## 3 Results and Discussion
### The Effect of the Heating Rate on the Pore Pressure  
#### 3.5.1 The Effect of the Heating Rate on the Pore Pressure of the Oven-Dried Specimens  
The literature shows that an increase in the heating rate tends to lead to a higher risk of spalling [19]. This is because the higher thermal gradient accelerates the build-up of pore pressure near the exposed surface. The evolution of the pore pressure at a depth of 10 mm from the heated surface of the oven-dried specimens at a target temperature of 600 \({}^{\circ}\)C is shown in Figure 20a. With the increase in the heating rate (from 5 to 10 \({}^{\circ}\)C/min), the maximum pore pressure of the LWAC slab increased significantly from 102 to 126 kPa, while the maximum pore pressure of the NWC slab increased from 85 to 94 kPa. This is consistent with the view of classical theory. That is, in the absence of surface cracking and severe concrete damage, an increase in the heating rate should lead to an increase in the vapor transport rate and an increase in the accumulation of moisture vapor, leading to the formation of a saturated zone and, thus, an increase in the pore pressure [46]. From the pore pressure curve of the LWAC specimen with a heating rate of 10 \({}^{\circ}\)C/min, the pressure value jumped greatly after being heated for 3 h. This means that cracks were created inside it, resulting in a decrease in the pore pressure. If the pressure curves are compared, it can be clearly seen that the faster the heating rate, the faster the pressure curve rises. This is because a faster heating rate causes the internal temperature of the specimen to rise faster, resulting in a faster rise in the pore pressure. Furthermore, under the same heating rate, the pore pressure of the LWAC slab at a depth of 10 mm was higher than that of the NWC slab. As for the depths of 30 and 50 mm from the heated surface of the concrete slab, the evolution of the pore pressure is shown in Figure 20b,c. These figures show that with the increase in the heating rate, the pore pressure of the LWAC slab still increased significantly, and the same was true for the NWC slab. For example, when the heating rate was raised from 5 to 10 \({}^{\circ}\)C/min, the maximum pore pressure at a depth of 30 mm for the LWAC slab increased significantly from 82 to 99 kPa, while the maximum pore pressure at a depth of 30 mm for the NWC slab increased from 138 to 150 kPa. The above results show that a faster heating rate led to a higher pore pressure and pressure rising rate. This is in agreement with the findings of [PERSON] and [PERSON] [46]. The evolution of the pore pressure at a depth of 10 mm from the heated surface of the oven-dried specimens at a target temperature of 800 \({}^{\circ}\)C is shown in Figure 21a. It can be seen that the pore pressure generated by the LWAC slab with a heating rate of 10 \({}^{\circ}\)C/min was higher than that with a heating rate of 5 \({}^{\circ}\)C/min. In addition, the pressure curve showed a steep rise when heated for 1 h. However, the NWC slab had a higher pore pressure with a heating rate of 5 \({}^{\circ}\)C/min. At a depth of 30 mm from the heated surface (as shown in Figure 21b), both the LWAC and NWC slabs generated a large pore pressure when the heating rate was 10 \({}^{\circ}\)C/min, and the pressure in both cases rose rapidly. The peaks were evident from these pressure curves but then dropped rapidly. This result indicates that there was an internal crack due to the high temperature, which caused the pressure to dissipate quickly. At a distance of 50 mm from the heated surface (as shown in  
Figure 20: Effect of heating rate on the pore pressure of oven-dried specimens performed at a target temperature of 600 \({}^{\circ}\)C: (**a**) 10; (**b**) 30; (**c**) 50 mm depths. Figure 21c), both the LWAC and NWC slabs generated larger pore pressures at a heating rate of 5 \({}^{\circ}\)C/min. This is consistent with [PERSON]'s findings [62].