Low-Temperature Oxidation Kinetics of Asphalt Binders

Abstract

A simple dual-mechanism model successfully fits the oxidation of 12 unmodified asphalt binders originating from a wide variety of sources. The kinetic formulation includes fast and slow reaction paths in parallel with free radical interactions between the two reaction pathways. The same Arrhenius parameters are used for all 12 binders studied. The differences in asphalt binder oxidation rates can be explained with the use of only one adjustable parameter, the amount of reactive material available for the fast reaction. This result suggests that unmodified asphalt binders oxidize with essentially the same chemical mechanisms. Because the Arrhenius parameters apply universally, a simple test may be performed to characterize the oxidation kinetics for unmodified binders without expensive, long-term oxidation experiments at multiple temperatures. A rheological study of the materials generated in the aging of the 12 binders using dynamic shear rheometry was also performed to investigate the relationship of rheological changes with chemical changes as binders oxidize. The rheometry consisted of the generation of a series of isothermal frequency sweeps, followed by fitting the resulting master curve with the Christensen–Anderson model. Simple shifting cannot account for the master curve changes, but changes in the model parameters follow a log-linear relationship for oxidation chemical changes. These fits appear to be source dependent, suggesting that a method with two aging time conditions would be required to characterize the rheological property changes in an unmodified asphalt binder as it ages. Such a method would produce a complete master curve–shift function set at any extent of aging, suitable for input into rational performance models.