Investigating the structural condition of pavement is one of the most important parts of road improvement studies. The most important part in the structural eva‎luation of the pavement is to perform tests with the aim of identifying the actual condition of the different pavement layers in order to prioritize the restoration. Falling Weight Deflectometer (FWD) is the common non-destructive testing method for in-situ eva‎luation of pavement condition. The surface deflection measured under the FWD test is used to calculate the structural capacity of the pavement system and back-calculation of its layers moduli. Therefore, accurate mechanical models considering the relationship between loading, environment and pavement structure are needed to analyze FWD test results. The objective of this study is to develop finite element (FE) models that can simulate FWD loading on pavement structure and considering the boundary condition, layer interface and the complexity in material properties in the form of relevant structural models. In this research, a finite element model was developed for the dynamic simulation of flexible pavement responses under the FWD test in general-purpose finite element software ABAQUS. Due to the existence of some properties such as nonlinearity of unbound materials, a user material subroutine (UMAT) was coded with the Fortran programming language to calculate the responses of the pavement considering the desired properties in the ABAQUS. Then the model was validated using field data from other studies. While providing explanations about the selection of uncertain parameters in the analysis, the importance and influence of a number of factors and conditions such as dynamic analysis, asphalt layer viscoelasticity, cross-anisotropic of granular material, nonlinearity of unbound materials and the presence and depth to bedrock, were eva‎luated in the analysis of the flexible pavement responses. According to the results, the dynamic analysis approach has a better match with the responses obtained from the FWD measurements compared to the static analysis and the assumption of viscoelasticity for the asphalt layer has shown a better performance in predicting the responses than the analysis in the elastic state. On the other hand, the assumption of the cross-anisotropic increased the pavement deflections up to 15% compared to the analysis in the isotropic mode. Also, the assumption of the nonlinearity of unbound materials made the pavement responses different compared to the linear state. In such a way that the assumption of the base layer as a cross-anisotropic and nonlinear material led to a 37% reduction in pavement surface deflections compared to assumption of a linear and isotropic base. This value showed a decrease of up to 3.5% in the investigation carried out for the subgrade. It should be noted that the increase in temperature will increase the effect of the mentioned factors. Although the parametric analysis findings vary depending on the desired materials, but it can determine the importance of the eva‎luated factors in the dynamic simulation of flexible pavement responses under FWD loading and be taken into account in performance eva‎luation of back-calculation software for FWD devices. In this research, the modeling of flexible pavement under FWD loading was done with high accuracy according to the environmental conditions and the complexity of pavement material properties, and the analysis results will be more reliable. Also, the dynamic analysis that has been done provides the possibility of quick and easy access to the detailed investigation of the behavior and performance of the layers in each element of the pavement structure during the loading time and after (according to the viscoelastic properties of the asphalt layer).

محمدرضا خان محمدي , مهدي نسيمي فر

مهدي ابطحي فروشاني , پوريا حاجي كريمي