## 4 Mechanistic-Empirical Pavement Analysis Considering Dynamic Axle Loads
### Impact of Dynamic Loads on Pavement Performance  
In general, the new asphalt overlay shows similar cracking pattern, which matches the fatigue cracking in the existing asphalt layer after opening to traffic. The cracking propagation from the existing AC layer into and through the new AC overlay is known as reflective cracking. The occurrence of reflective cracking tends to cause water infiltration, rapid deterioration of the underlying pavement structure, and reduction in pavement service life. Reflective cracking is the result of strain concentration in the AC overlay due to the movement induced by shearing or bending action from traffic loading and daily/seasonal temperature changes in the existing asphalt pavement at the vicinity of cracks. Thus, reflective cracking is one of the failure criteria for asphalt overlay. Regarding AC overlay rutting, it is well accepted that rutting or permanent deformation is the combined result of densification due to the volume change of asphalt mixture, and repetitive shear deformation is the result of plastic flow without volume change. Rut depth in the AC layer is another failure criterion for AC overlay design. Rutting depth is impacted by many factors, such as binder content, binder type, mix type, temperature, load level, and initial density after compaction [29]. In M-E pavement design, reliability is a combined indicator of probability that each performance measure will be less than a specific level over the analysis period [1]. The reliability level of 90% was considered in the analysis. In the M-E analysis, initial IRI values were assumed to be 1 m/km (63 inch/mile) for both types of highway, and the terminal IRI values at the 20 th year of service period were found to be 2.25 m/km (142 inch/mile) and 2.32 m/km (146 inch/mile) for major and minor highways, respectively. When the initial IRI was 2 m/km (126 inch/mile), the terminal IRI values at the 20 th year of service  
Figure 5: Dynamic modulus of asphalt mixture for overlay. period were 3.97 m/km (250 inch/mile) and 4.03 m/km (254 inch/mile) for major and minor highways, respectively. Besides, the initial IRI was found to have no influence on the propagation of pavement distress, except for terminal IRI value. Therefore, the IRI values of 1 m/km, 2 m/km, and 3 m/km, which included a reasonable range of IRI values during pavement service period, were applied to derive the dynamic axle load spectra and evaluate the effect of dynamic loads on pavement distresses. M-E outputs indicated that dynamic loads increased IRI, rut depth (total and AC only), and fatigue cracking (reflective) during the pavement service period. The effect of dynamic loads on reflective fatigue cracking was relatively more significant. Figure 6 illustrates the comparison of reflective fatigue cracking and rut depth under the static and dynamic loads in pavement structures on major highway. Reflective fatigue cracking of AC overlay pavement sections includes reflective cracking and bottom-up cracking in AC layer. The AC overlay shows a similar cracking pattern which matches existing cracking. Reflective fatigue cracking increased rapidly after a 2-year service period, and approached a specific value after a 10-year analysis period. The rut depth in the AC layer accumulated slowly over time. It indicated that the impact of dynamic loads on reflective cracking varied during the cracking propagation. The influence became more significant from the 3 rd to 7 th year, while a slight influence was observed at the end of the cracking propagation. The impact of dynamic loads on rut depth was found to slightly increase with the accumulation of rut depth with pavement age, but the overall impact was not significant. Figure 7 illustrates the reflective fatigue cracking and rut depth in pavement structure on minor highway under static and dynamic loads. Reflective fatigue cracking propagated rapidly after a 2.5-year service period and approached constant value after a 15-year analysis period.