## 1 Introduction
[PERSON] and [PERSON] [17], through a comprehensive data-driven inspection of actual fire tests, proved that artificial intelligence technology can provide an attractive tool capable of predicting fire-induced spalling phenomenon with high precision. [PERSON] and [PERSON] [19] studied the residual strength of HSC after exposure to high temperatures. The test results showed that HSC is prone to exploding after exposure to high temperatures, and the first spalling occurred when the temperature was close to 400 \({}^{\circ}\)C. For HSC doped with high melting point fibers, the first spalling occurred when the temperature reached about 800 \({}^{\circ}\)C; the HSC with low melting point polypropylene (PP) fibers did not explode when exposed to high temperatures. The properties of HSC mixed with a high melting point fiber (carbon or steel fiber) and a low melting point fiber (polypropylene fiber) can be considerably improved after exposure to high temperature. [PERSON] et al. [20] reviewed the mechanical properties of steel fiber-reinforced concrete (SFRC) subjected to high temperature, including its residual compressive strength, flexural strength, tensile strength, elasticity, fracture characteristics, and stress-strain relationship. The residual mechanical properties of the SFRC and the mechanism of action of steel fibers were reviewed in detail. The results showed that, in general, SFRC exhibited better residual mechanical properties than plain concrete when exposed to high temperatures and can more effectively prevent the risk of explosive spalling. [PERSON] and [PERSON] [21] confirmed that concrete spalling is directly related to its permeability and has an adverse effect on the fire resistance of RC beams. Due to spalling caused by fire in concrete, the fire resistance of HSC beams with extremely low permeability can be significantly reduced by more than 50%. Many researchers [3; 4; 5; 9; 10; 17; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32] studied the residual mechanical properties of LWC at room temperature after exposure to high temperatures because, to some extent, they represent conditions present after a fire event. Compared with ordinary aggregates, LWA has a larger porosity, so its water absorption is relatively higher. [PERSON] et al. [26] found that under normal circumstances, when the moisture content of NWC is less than 75%, spalling caused by high temperature does not occur. However, when the moisture content of the LWC is higher than 25%, spalling at high temperatures may occur. This shows that LWC spalling at high temperatures is much more sensitive to moisture than NWC. In practice, to minimize the water absorption of LWA and its effect on the workability and subsequent setting and hardening of the concrete produced, LWA is usually pre-saturated before mixing. However, this treatment allows excess water to enter the concrete to increase its moisture content, thereby increasing the possibility of spalling at high temperatures. To improve the spalling resistance of LWC at high temperatures, [PERSON] et al. (2017) used two types of modified materials to modify the LWA using surface coating modifications. During the temperature increase, the spalling behavior of concrete samples formed with modified aggregates was observed. Their test results showed that the modified concrete specimen remained intact at 1200 \({}^{\circ}\)C, and retained about 25% to 38% of the residual compressive strength at this temperature. [PERSON] et al. (2017) investigated the mechanical behavior and microstructure of a new type of ultra-lightweight cement composite (ULCC), which used nanospheres as the LWA and was exposed to high temperatures up to 900 \({}^{\circ}\)C. To prevent the spalling of ULCC material at high temperatures, synthetic fibers were used, with different contents of polypropylene fiber, steel fiber, mixed fiber, and fly ash instead of cement.