Validation of a Bitumen Oxidation Rate Model

Abstract

This paper reports on experiments aimed at validating a model for predicting the rate of oxidation of bitumen, in asphalt mix and chip seal road surfacings, over time periods of practical significance. Experiments were conducted in which bitumen films of varying thickness were oxidized in the field for 20 years and in the laboratory for up to 5.5 years. Bitumen oxidation in the laboratory was carried out at 50°C in air at atmospheric pressure and resulted in an extent of oxidation approximately equivalent to 11.5 years in the field. Oxidation of bitumen in the films was followed for changes in carbonyl, sulfoxide, and hydroxyl group concentrations measured by infrared spectroscopy, weight change, and viscosity. The data were fitted to an earlier derived rate equation based on a model in which the overall reaction was approximated as the sum of a fast and slow reaction. The model had been found to describe oxidation product formation very well over short time frames. The current work has shown that the model also applies over time frames (extents of oxidation) relevant to actual surfacing lives. A key assumption in the model that depletion of bitumen reactants in the slow reaction phase was negligible was also supported over long time frames. Both fast and slow phases of the reaction were found to be dependent on oxygen concentration, as assumed in the model. The oxygen dependence of the rate is discussed as a two-stage reaction mechanism previously described in the literature.