Integrating the Dynamic Modulus of Asphalt Mixes in the 1993 AASHTO Design Method

The AASHTO Guide for Design of Pavement Structures 1993 (1993 Design Guide) remains the most widely used pavement design manual by highway agencies and design consultants around the world. As defined in the 1993 Design Guide, the structural coefficient of a pavement layer (a_i) is an abstract measure of the relative ability of a unit thickness of a given material to function as a structural component of the pavement. Nevertheless, the assumed a_i values of the asphalt layers and a proposed relationship between a_i and the resilient modulus do not account for the mechanical and physical properties of asphalt materials, traffic volume and speed, layer thicknesses (thin versus thick pavements), climate, and unbound layer properties. The purpose of this research was to enhance the design methodology incorporated in the 1993 Design Guide by integrating asphalt mixture properties in the design process. The objective was to devise a relationship between the structural coefficient (a_i) of the asphalt layer and the effective dynamic modulus (|E*|_eff.) of the corresponding asphalt mix to yield a more realistic estimate of the structural capacity of the asphalt layer. The paper illustrates the development of a multilinear relationship between a_i, (|E*|_eff.), and the resilient modulus of the aggregate base layer. Pavement structural designs for various asphalt mixes and design inputs using the developed a_i–(|E*|_eff.) relationship yielded asphalt layer thicknesses that were generally smaller than those obtained using the typical a_i value of 0.44 for the asphalt layer and closer to thicknesses obtained with the AASHTO mechanistic–empirical design method using the Pavement ME software.