Unified approach to fatigue study of high performance recycled asphalt concretes

The paper analyses the results of a laboratory investigation concerning the design and the energy based fatigue characterization of high performance recycled asphalt concretes for flexible road pavements. Six different bituminous mixtures were optimized with both crushed limestone and reclaimed asphalt pavement (RAP) materials, at variable proportions (up to 40% by weight of the aggregate), using a conventional bitumen as well as a polymer modified binder. A combined volumetric-mechanical mix design procedure, based on the gyratory compaction and the indirect tensile strength test, was adopted. The fatigue investigation was performed by means of four point bending tests, conducted at 20 °C and 10 Hz. The fatigue resistance was studied by means of the empirical stiffness reduction approach, but also using a more rational method, based on the dissipated energy concept and related to the macro-structural damage state of the asphalt concretes. Damage curves, associated to the Plateau Value of the Ratio of Dissipated Energy Change, for both the controlled stress and strain mode, have been developed and statistically studied, in order to unify the fatigue investigation. With respect to the control mixes, characterized by a limestone aggregate structure, an improved fatigue behaviour has been observed for the asphalt concretes made with RAP, even when the polymer modified bitumen has been used, regardless of the method used for the data analysis. The energy based approach allowed to obtain a unique fatigue life prediction, irrespectively of the loading mode.