Investigation of damage evolution in asphalt concrete under coupled ultraviolet aging and freeze-thaw cycles

To clarify the degradation law of crack resistance of asphalt concrete under the coupled action of UV aging and freeze-thaw cycles, semi-circular bending tests were conducted on unaged and UV-aged specimens subjected to varying numbers of freeze-thaw cycles. Key parameters, including air void content, flexural tensile strength, flexural stiffness, failure displacement, and fracture energy, were measured, and their effectiveness in characterizing coupled damage was evaluated using a damage index and correlation analysis. The results indicate that increasing freeze-thaw cycles lead to a gradual rise in air void content, accompanied by continuous reductions in flexural tensile strength, pre-peak fracture energy, and total fracture energy, demonstrating a significant deterioration in internal compactness and crack resistance. Under identical freeze-thaw conditions, specimens exposed to combined UV aging and freeze-thaw effects exhibit higher air void content and further reductions in strength and fracture energy compared to those subjected to freeze-thaw cycles alone, indicating that UV aging exacerbates freeze-thaw damage. Although flexural stiffness shows an overall decreasing trend, coupled-damage specimens maintain relatively higher values than unaged specimens, reflecting the stiffening effect induced by UV aging. Damage index analysis identifies flexural tensile strength as the most sensitive indicator of both freeze-thaw and coupled damage, followed by fracture energy and air void content. Correlation analysis reveals a general negative relationship between air void content and major crack-resistance indices, while flexural tensile strength shows a strong positive correlation with pre-peak fracture energy. Overall, flexural tensile strength and pre-peak fracture energy are recommended as key indicators for evaluating the evolution of crack resistance under coupled environmental effects.