Evolution of the microstructure of warm mix asphalt binders with aging in an accelerated weathering tester

This study investigated the microstructural evolution of two binders, Pen 60=70 and PG 76-22, modified with two warm mix asphalt (WMA) additives, Advera (PQ Corporation, Malvern, Pennsylvania) and Sasobit (Sasol Wax, Sasolburg, South Africa), due to aging performed with an accelerated weathering tester. A new long-term aging protocol was utilized, which applies direct aging on the exposed surface of the binders. The aging protocol considers the effects of ultraviolet (UV) radiation (with alternation of UVand dark cycles), presence of oxygen, and temperature. After varying aging durations, different surface morphologies were detected using both optical photos and atomic force microscopy (AFM). New microstructures in the shape of rods, squares, spheres, ellipsoids, and beans and flower or leaf structures were observed on the aged surfaces. The observed microstructures are probably composed of the product of the oxidation and polymerization reactions occurring on the binder surface due to direct aging. The evolution of the microstructure due to direct aging in Advera modified binders was similar to that of the original binders, displaying flower or leaf structures after the longest aging durations. The microstructural evolution observed on the Sasobit modified binders differed from that of the original and Advera modified binders. The discrepancy is attributed to the dissimilar microstructure and chemical composition of the unaged surface, which reacts and forms compounds having different compositions and metastable configurations.