# Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures
## 3 Test Results and Discussions
### Failure Modes after Axial Compression and Compressive Stress-Strain Curves  
The failure modes of different concrete mixes after axial compression tests are studied and shown in Figure 8. The transverse deformation of ALWC was larger than that of SLWC and NWC during compression at room temperature. Both the steel fiber types employed in this work significantly helped to improve the toughness of LWC, and more fine cracks appeared on the surface of LWC specimens reinforced with steel fiber during compression. Under similar heat treatments, failure modes and processes of plain LWC specimens with no steel fibers (specimens ALWC and SLWC) were characterized by inclined and tortuous failure planes, while adding steel fiber prevented the overall disintegration of concrete prisms when treated at high temperatures. On the failure surface of heated samples, lightweight aggregates were crushed and many CFs were pulled out. For samples exposed to 600 and 800 \({}^{\circ}\)C, HF-reinforced samples had more multiple thinner cracks after compression than CF reinforced samples. Such a phenomenon was attributed to the higher bonding capacity between HF and concrete matrix, which is in accordance with the conclusions that HFs can more effectively reduce the degradation of strength and toughness of concrete under elevated temperatures. Figure 8: Failure mode of mixes after compression: (**a**) All-lightweight concrete (ALWC); (**b**) Semi-lightweight concrete (SLWC); (**c**) All-lightweight concrete with crimped shape steel fiber (CFAL); (**d**) All-lightweight concrete with Hooked end steel fiber (HFAL); (**e**) Semi-lightweight concrete with crimped shape steel fiber (CFSL); (**f**) Semi-lightweight concrete with hooked end steel fiber (HFSL). Figure 7: Temperature effect on splitting tensile strength: (**a**) absolute splitting tensile strength; (**b**) relative splitting tensile strength. Figure 9 presents the typical compressive stress-strain curves for different concrete mixtures under different high temperatures. Each set of curves included the axial stress-axial strain and corresponding axial stress-transverse strain curves. Figure 9 shows that the axial peak strain and transverse deformation of LWC are significantly higher than those of NWC at room temperature. [PERSON] et al. (2018) found that higher volume contents of lightweight aggregate in LWC resulted in longer linear ascending branches of the stress-axial strain curve and a steeper descending section, which means that the brittleness would increase with a decreasing concrete mixture density. Test results prove that LWC was more brittle than NWC of the same strength grade. Compared with SWLC, the stress-strain curve of ALWC was smoother in the ascending section, while it was steeper in the descending section, with relatively larger axial and transverse peak strain. This denoted that ALWC was more brittle than SLWC. The shapes of stress-strain diagrams up to 200 \({}^{\circ}\)C were similar to those obtained at room temperature. From 400 to 800 \({}^{\circ}\)C, the peak strain increased with increasing temperature, but the peak stress showed a decreasing trend, with a gradually decreasing slope. As a result, the stress-strain curves of all concrete gradually leveled off. Above 400 \({}^{\circ}\)C, the loss of strength and stiffness for NWC samples became much greater than those of SLWC and ALWC, and the area under the descending part of the stress-strain curves also decreased more sharply than that of SLWC and ALWC. By adding steel fiber, the peak stress and strain of the ascending curve increased slightly, and the area under the stress-strain curves also increased in the 25-800 \({}^{\circ}\)C temperature range. The above results showed that both steel fiber types could enhance the compressive energy absorption capacity of the LWC before and after being exposed to high temperatures. Obviously, the HFs could improve the energy absorption capacity of LWC more effectively.