Low Temperature Oxidation Kinetics of Asphalt Binders

摘要

A simple dual mechanism model successfully fits the oxidation of 12 unmodified asphalt bindersoriginating from a wide variety of sources. The kinetic formulation includes fast and slow reaction pathsin parallel with free radical interactions between the two reaction pathways as first proposed by Petersen.The same Arrhenius parameters are used for all 12 binders studied. Using only one adjustable parameter,the amount of reactive material available for the fast reaction, the differences in asphalt binder oxidationrates can be explained. This result suggests that unmodified asphalt binders oxidize with essentially thesame chemical mechanisms. Since the Arrhenius parameters apply universally, a simple test may beperformed to characterize the oxidation kinetics for unmodified binders without doing expensive longterm oxidation experiments at multiple temperatures. A rheological study of the materials generated in theaging of the 12 binders using dynamic shear rheometry was also performed to investigate the relationshipof rheological changes with chemical changes as binders oxidize. The rheometry consisted of thegeneration of a series of isothermal frequency sweeps, followed by fitting the resulting master curve withthe Christensen-Anderson model. Simple shifting cannot account for the master curve changes, butchanges in the Christensen-Anderson model parameters follow a log-linear relationship with regard tooxidation chemical changes. These fits appear to be source dependent, suggesting a two aging timemethod would be required to characterize the rheological property changes in an unmodified asphaltbinder as it ages. Such a method would produce a complete master curve/shift function set at any extentof aging, suitable for input into rational performance models.