Investigation of Aggregate Shape Effects on Hot Mix Performance Using an Image Analysis Approach

The objectives of this Federal Highway Administration (FHWA) pooled fund research project included the measurement of imaging based volumetric and morphological indices of coarse aggregates and their correlations with laboratory and field performance results of asphalt concrete mixes as a wave of future in the development of asphalt pavement science and technology. The study partners were the National Center for Asphalt Technology (NCAT), state highway agencies of Alabama, Georgia, Indiana, Minnesota, Mississippi, Missouri, Montana, and South Carolina, and the FHWA Central Federal Lands and Highways Division. In Phase I of the study, the readily available image analysis device, University of Illinois Aggregate Image Analyzer (UIAIA), was used for validation and development of imaging based morphological indices, i.e., flat and elongated (F&E) ratio, angularity index (AI) and surface texture (ST) index, of the coarse aggregate used in the national NCAT Pavement Test Track rutting study and in typical asphalt mixes obtained from the pooled fund participating agencies. In Phase II, all UIAIA determined NCAT coarse aggregate shape indices indicated good correlations individually with the field rutting data from the NCAT Pavement Test Track with the ST index giving the best correlation and the AI giving the next best correlation. The UIAIA determined coarse aggregate shape indices were also correlated to the laboratory resilient modulus and permanent deformation test results of the participating agency specimens. For a total of 18 Superpave asphalt mix designs studied, the effects of the AI and ST indices on the hot mix asphalt resilient moduli and permanent deformations were especially significant from the test results when evaluated according to the below the restricted zone (BRZ) aggregate gradations. The resilient modulus test data, when grouped according to asphalt binder grade and/or stiffness, generally demonstrated a much better relationship with the coarse aggregate morphology. The increased stability and reduced permanent deformation or rutting potential trends of the most dense graded asphalt mixtures studied herein using the UIAIA approach were more favorably influenced primarily by the increased surface texture or roughness property of coarse aggregate particles.