Providing Sufficient Void Space for Asphalt Cement: Relationship of Mineral Aggregate Voids and Aggregate Gradation

Abstract

Since the 1960's, the Asphalt Institute has recommended voids in the mineral aggregate (VMA) to control minimum asphalt content in asphalt aggregate mixtures. VMA criteria are based on aggregate nominal maximum sieve size. Use of VMA criteria requires a clear understanding of the relationship of VMA to aggregate gradation. Nominal maximum size and maximum density lines must also be understood. In the asphalt industry today, significant confusion exists concerning different methods used to draw “maximum” density lines. A need exists to determine which method represents the maximum packing of aggregate materials. Maximum density lines are required to modify gradations and change VMA. Closely related to maximum density lines is the definition of aggregate nominal maximum size which also requires clear definition. This paper reports the relationship of VMA to aggregate gradation and particle characteristics for a controlled experiment. Using results from this experiment plus three other databases of mix designs, different methods of drawing maximum density lines are evaluated. A definition of nominal maximum sieve size which is more specific than the current ASTM definition is proposed.