## 3 Experimental Results  
### Macroscopic Phenomena and Failure Characteristics  
The macroscopic phenomena and failure characteristics of the two specimens were similar. At the beginning of the test, slab angles C and D begin to tilt slightly. After approximately 5 min, crack 1 developed along the axis of clamped edge AB, as shown in Figure 6a. At 13 min, the warpage of slab angles C and D increased continuously (as shown in Figure 6b), cracks 2 and 3 inclined at L/4-L/2 from the clamped edge at approximately 45\({}^{\circ}\) to the simply supported edge that appeared near slab angles A and B, and main cracks 4 parallel to the short span direction and from the middle to the supporting axes on both sides appeared in mid-span. After the concrete cracks at the axis of the clamped support side, the negative bending moment of the inner plate of the axis increases, and crack 5 (as shown in Figure 6c) appears 160 mm away from the axis of the clamped support side after 18 min. Several cracks along the short span direction (such as cracks 6 and 7, as shown in Figure 6d) appear on both sides of main crack 4. At approximately 21 min, there is a depression in the center of the plate, arc diagonal cracks increase near corners A and B of the clamped side slab, and diagonal cracks 8 appear. At 30 min, the vertical cracks on the side of each slab increase (as shown in Figure 6e), and the axial direction of both sides appears in turn. The test is stopped at 180 min for slab CS1 and 210 min for slab CS2. Four hours after the test, the width of the cracks on the surface of the slabs were reduced compared with the experimental results. The middle part of the board was depressed, and the four corners of the board rebound. The cracking form of the top crack of the two slabs is shown in Figure 7, and the failure mode that resembles the bottom of a shallow bowl is finally formed. During the heating stage, water stains began to appear at the cracks on the top of the slab at approximately 18 min; at approximately 26 min, the rate of water increase at the top of the slab was obviously fast, and water evaporation began to appear at the crack along the thickness of the slab. At approximately 71 min, the water that accumulated in the concave part of the slab surface reached the maximum, accompanied by a large amount of water evaporation (as shown in Figure 6f); at approximately 140 min, the puddles on the top of the slab began to decrease. Figure 5: Arrangement of the deflections and tilt transducers (dimensions in mm). Figure 6: Macroscopic phenomenon of the slab surface: (**a**) crack developed along the axis of clamped edge, (**b**) tilt of the slab angle, (**c**) cracks along the short span direction, (**d**) cracks along the short span direction, (**e**) cracks on the side of the slab and (**f**) surface water vapor. Figure 7: Cracking development during the test (top surface of slab, dimensions in mm). (**a**) Slab CS1. (**b**) Slab CS2. When the temperature inside the furnace rises, due to the action of fixed end bending moment, the negative bending moment at the fixed support axis is large. When the ultimate tensile strength of concrete is less than its value, the concrete will crack first. As the temperature rises, the concrete strength decreases sharply. The bending moment in the middle of the slab span is related to the length of the slab span. The bending moment in the middle of the long span of the concrete slab is large, and the concrete in the slab is compressed and cracked along the short span direction, resulting in cracks. Due to the temperature rise, the concrete slab expands, the slab angle is warped, the slab angle force is upward, and the slab surface is subjected to downward uniformly distributed load. Under the coupling of temperature and load, oblique annular cracks are produced in the corner of the plate.