freeze–thaw cycles, both the peak shear strength and residual shear strength models were developed based on the Mohr–Coulomb criterion. Strength and stiffness expressions based on porosity increments were also derived and a piecewise constitutive model capable of fully describing the entire shear process was further established. Model predictions showed good agreement with experimental data, with correlation coefficients exceeding 0.91, validating its high predictive accuracy. These findings provide a theoretical basis for the durability design and safety assessment of heterogeneous concrete structural surfaces in water conservancy projects within seasonal freeze zones.