## 2 Mechanical Properties of SFRC Exposed to High Temperature
### Residual Tensile Strength  
The tensile strength of concrete is often disregarded in the calculation of strength, because the tensile strength of concrete is significantly lower than the compressive strength [61]. Furthermore, a higher temperature would result in a more significant reduction in the tensile strength of concrete than the compressive strength. This may be because the tensile strength is more susceptible to microcracks in the interface transition zone caused by elevated temperatures compared with the compressive strength [62; 63]. Its importance is crucial when concrete is heated because the tensile properties can inhibit crack expansion inside concrete [64]. In addition, the tensile properties of concrete can resist the internal vapor pressure generated by high temperatures to prevent explosive spalling in concrete [65]. Hence, to improve the tensile strength of concrete after exposure to high temperatures by preventing and minimizing the explosive spalling caused by fires, it is critical to add steel fibers to concrete. A large number of studies [27; 28; 56; 63; 66; 67; 68] have demonstrated that steel fibers contributed positively to improving the residual tensile strength of concrete. This may be because steel fibers can maintain  
Figure 4: Residual flexural strength of fiber-reinforced concrete (FRC) [56].  
their mechanical properties at high temperatures; therefore, they could still bridge cracks under tensile loading and improve the microstructure of heated concrete owing to the limited expansion of defects in the concrete interior [58]. In addition, adding steel fibers could improve the thermal conductivity of concrete owing to the higher thermal conductivity of steel fiber than those of cement matrix and aggregates; therefore, SFRC exhibited a more uniform internal thermal stress after heating, thereby reducing the internal damage caused by thermal gradients in concrete [37; 68; 69]. Ultimately, the residual tensile strength of steel-fiber-reinforced concrete subjected to high temperatures would be improved.  
It has been discovered that the dosage, geometry type, and aspect ratio of steel fibers are main factors affecting the residual tensile strength of SFRC. According to the study of [PERSON] et al. [28], as the content of steel fiber increased from 0% to 2%, the residual splitting tensile strength of ground granulated blast furnace slag concrete after being exposed to 400 \({}^{\circ}\)C increased correspondingly. However, in terms of the relative tensile strength, 1.0% steel fiber was the optimal amount of concrete. [PERSON] et al. [27] studied the relation between the relative tensile strength of reactive powder concrete and steel fiber content and discovered that the relative tensile strength increased as the steel fiber content increased from 1% to 3% when the temperature was below 600 \({}^{\circ}\)C; however, the relative tensile strength decreased at temperatures from 600 to 800 \({}^{\circ}\)C with an increase in steel fiber dosage. [PERSON] et al. [63] investigated the effects of steel fibers of different aspect ratios (J/d = 60 or 80), geometry types (twisted or hooked), and volume content (0%, 0.25%, 0.5%, or 1%) on the residual tensile strength of specimens using double-impact tests when the specimens were exposed to 15 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 700 \({}^{\circ}\)C. Figure 5 shows the effects of steel fiber types, the aspect ratio, and content on the residual tensile strength of concrete. As shown, the dosage and aspect ratio of steel fiber exerted a greater impact on the residual tensile strength of SFRC compared with its geometric type. At all test temperatures, the residual tensile strength of SFRC increased with the amount of steel fiber, regardless of its type and the aspect ratio. The residual tensile strength of SFRC was barely affected by the geometrical type of steel fiber, whereas the higher aspect ratio of steel fiber exerted a better effect on the tensile strength of SFRC.