Experimental and Theoretical Investigation on Interfacial Shear Behavior between Concrete and Crushed Rock–Soil Mixture under Freeze–Thaw Cycling

The durability evaluation of permafrost infrastructure heavily relies on interfacial strength characterization. However, existing constitutive models systematically underestimate the damage accumulation rate during freeze–thaw cycling while overestimating residual strength, leading to significantly increased structural safety risks and severely shortened engineering service life. This study investigates shear behavior at concrete–crushed rock soil interfaces under freeze–thaw cycles through laboratory direct shear tests. The shear stress–displacement relationship is analyzed as a function of cycle number, with nuclear magnetic resonance quantifying interfacial pore structure evolution. A four-parameter modified Duncan–Chang model is developed to establish a higher-order nonlinear constitutive framework that integrates freeze–thaw damage effects. Unlike traditional one- or two-parameter hyperbolic models, the proposed model captures complex deformation phases including plastic hardening and incipient strain softening, which are empirically observed in freeze–thaw-damaged interfaces. A two-stage energy decoupling mechanism was proposed to separately describe interfacial debonding energetics and particulate friction thermodynamics, establishing a direct correlation that correlates microstructural ice cementation rupture patterns with continuum-scale elastoplastic deformation characteristics under freeze–thaw cycles. Interfacial shear strength exhibits dual dependence on normal stress magnitude and freeze–thaw history, showing a 40.5% increase in strength at 300 versus 100 kPa normal stress after 15 cycles, followed by stabilized degradation rates attributed to self-organized ice recrystallization patterns. Porosity progressively expands by 0.9%–2.3% with cycling, driven by phase transition–induced microcrack bifurcation and bidirectional pore restructuring (micropore coalescence/macropore fragmentation), which inversely correlates with cohesion reduction. The four-stage constitutive model with cubic–hyperbolic cyclic damage corrections achieves R2 > 0.95 via nonlinear least-squares validation. The model can explain the stress–displacement process of the interface and the strain softening phenomenon in detail, which can provide a basis for the numerical simulation and theoretical calculation of the structure in the frozen soil ground.