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This study investigates the affecting pattern of cooling rates on the physical and mechanical properties of granite at high temperature. The different cooling rates are realized by cooling the heated granite specimens in the air at room temperature, −60 °C, and −100 °C. Slow cooling in the unplugged furnace is also performed on an additional set of specimens as a reference group. Physical and mechanical tests are performed on the granite specimens after thermal shock treatments. The results indicate a decreasing trend of the dry density, P-wave velocity, strength, and fracture toughness, and an increasing trend of the porosity, as the heating level or the cooling rate ascends. The microscopic observation on the fracture surface of the tested specimens manifests the deteriorating effect of thermal shock with a higher cooling rate, where the transition from trans-granular and intra-granular fracturing to intergranular fracturing serves to explain the variation pattern of the properties obtained in the lab tests.

In order to explore the damage and mechanical properties of ballastless track after a fire, the uniaxial compressive strength, shear strength, peak strain, and elastic modulus changes due to temperature were obtained through uniaxial compressive and shear tests of concrete after exposure to high temperatures. The test results showed that with increases in temperature, the uniaxial compressive strength, shear strength, and elastic modulus of concrete all presented a decreasing trend, while the peak strain had an increasing trend. Then, based on the classical damage theory model and the strength probability distribution function of concrete micro-units, the high-temperature damage constitutive equation for concrete was established, and the compressive stress–strain curve of concrete after exposure to high temperature was reproduced. Finally, using the CFD numerical simulation software, the temperature field of a ballastless track structure in a tunnel during a fire was obtained, and the temperatures at different positions of ballastless track bed were acquired. Combined with the high-temperature damage constitutive equation for concrete deduced from tests and theoretical analysis, the strength and damage values of the ballastless track bed at different positions after a tunnel fire were obtained.

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