A 2D numerical model of 3D concrete printing including thixotropy

Additive Manufacturing (AM) techniques are increasingly drawing interest in the construction sector since they can offer new architectural possibilities while improving the accuracy and the sustainability of the construction process. Among these techniques, 3D Concrete Printing (3DCP) is probably the most established and its future role in lowering the environmental impact of the building industry is under deep investigation. 3DCP main advantages, which are linked to the capacity of building optimized structural shapes, without the need for formworks and in very short times, are however still limited by the lack of knowledge and regulation. It is therefore necessary to provide designers with more advanced design tools to fully unlock the potential of 3DCP. This work presents a numerical model of 3DCP, which realistically simulates the extrusion and layer deposition phases. The model assumes that fresh concrete can be treated as a homogeneous viscous fluid. The problem is then governed by the Navier-Stokes equations, which are solved in a Lagrangian framework with the Particle Finite Element Method (PFEM). A Bingham law, modified to include thixotropic effects, is employed to accurately reproduce the material rheological behaviour at the early ages. The model is then applied to simulate a simple printing scenario, for which the fundamental experimental data is available in the literature, to assess the role of thixotropy in the printing process. The results show how thixotropy has a crucial effect in limiting the overall shape deformation and in avoiding the early compression failure of the bottom layers. In this view, the developed model can be applied to optimize the printing parameters with respect to the rheological properties and could also suggest how to improve the toolpath and the printing times for a given structure.