Vertical and radial inhomogeneity of asphalt mixture components in laboratory-fabricated specimens have been of concern in asphalt mixture testing because of their potential effect on the mechanical response of the materials. Two important questions needed to be answered. First, can the existence of inhomogeneity in laboratory specimens definitively be distinguished? Second, if inhomogeneity exists, what effect would it have on the performance of asphalt materials?

Several new indices were developed to assess the extent of inhomogeneity. The level of accuracy of the suggested indices was evaluated by testing virtual and real specimens. Computer simulation was used to fabricate virtual specimens with various aggregate structures and to test the indices. The statistical power of the tests and the critical values for tests on the proposed indices were computed. The computed power of the tests indicated that the proposed tests are accurate for the measurement of both vertical and radial inhomogeneity.

Actual specimens, both homogeneous and inhomogeneous, were fabricated to validate the simulation results. The indices of homogeneity were computed from the x?ray computed tomography images of the specimens. Among the proposed indices, the z index on frequency proportion most clearly distinguished between the homogeneous and inhomogeneous specimens.

The specimens were then subjected to mechanical testing to examine the effect of inhomogeneity on the mechanical performance of the material. The effect of vertical and radial inhomogeneity was examined on compressive and shear properties of the mixtures, respectively. Statistical analyses on the results indicated that the compressive modulus (E*) of homogeneous specimens were slightly but not significantly higher than those of vertically inhomogeneous specimens, and the shear modulus (G*) of homogeneous specimens were significantly lower than those of radially inhomogeneous specimens.

A correlation analysis indicated insignificant correlation between the compressive properties and the index of vertical homogeneity but significant correlation between the shear properties and the index of radial homogeneity. The asphalt mixture was not sensitive to extreme level of vertical inhomogeneity when loaded axially but was responsive to radial inhomogeneity when loaded in shear.