Simple methods to estimate inherent and stress-induced anisotropy of aggregate base

Simple methods to estimate cross-anisotropic properties of unbound aggregate assemblies on the basis of aggregate physical properties are presented. A regression model for the cross-anisotropic material properties was developed from a database consisting of aggregates from six sources. Aggregate specimens from each source were tested with the use of different gradations and compaction moisture contents. The results demonstrate that aggregate shape and gradation influence the level of anisotropy, which has a substantial effect on the pavement responses that affect pavement design. The level of anisotropy, defined as the ratio of the horizontal modulus to the vertical modulus, was calculated from the regression model and compared with the results from a micromechanics model. This micromechanics model accounted for the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles on inherent anisotropy. Horizontal-to-vertical modulus ratios ranging from 0.4 to 1.0 were calculated with the micromechanics model. However, the experimental results and regression model show that the level of anisotropy can drop to as low as 0.15 because of the additional anisotropy induced by repeated loading. The effect of this increased anisotropy on the performance of a pavement with an unbound aggregate base is substantial.