## 4 Mechanistic-Empirical Pavement Analysis Considering Dynamic Axle Loads
### Impact of Overweight Traffic on Pavement Performance under Dynamic Loads
Figure 9 presents reflective fatigue cracking in pavement structures on minor highway under static and dynamic loads of total and non-overweight traffic. Taking into account the fact that dynamic axle load spectra accelerated pavement distress under total traffic and decelerated pavement distress under non-overweight traffic, the impact of overweight traffic on pavement distress would be significant when dynamic loads are considered. It suggested that the negative influence of overweight traffic on pavement distresses was underestimated if static axle load spectra were applied in M-E design. Figure 8: _Cont._  
Figure 8: Static and dynamic axle load spectra of non-overweight traffic: (**a**) single axle (major); (**b**) tandem axle (major); (**c**) tridem axle (major); (**d**) single axle (minor); (**e**) tandem axle (minor); and (**f**) tridem axle (minor). Design reliability is supposed to be selected in balance with performance criteria. The recommended reflective fatigue cracking (alligator cracking) of AC overlay on Interstate and primary highways are 10% and 20%, respectively. The recommended run depth on Interstate and primary highways are 10.16 mm (0.4 inch) and 12.7 mm (0.5 inch), respectively. The recommended reliability level of principal arterial is 90% [35]. The considered criteria of fatigue cracking were 10% and 5% due to the limit of cracking propagation, as shown in Figures 6a and 7a. To quantify the effect of overweight traffic loading on pavement distress, the reduction ratio of pavement service life was defined by Equation (4). \[R_{r}=\frac{L_{T}-L_{N}}{L_{N}} \tag{4}\]  
Figure 9: Fatigue cracking (reflective) of (**a**) major highway and (**b**) minor highway under total and non-overweight traffic (10% existing cracking). where, \(R_{r}\) is the reduction ratio of pavement service life; \(L_{T}\) is the service period when a given level of pavement distress occurs under total traffic; and \(L_{N}\) is the service period when a given level of pavement distress occurs under non-overweight traffic. Since the terminal rut depth at the end of design life was much smaller than the recommended values as shown in Figures 6b and 7b, the pavement service life determined by rut depth and corresponding reduction ratio due to dynamic loads were not considered in this study. Figure 10 shows the impact of overweight traffic on the reduction ratio of pavement life, considering different percentage of existing fatigue cracking in the AC layers. The considered threshold of reflective fatigue cracking was 5%. The analysis results indicated that the dynamic axle load spectra increased the impact of overweight traffic on reflective fatigue cracking, and the pavement structure on major highway tended to be more sensitive to overweight traffic loading. One of the reasons would be greater DLC values corresponding to the higher operational speed on major highways. Pavement service life was found to reduce as existing fatigue cracking increased, and the reduction ratio of pavement service life increased as existing fatigue cracking increased. The effect of existing cracking had a more significant impact on the pavement structure of major highways. The service life of the pavement structure with 15% existing fatigue cracking was reduced by an additional 12%, which increased the reduction ratio of pavement life up to 46%.