Zero shear viscosity of asphalt binders. Discussion. Authors' closure

Recently there has been considerable interest, especially in Europe, in the use of zero shear viscosity (ZSV) as a specification criterion for asphalt binders. This interest is precipitated by the apparent inability of the current Superpave® criterion, G*/sin δ, to capture the contribution to rutting resistance afforded by polymer modification. ZSV can be determined directly from long-term creep tests, but such tests are time-consuming and are often very difficult to perform. Several alternative methods for determining the ZSV have been proposed in the literature, including extrapolating the dynamic viscosity to zero frequency; applying the Cross model to dynamic data; and superimposing multiple short-term, non-steady-state creep tests. A number of methods for determining the ZSV from both creep and dynamic data were evaluated. Laboratory test data for 10 unmodified and modified binders were obtained through a series of creep and dynamic experiments. ZSV values obtained from two of the more promising methods were compared, along with a comparison of the ZSV ranking with the Superpave grading temperature. Two of the methods provided very similar values for the ZSV when applied over a considerable range in test temperature, and the results from the two methods could be used interchangeably for the materials that were tested. The binders ranked quite differently when ranked according to their Superpave grading temperature or their ZSV.