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

An oxidative aging treatment in thelaboratory by a pressureagingvessel (PAV) is applied on fresh 10/20 and 35/50 bitumens with differentthermomechanical properties, in order to simulate long-term agingof bitumen on road pavements. The influence of the aging treatmenton 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-angleX-ray scattering (SAXS) techniques allows us to confirm that lesspolar aromatic species are oxidized onto more polar content, and theasphaltene family content is found to increase significantly for morethan 75% after a 45 h PAV treatment. SAXS patterns exhibit a progressiveincrease in the average size of nanoaggregates (from similar to 1.8 to2.3 nm) and primary clusters (from similar to 4.7 to 5.4 nm) after suchaging treatment. This is coherent with both the size increase of smallerasphaltene nanoaggregates and the formation of new nanoaggregatesby auto-association of new oxidized asphaltene monomers. As expected,such evolutions of both the asphaltene content and its nanostructurehave an influence on macroscopic rheological properties. An analysisof black diagrams and bending beam rheometer (BBR) measurements showthat aging induces a shift toward higher moduli at low frequenciescoupled with a loss of curvature at intermediate frequencies, meaninglesser susceptibility of aged bitumens regarding thermomechanicalsolicitation, a loss of viscous behavior at the profit of elasticbehavior, and a decrease of the ability to relax stress at low temperatures.