Laboratory Determination of Anisotropic Aggregate Resilient Moduli Using an Innovative Test Device

Abstract

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.

Combined effect of asphalt concrete cross-anisotropy and temperature variation on pavement stress–strain under dynamic loading

“…Before 2000, study of cross-anisotropy was mostly concentrated to the unbound granular aggregate layer [1,2]. The possible presence of cross-anisotropy in asphalt concrete (AC) compacted by http the Superpave Gyratory Compactor was first studied by Masad et al [3].…”

“…Flexible pavements are constructed by compacting hot mix asphalt (HMA) in vertical direction, which results in unequal material stiffness, i.e., defined by the modulus of elasticity, E, in horizontal and vertical direction [1]. Asphalt concrete (AC) can be isotropic if its stiffness property (i.e.…”

In-situ assessment of the stress-dependent stiffness of unbound aggregate bases: application in inverted base pavements

“…Several laboratory tests have been developed to determine the resilient modulus of granular bases (FHWA 1996, Tutumluer and Seyhan 1999, NCHRP 2002, Puppala 2008. These tests simulate material compaction and loading history relevant to field conditions.…”

Stress-path dependency of resilient behaviour of granular materials

“…Several researchers have studied the anisotropic resilient behaviour of aggregates , Tutunnluer 1995, Tuturmluer and Seyhan 1999, Karasehin and Dawson 2000, Adu-Osei et al 2000, Masad et al 2004, Kim et al 2005). …”

“…Nevertheless, the determination of the parameters is not an easy task, and therefore has not been received well by design engineers, who prefer simple and reliable procedures. Pappin andBrown (1980, 1985) (Brown and Pappin, 1985) With regard to modelling cross-anisotropic resilient behaviour of unbound aggregates, Tuturmluer and Seyhan (1999, Karasehin and Dawson (2000) and Adu-Osei et al (2000) assumed that the resilient modulus in different directions obeys the Uzan equation and thus, can be represented by the same stress-dependent formulation with different values of coefficients.…”