# Impacts of Material Orthotropy on Mechanical Behaviors of Asphal Pavements
## 1 Introduction  
Asphalt pavement is a type of vertically compacted material during construction that suffers vertical traffic loads in service. Therefore, each layer of pavement material should be considered anisotropic [1, 2, 3]. An anisotropic material indicates that the material properties (the stiffness, modulus, etc.) vary in three directions, i.e., the vertical, longitudinal, and transverse [4, 5]. However, most current asphalt pavement design methods only consider the material properties as isotropic, which can significantly skew the mechanical behaviors of asphalt pavements [6, 7]. In general, the anisotropic properties of asphalt pavement materials include transverse isotropy and orthotropy [8; 9; 10; 11; 12]. The transverse isotropy of asphalt concrete (AC) was first investigated by [PERSON] et al. [13]. After that, [PERSON] et al. [11] verified transverse isotropy in field samples using the triaxial test. The results showed that the degree of anisotropy of the AC samples was in the range of 20%-50%. Then, the dynamic moduli of the AC samples along the vertical and longitudinal directions were measured by [PERSON] and [PERSON] [14], showing that the longitudinal moduli and stiffness were 40% and 30% of that in vertical. Also, the vertical modulus and out-of-plane Poisson's ratio were higher than the longitudinal modulus and in-plane Poisson's ratio, respectively [15]. Moreover, [PERSON] et al. [16] stated that the anisotropy decreased with the increasing density. Without considering the transverse isotropy of pavement materials, an error prediction in pavement fatigue cracking and potential rutting would occur [17; 18]. In addition, previous studies showed that the damage of asphalt pavement increased with the growing transverse isotropy [4]. The mechanical behaviors of asphalt pavement between transverse isotropic and isotropic materials were compared [19]. We found that, when considering the transverse isotropy, the tensile and compressive strains were higher than those when considering the material as isotropic. In addition, the vertical strain obtained by the transverse isotropy model agreed well with the measured value [20]. [PERSON] et al. [21] demonstrated that when the degrees of anisotropy of pavement materials were 0.80 and 0.87, the FE-based vertical and horizontal stress matched well with field data. In addition, the transverse isotropy effects of the separated layer on the pavement mechanical behaviors were discussed in several studies, and detailed information for these layers can be found in the literature, such as surface layer [22; 23], semi-rigid base layer [24; 25; 26], sub-base layer [27; 28], and subgrade layer [29; 30; 31]. [PERSON] et al. [4] investigated the transverse isotropy effects of the combined layers, including hot-mix asphalt (HMA), base, sub-base, and subgrade, on the pavement mechanical behaviors. They argued that the transverse isotropy effects on the HMA and all-layer combination structures were significant in the pavement response. Materials with three mutually orthogonal elastic symmetry surfaces are defined as orthotropy [32]. According to this definition, the elastic modulus, Poisson's ratio, and shear modulus of pavement material are different in the transverse and longitudinal directions. [PERSON] et al. [33] showed that the surface deflection and tensile stress at the bottom of the semi-rigid base layer were significantly affected by the anisotropy in two directions of the horizontal plane. There is a need to include material anisotropy in pavement design and evaluation [34]. However, most researchers only investigated the effects of transverse isotropy on pavement mechanical behaviors [12; 28]. In general, the transverse isotropy is a simplified case, since the material properties are identical in the transverse and longitudinal directions [35; 36]. This case does not conform to the actual anisotropic properties of asphalt pavement materials [4].