## 3 Results and Discussion
### Observation of the Explosive Spalling  
Spalling of the specimen is not observed when it is in the electric resistance furnace. However, the occurrence of spalling can be recognized by the sound of spalling and the sound intensity during the heating procedure. It was observed that no spalling occurs in the cubic specimens when the test temperature is lower than 400 \({}^{\circ}\)C. This result was obtained from the heating procedure and after opening the furnace. However, for prismatic specimens made of SCC without fibers and specimens with relatively low dosages of SF, such as specimens SF20 and SF40, explosive spalling occurs. When the heating procedure began, explosive spalling of specimens NC, SF20, and SF40 occurred at the 38 th, 42 nd, and 43 rd min, respectively. Figure 7 illustrates the explosive spalling of the prismatic specimens. No spalling was observed for all specimens with PF, including the cases of SCC with single PF or the combination of SF and PF. It is worth noting that this phenomenon is consistent with [PERSON] et al. 's findings [20]. However, regarding SF specimens, as the fiber content increased, the spalling depth and volume decreased gradually. When the fiber content increased to 50 kg/m\({}^{3}\), no significant spalling was observed. This phenomenon can be illustrated by the fact that randomly dispersed SF in the concrete can bridge the internal cracks induced by high temperatures. Moreover, higher residual flexural tensile strength is observed in the following section. Therefore, the beneficial effect of the SF on the spalling behavior of SCC can be obtained. Figure 8 shows that cubic specimens NC, SF20, and SF40 also suffer explosive spalling when the target temperature rises to 600 \({}^{\circ}\)C. Three NC specimens all suffered severe spalling to different extents. Although SF specimens suffered extensive spalling, not all three specimens suffered explosive spalling. This demonstrates that the addition of SF can help improve the fire resistance of the concrete. The spalling time mainly focuses  
Figure 7: Explosive spalling of bending specimens after exposure to 400 \({}^{\circ}\)C: (**a**) NC; (**b**) SF20; (**c**) SF40. on the periods from the 45 th to the 53 rd min after the heating process begins. Moreover, there is no significant spalling sound in the following constant temperature procedures. For bending specimens, they are almost the same as the cubic specimens in the case of spalling. However, the furnace test shows that specimen SF50 suffered significant explosive spalling and specimen SF40 suffered layered explosive spalling. Figure 9 illustrates that the peeling-off depth is 8 to 14 mm at the bottom of the beam. It was found that for target temperatures of 800 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, spalling of the cubic specimens is similar to the case of specimens exposed to the high temperature of 600 \({}^{\circ}\)C. The spalling of the specimens occurs almost at the 51 st to 63 rd min from the beginning of the heating process, and severe spalling sounds were heard. It should be indicated that the sound intensified and then a great explosion occurred. Then, the spalling sound reduced sharply. Figure 10 shows the layered color of the steel-fiber-reinforced specimens after exposure to the high temperature of 1000 \({}^{\circ}\)C. It was observed that the layer depth from the outmost fiber of the specimen is about 55 to 65 mm. Moreover, Figure 10 illustrates that SF was severely oxidized, and almost no complete SF exists in the concrete. From the residual strength test results illustrated in the next section, it was found that nearly no strength can be reserved after heating to 1000 \({}^{\circ}\)C.  
In summary, the SCC's explosive spalling resistance can be effectively enhanced with the incorporation of fibers, especially for the inclusion of PF in the form of a single PF or combined with SF. On the one hand, it is beneficial from the uniform distribution of PF in the concrete matrix and lower melting temperature (165-173 \({}^{\circ}\)C).