Modelling of aggregates' contact mechanics to study roles of binders and aggregates in asphalt mixtures rutting

Rutting resistance is one of the essential needs in the design of asphalt mixtures. Volumteric properties, binder modification, aggregates properties, and performance tests such as the triaxial test and wheel tracking test are used today to qualify the rutting resistance in practice. However, these tools are used on a trial and error basis and are mostly based on experience to achieve better resistance. With all the experience available today, it is still not clear what is the contribution of aggregate structure, what air-voids content should be used, and if plastomeric or elastomeric modifiers are preferred for better resistance. While compiling larger databases and expanding the types of aggregates and binders used in testing could help, a more fundamental understanding of how aggregate contacts are formed and how binders can contribute to actual mechanics of rutting resistance is a better path for designing of mixtures. In the last decade, a significant improvement in imaging and multi-scale modelling has been achieved, which allows looking closer at how binders and aggregates could affect rutting resistance. One of the most challenging aspects of this modelling effort is dealing with aggregates proximity zones or contacts and how the response of mixture changes when aggregates come into contact. This paper includes a proposal for addressing this challenge and provides a solution that is simple yet effective in better simulating of measured rutting in the laboratory. The paper also includes an application of the model to simulate mixtures produced with plastomeric and elastomeric modifiers with different viscoelastic properties. The mixtures include different gradations to represent fine and coarse gradations of aggregates. Results demonstrate that packing of the aggregates is significantly affected by the binder modifiers as some additives can result in a better-packed aggregate internal structure that is much more effective in improving rutting resistance of mixtures. It is expected that the improved modelling of contact zones will aid in more rational selection of aggregates and modification techniques in the mix design phase to enhance the rutting resistance of asphalt pavements.