Laboratory determination of anisotropic aggregate resilient moduli using an innovative test device

The importance of anisotropic aggregate behavior is presently not considered in material characterization for pavement design because of the lack of proper laboratory equipment and testing capabilities. A newly acquired, innovative triaxial testing machine, referred to as the University of Illinois FastCell (UI-FC), was used to determine in the laboratory the anisotropic resilient properties of granular materials. Since the UI-FC permits stresses to be cycled independently in the vertical and horizontal directions, it is ideally suited for simulating dynamic stresses on a sample and for studying the effects of anisotropic stress-path-dependent aggregate behavior. Preliminary results obtained from four aggregates tested using UI-FC indicated definite directional dependency (anisotropy) of aggregate moduli. The resilient moduli computed in the vertical and radial directions varied pronouncedly with the applied stress states. However, when the same procedure was used to test a synthetic specimen having isotropic material properties, the axial and radial stiffnesses computed from the measured resilient deformations remained unchanged. The vertical moduli were typically higher than the horizontal moduli for most aggregates tested, except for a sandy gravel having a significant amount of fines. The testing procedures that were followed, that is, the order of vertical and horizontal pulsing on the specimens, also seemed to influence the computed anisotropic moduli. More anisotropic testing of this nature should be undertaken to investigate the directional dependency of aggregate stiffnesses.