## 5 Conclusions and Perspectives  
### Conclusions  
The effect of steel fibers on the mechanical properties, explosive spalling behavior, and microstructure of concrete subjected to high temperatures was reviewed based on the findings of previous studies. The residual compressive strength, flexural properties, tensile strength, elastic  
Figure 13: SEM micrograph of CRFC (Steel Fiber Reinforced Concrete) after 0 min (**a**), 60 min (**b**), and 300 min (**c**) of a high temperature of 300 \({}^{\circ}\)C [101].  
properties, fracture properties, explosive spalling, and microstructure of SFRC were discussed. The following main conclusions were obtained based on the review:  
1. The residual compressive strength of concrete exposed to high temperatures improved significantly by the addition of steel fibers owing to the bridging crack effect and inherently high melting temperature. The residual compressive strength of SFRC was affected by the volume content of steel fibers and the temperature history of the sample, including the heating rate and cooling regime. Most studies have shown that the most significant effect was exerted when the amount of steel fiber was 1%.
2. Steel fibers significantly improved the residual flexural strength of concrete when the exposure temperature was less than 800 \({}^{\circ}\)C. Beyond 800 \({}^{\circ}\)C, the steel fiber lost its effect gradually owing to oxidation and corrosion. Compared with hybrid steel and polypropylene FRC and single polypropylene FRC, the residual flexural strength of single steel FRC was the highest.
3. The tensile strength of concrete was closely related to its explosive spalling behavior at elevated temperatures. SFRC maintained its good tensile properties at high temperatures. With the increase in steel fiber dosage, the residual tensile strength of SFRC improved significantly. Compared with type, the aspect ratio and volume fraction of steel fiber exerted greater effects on the residual tensile strength.
4. High temperatures exerted the most significant effect on the elastic modulus of concrete. The elastic modulus of SFRC decreased approximately linearly. In addition, steel fibers could enhance the residual elastic properties of RAC.
5. With the increase in exposure temperature (below 400 \({}^{\circ}\)C), the residual fracture energy of concrete increased gradually. The presence of steel fiber further increased the residual fracture energy and significantly improved the fracture toughness of concrete after elevated temperature exposure. In addition, the enhancement of unstable fracture toughness was approximately twice that of initiation fracture toughness.
6. With the increase in temperature, both the pre-peak and post-peak stages of the stress-strain curve of SFRC became gentler gradually, the peak stress decreased gradually, and the strain at the peak stress increased gradually. Additionally, the presence of steel fibers improved the toughness of heated concrete.
7. Water vapor pressure and thermal stress caused by the temperature gradient inside the concrete at high temperatures were the main causes of explosive spalling. Most studies have reported that the addition of steel fiber reduced the risk of concrete explosive spalling. The presence of steel fiber could decelerate the accumulation of steam pressure inside concrete, reduce the temperature gradient, bridge the microcracks inside concrete, and improve the tensile strength of concrete.