Multiscale Study of the Damage Evolution Mechanism of Polyurethane Concrete under Freeze–Thaw Conditions

The macroscopic physical and mechanical behavior, mesocosmic pore size distribution, and microscopic particle morphology of polyurethane concrete (PUC) during freeze–thaw conditions are thoroughly examined in this research. The results reveal that as the number of freeze–thaw cycles (F-T cycles) increases, the P-wave velocity of PUC drops constantly, the peak shear strength reduces gradually, but the porosity increases. A four-peak exponential function can be used to describe the pore dispersion features of PUC. With the increase in the number of F-T cycles, PUC showed an increasing trend in all characteristic pore radii, but only the volume fraction of macropores increased. The compressive and shear strength of PUC were positively correlated with the volume fraction of nanopores and micropores and negatively correlated with the volume fraction of macropores. It was proved that the macroporous content is the main reason for the decrease in the mechanical properties of PUC. The fractal dimension of PUC decreases with the increase in the number of F-T cycles, indicating that the complexity of the PUC pore structure decreases after F-T cycles. At the micro-meso-macro scale, the damage progression of PUC under freeze–thaw conditions demonstrates a gradual and mutually feeding connection. The damage evolution process can be broadly divided into three stages, beginning with the formation of micropores in the polyurethane agglomeration, progressing to the gradual germination and expansion of cracks in the polyurethane agglomeration, and finally to the connected sum expansion of cracks between the polyurethane agglomeration and the aggregates, resulting in significant deterioration of the macromechanical properties of PUC.