## 3 Results and Discussion
### Compressive Strength
After exposure to 600-800 \({}^{\circ}\)C, the residual compressive strength of each concrete mixture decreased significantly because the strength loss was mainly caused by physical changes below 400 \({}^{\circ}\)C. However, at 600-800 \({}^{\circ}\)C, the decrease in strength was mainly due to chemical degradation in hydrated products and aggregates, which was basically unrelated to the amount and type of fiber [46]. [PERSON] et al. [47] showed that after exposure to 800 \({}^{\circ}\)C, the  
Figure 4: Results of compressive strength test. residual strength of LWC decreased to 40% of the room temperature strength. However, in this study, the residual strength of the two groups of LWC after exposure to 800 \({}^{\circ}\)C was about 50%-66% of the initial strength. The degree of variation in strength after exposure to high temperatures can be understood through the residual compressive strength ratio of the specimen, which is defined as the ratio of the strength after exposure to the target temperature to the original strength at room temperature. Figure 5 shows that the variation in the residual compressive strength ratio of different LWC mixtures is a function of temperature. Notably, the residual compressive strength ratio of the two series of concrete after exposure to 400 \({}^{\circ}\)C significantly improved, as shown in Figure 5. In the case of low-strength LWC, Figure 5 shows that the residual compressive strength ratio of the experimental group after exposure to high temperatures was higher than that of the control group. Similarly, for medium-strength LWC, Figure 5 shows that the residual compressive strength ratio of the experimental group after exposure to 400-600 \({}^{\circ}\)C was higher than that of the control group due to the bridging effect of the fibers. Figure 5, in the experimental group, shows that the residual compression of the specimens with steel fibers (E50-S and E50-M) produced obvious changes in the residual compressive strength ratio with increasing temperature between 400 and 600 \({}^{\circ}\)C. By comparison, specimens with polypropylene fiber (E50-P) underwent a moderate change in the residual compressive strength ratio with increasing temperature because the addition of polypropylene fibers in concrete led to improved retention of compressive strength at moderately high temperatures due to the melting of polypropylene fibers, which created pathways within the concrete microstructure for the release of water. As can be seen in Figure 5, the residual compressive strength ratio of the E50-P mix was lower than that of the control group after exposure to 800 \({}^{\circ}\)C. After this temperature, the specimens with polypropylene fibers had the lowest residual compressive strength ratio among all tested concretes because the pyrolysis process of polypropylene fibers destroyed the internal structure of the material [13]. In other words, the burning of polypropylene fibers led to creating additional voids that further weakened the microstructure, which could facilitate collapse. In contrast, the residual compressive strength ratio of the E50-M mix was higher than that of the control group because the inherent high melting temperature of the steel fiber guaranteed its good performance under high temperature conditions, thus allowing the concrete to retain better residual compressive strength. According to the above results, the effect of the added fiber on the residual compressive strength of different grades of the LWC was also different. Overall, the hybrid fiber reinforced concrete showed better ability to resist the loss of compressive strength caused by high temperature. This is consistent with the test results reported by [PERSON] and [PERSON] [48]. Figure 5: Comparison of residual compressive strength ratio of LWC specimens: (**a**) low-strength and (**b**) medium-strength concrete.