Modeling Stress-Dependent Anisotropic Elastoplastic Unbound Granular Base in Flexible Pavements

Abstract

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 horizontal tensile stresses at the base bottom and introduced compressive stresses in the middle and top of the base course. This is caused by an increasing confinement resulting from a horizontal plastic dilation in the base course, which cannot be modeled without considering plasticity. The stress-dependent modulus for the UGB material decreases with depth and the distance from loading centerline. Compared to a nonlinear anisotropic elastic model, the NAEP model predicted the same tensile strain at asphalt layer bottom, a higher base modulus and a higher subgrade compressive strain. Thus the nonlinear anisotropic elastic UGB model results in the same fatigue life as the NAEP model, but may riskily under-predict the rutting damage.Zhang et al.

Characterizing stress-dependent complex and relaxation moduli of dense graded asphalt mixtures

“…The authors understand that the application of the result of this paper into pavement structure is important and some preliminary application of the results can be found elsewhere [19]. In this paper, however, due to the paper length limit, the effects of stress dependent properties on pavement performances were not included but they will be investigated in a following paper, in which the stress dependent viscoelastic parameters will be used to determine the pavement performances under different traffic loading levels with a verified finite element model.…”

In-situ and numerical investigation on the dynamic response of unbounded granular material in permeable pavement

“…The resilient modulus of unbound aggregates has been widely recognized as the primary mechanical property of the base course required in the mechanistic-empirical design of pavement structures. In order to determine the resilient modulus of unbound aggregates, various models have been developed [19].…”

“…It has been widely researched and proven that the cross-anisotropic model of UGB should be considered in the numerical investigation of pavement performance under consideration of non-linear material [19,28].…”

“…Consequently, the UGB layer is more sensitive to moisture, which in addition to the significant plastic behavior and anisotropic behavior, is one of the serious problems affecting the performance of the whole FPP structure [19,20]. In particular, hydro-mechanical interaction can significantly change the modulus of UGB, which in turn affects the service life of FPP, and results in completely different mechanical distributions in the pavement layers [21,22].…”

“…In particular, hydro-mechanical interaction can significantly change the modulus of UGB, which in turn affects the service life of FPP, and results in completely different mechanical distributions in the pavement layers [21,22]. Previous studies have developed models to simulate the stress-dependent anisotropic and plastic behaviors of the UGB material [19]. However, few studies can be found having coupled stress-dependence, moisture sensitive, and cross anisotropic elastoplastic (SMAEP) models for this material.…”