## 1 Introduction  
Aggregates used in cement concrete are generally classified into three categories: light-, normal-, and heavy-weight. Lightweight aggregate (LWA) is a general term for natural or artificial aggregates with a bulk density less than 1200 kg/m\({}^{3}\)[1]. Due to the increasing demand for LWAs and the unavailability of natural LWAs worldwide, techniques have been developed to produce them in modern factories [2]. LWA can be used to produce lightweight aggregate concrete (LWC) [1, 2], which has a lower unit weight and can significantly reduce the cross-section of load-bearing members, thereby reducing the size of the foundation and making it more suitable for structural engineering [2, 3]. Compared with normal weight aggregate concrete (NWC), LWC has practical advantages, such as good seismic performance, fire resistance, and durability [2]. Therefore, in recent years, LWC has become an important structural material, and the demand for it is increasing [1, 2]. However, LWC generally has higher brittleness and lower mechanical properties than NWC with the same compressive strength [4, 5]. Many studies showed that the use of fibers in LWC is a solution to resolving these problems [6, 7, 8]. In fiber-reinforced concretes, the fiber acts as a crack arrester that can dissipate the local internal stress and cut off the cracks caused by the internal stress and the propagation path of the original crack to prevent plastic dry shrinkage cracks in the initial solidification of concrete. The addition of fibers to LWC can improve its mechanical properties and significantly increase its toughness, ductility performance, and energy absorption, while decreasing its workability [5; 9; 10]. High temperatures cause the deterioration of concrete materials, which is a special failure type of concrete structures and considerably impacts on the overall safety of the structure. Thermal degradation of concrete mechanical properties is critical in assessing the fire resistance and post-fire capacity of reinforced concrete structures. Once concrete is exposed to fire and high temperature, the microstructure and properties of the cement hydration product change with temperature, which directly or indirectly destroys the macroscopic properties of the matrix, thereby affecting the overall behavior of the concrete at high temperatures [11]. When the firing temperature reaches 200 \({}^{\circ}\)C or above, the decomposition of cement hydrate and the destruction of aggregates gradually occur in the concrete. The difference in thermal deformation between the cement matrix and the aggregate leads to stress concentration, which causes the concrete strength to decline significantly, and the degree of strength reduction mainly depends on the characteristics of the aggregate and the temperature to which the concrete is subjected [8]. Especially in environments where the temperature rises rapidly, high strength concrete (HSC) is more susceptible to explosive spalling due to the combined effect of pore pressure and thermal stress caused by temperature gradients during heating [12; 13]. The structural integrity of a reinforced concrete structure is severely damaged. Therefore, the spalling behavior of HSC at high temperatures has attracted the interest of many researchers [14; 15; 16; 17; 18; 19; 20; 21]. With fiber concrete, the thermal interface between the fiber and the concrete matrix can lead to thermal cracking at high temperatures. [PERSON] and [PERSON]'s research [14] pointed out that even if the added amount is 1.0% (volume), steel fiber cannot effectively prevent the spalling of ultra-high performance concrete. It was found that polypropylene fibers with a dosage of 0.1% can be effective at temperatures up to 800 \({}^{\circ}\)C because the polypropylene fibers melted and then left voids to release steam.