Effect of colloidal instability on the rheological and fatigue properties of asphalt binders

Abstract

The main objective of the study described in this paper was to identify possible relationships between the chemical composition and the rheological and fatigue properties of asphalt binders.In order to address such an issue, two asphalt binders of different penetration grade were subjected to four ageing treatments of increasing severity, with the consequent change of their chemical composition. Chemical analyses were performed by means of thin layer chromatography, with the evaluation of the relative percentages of saturates, aromatics, resins and asphaltenes (SARA fractions) and with the corresponding calculation of the colloidal instability index. For the assessment of their rheological properties, virgin and aged binders were subjected to rotational viscosity and frequency sweep tests. Fatigue behavior was evaluated by means of linear amplitude sweep tests and corresponding results were analyzed by means of the simplified viscoelastic continuum damage model. Obtained results, which were also used for the calculation of purposely defined ageing indices, showed that there is a strong correlation between the colloidal instability of asphalt binders and their properties evaluated both in flow conditions and in the linear viscoelastic region. In particular, higher levels of instability are associated to a more viscous and stiffer response regardless of the type of considered original binder. Results from fatigue tests demonstrated that damage endurance is also affected by the degree of instability. However, it was found that correlations exist only when the degree of colloidal instability is below a certain threshold. Above such a limit, fatigue resistance collapses and no prediction of endurance is possible. Although the outcomes of this study need to be supplemented by further research, they are useful to pavement engineers for a better understanding of the behavior of aged asphalt binders and of the mechanisms which underly chemical ageing.