## 3 Results and Discussion
### Stress-Strain Curve  
The stress-strain curves for low- and medium-strength LWC after exposure to different temperatures are shown in Figures 6 and 7, respectively. At room temperature, the ascending branch of the stress-strain curve of the low-strength LWC exhibited a linear relationship, as shown in Figure 6. After exposing the specimen to temperatures above 400 \({}^{\circ}\)C, the stress-strain curve was characterized by atypical non-linearities in the elastic region, that is, at the beginning of loading, which formed a concave-up curve. This is consistent with the results of [PERSON] et al. [49]. During the period of pre-elastic hardening, the specimen stiffened as it approached the linear elastic region. As the exposure temperature increased, this behavior became more apparent. [PERSON] et al. confirmed that the closing of pre-existing cracks due to heating and cooling can cause this phenomenon. Dehydration of calcium hydroxide can lead to cracking, which led to the shrinkage of the matrix, thermal incompatibility between the aggregate and cement paste, and the expansion of steel fibers [50]. After exposure to 400 \({}^{\circ}\)C, as mentioned above, due to the high temperature drying effect, the ascending branches of the stress-strain curves of each concrete group were approximately linear, and the slope was steeper than that at room temperature. However, after exposure to 600-800 \({}^{\circ}\)C, the ascending branch of the stress-strain curve of the experimental group was approximately linear and the slope was relatively small, which was caused by the difference in expansion between the fiber and the matrix during the heating process, resulting in interface peeling. In this case, the position where the fiber was located became a potential crack. As a result, as the temperature increased, the stiffness of the concrete specimens gradually decreased. After being exposed to 800 \({}^{\circ}\)C, the slope of the descending branch of the stress-strain curve reduced, and the ultimate strain value markedly increased, that is, the stress-strain curve became quite gentle. As a whole, the residual stress-strain curve of the experimental group exposed to high temperatures was relatively flat due to the incorporation of fibers. This also confirmed that the residual stress-strain behavior of the experimental group was superior to that of the control group. Figure 6: Stress versus strain curves of low-strength LWC specimens: (**a**) control group C30 and (**b**) experimental group E30-S.  
As for the medium-strength LWC, taking the E50-M mix as an example, as shown in Figure 7, a linear relationship was found with the ascending branch of the stress-strain curve before the fire damage, and the slope was quite steep. Similarly, after exposing the specimen to temperatures above 400 \({}^{\circ}\)C, the stress-strain curve was also characterized by atypical non-linearities in the elastic region. The ascending branch of the stress-strain curves after exposure to high temperatures still remained linear, but the slope decreased significantly with increasing temperature. Within 400 to 600 \({}^{\circ}\)C, the damage caused by high temperature was not obvious, but within 600 to 800 \({}^{\circ}\)C, the effect of high temperature was more pronounced. The reason may be that in the case of low temperature damage, the temperature was transmitted through pore water, colloids, and the crystal skeleton. As a result, the transfer of heat energy to the center of the concrete to attenuate its strength was slow. However, in the case of high temperature fire, the transmission of heat energy mainly occurred through radiation, and the heat transfer was relatively rapid, so cracks formed faster inside the concrete. The test results of the elastic modulus of each LWC specimen at 28 days of age are depicted in Figure 8. It shows that at room temperature, the elastic modulus of the control group and the experimental group of low-strength LWC were 13.62 and 11.64 GPa, respectively.