Simulating hot asphalt compaction in Bullet Physics

Abstract

Asphalt concrete is the most used material in road construction. Analysing the behaviour and properties of this material is vital for modern day society. Currently, this is mostly done in labs using actual asphalt mixtures. However, this is a very expensive and time consuming process. An upcoming alternative approach is the use of computational models. The finite (FEM) and discrete element methods (DEM) have been used in the past, but both of these have shown significant disadvantages regarding the modelling of discrete particle movement and shape, respectively. An upcoming alternative is the use of physics engines, such as Bullet Physics, to model porous media. This method can have substantial benefits in terms of costs and research time. Also, phenomena can be visualised that cannot easily be observed in experiments.

This research focusses on the utility of Bullet Physics for modelling hot asphalt compaction. Therefore, a complex contact model is implemented which can describe the contact forces of the bituminous mixture. The superpave gyratory compaction process is digitally modelled. The model has been programmed in PyBullet, an open source physics engine, programmable in Python. A parametric study has been performed, which reveals the significance of certain properties, which cannot easily be investigated during laboratory compaction.

Bullet Physics was not initially designed for scientific research in the field of structural engineering. Therefore, alterations are needed to make the software usable. The implementation of a complex contact model is challenging. Although the Burgers’ contact forces can be correctly described, it proves difficult to implement custom contact forces directly in PyBullet. Two attempts have been made. In the case of a custom integration scheme, the computation time proved too long to be applicable for large scale simulations. In case of a direct implementation in Bullet Physics with the application of external forces, instability occurs. The only correct way to implement a custom contact model is by altering the source code.

During the simulations with a simpler contact model, substantial improvements of digital simulations over actual experiments are presented. The consistency proves far better than the prescribed minimum. The influence of inertia and friction can be assessed. It turns out that inertia, as well as the friction of the mould, can be disregarded. Another phenomenon that can be clearly illustrated is the revolving of aggregates inside the mould. Further analysis has shown that the average contact area depends on degradation, but in a typical asphalt mixture does not depend on the size of the individual elements, and could rather be treated as a constant. Further analysis shows that Bullet Physics is incredibly efficient, thus yielding the possibility of performing large scale simulations within reasonable time.