Macro- and Microdamage Characteristics of CA Mortar under Multiple Alternations of Freeze–Thaw Cycles and Fatigue Loading

Abstract

The combined effects of freeze–thaw (FT) cycles and train-induced fatigue (F) loading in cold regions accelerate the degradation of cement asphalt (CA) mortar in ballastless tracks, thereby compromising track operational safety. Employing four experimental regimes, including single FT cycles, single fatigue loading, and their alternating action (F-FT and FT-F, in which F-FT refers to successive alternations of fatigue loading followed by FT cycles, repeated until a predetermined number of cycles is reached; FT-F follows the reverse sequence), this study investigates the deterioration mechanisms of CA mortar under the alternating action of FT cycles and fatigue loading. The results show that when the number of alternating action cycles of FT cycles and fatigue loading reached three, the residual compressive strength loss ratios of CA mortar under F-FT and FT-F regimes were 38.3% and 36.2%, respectively, significantly exceeding the 13.1% observed under individual fatigue loading and 16.3% under sole FT cycles. Furthermore, the porosity of CA mortar under F-FT and FT-F conditions reached approximately 20% and 14%, respectively, compared with < 7% under single FT cycles. Similar trends were detected in other performance indices, including mass loss ratio and relative dynamic elastic modulus. Additionally, the damage severity of CA mortar was dependent on the alternating sequence: the F-FT regime induced more severe damage than the FT-F regime. These findings provide a theoretical foundation for improving the durability of ballastless tracks in cold regions.