Loss of volatiles by evaporation
The evaporation of volatile components depends mainly on temperature and the exposure conditions. The rate of evaporation is controlled by the diffusion rate and the length or thickness of the diffusion path. Well compacted dense asphalt will have a slow diffusion process, whereas open grade asphalts and surface dressings will present more rapid evaporative conditions. However, paving grade bitumens are relatively involatile, and therefore the amount of hardening resulting from evaporation of volatiles is usually fairly small.
Steric or physical hardening
The phenomenon of steric hardening was first reported in 1944 by Traxler, when it was observed that bitumen samples stored at 258C showed an increase in viscosity. This physical hardening is usually attributed to a combination of reorientation or restructuring of molecules within the bitumen and the slow crystallisation of waxes. The molecular reorientation that results in the hardening effect is reversible on the application of heat, with Traxler noting that raising the storage temperature to 708C reversed the changed structure, with the bitumen sample returning to its original viscosity.
Loss of volatiles by exudation
Exudative hardening results from the movement of oily components that exude from the bitumen into the mineral aggregate (Van Gooswilligen et al., 1989 ). It is a function of both the exudation tendency of the bitumen and the porosity of the aggregate. Durability of bitumens and asphalts