## 3 Results and Discussion
### Flexural Strength  
The flexural strengths at 28 days of age of each group of LWC specimens at room temperature were higher than 5 MPa, as shown in Figure 10. Of them, the flexural strength of the E50-M specimen with hybrid fibers was the largest (6.54 MPa). For the low-strength LWC, the addition of steel fiber did not increase its flexural strength. For the medium-strength LWC, due to the bridging effect of the steel fibers, the flexural strength of the steel fiber-added experimental group was higher than that of the control group. This is consistent with the test results reported by [PERSON] and [PERSON] [52]. However, the flexural strength of the polypropylene fiber-added experimental group was less than that of the control group. With the increase in temperature, taking the specimens mixed with steel fiber as an example, although the thermal energy generated by high temperature led to the formation of cracks between the cement paste and the steel fiber, the steel fiber still prevented the crack from propagating and maintained the flexural strength to a certain extent. As for the specimens mixed with polypropylene fiber, at 400-600 \({}^{\circ}\)C, the pores formed by the melting of the polypropylene fiber dissipated the water vapor pressure, thereby maintaining the flexural strength. However, above 800 \({}^{\circ}\)C, due to the melting of polypropylene fibers, additional voids were created that further weakened the microstructure and increased the likelihood of collapse. The residual flexural strengths of each LWC mixture after exposure to temperatures of 400-800 \({}^{\circ}\)C are also shown in Figure 10. In the control group, the residual flexural strengths of the C30 mix and C50 mix were between 2.14 and 5.71 MPa and 2.85 and 5.51 MPa, respectively. In the experimental group, the residual flexural strength of the E30-S, E50-S, E50-P, and E50-M mixes at different fire temperatures were 2.54-5.36, 3.15-5.83, 2.48-5.17, and 3.89-6.72 MPa, respectively. Figure 11 depicts the residual flexural strength ratio of each LWC mixture at different temperatures. This parameter is an important property from the viewpoint of fire resistance due to the significant influence of tensile strength on concrete cracking. Figure 11 shows that the flexural strength of each group of LWC was not influenced by the temperature rising up to 400 \({}^{\circ}\)C. However, after exposure to an elevated temperature of 600 \({}^{\circ}\)C, the flexural strength of each group of concrete decreased, but the residual flexural strength ratio after exposure to elevated temperatures can still be maintained above 0.8. After exposure to 800 \({}^{\circ}\)C, a dramatic loss was observed in the flexural strength of each group of LWC, and the residual flexural strength ratio after high temperatures was between 0.38 and 0.59. In the low-strength LWC (Figure 11a), when the temperature was 600 to 800 \({}^{\circ}\)C, the residual flexural strength ratio of the control group and the experimental group decreased sharply as the temperature increased, and the difference between the two was quite obvious. In the medium-strength LWC (Figure 11b), the residual flexural strength ratio of both groups slightly increased after exposure to 400 \({}^{\circ}\)C. At 600 \({}^{\circ}\)C, the residual flexural strength ratio of the control group and the experimental group significantly reduced. Of them, the residual flexural strength ratios of the experimental group and control group were between 0.90 and 0.91, and 0.83, respectively. Since steel fibers do not melt at temperatures  
Figure 10: Results of flexural strength. below 1300 \({}^{\circ}\)C, they maintain their beneficial effects of bridging cracks over a wide temperature range. The results of this study showed that after being subjected to 600 \({}^{\circ}\)C, the residual flexural strengths of the E50-S and E50-M specimens increased by 7%-8% compared with the control group.