Thermal Insulation Performance of Lightweight Aggregate Concrete for High-Geothermal-Temperature Highway Tunnels

The construction efficiency of geothermal tunnels is low, and operating lining problems occur frequently, which critically affect tunnel construction and operational safety. Traditional insulation methods primarily involve laying a sandwich thermal insulation layer; however, disadvantages of such methods include high cost, flammability, and poor durability. Therefore, based on the Nige Tunnel project in Yunnan Province, which is the highest-temperature highway tunnel in China, a lightweight aggregate concrete (LAC) with both supporting and thermal resistance effects was developed herein. The proposed concrete exhibits load-bearing capabilities and provides thermal insulation. Through an orthogonal test, it was identified that the shale ceramsite content was a significant factor affecting the mechanical properties and thermal conductivity of the LAC, followed by vitrified microspheres. When the shale ceramsite content was 60%, the vitrified microsphere content was 80%, and the curing temperature was 40°C; it is the best mixing ratio for LAC to consider both strength and thermal insulation performance. Moreover, a LAC thermodynamic performance index prediction model was established. A numerical simulation method was utilized to optimize the LAC thermal insulation structural parameters in the geothermal tunnel, and a field spraying test was conducted. The results demonstrated that as the thermal conductivity decreased or the spray layer thickness increased, the surrounding rock temperature at each radial depth increased. This influence weakened with an increase in the radial depth of the surrounding rock. When applying the LAC spray layer to high-geothermal-temperature tunnels, priority should be afforded to reducing the thermal conductivity of the material, followed by increasing the LAC spray layer thickness. For the Nige Tunnel, the concrete compressive strength was 18.27 MPa, the thermal conductivity was 0.69 W/(m·K), and the spray layer thickness was 200 mm. The air temperature in the tunnel ranged between 27.8 and 30.1°C, which simultaneously meets heat insulation and stress requirements.