Critical Problems with Using the Asphalt Ductility Test as a Performance Index for Modified Binders

Abstract

Despite the adaptation of advanced binder rheology characterization methods by many agencies, the asphalt ductility test is still being used in some specifications in the United States and a few other countries as a performance indicator for asphalt modification. In this study, binder properties known to reflect rutting and fatigue resistance were examined in a set of binders modified with two types of commonly used elastomeric polymer modifiers. The results showed no correlations with binder ductility. Additionally, a test procedure was developed by using a dynamic shear rheometer (DSR) as a surrogate to the conventional ductility test, results of which showed that both elastomeric modified binders are much more ductile than conventional binders, even when conventional ductility showed a loss of ductility. Finite element modeling was used to show the significant effects of decreasing true strain rates with elongation on the sample's stress and strain state attributable to the constant crosshead speed in the conventional ductility test. Because of the well-known dependency of failure stress and strain of viscoelastic material on strain rate and temperature, comparing binders with varying ductility values measured in the conventional test is essentially flawed because it is equivalent to comparing them at different temperatures and is thus fundamentally unreliable as an indicator of the asphalt's performance in the pavement. It is therefore strongly recommended that the practice of using low-temperature conventional ductility be removed from modified binder specifications, or the DSR procedure proposed in this paper be used to evaluate modified asphalts as a preferable test method.