Freeze–thaw durability and thermal conductivity of foamed concrete for high-altitude cold-region tunnels

Tunnel structures in high-altitude cold regions are vulnerable to freeze–thaw damage, which can reduce their stability and long-term durability. To improve tunnel performance in such environments, this study investigates the freeze–thaw resistance and thermal conductivity of foamed concrete under a simulated low-pressure condition. Foamed concrete is lightweight and has relatively low thermal conductivity, making it a potential insulating material for cold-region tunnel structures. The results show that foamed concrete with a dry density of 1200 kg/m 3 exhibited the best freeze–thaw resistance. After 28 days of curing and 15 freeze–thaw cycles, its compressive strength loss was only 1.9%. Compared with ordinary concrete, foamed concrete showed lower strength loss and much lower thermal conductivity, while air-entrained concrete performed best among the conventional concrete mixtures. The thermal conductivity of foamed concrete increased with dry density, ranging from 0.6795 W/(m·K) at 800 kg/m 3 to 0.9487 W/(m·K) at 1200 kg/m 3 , indicating better insulation performance than ordinary concrete. This study clarifies the coupled frost-resistance and thermal-insulation behavior of foamed concrete under a simulated 60 kPa low-pressure environment, which approximately corresponds to the atmospheric pressure at an altitude of about 4000–4500 m based on the standard atmosphere relationship. The findings provide experimental support for using high-density foamed concrete as an insulating filling material in high-altitude cold-region tunnels and offer a reference for the development of durable tunnel materials in cold-region infrastructure.