Macroscale to Microscale Strength Evolution of Carbonaceous Sandstone under Freeze–Thaw Cycles

This study investigates the degradation behavior and damage mechanisms of carbonaceous sandstone in seasonally frozen regions subjected to freeze–thaw (F-T) cycles. A series of uniaxial compressive strength (UCS), triaxial compressive strength (TCS), and Brazilian tensile strength (BTS) tests were performed, complemented by multiscale analyses, including X-ray diffraction, thin-section petrography, and scanning electron microscopy, which were used to characterize the mineralogy, cementation features, and crack evolution. The results demonstrated that both the strength and stiffness parameters decayed exponentially with increasing F-T cycles. After 60 cycles, UCS, TCS, and BTS decreased by 58.2%, 58.6%–61.0%, and 61.7%, respectively, while the cohesion, friction angle, and elastic modulus decreased by approximately 39%, 48%, and 38%, respectively. To quantitatively characterize the deformation evolution induced by F-T cycling, a strain ratio index system was proposed based on the segmentation of the stress–strain curve into compaction, elastic, and postpeak stages. At the macroscopic scale, UCS and TCS rock specimens progressively evolved from tensile splitting and single shear failure to composite failure modes characterized by multiple crack propagation and particle spalling, whereas BTS specimens shifted from typical throughgoing fractures to complex patterns involving crack branching, bending, and localized fragmentation. At the microscopic scale, the high abundance of quartz and feldspar (70.4%) imparted significant brittleness to the rock specimens, whereas kaolinite (16.9%) increased the saturation water content to 7.56%, thereby enhancing both the water sensitivity and freeze susceptibility. Repeated water–ice phase transitions induced interfacial debonding and pore expansion within clay and carbonaceous cementation zones, facilitating crack connectivity, particle detachment, and cumulative defect growth, ultimately driving progressive degradation of the macroscopic mechanical performance. These findings demonstrate the intrinsic relationship between F-T induced strength loss and microstructural damage, offering new insights for durability assessment and frost damage mitigation of tunnel surrounding rocks in seasonally frozen regions.