## 1 Introduction  
With the swift global development of tunnels, mines, subways, water conservancy and hydropower projects, and so on, shotcrete, as an advanced support method, is widely applied to surrounding rock control and roadway closure. Shotcrete is a kind of concrete formed by mixing concrete materials, such as gel material, aggregate, and so on, into the ejection equipment, by means of compressed air or other power transmission, and sprayed onto the spray surface at high speed [1]. Shotcrete technology was first used in mining and civil engineering by the United States in 1914. It has a history of more than 100 years. This method is used in underground and tunnel support, infrastructure repair and rehabilitation, slope stabilization, and in areas difficult to reach with conventional concrete, such as bridge piers and beam soffits [2]. Shotcrete has the characteristics of high compressive strength, good durability, and wide range of strength grades [3, 4, 5]. As a supporting material of roadway, shotcrete can not only prevent the oxidation of surrounding rock, but also plays a supporting role for the roadway. With the development of deep mines and deep tunnels, shotcrete is facing great challenges. Especially deep underground, high temperature accelerates the deterioration of shotcrete [6; 7; 8; 9]. According to relevant reports, the rock wall temperature of Sangzhuling Tunnel in the Yarlung Zangbo River Canyon in China reached 89 \({}^{\circ}\)C, and it was identified as a class I risk tunnel. The high temperature of 75 \({}^{\circ}\)C and the extreme temperature of 170 \({}^{\circ}\)C were encountered in the construction of Anfang tunnel and the third hydropower station of Heibu in Japan. The highest original rock temperature is 46.8 \({}^{\circ}\)C, at \(-\)980 m level in Sanhejian coal mine. The underground rock temperature of the Mbonig gold mine in South Africa reaches 65.6 \({}^{\circ}\)C.  
Many scholars have shown that there is a certain relationship between temperature and depth, and the rock temperature gradient is about 3-4 \({}^{\circ}\)C/100 m [10; 11; 12]. However, in a deep environment, due to the different rock properties and the possible existence of large fault zones, the change of temperature with temperature gradient is not obvious. Figure 1 shows the distribution of temperature with depth in deep tunnels and deep mines in some projects. From the figure, it could be concluded that the variation between temperature and depth in underground projects was a nonlinear relationship, that is, the ground temperature in deep environments shows an abnormal pattern [13; 14]. In Figure 1: a--Albert tunnel in Austria; b--Anfang highway tunnel in Japan; c--Sanhejian coal mine in China; d--Sangzhuling tunnel in China; e--Qinling tunnel of Xikang railway in China; f--Franco-Yibulangfeng highway tunnel; g--Kolar gold mine in India; h--Mponig gold mine in South Africa. When shotcrete as support structure contacts with high-temperature rock wall, the performance of shotcrete may change. High temperature leads to the deterioration of shotcrete structure and the weakening of roadway stability. In recent years, many scholars have studied the mechanical properties and micro-morphology of shotcrete under high temperature. For example, [PERSON] and [PERSON] et al. [15; 16] studied the variation of shear properties of shotcrete with granite cementation surface roughness and temperature. The results showed that there were critical values for the effects of granite cementation surface roughness and temperature on shear strength. Moreover, the temperature was the most important factor affecting the shear performance of shotcrete. [PERSON] et al. [17; 18] studied the impermeability of shotcrete under standard working conditions and variable temperature conditions. The results showed that the temperature destroys the dense structure inside the shotcrete, resulting in an increase in the impermeability of the shotcrete. [PERSON] and [PERSON] et al.