Aggregate morphology affecting resilient behavior of unbound granular materials

The resilient modulus (M_R) defines the nonlinear elastic response of pavement geomaterials, such as the unbound aggregate base-subbase materials, under repeated traffic loading. The resilient behavior of unbound aggregate layers is affected by the stress states experienced because of wheel loading and physical properties of aggregate materials. Among the physical properties, the influence of aggregate morphology on the resilient behavior of base-subbase materials is not yet well understood because of a lack of quantitative measurements of shape, angularity, and surface texture characteristics of aggregate particles. This paper focuses on investigating the influence of aggregate angularity and surface texture properties on the resilient behavior of unbound granular materials expressed by a nonlinear, stress-dependent resilient response model. With an image analysis approach, two important aggregate morphological indices [the angularity index (AI) and the surface texture index (ST)] were defined to quantify accurately the angularity and surface texture properties of six types of commonly used aggregate materials with varying surface irregularities. Twenty-one unbound specimens were prepared with the blending of six aggregate materials and tested for resilient moduli in a laboratory triaxial setup as per the AASHTO T 307-99 protocol. Both AI and ST were found to be closely linked to M_R values of the aggregate specimens blended in different percentages. As the aggregate angularity and surface roughness increased, the resilient moduli were considerably improved, which was primarily because of the increased shear strength with better aggregate interlock and frictional properties and the increased confinement levels expressed by higher bulk stresses.