Hydraulic concrete in cold regions is prone to spatially heterogeneous deterioration under semi-immersed freeze-thaw (F-T) conditions due to nonuniform moisture supply, while the underlying damage evolution mechanism and service-life assessment methods remain to be further clarified. In this study, fly ash cement concrete was subjected to semi-immersed rapid F-T cycles, and its coupled evolution in macroscopic performance, electrical response, and pore structure was characterized using mass loss rate, relative dynamic elastic modulus (RDEM), relative volume resistivity (RVR), residual moisture content, and low-field nuclear magnetic resonance. The results indicate that differences in moisture supply are an important factor governing the formation of spatial damage gradients under semi-immersed conditions. Continuous water supply in the immersed region promotes pore coarsening and crack interconnection, thereby accelerating F-T durability degradation. Adding sodium silicate can, to some extent, suppress the transformation of fine pores into harmful pores and microcracks, thereby improving pore-structure stability. RVR exhibits an evolution trend consistent with F-T durability degradation and reflects the combined effects of moisture state, pore connectivity, and damage accumulation, showing a significant positive correlation with RDEM (r = 0.82, p < 0.001). Using RVR alone as the damage indicator in the Weibull model is insufficient to fully characterize the coupled deterioration under semi-immersed F-T conditions, with a prediction error of 54.90%. After incorporating support vector regression to integrate RVR, moisture content, and other state-related features, the maximum prediction error was reduced to 16.78%. These findings provide a reference for non-destructive durability assessment and service-life prediction of concrete under semi-immersed F-T conditions.
