Freeze–thaw deterioration mechanisms of concrete under low-temperature high-frequency cycling in high-altitude mountainous regions

Concrete structures in high-altitude mountainous regions of western Sichuan, China, are subjected to prolonged low temperatures and frequent diurnal freeze–thaw cycles, with annual occurrence rates of −5±2 °C conditions reaching 30%. To elucidate the deterioration mechanisms of concrete under these conditions, rapid freeze–thaw cycle tests were conducted within the temperature ranges of −5 °C to 5 °C and −18 °C to 5 °C. The effects of cycle frequency on mass loss, compressive strength degradation, and microstructural evolution were evaluated, and a comparative analysis between the two thermal regimes was performed. Results showed that deterioration at −5 °C to 5 °C progressed slowly before 300 cycles but accelerated thereafter, with a 28.59% strength loss after 900 cycles. In contrast, deterioration at −18 °C to 5 °C intensified after 200 cycles, with a 29.11% strength loss at 600 cycles. Freeze–thaw damage models based on mass loss and strength loss were developed using a two-parameter Weibull distribution, revealing lower limit cycles when evaluated by strength loss. Microstructural analyses (air-void system, pore size distribution, X-ray CT, XRD, and SEM) indicated that high-frequency cycling enlarged average pore size, reduced <100 μm pores, increased >200 μm pores, and promoted pore coalescence. These changes were accompanied by hydration product leaching and crack propagation, ultimately accelerating macro-scale deterioration. The findings provide a theoretical and technical basis for enhancing the long-term freeze–thaw resistance of concrete in high-altitude regions with frequent temperature fluctuations. • Experimental design targets plateau climate conditions. • Investigates damage mechanisms in concrete under high-frequency freeze-thaw cycles. • Incorporates multiple pore structural factors using CT and air-void analyses. • Develops a physical damage model of concrete under high-frequency freeze-thaw cycles.