## 2 Mechanical Properties of SFRC Exposed to High Temperature  
### Residual Compressive Strength  
The residual compressive properties of concrete exposed to elevated temperatures are particularly important not only for the fire protection design of the structure, but also for the evaluation and repair of the structure. A considerable number of experimental studies regarding the residual compressive strength of SFRC have been conducted. Some experimental data regarding the residual compressive strength of SFRC are presented in Figure 1. As shown, the relationship between the maximum exposure temperature and the residual compressive strength can be classified into three different stages:(1) In the primary stage, the residual compressive strength of SFRC decreased slightly and may even increase slightly as the high temperature caused the cement hydration reaction to occur more fully when the temperature range was between the room temperature and 400 \({}^{\circ}\)C. (2) In the second stage, the residual compressive strength of SFRC decreased significantly as the temperature increased from 400 to 800 \({}^{\circ}\)C. (3) In the final stage, the residual compressive strength of SFRC was almost completely lost when the temperature was beyond 800 \({}^{\circ}\)C.  
As shown in the comparative test data, the attenuation of the residual compressive strength of concrete is effectively suppressed because of the utilization of steel fibers. [PERSON] and [PERSON]  
With these expansions and shrinkages occurring inside the concrete, cracks began to appear and develop gradually, which caused the compressive strength of the concrete to decline [40; 46]. Owing to the bridging crack effect of the steel fibers, after the steel fibers were applied to the concrete, the volume change behavior of the concrete under a rapid temperature change or high temperature environment was limited, and the initiation and expansion of the defects that occurred inside the concrete reduced [37; 38; 47]. Meanwhile, its inherently high melting temperature guaranteed its good performance under high-temperature conditions. Therefore, the deterioration of the residual compressive strength of concrete can be reduced using steel fibers. The residual compressive strength of concrete is affected by the volume content of steel fibers. [PERSON] et al. [39] observed the variation law of residual compressive strength of samples prepared with different steel fiber contents (0, 40, 80, 120, and 160 kg/m\({}^{3}\)) at different maximum temperatures through experiments. As illustrated in Figure 2, with the increase in steel fiber dosage, the residual compressive strength of SFRC increased between the room temperature and 300 \({}^{\circ}\)C. However, from 500 to 800 \({}^{\circ}\)C, when the steel fiber content was less than 80 kg/m\({}^{3}\), the compressive strength of SFRC increased with the steel fiber content; furthermore, when the steel fiber dosage continued increasing to 160 kg/m\({}^{3}\), the compressive strength of SFRC decreased with the increase in steel fiber dosage. This changing trend is supported by the conclusion of [PERSON] et al. [27; 48]. When the concrete was exposed to a temperature beyond 900 \({}^{\circ}\)C, the variation trend of the residual compressive strength of SFRC as shown in Figure 3, showing that when the steel fiber content is less than 1%, the residual compressive strength of SFRC increases with the steel fiber content; however, when the steel fiber content surpasses 1%, the residual compressive strength decreases with the increase in steel fiber dosage, and the age of concrete exerted little effect on this trend [40]. When the temperature increased beyond 500 \({}^{\circ}\)C, the effect of the steel fibers began to become more significant; however, a higher steel fiber content caused coagulation in the steel fibers, resulting in reduced usable area.