# Experimental Investigation into Permeable Asphalt Pavement Based on Small-Scale Accelerated Testing
## 2 Materials and Methods
### Test Procedure  
In this study, the effects of dry and wet subgrade on the mechanical properties were explored. Wheel track 1 was firstly loaded under dry condition as shown in Figure 5. After 0.3 million cycles of loading, the MMLS3 was moved to wheel track 2. At the same time, water was poured into the inner pavement through a pre-set pipe. In addition, during the entire loading, water was poured into the pavement to maintain the wet condition of the subgrade by checking the water level in the pipe. From Figure 4b, it can be seen that a PVC pipe was installed in the pavement as an observation well. In this study, these 3 types of structures were loaded for 0.3 million cycles at wheel track 1 and 0.85 million cycles at wheel track 2. In fact, the structures of 4 PA-CTPB-D and 7 PA-CTPB-D were loaded for 0.35 million cycles, where the deformation and strain barely increased with increasing cycles after 0.35 million cycles. The structure of 7 PA-CTB-D was loaded for 0.3 million cycles. Moreover, since the length of the wheel track was only 1260 mm, it was impossible to load three structures at the same time. Therefore, 4 PA-CTPB and 7 PA-CTPB were firstly loaded for 0.35 million cycles under dry condition, then 7 PA-CTB was loaded for 0.3 million cycles. After tests under dry conditions, water was poured into the inner structure of 4 PA  
Figure 3: The master curve of the porous asphalt mixture.  
CTPB and 7 PA-CTPB to maintain the wet state of the top of the subgrade. 7 PA-CTPB and 7 PA-CTB were loaded for 0.85 million cycles in this study. More loading cycles were chosen to investigate the effects of water on the structural mechanical properties.  
During the period of testing, MMLS3 was used to load the experimental pavement, and no temperature control was used during the entire testing. The strain responses, rut depth, and temperature data were measured during the entire MMLS testing. The model mobile load simulator was running at daytime and stopped at nighttime. The strain data were stored for 30 s every hour. Due to the stopping of the MMLS3 device (MIS, Randburg, South African) at the nighttime, the entire testing lasted several days. The data collection frequency was 150 Hz.  
Figure 4: Summary of work sequence: (**a**) steel tank; (**b**) compacted base; (**c**) strain gauge; (**d**) compacted asphalt mixture; (**e**) the loading device.  
Figure 5: Plane graph of positions of wheel track and strain gauges.  
In this study, the permeability coefficient of pavement before and after loading was tested according to ASTM C1701 [34]. Before loading, the permeability coefficients of 4 PA-CTPB, 7 PA-CTPB, and 7 PA-CTB were 0.18 cm/s, 0.17 cm/s, and 0.14 cm/s, respectively. After loading, the permeability coefficients of the three pavement structures were 0.17 cm/s, 0.15 cm/s, and 0.12 cm/s, respectively. The load resulted in a decrease in the air void of the asphalt mixture in the wheel track and a decrease in the permeability coefficient of the pavement structure. Because the accelerated loading test was an indoor test, no excess soil or contaminants blocked the air void, and the effect of clogging on the long-term performance of permeable pavement is not discussed.