# Simulation of the Cooling Effect of Porous Asphalt
## 4 Sunshine Experiment
### Data Analysis  
Asphalt concrete is a temperature-sensitive material. Nonlinear viscoelastic response will occur inside asphalt concrete at high temperatures [19; 20; 21]. Porous asphalt pavement performs similarly at high temperatures. The surface temperature variation curves of each specimen in the sunshine experiment under windless conditions are shown in Figure 4, including two kinds of curves. The curves marked with \"(test)\" represent the results of the laboratory sunshine experiment, and the others marked with \"(simulation)\" are the results of the virtual simulation experiment. Figure 4: _Cont._  
It was shown that the temperature variation of the AC specimens and the PAC specimens was similar at the same depth. At the beginning of the experiment, the surface temperature of all specimens rose rapidly. As time went by, the surface temperature curves gradually become gentle and tended to be stable. By comparing temperature values at different depths, it can be concluded that the temperature conduction in asphalt pavement has hysteresis on the vertical direction. With increasing depth, this hysteresis becomes more obvious. About half of the temperature increment was completed within a few minutes after the experiments began. However, at the bottom of the specimens, almost no temperature change was observed in the first 25 minutes. Therefore, with increasing depth, the temperature variation will be less affected by the temperature variation of the pavement surface. Besides this, the results of the laboratory experiment and the virtual simulation experiment are very close, indicating that the accuracy of the virtual experiment is very reliable. The temperature values of the specimens at different depths in the windless laboratory sunshine experiment are listed in Table 3. As shown in the table, the surface temperature of PAC specimens was slightly higher than that of the AC specimens. The reason for this phenomenon is that after absorbing heat on the surface in a windless environment, there is no air flow between adjacent voids to take away heat and reduce temperature. Moreover, as mentioned above, the thermal diffusion coefficient of the porous asphalt mixture decreases with increasing proportion of air voids. Therefore, larger air voids will accumulate more heat on the surface, so the temperature will be higher. In addition, when the depth was 4 or 10 cm (corresponding to the top surface and the bottom surface of the middle layer), the temperature of the PAC specimen was lower than that of the AC specimen, indicating that when the wind and pavement are all dry, the temperature of the middle layer of porous pavement is still lower than that of the dense-graded pavement. Putting mainly occurs in the middle and lower layers. Therefore, porous asphalt pavement can reduce the temperature of the  
Figure 4: Temperature variation curves of each specimen. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimen Number**} & **Upper-Layer Air** & \multirow{2}{*}{**Lower-Layer Material**} & \multicolumn{3}{c}{**Equilibrium Temperature (\({}^{\circ}\)C)**} \\ \cline{3-6}  & **Voids (Material)** & & **Surface** & **4 cm** & **10 cm** \\ \hline AC & AC-13 & AC-20 & 65.6 & 41.3 & 30.3 \\ S1 & 16.68\% (PAC-13) & AC-20 & 64.8 & 40 & 29.2 \\ S2 & 18.11\% (PAC-13) & AC-20 & 67.4 & 41.3 & 28.9 \\ S3 & 19.66\% (PAC-13) & AC-20 & 66.45 & 39.3 & 28.8 \\ S4 & 20.79\% (PAC-13) & AC-20 & 67.2 & 38.8 & 28.6 \\ S5 & 23.05\% (PAC-13) & AC-20 & 67.5 & 37.15 & 27.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specim equilibrium temperature. middle layer and lower layers, reduce the probability of rutting under extreme conditions, and thus improve the safety performance of the pavement. It can also be concluded from Table 3 that the larger the proportion of air voids, the lower the temperature of the lower layer in the PAC specimen, and the temperatures at both depths were lower than those in the AC specimen.