## 4 Conclusions  
In order to investigate the effects of asphalt migration on the dynamic modulus of asphalt mixture, the asphalt mixture samples with five different asphalt contents layers were prepared for the dynamic modulus test with a Simple Performance Tester (SPT). By constructing the master curves of dynamic modulus and phase angle, the effects of asphalt migration on the dynamic modulus and the variation laws of the dynamic modulus ([E*]) and phase angle (\(\delta\)) at different testing temperatures and loading frequencies were analyzed in this paper. Additionally, the dynamic modulus ([E*]) and the stiffness  
Figure 9: The variations of dynamic modulus and the stiffness parameter of five groups of samples (10 Hz, 50 \({}^{\circ}\)C): (a) \(|\)E*\(|\); (b) \(|\)E*\(|\)/sin\(\delta\). parameter (\(|\)E*\(|\)/sin\(\delta\)) at the loading frequency of 10 Hz and testing temperature of 50 \({}^{\circ}\)C were also studied to further investigate the effects of asphalt migration on the rutting resistance of asphalt mixture at relatively high temperature. Based on the laboratory test results, the following conclusions were drawn:  
By simulating the state after the occurrence of asphalt migration amid asphalt pavement layers and constructing the master curves with the WLF equation and Sigmoidal function based on the time-temperature superposition principle (TTSP), the effects of asphalt migration on the dynamic modulus and the variation laws of the dynamic modulus of asphalt mixture after the occurrence of asphalt migration were investigated at the quantitative level. The dynamic modulus test makes it apparent that the dynamic moduli of asphalt mixtures increase with the increasing loading frequency, while they decrease with the increasing testing temperature. The dynamic modulus of the first group of asphalt mixture samples (with the optimum asphalt content of 5.0% in upper half layer and lower half layer) are the highest and then become smaller with the incremental differences of asphalt content in the upper and lower half portions. This indicates that the asphalt migration has compromised the mixture's mechanical structure and the more asphalt migrates, the weaker the mechanical properties of asphalt mixture will be. Different from the master curves of dynamic modulus, the master curves of phase angle firstly increase with the increase of loading frequency to the highest point, and then decrease with the further increase of loading frequency. The master curves of phase angle are not as smooth as that of dynamic modulus. Moreover, the phase angle of the first group of samples (5.0% ACU and 5.0% ACL) are the smallest, indicating that the asphalt migration has compromised the aggregates skeleton and aggravated strain response lag. In the analysis of the dynamic modulus and stiffness parameter of asphalt mixture, the dynamic modulus and stiffness parameter of five groups of asphalt mixtures decrease with the incremental difference of asphalt content in the upper and lower half layers and have the highest values at the optimum asphalt content (5.0% ACU and 5.0% ACL). This suggests that the asphalt migration has weakened the rutting resistance of asphalt mixture at relatively high temperature and the more asphalt migrates, the less rutting resistance of asphalt mixture. According to the research in this study, the effects of asphalt migration on the dynamic modulus and the variation laws of the dynamic modulus of asphalt mixture after the occurrence of asphalt migration can be better construed. This may help road engineers to figure out the correlation between asphalt migration and mechanical properties of asphalt mixture during the design of asphalt pavement, and to avoid asphalt migration and road rutting to a greater extent. For example, limiting the vehicle speed and axial load, as well as using some constructing materials with preferable high temperature resistance, such as stiffer asphalt or fiber mixture, etc., may be effective ways to reduce asphalt migration and road rutting.