Material factors that influence anisotropic behavior of aggregate bases

The directional dependency of material properties of aggregate systems have been the subject of study over the last 10 years. A procedure was established to determine the level of anisotropy of aggregate systems using simple aggregate properties and anisotropy level as an input to predict performance of aggregate bases. Stress-induced directional dependency of material properties were evaluated on the basis of multiple variable dynamic confining pressure stress path tests for 10 aggregate sources. Various gradations and saturation levels were considered for each aggregate source. Particle geometry was characterized using the Aggregate Imaging System. The cumulative Weibull distribution function was used to describe aggregate size and aggregate geometrical characteristics. The fine portion of the gradation was characterized by the Rigden voids test and methylene blue test to account for fine particle shape properties and the deleterious effect of plastic fines on the volumetric stability of aggregate layers. The cross-anisotropic modular ratios were used as indicators of the level of anisotropy. Aggregate characteristics (form, angularity, texture, and gradation) are in turn related to Weibull cumulative distribution parameters. A sensitivity analysis revealed that the level of anisotropy is very sensitive to aggregate form, angularity, and textural properties. It is also sensitive to the level of bulk stress and shear stress as reflected by the k₂ and k₃ parameters, Cross-anisotropic shear strength ratios for four pavement sections at the top of the base layer were calculated using a finite element program. The results indicated that the level of anisotropy significantly affected the shear strength ratio.