Characterization of binder, mastic, and FAM film thickness within asphalt concrete mixtures

Aggregate particles within an asphalt mixture are coated in a thin film of binder. Several studies and even mix design procedures have been based on an implied correlation between the binder film thickness (BFT) and the mechanical properties of these mixtures. Those studies generally define the BFT based on a predictive equation linked to one or more theoretical considerations, but without direct experimental determination of the BFT. However, recent developments of powerful scanning electron microscopes (SEMs) and digital image processing (DIP) techniques allow one to perform detailed analysis of microstructural images and thus directly measure the BFT value. This paper presents an analysis of BFT and mastic film thickness (MFT) within asphalt concrete (AC) mixtures using high-resolution SEM images and DIP. Based on this methodology, the measured mean BFT and MFT values ranged from 5.6 mu m to 10.8 mu m, and from 20.8 mu m to 28.2 mu m, respectively. Additionally, a BFT analysis considering fine aggregate matrix (FAM) phase within the ACs was pursued. A criterion to obtain FAM BFT is proposed and it can be useful in volumetric FAM mix design and sample fabrication.