The rheological response of the bitumen exhibits a variety of transitory responses from Newtonian to viscoelastic fluid to viscoelastic solid to elastic solid. Such responses, especially, during service temperatures are strongly influenced by the rate of loading. These transitions are not well-defined and hence, identifying the same and using such information within the context of material characterization and application for the quantification of damage is necessary. For instance, the intermediate pavement temperature, which is used to identify the critical fatigue regime of the material lies within the viscoelastic solid regime with disparate frequency dependence. The determination of the fatigue life of bitumen under repeated loading is often underestimated by the presence of thixotropy which has similar characteristics to that of fatigue in terms of evolution of material stiffness with loading but is a reversible phenomenon. The challenge is multi-fold here, and this includes the development of experimental techniques refined enough to capture such transitions, the use of appropriate linear/nonlinear constitutive models, and the quantification of fatigue and the associated thixotropy.
Since conventional test methods such as creep-recovery, stress relaxation or small amplitude oscillatory shear (time/frequency/strain sweep) experiments cannot capture the transition in material behavior to the required rigor at such a temperature regime, advanced rheological techniques that are prevalent in the field of polymeric science to capture transition in material behavior are proposed to be implemented on bitumen. In addition to this, appropriate linear/nonlinear constitutive models will be identified to predict the material response in this temperature regime. Finally, suitable test methods and post-processing techniques will be adopted to eliminate the effects of thixotropy from damage.
The expected contributions of this work include: (i) introducing rheological techniques in the field of bitumen rheology that could probe the material in a way conventional test methods cannot and predicting the material response when subjected to such techniques (ii) quantifying the ‘true’ fatigue resistance of the bitumen by separating reversible effects like thixotropy.