SIMPLE METHODS TO ESTIMATE INHERENT AND STRESS-INDUCED ANISOTROPY OF AGGREGATE BASE (05-1355)**
Dallas N. Little, Texas A&M Transportation Institute
Eyad A. Masad, Texas A&M University
Sung-Hee Kim, Southern Polytechnic State University

This paper presents simple methods to estimate cross-anisotropic material properties of unbound aggregate assemblies on the basis of physical properties of the aggregates. A regression model was developed from a database consisting of aggregates from six sources. Three gradations were selected for each source and three compaction moisture contents were selected for each gradation (6 x 3 x 3 = 54 combinations). Levels of anisotropy calculated from the regression analysis were compared to levels of anisotropy predicted by a micromechanics model that accounts for directional forces as a result of the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles. The effect of aggregate type, particle shape, and textural characteristics on the level of anisotropy of unbound aggregate bases is discussed. The results demonstrate that aggregate type, orientation, grading, and shape influence the level of cross-anisotropy, which has a substantial effect on the pavement responses that impact pavement design. The micromechanics analysis predicted a level of (inherent) anisotropy (ratio of horizontal to vertical modulus) ranging from 1.0 to 0.4; while experimental results demonstrate that the level of anisotropy can drop to as low as 0.1 due to the impact of repeated loading.