Influence of Processing Parameters on the Layer Geometry in 3D Concrete Printing: Experiments and Modelling

This paper presents the numerical simulation results of a computational fluid dynamics (CFD) model that describes the layer shape in extrusion-based 3D Concrete Printing (3DCP). The simulation outcome is validated through an experimental program in which we investigated the influence of 3DCP processing parameters on the geometry of a single layer. Specifically, a set of single layers were printed using a Ø25 mm nozzle mounted on an 6-axis industrial robotic arm travelling at different speeds and with different layer heights. A fresh concrete -- comprising CEM I 52,5 R - SR 5 (EA), limestone filler, fine sand, water, and admixtures (i.e. viscosity modifying agent, high-range water-reducing admixtures and a hydration retarder) -- was pumped and extruded at a fixed volumetric rate. Once hardened, the extruded layers were sliced to examine the resulting cross-sections. Specifically, the cross-sections' geometry were obtained by a custom image processing algorithm. Next, the extrusion flow was modelled with a CFD simulation using the software FLOW-3D®. The constitutive behavior of fresh concrete was modelled as a Bingham fluid, while the volume-of-fluid method was used to predict the free surface of the concrete and, thus, the layer geometry. The numerical results agree qualitatively with the experimental observations, enabling us to identify two non-dimensional 3DCP processing parameters that influence the overall cross-sectional shapes: 1) the geometric ratio between layer height and nozzle diameter, and 2) the ratio between the nozzle velocity and the extrusion volumetric flux. These findings -- when complemented with a model describing the overall deformation of stacked layers -- serve as the basis for correlating material rheological properties to 3DCP process parameters, promoting a better link between design and fabrication in a 3DCP context.