## 3 Results and Discussion
### Explosive Spalling Phenomenon of Concrete Slabs Exposed to High Temperatures  
Many factors affect concrete spalling or sloughing off including the moisture content, heating rate, thermal gradient, concrete strength, water-cement ratio, specimen size, aggregate type, and protective layer thickness [51; 52]. The failure modes can be divided into aggregate spalling, corner spalling, surface spalling, and explosive spalling [53]. When the temperature of concrete subjected to fire damage reaches a certain value, its important mechanical properties will begin to decay rapidly with the increase in temperature. This temperature is called critical temperature. Therefore, the target temperature must include the critical temperature. This is mainly because when the concrete is exposed to a temperature of 400-600 \({}^{\circ}\)C, the cement hydration products in the matrix, namely, the C-S-H gels and C-H crystals, will decompose, resulting in a sharp drop in concrete strength [1; 2]. However, when the high-temperature electric furnace is heated to the target temperature, the temperature it represents is the temperature in the furnace, not the actual temperature of the specimen. Therefore, the heating temperature of the electric furnace was increased to 600-800 \({}^{\circ}\)C as the target temperature of the high-temperature test so as to include the temperature range above and below the critical temperature. As for the high-temperature test, the furnace was heated to the target temperature at a rate of 5 \({}^{\circ}\)C/min and held for 1 h, and it was heated at a rate of 10 \({}^{\circ}\)C/min and held for 2 h.  
In Table 6, according to the different test variables (i.e., the type of concrete, specimen's moisture state, target temperature, and heating rate), the spalling of the specimen or lack thereof during the high-temperature test is indicated. The table shows that the specimens with corner spalling in the test were all LWAC, and the target temperature was 800 \({}^{\circ}\)C. Figure 6f-h show that the thermal gradient of the heated surface of these specimens was large. This is because LWAC has a low thermal conductivity and a strong heat resistance effect, which caused a large temperature difference between the inside and outside of the specimen, resulting in a difference in the thermal expansion. This caused the concrete to peel off due to the tension, as shown in Figure 12. Corner spalling is a nonviolent form of spalling characterized by the separation of the concrete corners. It usually occurs late in the high-temperature test, mainly due to the loss of tensile strength due to the prolonged exposure to high temperatures [54]. Figure 11: Comparison of the temperature gradients between the LWAC and NWC oven-dried specimens at a target temperature of 600 \({}^{\circ}\)C: (**a**) heating rate of 5 \({}^{\circ}\)C/min; (**b**) heating rate of 10 \({}^{\circ}\)C/min. As far as the influence of the state of the specimen and the heating rate is considered, when the target temperature was 800 \({}^{\circ}\)C, the LWAC spalled off only when the heating rate was 10 \({}^{\circ}\)C/min in the oven-dried state; in the air-dried state, spalling occurred regardless of the heating rate. This was due to the large thermal gradient formed on the surface of these specimens, which produced compressive stress in the direction parallel to the surface and tensile stress in the orthogonal direction [6]. [PERSON] et al. [52] found that an indicative thermal gradient value of approximately 14 \({}^{\circ}\)C/mm was critical for spalling to occur. Table 7 shows that the temperature gradient between each measuring point and the heated surface of these specimens had exceeded the above critical value, resulting in corner spalling. For example, at a target temperature of 800 \({}^{\circ}\)C and a heating rate of 10 \({}^{\circ}\)C/min, the thermal gradient of the air-dried LWAC specimen at a distance of 10 mm from the heated surface was 25.44 \({}^{\circ}\)C/mm.