Anisotropic modular ratios as unbound aggregate performance indicators

Standard repeated load triaxial tests commonly performed in the laboratory do not always apply the most damaging field loading conditions for predicting resilient and permanent deformation (rutting) responses of unbound aggregate layers due to moving wheel loads. An advanced triaxial testing machine, named the University of Illinois FastCell (UI-FastCell), was used in this study for determining the loading related directional dependency of granular material properties. Since stresses can be cycled independently in the axial and radial directions, UI-FastCell is ideally suited for simulating dynamic field stresses on the sample and for studying the effects of stress-induced anisotropy. Thirteen aggregates with varying material properties were obtained from eight different states in the United States and tested using the UI-FastCell under both vertical and horizontal dynamic loads. Anisotropic modular ratios were established as aggregate performance indicators, relating these ratios to the quality and strength properties. In general, much lower modular ratios were obtained for the "good quality" materials than for the "poor quality" ones at some representative low, intermediate, and high stress states. A shear stress ratio, which gives the level of applied shear stress as a fraction of the shear strength of the material, was also defined and successfully linked to the modular ratios of the 13 aggregates. A limiting value of this shear stress ratio is believed to control the permanent deformation behavior of aggregates.