## 3 Results and Discussion
### Stress-Strain Curve
For medium-strength LWC, the elastic modulus of the control group was 27.86 GPa and the elastic modulus of the experimental group was between 22.18 and 28.39 GPa (the highest was the E50-S mix). The residual elastic modulus of each LWC mixture after exposure to elevated temperatures of 400-800 \({}^{\circ}\)C is also shown in Figure 8. In the control group, the residual elastic modulus of the C30 mix was between 4.51 and 13.92 GPa; the residual elastic modulus of the C50 mix was between 8.79 and 29.51 GPa. In the experimental group, the residual elastic modulus of the E30-S, E50-S, EC50-P, and E50-M mixes at different fire temperatures were 6.15-12.13, 9.32-30.06, 6.52-22.96, and 11.24-24.74 GPa, respectively. In addition, the test results of the low-strength LWC (the C30 and E30-S mixes) indicated that the use of steel fiber slightly reduced the modulus of elasticity at 28 days of age. For medium-strength LWC (C50, E50-S, E50-P, and E50 M mixes), the elastic modulus of the E50-S mix was higher than that of the control group, but the difference between the specimens of the experimental group had no certain tendency. Figure 8 shows that when the temperature was 400 to 600 \({}^{\circ}\)C, the elastic modulus of both groups of LWC decreased slightly. However, after exposure to 800 \({}^{\circ}\)C, the residual elastic modulus of each group of LWC decreased significantly. This is mainly attributed to the increase in the volume  
Figure 7: Stress versus strain curves of medium-strength LWC specimens: (**a**) control group C50 and (**b**) experimental group E50-M.  
of the porous and the cracking of the interface zone between the paste and the aggregate after high temperature exposure. Figure 9 shows the residual elastic modulus ratios of each LWC mixture at different temperatures. The residual elastic modulus ratio tended to rise first and then fell as the temperature increased. Figure 9a shows that the residual elastic modulus ratios were only slightly reduced when the temperature was 400 to 600 \({}^{\circ}\)C. However, at 800 \({}^{\circ}\)C, the residual elastic modulus ratios attenuated sharply. In the control group, the residual elastic modulus ratio was 0.33. For the experimental group, the residual elastic modulus ratio was maintained at 0.53. Figure 9b shows that the residual elastic modulus ratios of the medium-strength LWC increased after exposure to 400 \({}^{\circ}\)C. When the temperature was 600 \({}^{\circ}\)C, the elastic modulus of the control group and the experimental group declined slightly. After exposure to higher temperatures (600 to 800 \({}^{\circ}\)C), the residual elastic modulus ratios of the control group and the experimental group significantly attenuated. Notably, as the temperature rose, the decreasing trend of the residual elastic modulus ratio was more obvious than that of the residual compressive strength because the elastic modulus is more sensitive to cracks at the macroscopic or microscopic scale induced by high temperatures [51]. At normal temperature, the higher the compressive strength, the higher the elastic modulus of concrete. The relationship in which the modulus of elasticity is proportional to the compressive strength has long been accepted. However, the test results showed that the aforementioned relationship should not be applied unconditionally to concrete affected by high temperature. Therefore, it is necessary to be particularly cautious when estimating the elastic modulus of concrete after fire based on the measured compressive strength [13]. This also specifically confirms that the parameter has a high sensitivity to heating at high temperatures. Figure 8: Results of elastic modulus. Figure 9: Comparison of residual elastic modulus ratio of LWC specimens: (**a**) low- and (**b**) medium-strength concrete.