Relationships between Chemical Composition, Asphaltene Nanostructures, and Thermochemical Properties of Bitumen before and after Accelerated Oxidative Aging

Abstract

An oxidative aging treatment in the laboratory by a pressure aging vessel (PAV) is applied on fresh 10/20 and 35/50 bitumens with different thermomechanical properties, in order to simulate long-term aging of bitumen on road pavements. The influence of the aging treatment on bitumen properties is studied as a function of the aging time. Combining Fourier transform infrared spectroscopy (FTIR), the saturate, aromatic, and resin-asphaltene determinator (SAR-AD), and small-angle X-ray scattering (SAXS) techniques allows us to confirm that less polar aromatic species are oxidized onto more polar content, and the asphaltene family content is found to increase significantly for more than 75% after a 45 h PAV treatment. SAXS patterns exhibit a progressive increase in the average size of nanoaggregates (from ∼1.8 to 2.3 nm) and primary clusters (from ∼4.7 to 5.4 nm) after such aging treatment. This is coherent with both the size increase of smaller asphaltene nanoaggregates and the formation of new nanoaggregates by auto-association of new oxidized asphaltene monomers. As expected, such evolutions of both the asphaltene content and its nanostructure have an influence on macroscopic rheological properties. An analysis of black diagrams and bending beam rheometer (BBR) measurements show that aging induces a shift toward higher moduli at low frequencies coupled with a loss of curvature at intermediate frequencies, meaning lesser susceptibility of aged bitumens regarding thermomechanical solicitation, a loss of viscous behavior at the profit of elastic behavior, and a decrease of the ability to relax stress at low temperatures.