# Experimental Investigation into Permeable Asphalt Pavement Based on Small-Scale Accelerated Testing
## 3 Results and Discussion
### Results of Rut Depth  
After each tens of thousands of loading cycles, the average rut depth was measured. Figure 10a shows the average rut depth of the three types of structures under dry conditions. In the primary phase of loading, the average rut depth increased significantly. With increasing cycles, the surface of the asphalt pavement was compacted, and the average rut depth had a lower rate of increase. In addition, after 0.3 million cycles, 4 PA-CTPB-D produced the highest rut depth, and 7 PA-CTB-D produced the lowest rut depth, which indicated the increasing thickness of the surface layer and modulus of the base layer could decrease rut depth. 7 PA-CTB-D had a thicker surface layer and higher modulus of base layer, which could be the reason why 7 PA-CTB-D produced the lowest rut depth. Generally, using less binder or a stiffer binder both improved pavements' rutting performance. In addition, pavements with a thin surface layer had less permanent compression than pavements with a thick surface layer, mostly because the thicker layer has more material to deform [48]. The rut depth of 4 PA-CTPB-D was higher than that of 7 PA-CTPB-D. In other words, the thinner surface layer produced a larger rut depth. The results could be affected by the scale and other environmental and test conditions. Figure 10b presents the average rut depth of 7 PA-CTPB-W and 7 PA-CTB-W under wet conditions. Compared with dry conditions, it was similar in that the average rut depth increased significantly in the primary phase of loading, and the average rut depth increased slowly in the secondary phase. In addition, the average rut depth had a more significant difference between 7 PA-CTPB-W and 7 PA-CTB-W, and the average rut depth of 7 PA-CTPB-W was higher than that of 7 PA-CTB-W. Compared with the rut depth under dry conditions, the average rut depth of 7 PA-CTPB-W increased to 2.65 mm only after 0.85 million cycles, whereas 7 PA-CTPB-D increased to 2.77 mm after 0.3 million. Similarly, the average rut depth of 7 PA-CTB-D was higher than 7 PA-CTB-W. Fully permeable pavements allow rainwater to infiltrate into the subgrade, which results in reduced stability due to the infiltration and scour of rainwater. The subgrade is mostly affected by rainwater [49]. Under the influence of water and repetitive load, the subgrade will easily lose its strength and stability, resulting in damage to the entire pavement structure. In this study, the initial permeability was 2.3 \(\times\) 10\({}^{-5}\) cm/s, and the initial compac \begin{table}
\begin{tabular}{c c c} \hline
**Types** & **Longitudinal Strain (\(\mu\varepsilon\))** & **Transverse Strain (\(\mu\varepsilon\))** \\ \hline
4 PA-CTPB-D & 28 & 10 \\
7 PA-CTPB-D & 12 & 94 \\
7 PA-CTB-D & - & 42 \\
7 PA-CTPB-W & 10 & 50 \\
7 PA-CTB-W & 28 & 36 \\ \hline \end{tabular}
\end{table} 
Table 6: Typical resilient strain of different types of structures. research indicated increasing the strength or compacting level could effectively improve the deformation resistance of the surface layer. The subgrade with a high modulus was able to change the stress-strain behavior of the pavement structure and decreased fatigue and rutting damage [50,51]. Finite element and mechanical analyses indicated increasing the elastic modulus of the subgrade could decrease the deflection of surface and tensile stress or strain at the bottom of the surface layer [52,53]. In this study, the initial compacting level of the subgrade was 90%; the subgrade could be recompaced after water immersion and being loaded. After the subgrade was soaked, the average rut depth of the surface layer decreased. This might be caused by re-compaction of the subgrade, which increased the strength of the subgrade. Moreover, from Table 6, the recoverable strain of 7 PA-CTPB-D is significantly higher than that of 7 PA-CTPB-W. The increase in the modulus of the subgrade soil was beneficial, leading to a lower tensile strain. This is consistent with the results of strain in this study.