# Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures
## 3 Test Results and Discussions
### Elastic Modulus and Compressive Peak Strains  
The absolute and relative values of residual elastic modulus of all mixtures under elevated temperatures are plotted in Figure 10. According to [39; 40], the elasticity modulus of each mixture is determined to be the secant modulus at one third of peak stress on the rising section of the corresponding uniaxial compressive stress-strain curve. The coefficient of variation of the elastic modulus tests was between 5.1% and 22.3%. However, the coefficient of variation of the tests were mostly less than 15%. From Figure 10a, before 400 \({}^{\circ}\)C, the elastic modulus of LWC was obviously smaller than that of NWC, and the elastic modulus of ALWC was smaller than that of SLWC. Additionally, the influence of steel fiber on the LWC elastic modulus was relatively small. It assumed that under only 1/3 of the peak load the micro-cracks were not well-developed, and the steel fiber stress was relatively small. Therefore, the characteristics of high elastic modulus and high tensile strength of steel fiber had not been fully developed. Figure 10b shows that: (1) with increasing temperature, the decrease in the elastic modulus of all mixtures was higher than that of compressive strength. This was because cracks propagate on the interface between cement mortar and coarse aggregate, resulting in loose internal structure, large deformation at high temperature, and a sharp decrease in stiffness at 400, 600 and 800 \({}^{\circ}\)C; (2) in the temperature range 200-800 \({}^{\circ}\)C, the stiffness loss of NWC was greater than that of LWC, including ALWC and SLWC. In the range of 400-800 \({}^{\circ}\)C, the stiffness loss of SLWC is greater than that of ALWC. The above phenomena can be explained by the bond between the aggregate and the mortar, the mineral composition of coarse aggregate, and cement mortar composition of different concrete mixtures. LWAs were prepared by high-temperature calcination and due to the hard enamel \"shell\" and high internal porosity of the particles, the thermal conductivity and thermal expansion coefficient were low. Therefore, these results led to better fire resistance, and less post-fire stiffness loss than that of NWC [28]. As for the ALWC, due to the same mineral composition of coarse and fine aggregate, the thermal compatibility of the coarse aggregate phase and cement mortar phase was better than that of NWC and SLWC. In addition, compared to the crushed shale ceramist's irregular appearance, the sintered fly ash ceramics used in SLWC was spherical, which led to the weaker adhesion between coarse aggregate and cement mortar in SLWC. For the above reasons, the stiffness loss of ALWC after the high temperature was less than that of NWC and SLWC. The absolute and relative values of compressive peak strains of all mixtures under elevated temperatures are plotted in Figure 11. The coefficient of variation of the compressive peak strain tests were between 1.5% and 8.0%. In Figure 11a, the average peak strain of the ALWC measured at room temperature was 2720 \(\mu\)e. Compared to that without fibers, when adding CF the corresponding strain of peak stress increased by 12.5%, and when  
Figure 10: Temperature effect on the elastic modulus: (**a**) absolute elastic modulus; (**b**) relative elastic modulus. adding HF the corresponding strain of peak stress increased by 14.7%. The average peak strain of SLWC was 2140 \(\mu\)\(\varepsilon\). Compared to that without fibers, the corresponding strain of peak stress increased by 28.5% with CF, while the corresponding strain of the peak stress increased by 32.7% with HF. For the same LWC matrix, HF had a significant influence on the peak compressive strain. In Figure 11b, the peak strain of the concretes with or without fibers increases gradually with the rise in temperature. Adding steel fiber did not influence the peak strain trend with temperature.