Shotcrete 3D Printing - Interaction of nozzle geometry, homogeneity and hardened concrete properties

Shotcrete 3D Printing (SC3DP) applies concrete layer by layer using a wet-spray process. The resulting hardened concrete properties of the applied SC3DP layers (e.g. height, width or mechanical strength) are largely dependent on the selected material and process parameters. In this context, the nozzle geometry is an important influencing parameter. During printing, the velocity of the shotcrete jet is significantly influenced by the nozzle outlet diameter. Therefore, in the present study, the effect of the nozzle outlet diameter (15 - 30 mm) is investigated with regard to the resulting layer homogeneity, i.e. local density and aggregate distribution in the cross-section, and hardened concrete properties, i.e. flexural strength. By analysing the manufactured specimens, an uneven distribution of the aggregate is observed horizontally across the cross-section of the layers. An accumulation of aggregate is present in the core of the layer resulting in a cement paste-rich region in the edge areas. This leads to increased local densities in the core of the specimen. The application of the concrete with small nozzle outlet diameters results in the highest local densities and the highest flexural strength.