Thixotropic Behavior of Paving-Grade Bitumens under Dynamic Shear

Abstract

A material exhibits thixotropic behavior if its apparent viscosity decreases under stress and if it recovers its initial viscosity when unloaded. This thixotropy is well defined for fluids, but it is not well understood for viscoelastic materials. For bituminous binders, it a priori corresponds to a decrease in material stiffness under cyclic loading and a recovery of this stiffness after rest. Rheological tests, in which the stress distribution in the material is simple and controlled, appear interesting to gain relevant observations concerning the thixotropic behavior of bitumens. The materials were tested under simple shear flows with the help of a controlled stress rheometer, and the influence of temperature, frequency, and conditioning time were studied. This paper proposes a new experimental procedure for studying binder thixotropy in cyclic mode by means of shear tests. The procedure relies on the observation of the breakdown curves in the nonlinear domain and modulus recovery curves at different frequencies and temperatures. Stiffness variations are demonstrated to be linked to a rearrangement of molecular structure and not to crack formation and healing. Moreover, fatigue performance appears better for the binder that shows the most important capacity to be destructured.