Quantification of asphalt binder ageing from apparent molecular weight distributions using a new approximated analytical approach of the phase angle

This paper aims to explore the effect of ageing on macromolecular organisation of pure petroleum asphalts by interpreting their rheological behaviours. Phase angle measurements are used to determine apparent molecular weight distributions (AMWD) assuming a special relationship between relaxation times and molecular sizes. This procedure is called the ‘delta method’. Four asphalt binders have been characterised in their original state and after lab ageing using conventional tests simulating ageing during the mixing process (RTFOT) and after several years of road service (PAV). Rheological properties have been measured with a visco-analyser METRAVIB under annular shearing and tension-compression mode of loadings. Measurements have been performed from –20°C to 60°C with a frequency sweep from 1 to 80 Hz. An analytical approximation of the phase angle, derived from the Huet-Such model, is used to calculate the AMWDs for each sample. A deconvolution procedure is then applied in order to identify four populations of molecular objects. This procedure allows to identify an increase of large molecular objects with ageing. The deconvolution procedure allows proving that it is the effect of smaller molecules that agglomerate themselves with the increase of the polar species content due to oxidation. This work shows that it is possible to get useful information on the asphalt macromolecular structure from linear viscoelastic measurements.