Fatigue cracking is one of the main distresses affects the service life of asphalt mixture. Recently, polyphosphoric acid (PPA) widely used as a modifier to improve mechanical and rheological characteristics of asphalt binders and mixtures especially at high and low service temperatures. While there are several studies devoted to investigate high and low service temperature of PPA modified asphalt binder and mixture, its fatigue behaviour needs to be studied in more detail. The main objective of this paper is to develop a multiscale heterogeneous numerical finite element (FE) model as well as experimental program to investigate the effect of PPA modification on fatigue behaviour of different bituminous composites from neat asphalt binder to asphalt mixture. The experimental program includes frequency sweep test at test temperature of 25°C by implementing dynamic shear rheometer (DSR) for neat and PPA modified asphalt binder and also indirect tensile fatigue (ITF) at frequency of 1 Hz (0.1 s loading time and 0.9 s rest time) under constant stress of 300 kPa at test temperature of 25°C by using a UTM-25kN. Three different contents of PPA is used for modifying neat asphalt binder including 0.5, 1 and 1.5 wt.% of neat binder to consider the variation of modifier extent on fatigue behaviour of neat asphalt binder and mixture. The mixture specimens were prepared with granite aggregate and asphalt binder of 85/100 penetration grade. The asphalt mixture is modeled as a biphasic medium composed of granular inclusions with linear elastic properties and bituminous matrix exhibiting linear viscoelastic behavior at small strain values. The generalized Maxwell rheological model was used to simulate viscoelastic behavior of the asphalt binder and PPA modified asphalt binder. In order to create the aggregate skeleton of the bituminous composites, a custom software (called "MOA", French acronym for Random Object Modeler) developed in University of Limoges, has been used to generate random inclusions of various sizes and shapes. The experimental results of asphalt binder and mixture indicated that modification with PPA has a significant effect on improving asphalt mixture's fatigue life especially in dosage of 1%. On the other hand, results of numerical simulation have shown a good agreement with experimental results.
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