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 µm to 10.8 µm, and from 20.8 µm to 28.2 µm, 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. •A procedure to characterize BFT using SEM images and deep learning segmentation techniques is presented.•A representative scanning area of 16 mm2 was identified as appropriate to perform BFT characterization.•BFT followed lognormal distributions and its mean values ranged between 5.6 μm and 10.8 μm.•MFT followed gamma distributions and its mean values ranged between 20.8 μm and 28.2 μm.•Fine aggregate matrix BFT can be estimated using asphalt concrete standardized sieves.