Influence of cooling rates on the fracture properties of asphalt concrete subjected to freeze-thaw cycles

Low‑temperature cracking is one of the main causes of asphalt pavement damage in cold regions. While many parameters influencing its low-temperature performance are well-studied, the specific effect of the cooling rate during freeze-thaw cycles (FTCs) on the fracture performance of asphalt mixtures remains largely unexplored. In this study, the effect of cooling rate on the fracture toughness (FT), fracture energy (FE), and cracking‑resistance index (CRI) of asphalt concrete is investigated. For this purpose, dry and saturated semi-circular bend (SCB) samples were subjected to seven FTCs at different cooling rates (0.1, 0.2, 0.5 °C/min, and the shock‑freeze state) and temperatures (- 5 °C, - 15 °C, and - 20 °C), after which they were fractured under a fixed mixed‑mode I/II loading conditions. The results of this study show that increasing the cooling rate reduces all three parameters: FE, FT, and CRI. For instance, applying FTCs with a cooling rate of 0.1 °C/min was significantly less damaging, resulting in a 16% and 20% reduction in FT and FE, respectively, compared to 23% and 48% for the shock‑freeze state. Furthermore, increasing the cooling rate reduces both the displacement and the peak load at the time of fracture. This degradation is attributed to insufficient time for viscoelastic stress relaxation at rapid cooling rates, which promotes a brittle response characterized by higher stiffness and a greater tendency toward elastic behavior. Other factors also contribute to the reduction in mechanical performance, such as the increased thermal gradient between the core and surface, rapid freezing of surface water compared to core water, and volumetric expansion of trapped water within the core. These phenomena collectively increase thermal stresses and accelerate the formation of microcracks. Finally, the influence of temperature in FTCs was characterized: decreasing the temperature reduced the displacement at failure, whereas the peak load first increased down to - 15 °C before decreasing at lower temperatures.