Damage evolution law and underlying mechanism of asphalt concrete based on micro- and macro-performance under long-term freeze-thaw action

Impermeability is paramount for asphalt concrete in ballastless track foundations, especially in cold regions where freeze thaw (FT) cycles critically threaten structural integrity. This study employed X-ray computed tomography (X-CT), digital image processing, direct shear and dynamic modulus test to analyze the evolution of macro and micro performance in asphalt concrete subjected to 0, 5, 15, 25, and 35 FT cycles. The results indicate a three-stage pore evolution process: initiation (<5 cycles), expansion (5∼ 25 cycles), and stabilization (>25 cycles). A statistically significant damage threshold was identified at 25 cycles, beyond which the pore structure attained dynamic equilibrium, evidenced by a stabilized median aspect ratio of approximately 9.25 and a markedly reduced evolution rate. Vertically, pore distribution was symmetrical due to compaction effects; and the middle layer exhibited the lowest pore density with an absence of continuous pores. FT cycling primarily enhanced permeability in the horizontal (XY) direction, while the material continued to satisfies waterproofing requirements after 35 cycles. The consistent trends observed in the degradation of asphalt-aggregate adhesion parameters and dynamic modulus with the evolution of porosity confirm a direct correlation between microstructural changes and macro-performance degradation. The degree of mixture damage exhibited a strong linear relationship with the pore comprehensive effect index (PCEI) and followed an exponential trend with FT cycle count. Random forest regression analysis confirmed that the expansion of connected and extreme-size pores is the dominant mechanism driving FT damage. These findings offer theoretical insights for optimizing the design of asphalt concrete in high-speed railway subgrades exposed to cold climates.