Origins and evolution of instabilities in asphalt binder




Hajj, Ramez Muhammad

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Asphalt binders are complex viscoelastic materials that serve as the bonding agent for mineral aggregates in flexible pavement layers. One of the most important distresses exhibited in flexible pavements is their tendency to crack due to repeated loading. The current body of literature on asphalt binders has yet to establish experimentally how these cracks are born at a microscopic level, and a proper test method for evaluating a binder’s resistance to forming cracks. This dissertation proposes answers toward a solution to both of the above gaps. Eight asphalt binders were evaluated experimentally using a novel poker chip test that realistically mirrors the state of stress experienced by a binder in an asphalt mix. Based on these results, the interplay between stiffness, strength, and ductility of the binder was evaluated and used as a screening tool to for outlier binders. In addition, the cavitation surfaces and their relationship with strength was observed. Subsequently, dark field microscopy was used to observe the origination of the cavitation instability in the binder. Cavitation was observed to occur at the interface between inclusions in the bulk of the binder and the matrix itself. A theoretical framework for the reasons for this based on classical solutions in mechanics is proposed. Finally, X-ray computed tomography was used to obtain images of an asphalt mixture microstructure at two different scales. These images were meshed to form a finite element model, which was used to determine critical points in a mixture for cracking. The findings indicated high triaxiality could be appropriately modeled in the laboratory using the poker chip test at high aspect (diameter/thickness) ratio. This high aspect ratio creates an approximate confined stress state for testing, and also allows for nearly uniform stress distribution in the test specimen. This was validated using finite element analysis to confirm the poker chip test as suitable for laboratory testing of asphalt binders.


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