Toward an Improved Model of Asphalt Binder Oxidation in Pavements
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Asphalt binder oxidation in pavements has been proven to be an ongoing process throughout a pavement's service life. Understanding the nature of the oxidation process is a critical step toward better pavement design to achieve greater pavement durability. The main component in asphalt binder oxidation in pavements is binder oxidative hardening. As the aromatic compounds in asphalt binders are oxidized, more polar carbonyl compounds are created, which results in stronger associations between asphalt components and eventually leads to an increase in asphalt elastic modulus and viscosity. Consequently, the performance of pavements is affected directly by asphalt binder hardening. Also, low levels of accessible air voids in pavements potentially relate to binder oxidation according to a recent research study. When the pavements have sufficiently high accessible air voids (4 percent or greater), the oxidation rate is largely determined by the temperature in the pavement. On the other hand, when the percentage of accessible air voids in the pavement is considerably lower (2 percent or less), the hardening rate of binders in pavements is reduced significantly. Field evidence is mounting that asphalt binder oxidization in pavements produces a binder that is more susceptible to thermal and fatigue cracking. While the fundamentals of this oxidation process are fairly well known, predicting quantitatively the rate of oxidation as a function of depth in the pavement, is not straightforward. A thermal and oxygen transport model, coupled with binder reaction kinetics, provides the basis for such calculations. A one-dimensional thermal transport model, coupled with site-specific model parameters and recent improvements in the availability of required input climate data, enables calculation of pavement temperatures throughout the year, which then is used in an asphalt binder oxidation and transport model to calculate binder properties in the pavement over time. Calculated binder property changes with depth and time are compared to measurements of binder oxidation in the field. The work in this study is aimed at understanding the oxidation kinetics of asphalt binders in pavements, determining the impact of accessible air void levels on asphalt hardening, and ultimately developing an improved model of asphalt binder oxidation in pavements.