Investigations on viscosity and flow behavior of polyphosphoric acid (PPA) modified asphalt at high temperatures

The rheological behavior of asphalt binders is composed of the deformation behavior at medium-low temperature and the flow behavior at high temperature, however, few researchers pay attention to the latter for polyphosphoric acid (PPA) modified asphalt. To analyze viscosity characteristics and flow behavior of PPA modified asphalt, six kinds of PPA modified asphalts with different mass fractions were prepared, and the effects of PPA dosage, test temperature and shear rate on the Brookfield rotary viscosity were discussed. The rheological parameters, viscous activation energy Eη, flow index n and consistency coefficient k were obtained based on Arrhenius law and the power law model, and the flow master curve was established by the time-temperature superposition principle. The results show that viscosity is positively correlated with PPA dosage, approximately, conforms to the exponential growth model. Meanwhile, viscosity is negatively correlated with test temperature and shear rate, respectively, in accordance with Arrhenius equation and power law equation. Under the test conditions, the flow index n of six kinds of asphalts goes up with the increase of PPA dosage or the decrease of temperature. However, all the n values are less than 1, indicating asphalts are pseudo-plastic fluid. The consistency coefficient k is consistent with the viscosity with the change of PPA dosage and test temperature, and may represent viscosity to a certain extent. The flow master curve with three parameters established can describe the effects of test temperature and shear rate on viscous flow characteristics. In this investigation, the non-Newtonian flow behavior of PPA modified asphalt was analyzed from the perspective of viscosity, proposing a method to estimate the dependence of viscosity on test conditions in a wider range through limited experimental data. This method is not only applicable to PPA modified asphalt, but also can provide references for the study of viscous flow characteristics of other asphalt binders.