Characterize packing of aggregate particles for paving materials: Particle size impact

Aggregate packing directly affects the way of aggregate particles forming a skeleton to transmit and distribute traffic loads, thus influencing the stability and mechanical performance of the mixtures. Although some efforts have been conducted to evaluate the quality of a designed aggregate structure, such as the Bailey’s method for hot mix asphalt (HMA) mixture, there is a lack of fundamental understanding of the aggregate packing properties. Also how the packing and degree of aggregate interlock can be related to mixture performance is not clear. Major reason is because the aggregate structure is a very complicated system whose packing characteristics can be affected by both particle size distributions and the particle shape (angularities) distribution. To understand this complex packing system, our study developed a two-step procedure. As the first step, the size distribution effect is evaluated and the results are presented in this paper. The second step will investigate the combined effect of size distribution and shape impact and the results will be presented in a later paper. Specifically, in this paper we conducted a particle packing analysis using a discrete element modeling (DEM) simulation method. An HMA mixture gradation typically used in the State of Washington was utilized as a case study example for the analysis. By correlating the gradation parameter to the volumetric properties of the structure, this paper theoretically demonstrated the roles of aggregate particles with different sizes in an HMA mixture. Contact force chains and mean contact force were calculated using PFC3D DEM simulation, which provided an indication of the capability of the aggregate structure to transmit stresses through aggregate skeleton, and thereby, to resist permanent deformation. The study conducted here demonstrated the aggregate size distribution played a significant role in the packing characteristics, affecting both volumetrics and the contact characteristics of a packed structure. Such findings are critical for evaluating the combined effect of size and shape distribution on packing, and achieving a performance based aggregate gradation design.