A large-scale fire test of an immersed tunnel under the protection of fire resistive coating

The damage caused by fire can seriously affect the durability and safety of an immersed tunnel, including the reduction of structural bearing capacity and the spalling and cracking of concrete. Fire resistive coating is one of the usual measures to protect the tunnel from fire. In this study, a large-scale (1/5) immersed tunnel was built to carry out a fire test of the immersed tunnel under the protection of fire resistive coating. During the fire test, the heating phase lasted for 365 min, and the cooling phase lasted for 120 min. In the heating phase, the average temperature of the fire in the tunnel reached 1113.7 °C. Under the protection of fire resistive coating, the maximum concrete temperature was 412.4 °C, which was 701.3 °C lower than the average temperature of the fire. During the fire test, the deformations of the tunnel were very small, and the maximum deformation was 6.8 mm. But the concrete cracked seriously on the outside of the tunnel under fire, and the maximum crack width reached 1.86 mm. The concrete cracking not only occurred in the right tube which was exposed to fire, but also occurred in the mid gallery and left tube which were not exposed to fire. According to the temperature of vapor and water in the cracks on the ceiling recorded by the infrared thermography, it was inferred that the depth of cracks in the ceiling had reached 18 cm at 245 min. The acoustic emission signals show that the concrete cracks were mainly formed in the early stage of the fire test (the first 100 min), which is consistent with the macro phenomena of cracks development. The results of the fire test indicate that under the protection of fire resistive coating, the concrete temperatures were high and there was no concrete spalling, and the deformations of the tunnel were small, but the concrete cracking of the tunnel was very serious. Because the immersed tunnel is buried in the seabed, concrete cracks have a great impact on the durability and safety of the immersed tunnel. In the fire resistance design of the immersed tunnel, more attention should be paid to the concrete cracking of the immersed tunnel.