Determination of oxygen diffusion coefficients of compacted asphalt mixtures

The oxidative aging of asphalt binder in asphalt mixture is a key factor that affects the long-term durability of asphalt pavement. The aging susceptibility of an asphalt mixture is dependent on oxygen transport and consumption in the mixture. In this study, an apparatus was developed to measure oxygen diffusion and consumption in compacted asphalt mixtures, and methods to compute oxygen diffusion and possible consumption of asphalt mixtures were evaluated and compared. Oxygen diffusion coefficients of samples prepared with different asphalt mixture designs and air void contents were determined, and correlations between the diffusion coefficients and samples’ volumetric characteristics were examined. The oxygen environment exposed by asphalt binders in pavements and its implication on pavement durability were also discussed based on simulated oxygen concentration profiles with time. The developed apparatus and test procedure were found to be suitable to measure the oxygen diffusion coefficients of compacted asphalt mixtures. It was also found that oxygen consumption during the diffusion test at room temperature is negligible, and both numerical and analytical methods perform equally well in calculating oxygen diffusion coefficients. The oxygen diffusion coefficients of the tested asphalt samples vary in orders of magnitude, dependent not only on air void contents but also on other volumetric characteristics. The traditional models to predict the oxygen diffusion coefficients of dry porous media are not fit for compacted asphalt mixtures. The oxygen diffusion coefficients of asphalt mixtures have significant implications on oxygen level exposed by asphalt binders inside of the pavements; therefore, they may be used as an indicator in asphalt mixture design and construction quality control.