To investigate the mechanical properties of tunnel lining concrete under different moderate-low strain rates after high temperatures, uniaxial compression tests in association with ultrasonic tests were performed. Test results show that the ultrasonic wave velocity and mass loss of concrete specimen begin to sharply drop after high temperatures of 600°C and 400°C, respectively, at the strain rates of 10‒5s‒1 to 10‒2s‒1. The compressive strength and elastic modulus of specimen increase with increasing strain rate after the same temperature, but it is difficult to obtain an evident change law of peak strain with increasing strain rate. The compressive strength of concrete specimen decreases first, and then increases, but decreases again in the temperatures ranging from room temperature to 800°C at the strain rates of 10‒5s‒1 to 10‒2s‒1. It can be observed that the strain-rate sensitivity of compressive strength of specimen increases with increasing temperature. In addition, the peak strain also increases but the elastic modulus decreases substantially with increasing temperature under the same strain rate.

A “rock bridge”, defined as the closest distance between two joints in a rock mass, is an important feature affecting the jointed rock mass strength. Artificial jointed rock specimens with two parallel joint fractures were tested under uniaxial compression and numerical simulations were carried out to study the effects of the inclination of the rock bridge, the dip angle of the joint, rock bridge length, and the length of joints on the strength of the jointed rock mass. Research results show: (1) When the length of the joint fracture, the length of the rock bridge, and the inclination of the rock bridge stay unchanged, the uniaxial compressive strength of the specimen gradually increases as the inclination of the joint fracture increases from 0° to 90°. (2) When the length of the joint fracture, the length of the rock bridge, and the inclination of the joint fracture stay unchanged, the uniaxial compressive strength of the specimen shows variations in trends with the inclination of the rock bridge increasing from 30° to 150° (3). In the case when the joint is angled from the vertical loading direction, when the dip angle of the joint fracture, the inclination of the rock bridge, and the length of the rock bridge stay unchanged, the uniaxial compressive strength of the specimen gradually decreases with an increasing length of joint fracture. When the dip angle of the joint fracture, the inclination of the rock bridge, and the length of the joint fracture stay unchanged, the uniaxial compressive strength of the specimen does not show a clear trend with an increase of the length of the rock bridge.

In this study, cubic and cylindrical cement mortar specimens were first subjected to high temperatures, then the cubic and cylindrical specimens were taken out and conducted with uniaxial compressive test and splitting tensile test, respectively. The effect of the length to side ratio on the uniaxial compressive properties and the effect of thickness-to-diameter ratio on the splitting tensile properties of cement mortar specimens after high temperature were studied. Test results show that: (1) With temperature increasing from 25°C (room temperature) to 400°C, the compressive strength and elastic modulus of cubic specimens with three kinds of side lengths decrease; the decreasing rates of compressive strength and elastic modulus of cubic specimen with side length of 70.7 mm is higher than those of cubic specimens with side length of 100 mm and 150 mm, and the strain at the peak stress of cubic specimens with three kinds of side lengths increase. (2) After the same temperature, the tensile strength of cylindrical specimen decreases with the thickness-to-diameter ratio increasing from 0.5 to 1.0. The decreasing rate of tensile strength of cylindrical specimen with thickness-to-diameter ratio is highest when the temperature is 25°C (room temperature), followed by that after the temperature of 200°C, and that after the temperature of 400°C is the lowest.