Analysis of Aggregate Gradation and Packing for Easy Estimation of Hot-Mix-Asphalt Voids in Mineral Aggregate

The voids in mineral aggregate (VMA) design parameter is one of the most important in the current Superpave mix design procedure that links hot-mix-asphalt (HMA) mix properties to field performance. The success of achieving the appropriate VMA at an early stage of the mix-design process not only significantly impacts the design time and effort but also can produce an economical and reasonable design with good field performance. Although several current mix and gradation design methods have provided some guidance on adjusting gradation to achieve target VMA, these approaches still have to rely on initial experimental results with some degree of trial and error, and provide no direct correlation between aggregate gradation and VMA. This paper presents an analysis of aggregate packing and discrete-element modeling (DEM) simulation to link the aggregate gradation property to the VMA of HMA. A new definition is proposed for characterizing traditional dense-graded aggregate gradations into three types: coarse-graded, medium-graded, and fine-graded. A gradation weighing factor, $f_v$, defined as the percentage of voids change by volume caused by unit additional aggregate, is developed to theoretically evaluate the packing of an aggregate structure. A method of VMA estimation based on the generated $f_v$ values is thus provided. Eleven mix designs from different states are used for verifying the accuracy of the prediction. Most predictions are within 1% difference of the design VMA. Using one nominal maximum aggregate size (NMAS) of 12.5 mm as an example, this study provided a promising method for a fast and accurate VMA estimation. It can be used directly in the HMA mix design to reduce the design time and effort, and contribute to the estimation of the HMA mixture performance at an early stage of the design period.