Advanced Characterization of Granular Materials for Mechanistic Based Pavement Design

The recent adoption and use of mechanistic concepts in pavement design made it possible to more realistically characterize the unbound aggregates component of the pavement by incorporating the new advances in characterization, i.e., anisotropic, stress path dependent behavior, and accounting for the most damaging field conditions. This paper mainly focuses on the use of a unique triaxial testing machine, referred to as University of Illinois FastCell (UI-FastCell), for determining in the laboratory the stress-induced anisotropic resilient properties of twelve aggregates with varying material types and properties. In the selection process, consideration was given to both "good" and "poor" performing granular base/subbase materials obtained from seven different states. With the applied AASHTO T294-94 stress states conveniently pulsed either in the vertical or horizontal direction on the same specimen, the anisotropy (directional dependency) of resilient moduli are adequately determined in the laboratory for the compression and extension type dynamic loadings. Nonlinear stress dependent models are developed to characterize the resilient moduli under these extreme loadings. The moduli obtained under extension stress states are in general lower than the ones computed from compression tests. Such lower moduli would result in higher critical pavement responses as obtained from the mechanistic analysis of a conventional flexible pavement and be associated with the reduced pavement life eventually. The most significant stress history effect is observed for the materials having moderate amount of fines. Finally, anisotropic (horizontal to vertical) modular ratios corresponding to horizontal (extension) and vertical pulsing (compression) conditions are shown to either increase or decrease with increasing applied dynamic stresses depending on the material properties, i.e., quality of an aggregate.