Experimental Testing and Analysis Procedure to Determine the Apparent Film Thickness of Asphalt Binder in Fine Aggregate Matrix Mixtures

The fine aggregate matrix (FAM) is an important constituent of an asphalt concrete mixture; the FAM is where some key damage phenomena such as cracking start and propagate. The proper design and fabrication of isolated FAM testing samples that are representative of the material existing within asphalt concrete mixtures requires the objective determination of key characteristics such as the apparent film thickness (FT) of the asphalt binder and the specific surface area of the aggregates. These relevant parameters facilitate the estimation of the binder content. This study presents an experimental testing and analysis protocol to determine the apparent FT that covers particles of fine aggregate in FAM mixtures. The method is based on tests using a scanning electron microscope and a digital image analysis procedure using the open-source Fiji/ImageJ software. The results indicated that apparent FT ranged between 0.5 µm and 30 µm. An additional validation effort was pursued and demonstrated the applicability of the proposed methodology, which can provide meaningful information to improve volumetric-based FAM mix design methods and generate materials that are more representative of those existing in the asphalt concrete mixtures.