DIRECTIONAL DEPENDENCY OF AGGREGATE STIFFNESSES: AN INDICATOR OF GRANULAR BASE PERFORMANCE

A new laboratory testing approach is introduced to characterize unbound aggregate base/subbase behavior by taking into account the directional dependency (anisotropy) of aggregate stiffnesses. An advanced triaxial testing machine named University of Illinois FastCell (UI-FastCell) is used for determining the vertical and horizontal resilient moduli of thirteen "good" and "poor" performing base/subbase materials received from eight different states. Anisotropic (horizontal to vertical) modular ratios are shown to take different values and either increase or decrease with increasing applied dynamic stresses depending on the material properties, i.e., quality of an aggregate. The modular ratios are also established as an indicator of the maximum allowable fines percentage in an aggregate gradation. The new approach introduced can be used to differentiate the good, marginal, and poor aggregates based on anisotropic properties, revise current aggregate gradation specifications, and if needed, facilitate the development of new improved specifications for unbound aggregate base/subbase design and construction.