# Experimental Investigation into Permeable Asphalt Pavement Based on Small-Scale Accelerated Testing
## 3 Results and Discussion  
### Dry Condition  
The strains at the bottom of the surface layer of the three structures were collected. Figure 6 presents the history of the longitudinal and transverse strain versus time at the initial loading stage. In general, the total strain response of the asphalt pavement includes two parts: permanent strain and recoverable resilient strain. The resilient strain is defined as the magnitude of the strain recovering after a complete loading and unloading cycle. It was calculated as the maximum recoverable strain in 30 s of data acquisition. As shown in Figure 6, the peak of the strain curve of each structure corresponded to a wheel load. It was obvious that the longitudinal strain had alternations of tension and compression, while the transverse strain only demonstrated a tension state [35; 36]. Each tension peak corresponded to a wheel load located right above the strain gauge. When the strain reached a peak, there was next a slow recovery process, which was the stress relaxation process duo to the viscoelastic property of the asphalt mixture. In addition, the effect of the material's viscoelasticity results in the asymmetric shape of the strain curves under transient tire loading, especially due to delayed recovery after wheel load has passed the strain gauge position. Due to damage to the longitudinal strain gauge at the bottom of 7 PA-CTB-D, it was impossible to obtain accurate longitudinal strain data under dry conditions. Therefore, the longitudinal strain of 7 PA-CTB-D is not discussed in this paper. From Figure 6a, the resilient tensile strain of 4 PA-CTPB-D is close to the compressive strain at a value of 28 \(\mu\varepsilon\). However, the resilient tensile strain of 7 PA-CTPB-D was 12 \(\mu\varepsilon\), which was obviously less than the 36 \(\mu\varepsilon\) of the resilient compressive strain. From Figure 6b, the transverse resilient tensile strain of 4 PA-CTPB-D is less than the other two structures. The resilient tensile strain of 7 PA-CTPB-D was 94 \(\mu\varepsilon\), which was significantly larger than that of 7 PA-CTB-D at a value of 42 \(\mu\varepsilon\). Figure 7 presents the variation in the resilient strain of the whole loading period and the first day of loading under dry conditions. From Figure 7a, the transverse resilient strain of 7 PA-CTPB-D is significantly larger than the other strains. If the thickness of the surface layer was thin enough, this could lead to more compression or even induce compressive strain. These strains were partially from the viscoelastic property and partially from the material's movement under the wheel track due to the permanent deformation [16]. And the effect of permanent deformation was much higher for the transverse strain gauges compared with the longitudinal strain gauges [37; 38; 39]. In addition, the edges of the steel tank were close to the wheel track, resulting in higher transverse resilient tensile strain. However, the longitudinal strain could be higher than the transverse strain, as 4 PA-CTPB-D shows. Some studies showed that when multiple axles and multiple wheels are used with a wander, the longitudinal strain can be higher than the transverse strain. However, only a single wheel and single axle were used in this study. Therefore, that was probably because a thinner surface layer was used in 4 PA-CTPB-D. In particular, stopping the machine resulted in typical relaxation curves between the stop and subsequent start time, indicating similar relaxation behavior for any loading period. In addition, during a day loading, the transverse resilient strain of 7 PA-CTPB-D increased with increasing cycles, and the range in the variation of strain was 80-100 \(\mu\epsilon\). However, the longitudinal resilient strain of 7 PA-CTPB-D did not change obviously. Moreover, with increasing loading cycles, the ranges in both the longitudinal and transverse resilient strains of 4 PA-CTPB-D were relatively small, which indicated that the thinner surface layer resulted in less tension than the thicker surface layer under load.