# Impacts of Material Orthotropy on Mechanical Behaviors of Asphal Pavements
## 5 Discussion  
Previous studies have shown that, with the increase in the anisotropy, the road surface deflection, tensile stress, and strain at the bottom of the layer increase, and the maximum change rate can reach more than 20% [5]. In addition, the maximum change rate of a mechanical index is related to the range of the anisotropy [26]. The larger the range, the greater the influence on the results. In this paper, the results showed that the orthotropy increased the surface deflection and compressive strain on the subgrade top by 21% and 19%, respectively, which is consistent with previous studies. Based on the test results of the material parameters, the minimum anisotropy values of surface course and base course materials were 0.76 and 0.81, respectively, which are not less than 0.7. However, most of the material parameters used by previous scholars were based on assumed values, and the anisotropy could range from 0.17 to 1 [48]. The FE results in this study showed that: (1) the maximum deflection change rate of the pavement surface was 3.8% under the transverse isotropy, (2) the maximum change rate of the maximum tensile stress at the base was 3.0%, (3) the maximum change rate of the maximum compressive strain on the top of the subgrade was 6.7%, and (4) the absolute value of the maximum change rate of the three mechanical indexes was not more than 7%. Given this, the reason for the small change rate of the above mechanical indices was that the anisotropy range of material parameters was small in the FE calculations. In addition, the effects of the transversely isotropic and orthotropic materials on the mechanical properties were identical in this study while their change rates of the material anisotropy effect on mechanical properties were different. By comparing the difference of the maximum change rate of each mechanical index between the two kinds of anisotropic materials, we can deeply understand the difference in their influence on the asphalt pavement performance. The more significant the difference between them, the more sensitive it is to reflect the difference of the two materials effect on the asphalt pavement mechanical properties. Three pavement mechanical indices were evaluated: the (1) maximum deflection of pavement surface, (2) maximum tensile stress at the bottom of pavement layer, and (3) maximum compressive strain on the subgrade top. In the three indices, the difference in the maximum deflection change rate of pavement surface was the most prominent (17.4%), indicating that the difference between the transversely isotropic and orthotropic materials was the most sensitive to the pavement bearing capacity. The difference in the change rate of the maximum compressive strain on the subgrade top was 12.3%, indicating that the difference in the properties of the transversely isotropic and orthotropic materials was more sensitive to the maximum compressive strain on the subgrade top. The difference in the change rate of tensile stress at the bottom of the pavement layer was the most minuscule (\(-7.6\%\)), indicating that the difference between the two materials was the least sensitive to the maximum tensile stress at the bottom of the pavement layer. The comparison results showed that considering only the material transverse isotropy was insufficient. This may underestimate the influence of material anisotropy on the mechanical properties of pavement. Thus, it is necessary to consider the material orthotropy further. Based on the influence of orthotropic materials on the mechanical behavior of pavement, three significant suggestions for controlling the material during the process of pavement construction are put forward, as follows:  
1. The increased surface deflection illustrates that those materials (in all layers or each layer) only considered as isotropic are unfavorable for the pavement bearing capacity. Therefore, we recommend that the homogeneity degree (especially for the filling subgrade) be strictly controlled during construction to assure adequate pavement capacity. 2.