## 2 Mechanical Properties of SFRC Exposed to High Temperature
### Residual Elastic Properties  
In fires or high-temperature environments, the excessive deformation of a structure will not only cause bearing capacity loss, but also structure damage. Elastic properties are important indicators of the ability of concrete to deform. Elastic modulus and Young's modulus are the indexes that are typically used for evaluating the elastic properties of concrete. In particular, [PERSON] et al. [70] reported that high temperatures exerted the most significant effect on the elastic modulus of all the mechanical properties of concrete. Therefore, various experimental studies regarding the residual elastic properties of SFRC after heating have been performed. According to the findings of [PERSON] and [PERSON] [33], after being exposed between 105 and 1100 \({}^{\circ}\)C, the loss of elastic modulus of a control concrete increased from 7% to 93%, whereas that of concrete reinforced with 1% steel fiber ranged from 14% to 94%; both decreased approximately linearly. Steel fibers can improve the residual elastic modulus of concrete when the maximum heating temperature is below 800 \({}^{\circ}\)C; however, above 800 \({}^{\circ}\)C, the elastic modulus of concrete and SFRC are almost similar. [PERSON] et al. [40] calculated the tangent and secant modulus of elasticity of SFRC specimens subjected to 900 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, 1100 \({}^{\circ}\)C, and 1200 \({}^{\circ}\)C. The results indicated that the tangent and secant modulus of elasticity of concrete were parallel to the change in compressive strength but insensitive to the change in steel fiber content at the test temperatures. The results of [PERSON] et al. [43] indicated that the elastic modulus of reactive powder concrete reinforced by different steel fiber contents (1%, 2%, and 3%) decreased rapidly with increasing temperature, but the rate of decline slowed at temperatures exceeding 600 \({}^{\circ}\)C. Furthermore, a higher steel fiber content implied that reactive powder concrete (RPC) exhibited a higher elastic modulus above 400 \({}^{\circ}\)C. Furthermore, the data showed that the residual elastic modulus observed in SFRC was higher than that in plain concrete; this may be because the addition of steel fibers increased the cohesion of the matrix, thereby providing the necessary bond for preventing crack development and improving the ductility of concrete. However, their effects were impeded by their being oxidized and softened above 800 \({}^{\circ}\)C [43; 71].  
The presence of steel fibers can enhance the residual elastic modulus of recycled aggregate concrete (RAC). The effects of steel fiber on the elastic properties of RAC are controversial when RAC is exposed to 25 \({}^{\circ}\)C or 200 \({}^{\circ}\)C. [PERSON] et al. [37] reported that the elastic modulus of 1%-steel-fiber-reinforced RAC increased by 3.7% at 25 \({}^{\circ}\)C and 2.7% at 200 \({}^{\circ}\)C compared with RAC without steel fibers. However, the findings of [PERSON] et al. [13; 42] indicated that the elastic properties of steel-fiber-reinforced RAC was not comparable to those of RAC, and the Young's modulus of the samples decreased with the dosage of steel fiber increased from 0.5% to 1.5% when the samples were exposed to 25 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. However, both results indicated that the addition of steel fibers enhanced the elastic properties of RAC when the exposure temperature exceeded 200 \({}^{\circ}\)C. In addition, [PERSON] et al. [37] showed that the low content of silica fume (substitution ratio of 4%) was beneficial for improving the elastic modulus of steel-fiber-reinforced RAC exposed to above 400 \({}^{\circ}\)C.