Influence of Rheology on Mixing Homogeneity and Mechanical Behavior of Twin-Pipe 3D Printable Concrete (2023-10)¶

The twin-pipe pumping technology has been developed for the stiffening control of 3D printable concrete, which is achieved by pumping two different mixtures separately and blending these mixtures using a static mixer before extrusion. The combination of the two mixtures undergoes a significant phase transition in the static mixer from liquid to solid due to accelerated hydration. The phase transition, as well as the mechanical performance of hardened elements, is strongly influenced by the mixing homogeneity of the mixtures. In this study, we examine how the rheological behavior of the two mixtures being combined in the static mixer affects the mixing homogeneity. The rheological behavior was studied by performing flow curve measurements using a dynamic shear rheometer. To characterize the mixing homogeneity, polished cross-sections of 3D-printed elements were prepared and characterized using an innovative macropixel-based image analysis procedure. Specifically, the image analysis-based procedure involves identifying the two mixtures by applying color segmentation, followed by the Poole index method to assess the mixing homogeneity. The results indicate that a low yield stress, especially for the main constituent mixture of which the flow rate is higher than the second constituent mixture, is advantageous to achieve a higher mixing homogeneity. The viscosity or the viscosity ratio of the two constituent mixtures is found to have a minor influence on the mixing homogeneity.