# Experimental Investigation into Permeable Asphalt Pavement Based on Small-Scale Accelerated Testing
## 3 Results and Discussion
### Wet Conditions  
Rainwater can infiltrate into natural soil for fully permeable pavement, which leads to the saturation status of the subgrade for long time during its service life. Therefore, when loading 7 PA-CTPB-W and 7 PA-CTB-W, water was added to maintain the water immersion status of the subgrade. However, it was found that water flowed out of the structures after a few minutes, and the subgrade of 4 PA-CTPB was thicker than the other two structures. In addition, the waterproof geotextile was used to separate 4 PA-CTPB, 7 PA-CTPB, and 7 PA-CTB. Therefore, water could not flow into 4 PA-CTPB. Water was poured into 7 PA-CTPB and 7 PA-CTB every hour to maintain the wet state of the top of the subgrade. The above operation could keep the top of the subgrade in a wet state or  
Figure 7: Resilient strain under dry conditions: (**a**) 7 PA-CTPB-D and 4 PA-CTPB-D; (**b**) 7 PA-CTPB-D and 4 PA-CTPB-D in a day; (**c**) 7 PA-CTB-D; (**d**) 7 PA-CTB-D in a day. saturated state. Figures 8 and 9 present the results of the resilient tensile strain of 7 PA-CTPB-W and 7 PA-CTB-W. As shown in Figure 8a, the longitudinal strain of 7 PA-CTPB-W and 7 PA-CTB-W consisted of both compression and tension, whereas the transverse strain showed tension only. In addition, 7 PA-CTPB-W presented a typical longitudinal strain curve (compression-tension-compression), whereas 7 PA-CTB-W had a variation from compression to tension only. From Figure 8a, the longitudinal resilient strain of 7 PA-CTB-W recovered slowly from the ending of one loading cycle to the beginning of the next loading cycle whereas 7 PA-CTPB-W recovered rapidly. The reason could be that the cement-treated base had a higher modulus or rigid nature. The transverse strains of the two types of structures showed higher resilient tensile strains compared with the longitudinal strains from Figure 9a. 7 PA-CTPB-D also followed the same rule. The transverse resilient tensile strain of both 7 PA-CTPB-W and 7 PA-CTB-W presented tension only, and 7 PA-CTPB-W showed a higher transverse resilient strain caused by the lower modulus of its base and the saturated subgrade. With increasing loading cycles, the longitudinal resilient tensile strain of 7 PA-CTPB-W changed little, while 7 PA-CTB-W's increased significantly. Figures 7a and 9a demonstrate resilient tensile strain versus cycles; these strain histories show an initial high rate of increase (primary phase), followed by a lower rate of increase (secondary phase). In general, the curve of fatigue life had a primary and secondary phase and finally a sudden high rate of increase (tertiary phase) just prior to the failure. Since the testing was intermittent, the strain recovered during relaxation. Therefore, the three types of structures produced no failures in this study. However, an increase in resilient strain in a day could indicate the damage development of the porous asphalt mixture under continuous loading. Figure 8: Wet conditions: (**a**) longitudinal and (**b**) transverse strains versus time (t). Moreover, as shown in Table 6, typical resilient tensile strains for the initial loading stage were chosen. The initial loading stage meant the corresponding structure just began to be loaded. The longitudinal resilient tensile strains of 7 PA-CTPB-D and 7 PA-CTPB-W were close to each other, whereas the transverse resilient tensile strain of 7 PA-CTPB-D under dry conditions was nearly twice as large as that of 7 PA-CTPB-W under wet condition. The tested results proved that the water sensitivity of subgrade soil could be an important factor because of a significant reduction in resilient tensile strain under wet condition. Therefore, this would suggest that the subgrade should comprise high-quality materials for the successful performance of permeable pavements. It might be necessary to treat the natural or low-compacting-level subgrade for specific requirements. However, treated subgrade will increase costs. Therefore, investors and operators should choose the types of structures flexibly.