## 3 Research Progress of High-Temperature Rock Mechanics in Large-Buried Tunnels
### Thermo-Mechanical Properties of Shotcrete  
In 1971, [PERSON] tested the compressive and tensile strength of concrete after curing in a high-temperature environment, and obtained that concrete with different water content had different strength [63]. [PERSON] et al. [64] conducted mechanical tests of high temperature concrete with different cement content, and discovered that there were fewereffects on cement content. [PERSON] et al. [65] used the microscopic detection technology to vibrate concrete in different ways, and obtained that the strength of concrete would degrade with time by observing the change of internal microstructure along with high-temperature curing time. [PERSON] et al. [66] studied the mechanical law of steel fiber reinforced concrete at high temperature by changing the size and the type of concrete, and established the corresponding relationship as presented in Equation (9), which can be used to roughly predict the strength of steel fiber reinforced concrete. \[f_{DPT,T}=\left\{\left[\left(0.0008\frac{l}{d}-0.03\right)F-0.113\right]T+100 \right\}f_{DPT,15^{\circ}C} \tag{9}\]  
where \(f_{DPT,T}\) is the \(DPT\) tensile strength (MPa) of the heated SFRC at temperature \(T\), \(F\) is the fiber volume fraction (\(\%\)), and \(T\) is the maximum temperature (\(C\)). \(f_{DPT,\,15^{\circ}C}\) is the \(DPT\) tensile strength (MPa) of SFRC at 15 \({}^{\circ}C\). The aspect ratio of fiber is denoted by \(l/d\). [PERSON] [67] simulated the 7d and 28d bond strengths of shotcrete and rock in the environment of high-temperature tunnel. It was concluded that the hydration speed was accelerated in the high-temperature environment through the three-dimensional video microscope. [PERSON] et al. [68] pointed out that the rock samples cured in high temperature environments can improve the early strength of concrete and promote the hydration reaction. Later, it is found that adding admixtures to concrete is helpful to improve the mechanical properties of concrete. [PERSON] and [PERSON] [69] conducted compressive tests on concrete by changing the values of temperature, water binder ratio and admixture. Through the analysis of the results and monitoring data, it was shown that the high temperature was helpful to improve the early strength of concrete, while reducing the water binder ratio and adding admixtures can improve the late strength of concrete, and the effect of silica fume, fly ash, and slag in the admixtures was reduced successively. [PERSON] [36] studied the influence of different admixtures on the mechanical properties and durability of concrete, which showed that proper addition of fly ash, slag powder, and water-reducing agent can effectively compensate the late compressive strength of concrete and improve its durability, but reduce the late tensile strength, elastic modulus, and carbonation resistance of concrete. [PERSON] [70] studied the influence of different temperatures, humidity, admixtures, and curing agents on the mechanical properties of concrete through experiments, further analyzed the action mechanism of admixtures on the mechanical properties of concrete in combination with microscopic analysis methods, and proposed the design method and maintenance scheme of shotcrete. Aiming at the performances of concrete support structure in deep-buried long tunnel under high temperature, [PERSON] et al. [71] studied the influence of temperature model and fly ash model on concrete strength by using regression analysis method, and summarized that fly ash was the main factor for the formation of concrete strength. Additionally, the high-temperature curing range of 46-95 \({}^{\circ}C\) can promote the growth of concrete strength. With the development of mathematical statistics technology, scholars generally believe that the study of mechanical properties of concrete should not consider the influencing factors of temperature only, but also take other relevant factors into account for diversified analysis.