## 3 Results and Discussion
### Compressive Strength  
At a test age of 28 days, the cylinders of investigated concrete mixtures were capped with gypsum capping compound and tested in compression to determine the compressive strength of concrete. As shown in Figure 4, the 28 day compressive strength of each concrete mixture at room temperature was higher than the designed 28 day compressive strength. Comparing the strength of the concrete  
Figure 3: Results of slump and unit weight test of lightweight aggregate concretes. Figure 2: Schematic diagram of specimens heating and cooling in the furnace. with low strength C30 and E30-S mixes, the incorporation of steel fiber did not significantly contribute to the increase in the 28 day compressive strength of the concrete; it rather decreased slightly by 2.1%. As for the medium-strength concrete, the compressive strength of the experimental group (E50-S, E50-P, and E50-M mixes) was 9.6%-11.7% lower than that of the control group (the C50 mix). The strength of the E50-S mix was slightly lower than that of the E50-P mix, whereas the strength of the E50-M mixture was the lowest. These results showed that regardless of whether the concrete is low- or medium-strength, the control group had higher strength. Although it was fully stirred during the mixing, the decrease in the compressive strength of the experimental group may be due to difficulties in scattering and condensing the fibers in the concrete. This phenomenon has been discussed elsewhere [5]. The residual compressive strength of investigated LWC mixtures after exposure to different elevated temperatures is shown in Figure 4. Taking the control group as an example, the residual compressive strength of the C30 and C50 mixes at different fire temperatures was 16.5-34.4 and 33.2-60.8 MPa, respectively. For the experimental group, the residual compressive strengths of the E30-S, E50-S, EC50-P, and E50-M mixes at the different fire temperatures were 19.1-35.4, 29.6-54.1, 28.5-54.7, and 32.8-52.8 MPa, respectively. Overall, the variation in the compressive strength of each group of LWC showed a trend of increasing first and then decreasing. After exposure to the lowest target temperature (400 \({}^{\circ}\)C), the residual compressive strength of all mixtures increased relative to the initial values determined at room temperature. The reason for the increase in strength is the drying effect of the concrete specimen due to the increase in temperature [41]. [PERSON] et al. [42] confirmed that a significant gap between the internal temperature of the specimen and the furnace temperature, which was only about half that of the furnace temperature. In other words, despite maintaining the furnace temperature for one hour, the temperature inside the concrete was still far below 400 \({}^{\circ}\)C. Therefore, under the action of high temperature drying, the water vapor in the specimen remained, which contributed to the improvement of strength. [PERSON] and [PERSON] [43] also demonstrated that the compressive strength of concrete after 200-300 \({}^{\circ}\)C increased with increasing temperature. [PERSON] et al. [44] showed that the compressive strength of fiber concrete after 300 \({}^{\circ}\)C was also higher than the initial value measured at 20 \({}^{\circ}\)C. Some scholars found that when the temperature range is between room temperature and 400 \({}^{\circ}\)C, high temperatures cause the cement hydration reaction to occur more fully, thus increasing the strength [20]. From this point of view, a small increase in temperature can produce favorable chemical changes inside the concrete microstructure and can even improve concrete's performance [45]. However, once the temperature exceeded a certain threshold, the mechanical properties of the two groups of concrete tended to deteriorate.