## 3 Results and Discussion
### Flexural Strength
[PERSON] and [PERSON] and [PERSON] and [PERSON] (2018) reported that after being subjected to 600 \({}^{\circ}\)C, the use of 1% steel fiber (based on the volume of the composite material) increased the residual flexural strength by about 15% compared to similar concrete with no steel fiber. After exposure to an elevated temperature of 800 \({}^{\circ}\)C, the residual flexural strength ratio of the control group and the experimental group significantly reduced; the residual flexural strength ratio of the E50-P mix was the lowest. This was attributed to polypropylene fibers melting at a relatively low temperature of 165 \({}^{\circ}\)C, so the crack-bridging ability was not maintained at higher temperatures. This is the same as the result of the compressive strength because the pyrolysis process of the polypropylene fiber damages the internal structure of the material. Overall, compared with the loss of compressive strength, the loss of flexural strength was relatively obvious because the flexural strength is more sensitive to macroscopic or microscopic cracks caused by high temperatures. The relationship between flexural load and midspan deflection of the low-strength LWC is shown in Figure 12. When the load reached the peak, the test was stopped immediately. Therefore, the post-peak behavior cannot be captured. At all temperatures, the flexural load of concrete specimens increased almost linearly with the increase in displacement, and sharply decreased after reaching the peak. Figure 12 shows that the peak load of the control group did not change significantly after exposure to 400 \({}^{\circ}\)C, whereas the peak load of the experimental group slightly increased. As mentioned above, this was attributed to the LWC specimen increasing in strength due to the drying effect of high temperatures. With high temperature drying, removing the evaporable water (capillary water, adsorbed water, and interlayer water) first was easier. The pore pressure formed by the high temperature thus lowered and the flexural load of the specimen increased. At different temperatures, the midspan deflection of the low-strength LWC specimens was relatively small, and the value was about 0.6-1.0 mm. The load-deflection curves of medium-strength LWC specimens at room temperature and each fire temperature are shown in Figure 13. Figure 13 demonstrates that the load-deflection curve of the experimental group was relatively ductile at room temperature or at various firing temperatures. Figure 13 shows that at room temperature, due to the bridging effect of the fibers, the experimental group generally showed better ductility than the control group. After being exposed to 400 \({}^{\circ}\)C, the slope of the ascending branch of the load-deflection curve of the C50 specimen in the control group had no obvious attenuation, whereas that of the E50-M sample in the experimental group significantly increased, as shown in Figure 13. In addition, after exposure to 600 \({}^{\circ}\)C, the initial slopes of ascending branches of the load-deflection of each specimen reduced but still maintained a certain degree (Figure 13). However, after exposure to 800 \({}^{\circ}\)C, the slope of the ascending branch of the  
Figure 11: Comparison of residual flexural strength ratio of LWC specimens: (**a**) low-strength concrete and (**b**) medium-strength concrete. load-deflection curve of the experimental group significantly reduced. Figure 13d shows that the failure deflection of the control group was quite close to the crack deflection; in contrast, the failure deflection of the E50-S and E50-M specimens reached several times the crack deflection, that is, the ductility of the load-deflection curve was more pronounced. Based on the above results, although polypropylene fibers had better strength and ductility at low to moderately elevated temperatures, steel fibers could provide fiber bridging effects at higher temperatures, allowing the E50-S and E50-M specimens to retain a large proportion of flexural strength and ductility.