In road construction for large amounts of traffic, asphalt is the main component used. When analyzing the internal structure of asphalt, bitumen is the "glue" that affixes the components of the rigid backbone, the grains. The grains, the bitumen, and the holes make asphalt a very complex and interesting system that – up to now – was not successfully and quantitatively modelled on the scale of the grains and the inter-grain bitumen. Neither are “microscopic” models available to describe the peculiar interplay between bitumen, holes, and the granular backbone at various temperatures, nor exist macroscopic models that describe the interesting and sometimes problematic behaviour of asphalt.
On the micro-scale, the interaction of the bitumen with the grains and between the grains can be modeled using a Discrete Element Method (DEM) with contacts models that account for the elasticity of the grains as well as for the viscous and adhesive properties of the bitumen.
Particle-based computer simulations like DEM reproduce the behaviour of the discrete material focusing on the particle-level. Particle simulations are a very powerful tool to investigate the different scales of the material. The interaction between pairs of grains is modelled by means of suitable contact laws that take in account dissipative, attractive, and frictional forces. Realistic asphalt particles are surrounded by a thin layer of bitumen/polymeer. To properly model the system, new, more advanced interaction models have to be considered, where pressure-, time- temperature-dependent mechanisms are included.
Simple interaction models for non-organic materials are well established and also for simple viscoelastic polymeric materials; however, the modelling of hard-core particles covered by soft material is an open issue.
Furthermore, many questions on how particles interfere during sintering along one (or many) temperature cycle(s) (heat-up – cool-down) have still no answers. A simple DEM model for self-healing of sintered material has been already developed and will be used to investigate the issue. This particle based model is able to show interesting properties of the material, which could not be obtained with other methods, e.g. (i) the crack evolution under compression/tension-stress tests could be simulated and analysed; (ii) the effect of sintering time and confining pressure on the final material parameters can be studied.
DEM simulation of a sintered sample before (left) and after (right) a vertical-compression / horizontal-tension stress test.
Thus, a new, micro-mechanical approach is proposed, focusing on the intrinsic characteristics of bitumen and asphalt, where the macroscopic behavior of the material is expressed in terms of system-, particle-, contact-properties and the network of the microscopic elements.
The goal is to investigate, understand and clarify its behaviour
- to propose new constitutive continuum models that allow to predict quantitatively the long-term behavior of asphalt;
- to enhance the material properties in order to minimize or prevent damage symptoms like rutting and ravelling;
- to improve the knowledge of new asphalts with self-healing capabilities.