# Experimental Investigation into Permeable Asphalt Pavement Based on Small-Scale Accelerated Testing
## 3 Results and Discussion
### Results of Temperatures of Bottom Layer  
In this study, the temperature at the bottom of the surface layer was measured, and the temperature differences from the initial temperature are demonstrated in Figure 11. The temperature differences of different structures during the entire loading are shown in Figure 11a-c. The temperature differences presented the same variation trend as strain versus cycles. The temperature differences increased with increasing loading cycles during the daytime, and the temperature difference dropped to a minimum during the nighttime. Figure 10: Rut depth: (**a**) dry conditions and (**b**) wet conditions. Therefore, the strain variation was caused by both load and temperature. For outdoor experiments, there exist many factors, especially temperature. Therefore, it was extremely difficult to maintain an exact target temperature for the entire duration of the test; the plots show that the variations were within 6 \({}^{\circ}\)C. As shown in Figure 11a, 4 PA-CTPB-D produced a slightly higher temperature at the bottom of the surface layer due to a thinner surface layer than 7 PA-CTPB-D. From Figure 11b, the temperature differences of 7 PA-CTPB-D were a little higher than those of 7 PA-CTB-D, which could be because porous base materials had a lower heat conductivity, leading to more heat accumulating on top of the porous base layer. Figure 11c demonstrates that the temperature differences of 7 PA-CTPB-W were a little higher than those of 7 PA-CTB-W. The denser cement-treated base had a higher heat conductivity, which resulted in the fact that heat could transfer more easily into the base and subgrade. Moreover, after a relaxation of one night, the temperature difference of 7 PA-CTB-W was a little lower than that of 7 PA-CTPB-W, which could be caused by porous base material with a low heat conductivity. Compared with a drainage pavement structure, fully permeable pavement with a porous base could produce a higher temperature at the bottom of the surface layer, potentially leading to more damage to the surface layer, because porous base materials have a lower heat conductivity compared with dense base materials. The heat transfer principle is mainly affected by the heat conductivity of materials. Heat transfer efficiency reduces when heat transfers from the surface layer to the base layer. Heat will accumulate between the two layers. In this study, the accelerated pavement testing was stopped every night, which resulted in typical relaxation behavior; the strain and temperature differences at the beginning of each day of loading are presented in Figure 11d-f. The transverse strain obviously varied with the temperature differences, while the variation in longitudinal strain with temperature difference was not obvious. From Figure 11d, 4 PA-CTPB-D induced a little less variation in strain that 7 PA-CTPB-D, which indicated that the thicker surface layer had greater fluctuations in temperature and strain. From Figure 11e, compared with the other two structures, 7 PA-CTB-D presents a similar change law. Especially, both the transverse and longitudinal strains of 7 PA-CTB-W obviously varied with temperature differences and present a similar change trend from Figure 11f. After the relaxation of a night, the strain recovered, and the remaining unrecovered strain left. From Figure 11d-f, both the transverse and longitudinal strains of the three structures varied with temperature, and no obvious permanent unrecovered strain was produced, which indicated that temperature was a critical factor affecting the strain. In this study, the environmental condition was uncontrollable, and further study should consider this factor. However, continuous full-scale accelerated pavement testing should be conducted on a field pavement. Therefore, if allowed, conducting continuous down-scaled accelerated pavement testing would obtain better results. The above results indicated that the temperature produced some effect on the viscoelastic asphalt layer.