# Impacts of Material Orthotropy on Mechanical Behaviors of Asphal Pavements
## 6 Concluding Remarks  
To thoroughly evaluate the impacts of material orthotropy on pavement mechanical behavior, this paper presented a new and efficient 3D FE model of anisotropic materials. Based on this model, nine combinations of material characteristics were designed to determine the contributions of each layer orthotropy to the pavement mechanical properties. Finally, the anisotropic FE model with experiment-based inputs was applied and compared with field measurements. Based on this study, the following comments are offered:  
* The proposed 3D FE model that considers material orthotropy can investigate the anisotropy impacts on pavement mechanical behaviors. The results showed that these impacts should not be ignored and that the all-layer orthotropy was the most unfavorable combination. The results also showed that the material orthotropy had larger impacts on the surface deflection than did the transverse isotropy.
* The anisotropy degrees of the HMA and semi-rigid base layers were determined experimentally. The results can be used as a basis for inputs to analyze pavement mechanical behavior considering the material anisotropy.
* Compared with the isotropic material, the orthotropy significantly increased the surface deflection and the compressive strain on top of the subgrade by 21% and 19%, respectively. The orthotropy impacts in the HMA layer were noticeable in the tensile stress at the bottom of the base layer, while the impacts on the surface deflection were slight. The subgrade orthotropy made the most significant contributions to the surface deflection and the compressive strain on top of the subgrade. Orthotropy in the base layer had slight effects on the mechanical behaviors.
* The developed 3D FE model was shown to be in good agreement with the FWD data of semi-rigid asphalt pavement, showing that this model can be used to predict the mechanical behaviors of semi-rigid asphalt pavements. The proposed 3D FE model, which considers the orthotropic characteristics of pavement materials, provides a practical measure for the mechanical behavior analysis of asphalt pavements. We recommend that the homogeneity degree of the filling subgrade should be strictly controlled to assure adequate pavement capacity and anti-rutting performance during construction.
* The material anisotropy determined in this paper was based on the typical semi-rigid pavement structure. In the future, material parameters under different conditions (e.g., temperatures, moisture, and load) should be tested to provide more accurate in puts for the proposed 3D FE model. Finally, the most unfavorable anisotropy combination should be determined considering different pavement structures (e.g., inverted and composite).  
Conceptualization, M.L.; Formal analysis, M.L.; Methodology, M.L. and Z.J.; Project administration, H.G.; Software, M.L.; Supervision, C.H., H.G., and S.M.E.; Validation, M.L.; Writing--original draft, M.L.; Writing--review and editing, S.M.E. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 52078062 and the Natural Science Foundation of Hunan Province, grant number 2020 JJ4604.  
Not applicable.  
Data Availability Statement: Data sharing is not applicable to this article.  
The authors are grateful to three anonymous reviewers for their constructive suggestions and comments that have improved the content and presentation of the paper.  
The authors declare no conflict of interest.