Aggregate Properties Affecting Shear Strength and Permanent Deformation Characteristics of Unbound-Base Course Materials

Resistance to permanent deformation is the primary performance indicator of unbound aggregate pavement layers. In relation to the applied wheel-load deviator stress, the shear strength property of a confined base/subbase controls the rate of permanent strain accumulation in that aggregate layer. The objective of this study was to quantify the impact of aggregate properties on the shear strength and permanent deformation characteristics of unbound aggregates. Sixteen different aggregates were studied in the laboratory for two different gradations: original quarry source and engineered midrange gradations. Aggregate specimens were prepared at these gradations to determine moisture-density relationships, and conduct monotonic and repeated load triaxial tests at the target maximum dry densities and optimal moisture contents. In addition, imaging-based aggregate particle shapes or morphological indices were obtained. The laboratory findings indicated that the particle size corresponded to 60% passing, and imaging-based angularity index contributed significantly to the increases in the shear strength properties of aggregates, which were also affected by density and moisture content. Permanent strain accumulations were primarily influenced by applied shear stress in relation to the shear strength of aggregates, and rutting model parameters obtained through regression analyses were influenced by applied stress, strength, and material properties. Further, an aggregate with plastic fines showed the highest permanent strain accumulation, hence clearly indicating the significant influence of plasticity of fines on the permanent strain accumulation.