# Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures
## 1 Introduction  
Lightweight concrete (LWC) enjoys the advantages of low density, good sonic and thermal insulation performance, and perfect fire resistance, and have been extensively used as structural and non-structural building materials [1, 2]. LWCs can be classified into two types: all-lightweight concretes (ALWCs) and semi-lightweight concretes (SLWCs). For SLWCs, merely coarse lightweight aggregates (LWAs, these can be characterized by porosity, weakness, and brittiness) are employed, while in ALWCs, fine and coarse LWAs are both applied to reduce the self-weight of the LWC structure. Therefore, LWCs have a greater brittleness than normal-weight limestone aggregates concrete (NWC) for similar mix proportions and compressive strength classes [3]. It has been proved that the steel fibers can be added to LWCs to reduce the brittleness of LWC [4, 5]. With regard to the influence of steel fibers on the compressive strength of concrete, [PERSON] et al. [6] reported that adding steel fiber with a volume content of 1-1.5% can increase the compressive strength of concrete by 10-25%. [PERSON] et al. [7] showed that adding steel fiber to LWCs increased the compressive strength by 20%. The experimental study by [PERSON] et al. [8]showed that the incorporation of steel fiber increased the compressive strength of the LWC by 60%. However, some studies showed that steel fiber had no significant influence on the compressive strength of LWC [9,10]. [PERSON] et al. [11] studied the influence of steel fiber with a volume content of 1% on the compressive strength of NWC and high-strength LWC, and the test results showed that steel fiber has little influence on the concrete compressive strength. The negative impact of steel fiber on the compressive strength of LWC has been reported in some references [5,12]. Therefore, it is necessary to enrich the test database with more test results about the influence of steel fiber on the compressive strength of concrete so as to understand the compressive behavior of steel fiber-reinforced lightweight concrete (SFLWC) better. On the other hand, researchers agreed that the incorporation of steel fiber significantly improved the tensile strength, flexural strength, and flexural toughness of LWC [6,7]. According to the existing literature, steel fiber can significantly improve the energy absorption capacity (toughness) of LWC under axial compression [8,13], which is due to the bridging mechanism of steel fiber between cracks in LWC. Studies on the influence of the steel fiber content on the performance of LWC have been quite sufficient. It is worth noting that the addition of steel fiber will reduce the workability of fresh concrete [14], and the slump of fresh LWC will significantly decrease with the increase in the steel fiber volume [15,16]. When the volume admixture is 1.25%, the slump of steel fiber-reinforced high strength self-compacted LWC is lower than the minimum requirement of 600 mm, as reported by [PERSON] et al. [17]. For SFLWC with a high content of steel fiber, [PERSON] et al. [18] pointed out that when the content of steel fiber was more than 2%, the compactness of SFLWC would be affected. Therefore, a high content of steel fiber will have a negative impact on the workability of fresh SFLWC and the compactness of hardened SFLWC. The high content of steel fiber also increases the unit weight of concrete, which goes against the intention of using light aggregate to reduce the dead load of the structure. On the contrary, steel fibers with too low a dosage may not be useful. [PERSON] et al. [19] studied the self-compacting concrete with different types of steel fibers, and the result showed that the improvement of the concrete flexural behavior (toughness and flexural strength) by hooked end steel fiber (HF) was better than that of straight steel fiber. Specimens with HF had a deflection hardening response after the peak point, while the load-deflection curves of specimens with straight fibers were close to those of a plain concrete specimen. [PERSON] et al.