# Simulation of the Cooling Effect of Porous Asphalt
## 5 Finite Element Simulation of the Pavement Temperature Field
### Finite Element Simulation Result Analysis  
Because the temperature of the upper layer and the middle layer has a great influence on the pavement, we selected three main depths in the temperature field analysis process to evaluate the cooling effect of the porous asphalt pavement. The three depths were located on the pavement surface and 4 and 10 cm below the pavement surface. The data from each depth in each analysis step were collected to obtain the daily variation of the pavement surface temperature at different depths, as shown in Figure 7.  
As shown in Figure 7, the temperature of the pavement surface changed dramatically within 24 hours, and with increasing depth, the temperature change extent decreased gradually. The temperature peaks at different depths (including surface, 4 cm, and 10 cm) appeared at 13:00, 14:00, and 16:00, respectively. As the depth increased, the temperature peak appeared with more lag. This phenomenon explains the hysteresis of heat transfer.  
Figure 7: _Cont._  
From the coordinate data where the temperature peak occurred, it can be seen that the temperature peak at each depth of the PAC pavement was lower than that of the dense-graded asphalt pavement. Among the PAC pavement models studied, the minimum value of surface temperature peak appeared on the surface of the S5 pavement model at 53.05 \({}^{\circ}\)C. Similarly, the minimum temperature peak values at 4 cm depth and 10 cm depth also appeared at the corresponding depths of the S5 pavement model; the values were 44.73 and 39.65 \({}^{\circ}\)C, respectively. In addition, it can be seen that with increasing proportion of air voids, the temperature peak at a certain depth of the PAC pavement model showed a downward trend, indicating that the cooling performance of porous asphalt pavement is obviously better than that of dense-graded asphalt pavement.  
In order to quantify the cooling effect of PAC pavement, AC pavement was used as a benchmark, and the difference between the peak temperatures of the PAC pavement and the AC pavement at different depths was used to characterize the cooling effect, as shown in Figure 8. Fitting models between air voids and the cooling effect were obtained. The fitting curves and Equations are shown in the figure. It can be concluded that with increasing proportion of air voids, the cooling effect of the porous asphalt pavement at each depth showed an increasing trend. When the upper layer air void was 16.68%, there was almost no cooling effect on the pavement surface. When the upper layer air void was 23.05%, the cooling effect was the most obvious, and the cooling effects at 0, 4, and 10 cm depth were 5, 5.49, and 3.85 \({}^{\circ}\)C, respectively. Through the fitting models, a good positive linear correlation was found between the cooling effect at each depth of the single-layer porous pavement and the proportion of air voids of the PAC-13 pavement.  
Figure 8: Fitting curves of air void proportion and the cooling effect of PAC pavement.  
Figure 7: Temperature variation curves of each specimen at different depths.  
The above research shows that PAC pavement can effectively reduce the temperature inside the pavement, thereby reducing the occurrence of rutting and tire burst caused by high-temperature pavement and improving the pavement safety performance. The fitting models can provide an important reference for PAC pavement design.