Deterioration of physical and mechanical properties of tunnel insulation materials under water–salt immersion and freeze–thaw cycles

The durability of thermal insulation materials constitutes a critical scientific challenge for tunnels in cold regions. Therefore, the deterioration of both the microstructure and macroscopic physical properties of polyphenolic foam (PF), extruded polystyrene (XPS), and polyurethane (PU) were investigated through water absorption tests under freeze–thaw conditions. The results showed that with an increasing number of freeze-thaw cycles (FTCs), Na 2 SO 4 crystals became widely distributed across the pore surfaces, which to some extent impedes the infiltration of water. However, Na 2 SO 4 crystals would damage the pore structure, and under long-term FTCs, the physical properties of the material would significantly deteriorate. The mass water absorption was negatively correlated with the salt concentration (C) and positively correlated with the number of FTCs and the water head (H). The thermal conductivity was highly correlated with both the mass water content and mass salt content, reflecting the compounded influence of salt on the heat transfer properties. Take the freezing situation as an example, When H=5 m and C=0%–1%, the thermal conductivities for the PF, XPS, and PU increased by 2.44–2.61, 1.29–1.43, and 2.62–2.74 times, respectively, compared to those in the absence of FTCs. The compressive strengths of the three materials decreased by 29.92%–24.37%, 46.11%–35.23%, and 35.88%–26.64%, respectively, compared to those in the absence of FTCs. Based on fuzzy mathematical analysis and the deterioration rates, PF is recommended for surface layers and XPS is recommended for intermediate layers. The findings offer an experimental basis for revealing the mechanism of frost damage and the durability performance of insulation systems.