## 3 Results and Discussion
### Pore Pressure Distribution of the Concrete Slabs Exposed to High Temperatures
This result suggests that the vaporization of water into the concrete was responsible for the increase in pore pressure. The diffusion of heat in the specimen and the resulting complex hygral behavior created a zone of saturation close to the exposed surface (i.e., moisture clog) [48]. Figure 15 compares the pore pressure and saturated water vapor pressure curves of the LWAC slabs. Essentially, the saturated water vapor pressure is a function of temperature, which is the maximum pressure of water vapor at a certain temperature. From the relationship between the pore pressure and the saturated water vapor pressure curves, it can be divided into three stages. The first is called the supersaturated state, which represents a state where the pore pressure is higher than the saturated water vapor pressure. The second is called the saturated state, which represents the state where the pore pressure is equal to the saturated water vapor pressure. The third is called the unsaturated state, which represents the state where the pore pressure is lower than the saturated water vapor pressure. By comparing the saturated water vapor pressure and the pore pressure in the specimen, its moisture state change, water vapor amount, and moisture migration could be determined [57; 58]. Figure 15 shows that the water vapor of the oven-dried specimen at 10 mm was unsaturated. As for the air-dried specimen, under the condition of a heating rate of 10 \({}^{\circ}\)C/min, whether the target temperature was 600 or 800 \({}^{\circ}\)C, the water vapor at 10 and 50 mm was supersaturated. This was due to the contribution of the partial pressure of the dry air enclosed in the pores of the specimen [59]. In contrast, at a heating rate of 5 \({}^{\circ}\)C/min, the water vapor at 50 mm was unsaturated. It is worth noting that spalling also occurred when the water vapor pressure in the specimen was in an unsaturated state. This was because the pores in concrete consist of water in the form of free water, water vapor, and dry air. If the temperature distribution inside the concrete is uniform, the water vapor pressure in the pores increases without creating a pressure gradient between the pores, thereby raising the boiling point of water [58]. Therefore, the water remained liquid and did not evaporate, resulting in an unsaturated state of water vapor pressure. Figure 14: The evolution with time of the temperature and pore pressure at a depth of 30 mm from the heated surface of the LWAC slab at a target temperature of 600 \({}^{\circ}\)C and a heating rate of 5 \({}^{\circ}\)C/min: (**a**) oven-dried state; (**b**) air-dried state. On the other hand, Figure 13 shows that the relationship between pore pressure and time basically consisted of an ascending branch followed by a descending branch, and the ascending and descending rates varied with the location of the measuring point. The ascending branch could be explained as a result of the formation of a fully saturated layer during the transport of moisture to the deeper, cooler regions of the heated concrete, which prevented further transport of moisture vapor, resulting in a build-up of pore pressure at the saturated front [60]. Due to the formation of the moisture clog, the vapor flow would only flow to the heated surface. In addition, the concrete located between the heated surface and the moisture clog continued to dry and dehydrate. Once the rate at which the vapor escaped from the pores was higher than the rate at which the vapor (from drying, dehydration, and transport) filled the pores, the pressure began to drop [48]. At the same time, cracking might also lead to a decrease in pore pressure by creating more usable volume in the matrix, especially by increasing the permeability of the material such that vapor can escape [61]. These two phenomena could explain the shape of the pore pressure curve.