Applications of Strain-Rate Frequency Superposition for Bituminous Binders

Nonlinear viscoelastic behavior of bitumen has a determinant effect on the performance of asphalt roads suffering permanent deformation due to traffic loads. Up to know, conventional rheological characterization of bituminous binders, such as the time-temperature superposition (TTS) method, only addresses the linear response of this material without considering the application of high strain amplitudes. The strain-rate frequency superposition (SRFS) is an analogous technique that can experimentally determine the flow behavior from nonlinear oscillatory shear experiments. This method was originally applied to soft materials in order to study the slow relaxation process of particular systems by shifting to higher frequencies the behavior usually found at very low frequencies during conventional measurements. In this work, the feasibility of the SRFS method for assessing the rheological properties of bituminous binders has been evaluated. Oscillatory shear measurements accomplished at different constant shear strain ampliture rates (ỳo = ωỳo) and test temperatures allowed analysing the influence of the nonlinear behavior of unmodified and polymer modified bitumen on their viscoelastic responses. The results showed that displacements in the responses due to different strain rates were not so significant as to extend the frequency range further than in conventional measurements. Differences in responses between both techniques were mainly observed for polymer modified binders, especially to high strain amplitudes which usually involve nonlinear behaviour. In addition, master curves obtained with constant strain rates, i.e. taking into account nonlinear response of the material, showed similar results to those constructed by using conventional methods with constant strain amplitude. From these results, a closer comprehension of the large deformations generated in asphalt pavements can be achieved by studying the nonlinear viscoelastic properties of the bituminous binder