Importance of Nonlinear Anisotropic Modeling of Granular Base for Predicting Maximum Viscoelastic Pavement Responses under Moving Vehicular Loading (original) (raw)
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This paper explains the importance of using a nonlinear anisotropic three-dimensional (3D) finite-element (FE) pavement model to simulate the granular base layer and predict viscoelastic pavement responses under moving vehicular loading. The FE model was built using the general-purpose FE software ABAQUS, and a user material subroutine was developed to implement the constitutive model of granular material using a modified Newton-Raphson approach with secant stiffness. The FE model utilizes implicit dynamic analysis and simulates the vehicular loading as a moving load with 3D contact stresses at the tire-pavement interface. The analysis results indicate that it is important to consider the viscoelastic nature of the asphalt layer and the moving load for accurately capturing the nonlinear granular base modulus in the mechanistic pavement model. The modulus distribution in the granular base layer is affected not only by wheel load and pavement structure (such as asphalt layer thickness and subgrade support) but also by temperature and vehicle speed because of the viscoelastic behavior of the asphalt surface layer. Excluding the cross-anisotropic stress-dependent behavior of the granular base layer could cause significant error in predicting fatigue cracking and rutting potential in thin asphalt pavements. In addition, the model results captured the trends of field measurements at different loading and temperature conditions.
A three-dimensional (3-D) finite element (FE) model was developed to investigate the dynamic responses of thin, flexible pavement under impulsive loading similar to a falling weight deflectometer test. The FE model simulated the hot-mix asphalt (HMA) surface layer as a linear viscoelastic material and considered the cross-anisotropic stress dependent modulus for the unbound base layer. Implicit dynamic analysis was used to consider the effect of inertia on pavement structural responses. Using two thinpavement structures of different HMA layer thicknesses, 76 and 127 mm, the study analyzed the effects of cross-anisotropic stress-dependent aggregate base modulus and dynamic analysis on pavement responses, including surface deflection, tensile strain at the bottom of the HMA layer, deviator stress in the base layer, and compressive strain on top of the subgrade. Results showed that use of the cross-anisotropic stress-dependent modulus for the unbound base layer resulted in greater predicted pavement responses and, hence, less estimated pavement life for rutting and fatigue cracking. It was found that as the thickness of HMA surface layer or the ratio of horizontal modulus to vertical modulus decreases, the effects of stress dependency and cross anisotropy become more significant. Analysis-predicted surface deflections were compared to field-measured values and they were in agreement when the stress dependency and cross anisotropy of the base layer and subgrade were considered.
Effects of Interface Condition on Performance of Road Pavements with Non-linear Granular Materials
The main results of a numerical investigation into the effects of interface condition on the critical response and predicted performance of flexible pavements with granular bases are presented. In addition, the influence of using linear elastic and non-linear elastic models for granular base characterization, on the design life of typical road structures with granular bases under various combinations of layer interface conditions is examined. Repeated load triaxial tests are carried out and non-linear regression analyses on tests results are performed to determine the constitutive model parameters. The Asphalt Institute distress models for fatigue cracking and rutting using both linear and non-linear analyses are utilized to estimate the pavement design life. It is shown, among others, that interface condition affects significantly the critical response of pavement structures and hence their predicted performance. Furthermore, the results indicate that the use of linear assumption to characterize granular base and sub-base behaviour, grossly over-estimates the design life of pavement structures. This effect strongly depends on interface conditions used between the pavements layers.
2011
Resilient modulus is an important input for computation of the structural response of pavements in Mechanistic -Empirical Methods. It has a significant effect on computed pavement responses and predicted pavement performance. The main objective of the research study that is introduced here is to develop a material model subroutine applicable to Finite Element program to simulate the nonlinear behavior of unbound granular materials, and evaluate the influence of using different constitutive models for characterization of unbound granular materials on critical responses of Flexible pavements. These constitutive models include Linear Elastic, Uzan and Universal models. For this purpose, three different pavement sections are assumed and FE models for these sections were verified by comparing elastic responses with KENLAYER Program. A granular base with known material constants for these three models was selected and sections were analyzed using different constitutive models. The results...
Mathematical Problems in Engineering, 2020
This study developed two-and-half dimensional (2.5-D) finite element method (FEM) to predict viscoelastic pavement responses under moving loads and nonuniform tire contact stresses. The accuracy of 2.5-D FEM was validated with two analytical solutions for elastic and viscoelastic conditions. Compared to three-dimensional (3-D) FEM, the computational efficiency of the 2.5-D method was greatly improved. The effects of loading pattern and speed on pavement surface deflection and strain responses were analyzed for asphalt pavements with four different asphalt layer thicknesses. The analyzed pavement responses included surface deflections, maximum tensile strains in the asphalt layer, and maximum compressive strains on top of subgrade. The loading patterns have influence on the mechanical responses. According to the equivalent rule, the point load, rectangle type, and sinusoid-shape contact stresses were studied. It was found that the point load caused much greater pavement responses tha...
Modeling Stress-Dependent Anisotropic Elastoplastic Unbound Granular Base in Flexible Pavements
Transportation Research Record: Journal of the Transportation Research Board
Unbound granular base (UGB) has a cross-anisotropic and nonlinear (stress-dependent) modulus with a plastic behavior. Existing UGB models address nonlinear cross-anisotropy and plasticity separately. It is unknown how the two characteristics are coupled into a finite element model (FEM) and how this will affect the pavement responses. This study presents a coupled nonlinear cross-anisotropic elastoplastic (NAEP) constitutive model for the UGB and implements it in a weak form equation-based FEM. No material subroutine is needed to address the circular dependence between the stress-dependent anisotropic modulus, structural stress responses, and elastoplastic deformation. The NAEP model was calibrated by triaxial resilient modulus and strength tests and validated using laboratory measurements in a large-scale soil-tank pavement structural test. It is found that the NAEP model is valid and effective in predicting the UGB responses in flexible pavements. The model predicted less horizont...
Impact of Truck Loading on Design and Analysis of Asphaltic Pavement Structures- Phase II
In this study, Schapery’s nonlinear viscoelastic constitutive model is implemented into the commercial finite element (FE) software ABAQUS via user defined subroutine (user material, or UMAT) to analyze asphalt pavement subjected to heavy truck loads. Then, extensive creep-recovery tests are conducted at various stress levels and at two temperatures (30oC and 40oC) to obtain the stress- and temperature-dependent viscoelastic material properties of hot mix asphalt (HMA) mixtures. With the viscoelastic material properties characterized and the UMAT code, a typical pavement structure subjected to repeated heavy truck loads is modeled with the consideration of the effect of material nonlinearity with a realistic tire loading configuration. Three-dimensional finite element simulations of the pavement structure present significant differences between the linear viscoelastic approach and the nonlinear viscoelastic modeling in the prediction of pavement performance with respect to rutting a...
In the past few years, LCPC has been implementing in its finite element code CESAR-LCPC a module for pavement modelling, including non-linear models for asphalt materials and unbound granular materials. This program has been used to model results of a full scale experiment on a flexible pavement, with a granular base, performed on the LCPC accelerated pavement testing facility. The experimental results indicated that the response of the pavement depends strongly on the level of load and on the water content of the unbound granular layers. The modelling of the pavement was performed in 3D, and several modelling hypotheses were successively tested : linear elasticity, a non-linear elastic model for the unbound layers (Boyce model) and a visco-elastic model for the asphalt concrete (Huet-Sayegh model). The most complete model, coupling non-linear elasticity for the unbound materials and visco-elasticity for the asphalt concrete, led to realistic predictions of the pavement response for different levels of load and different loading speeds.
This paper discusses the distribution of contact stress at the tire-pavement interface and how to quantify its impact on viscoelastic pavement responses using a decoupled modeling approach. The authors developed a tire-pavement interaction model to predict the three-dimensional (3-D) contact stresses under various loads and inflation pressures. In this model, an air-inflated radial-ply ribbed tire was loaded on a non-deformable pavement surface. The predicted contact stresses are consistent with previous measurements and validate the non-uniformity of vertical contact stresses and localized tangential contact stresses at the tire-pavement interface. The load primarily affects the vertical contact stress at the edge of tire contact area and the longitudinal contact stress; while the inflation pressure primarily controls the vertical contact stress in the center region of tire contact area and the transverse contact stress. Statistical models were developed to predict the 3-D contact stresses at each rib under various loads and inflation pressures. Utilizing the realistic contact stress distribution at the tire-pavement interface, a 3-D finite element (FE) model was built to analyze the critical pavement responses under moving tire loading. The FE model simulated the asphalt mixture layer as a linear viscoelastic material and considered the cross-anisotropic stress-dependent modulus for the unbound base layer. The authors concluded that when 3-D tire contact stresses are used in the analysis, the longitudinal fatigue cracking, primary rutting, and secondary rutting potential in thin asphalt pavement are increased, compared to when uniform contact stress distribution is applied. The heavy load causes increased responses in the base layer and subgrade; while high tire pressure causes increased response impact in the asphalt mixture layer, especially the shear stress at high temperature. The results of the analysis illustrate the importance of considering the realistic contact stress distribution when analyzing pavement responses under various loading and tire pressure conditions.
Modelling granular pavement materials has a significant role in the pavement design procedure. Modelling can be through an experimental or numerical approach to predict the granular behaviour during cyclic loading. The current design process in Australia is based on linear elastic analysis of layers. The analysis is performed through a well-known program CIRCLY which is applied to model bound pavement material behaviour. The KENLAYER is one of the common pavement software models used for pavement design in the United States which performs non-linear analysis for granular materials. Alternatively, a general Finite Element program such as ABAQUS can be used to model the complicated behaviour of multilayer granular materials. This study is to compare results of numerical modelling with these three programs on two sample pavement models.