The influence of porosity on the damage induced by freeze-thaw cycles on fracture toughness

Freeze-Thaw (FT) cycles can affect rock slope stability in periglacial environments, where temperature fluctuations near 0°C may promote progressive damage in fractured rock masses. It is hypothesized that FT may trigger crack-tip propagation and the failure of rock bridges, thereby reducing rock slope stability. Although FT effects in porous rock have been extensively studied, the response of very low-porosity rocks remains poorly constrained, particularly with respect to fracture toughness. This study investigates the influence of FT cycling on Mode I fracture toughness K<inf>Ic</inf> and fracture processes in two lithologies with contrasting microstructure: a low-porosity, fine-grained quartzite of an alpine periglacial environment and a higher-porosity, coarser-grained sandstone. Semi-Circular Bending (SCB) tests were conducted on specimens in both their natural state and after 60 FT cycles, performed in an environmental chamber between −12°C and room temperature. The results show that quartzite exhibits a limited, though not negligible (only 1.3%), reduction in K<inf>Ic</inf> after 60 FT cycles, whereas for the sandstone a 7.2% reduction was observed. Digital Image Correlation (DIC) was used to analyze the Fracture Process Zone (FPZ) development in the quartzite specimens, and post-failure thin sections were examined to characterize fracture paths and microstructural damage. Only minor changes are observed in quartzite, whereas sandstone exhibits increased crack tortuosity and intragranular cracking. These differences highlight the role of porosity, grain size, and microstructural arrangement in controlling FT-induced damage. A simplified linear damage model was applied to estimate the potential long-term evolution of the quartzite fracture toughness after the FT cycles. Despite the limited experimental damage, the result of the model suggests that even low-porosity rocks may experience a significant degradation in years, with potential implications for the long-term stability of rock slopes in periglacial environments.