Assessing the Asphalt Binder Film Thickness in Recycled Asphalt Mixtures Using Micro-Level Techniques

Adequate asphalt binder film thickness (ABFT) delivers skeletal integrity in recycled asphalt mixtures, resulting in long-lasting roadways when exposed to traffic and environment. The inaccurate measurement of ABFT and the consequences of not having adequate film thickness model has substantially introduced discrepancies in predicting actual performance of recycled asphalt mixtures. Expansion of the ultra-modern expertise and SuperPave requirements necessitate the revision of authentic ABFT at micro-level. The current study identifies the weaknesses of the current methods of estimating ABFT and provides results that are reliable and useful, using modern measurement methods. Using scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS), this study measures the ABFT around the tiniest particle of 0.2 μm magnitude, entrenched in asphalt mastic in recycled asphalt mixtures. The ABFT, obtained through image analysis, is compared with those obtained through available analytical models. The study utilizes different asphalt mixtures, containing varying proportions of recycled asphalt mixture and rejuvenators. The aggregate, virgin, and recycled binders were characterized in terms of physical and rheological properties, respectively. Marshall mix design was carried out for the conventional and recycled mixture, containing 40%, 50%, and 60% recycled materials, rejuvenated with 3%, 6%, 9%, and 12% waste engine oil (WEO) at a mixing temperature of 160 °C, based on viscosity of the virgin and rejuvenated binder. ABFT was assessed through analytical models and image analysis for the aforesaid recycled asphalt mixtures, prepared at optimum binder and rejuvenator content as per protocol outlined in ASTM D1559. The analytical estimation of ABFT, in the aforesaid recycled asphalt mixtures, revealed that the ABFT fluctuates from 6.4 μm to 13.7 microns, with a significant association to recycled asphalt mixture and rejuvenator content. However, the image analysis revealed that the ABFT, in the aforesaid recycled asphalt mixtures, fluctuates from 0.4 μm to 2 microns, without any association to recycled asphalt mixture or rejuvenator content. The image analysis indicated that the recycled asphalt mixtures typically comprise of mortar, happening in uneven shape, and are used to grip large aggregates. The asphalt mastic, a blend of bitumen and mineral filler, was found to be an interlocking agent, used to grasp only fine particles in asphalt mortar. The asphalt binder film was discovered to be a deviating stand-alone entity that only exists around the mineral fillers in the asphalt mastic as a non-absorbed binder, occupying an imprecise space of 0.4 μm to 2 microns, among the filler particles. The current findings will be useful to design asphalt pavements through the aforesaid precise limit of SEM-based ABFT rather than traditionally measured ABFT to predict the actual performance of recycled asphalt mixtures.