Evaluating the Strength Behavior and Preliminary Mechanical Suitability of Foam Concrete for Tunnel Inverted Arch Backfilling Under Reduced Atmospheric Pressure

The safe service of tunnel inverted arch structures in high-altitude cold regions is heavily restricted by the performance of backfilling materials, which need to simultaneously adapt to low-temperature, low-pressure extreme environments and meet the long-term mechanical requirements of underground building structures. However, the strength development and preliminary mechanical applicability of foam concrete for tunnel inverted arch backfilling under reduced atmospheric pressure remain insufficiently understood. To this end, this paper carries out mix proportion optimization and mechanical performance testing of foam concrete, focusing on the strength behavior under different dry densities and simulated high-altitude low-pressure conditions. The test results show that the compressive strength of foam concrete is positively correlated with dry density, and the growth rate accelerates when the dry density is above 1000 kg·m−3. Specifically, the developed high-performance foam concrete with a dry density of 1200 kg·m−3 achieves a 28-day compressive strength of 27.1 ± 1.2 MPa under 60 kPa atmospheric pressure, indicating stable mechanical performance with low variability. The results indicate that, within the tested dry-density range and under the adopted curing and pressure conditions, the developed foam concrete can meet the basic compressive-strength requirement for tunnel inverted arch backfilling. This study provides a reference for material selection and structural design in high-altitude cold-region tunnel engineering and highlights the potential applicability of lightweight foam concrete in underground structures.