Prediction of the hardening of the bitumen in pavement surfacings by reaction with atmospheric oxygen

The major cause of the long term hardening of the bitumen binder in a pavement surfacing is the chemical reaction with atmospheric oxygen. This slow reaction is controlled by the intrinsic reactivity of the bitumen, the temperature regime at the pavement surface and the rate of diffusion of oxygen into the binder films in the interior of the surfacing.A model has been developed for the prediction of the rate of this process and the possible life of a surfacing at a particular location. It is based on hardening data obtained from the observation of experimental sprayed seals and thin dense hot mix surfacings laid at sites covering most of the climatic conditions encountered in mainland Australia. These surfacings have been monitored for periods of up to 14 years.Data collected on the yearly temperature distributions of experimental surfacings at Australian Capital city sites was used to establish a relationship between the yearly mean maximum daily air(screen) temperature(YMMT) at a particular location and the(constant) oxidation temperature used to evaluate hardening in the model film.The complex geometry of the films in a surfacing is described, in the model, by an equivalent “average” uni-directional diffusion path(model film thickness). For sprayed seals, this was related to the rate of binder application at construction and the increase in diffusion path produced by traffic action to form a cover aggregate mosaic during the first few years of its life. For a dense hot mix surfacing the model diffusion path was used to translate hardening data obtained at one site to another where the temperature regime and/or bitumen reactivity are different.