## 3 Results and Discussion
### Pore Pressure Distribution of the Concrete Slabs Exposed to High Temperatures  
In addition to the thermo-mechanical process, the high-temperature spalling of concrete is also related to the thermo-hygral process, which is directly associated with the mass transfer of vapor, water, and air in the porous network [40; 55]. In this study, pore pressure was a local measurement whose value depended on the location of the tube (in a cement matrix, near an aggregate, or in an air void). When the concrete slab was heated, three regions of dry/dehydrated, drying/dehydrating, and quasi-saturation were formed inside [40; 56]. Due to the different temperature distributions in these regions, the corresponding pore pressures were also different, resulting in different degrees of cracking, flaking, or even spalling on the surface. Figure 13 shows the comparison of the pore pressure-time curves of the three measuring points in the LWAC slab under different experimental variables. Overall, under different test variables, the pore pressure measured at each measuring point of the specimen was also different. The measured pore pressures varied approximately between 77 and 2121 KPa, which were lower values compared to the data in the literature. In the case of Figure 13d, a pressure of 2121 kPa was measured at a depth of 50 mm from the heated surface of the air-dried specimen, even though there was no spalling. Furthermore, under certain experimental conditions, the pore pressure exhibited a sudden drop, as shown in Figure 13d,f-h. This was caused by the rapid escape of moisture vapor into the atmosphere as the concrete fragments peeled off the heated surface. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Type of Concrete**} & \multirow{2}{*}{**Specimen**} & \multirow{2}{*}{**Target Temperature**} & \multirow{2}{*}{**Heating Rate**} & \multicolumn{2}{c}{**Maximum Thermal Gradient**} \\  & & & & & (\({}^{\circ}\)C/mm) & \\ \cline{5-6}  & & & & & **Depth from Heated Surface** \\ \cline{5-6}  & & & & **10 mm** & **30 mm** & **50 mm** \\ \hline \multirow{6}{*}{LWAC} & \multirow{2}{*}{Oven-dried} & \multirow{2}{*}{600 \({}^{\circ}\)C} & 5 \({}^{\circ}\)C/min & 12.41 & 7.44 & 5.47 \\  & & & 10 \({}^{\circ}\)C/min & 3.69 & 5.87 & 4.00 \\ \cline{3-6}  & & & 5 \({}^{\circ}\)C/min & 10.15 & 7.82 & 7.30 \\  & & & 10 \({}^{\circ}\)C/min & 11.61 & 12.27 & 8.60 \\ \cline{2-6}  & & & 5 \({}^{\circ}\)C/min & 0.02 & 2.59 & 3.77 \\  & & & 10 \({}^{\circ}\)C/min & 16.00 & 5.42 & 5.48 \\ \cline{2-6}  & & & 5 \({}^{\circ}\)C/min & 178.28 & 28.03 & 27.67 \\  & & & 10 \({}^{\circ}\)C/min & 25.44 & 10.38 & 6.98 \\ \hline \multirow{6}{*}{NWC} & \multirow{2}{*}{Oven-dried} & \multirow{2}{*}{600 \({}^{\circ}\)C} & 5 \({}^{\circ}\)C/min & 10.43 & 3.39 & 3.40 \\  & & & 10 \({}^{\circ}\)C/min & 7.17 & 5.04 & 4.31 \\ \cline{1-1} \cline{3-6}  & & & 5 \({}^{\circ}\)C/min & 6.56 & 6.45 & 4.86 \\ \cline{1-1} \cline{3-6}  & & & 10 \({}^{\circ}\)C/min & 7.28 & 6.42 & 4.69 \\ \cline{1-1} \cline{3-6}  & & & 5 \({}^{\circ}\)C/min & 13.70 & 9.12 & 7.65 \\ \cline{1-1} \cline{3-6}  & & & 10 \({}^{\circ}\)C/min & 4.25 & 6.01 & 5.03 \\ \cline{1-1} \cline{3-6}  & & & 5 \({}^{\circ}\)C/min & 12.97 & 8.65 & 7.72 \\ \cline{1-1} \cline{3-6}  & & & 10 \({}^{\circ}\)C/min & 25.80 & 10.46 & 8.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Maximum thermal gradient. Figure 13: The build-up of pore pressure with time for the LWAC slabs: (**a**) L-OD-600-R5; (**b**) L-OD-600-R10; (**c**) L-AD-600-R5; (**d**) L-AD-600-R10; (**e**) L-OD-800-R5; (**f**) L-OD-800-R10; (**g**) L-AD-800-R5; (**h**) L-AD-800-R10. The evolution with time of the temperature and pore pressure at a depth of 30 mm from the heated surface of the LWAC slabs at a target temperature of 600 \({}^{\circ}\)C and a heating rate of 5 \({}^{\circ}\)C/min is shown in Figure 14. The figure shows that the peak value of the measured pore pressure was reached in the vaporization temperature range.