Evaluation of Aggregate Distribution Heterogeneity in 3D Printed Concrete by Means of X-ray CT

The mechanical performance of a printed object in 3D concrete printing is influenced by the interfacial bonding strength between the deposited filaments. Hence, the physical properties of the layer interface and the influential factors have been studied. This study aims to clarify the mechanism of aggregate distribution heterogeneity as well as the influence of printing material extrusion speed on the heterogeneity inside the filament. A laboratory-scale material extruder is developed and used in this study. The aggregate distribution is evaluated in a quantitative manner with the cross-sectional image obtained by X-ray computed tomography. The images were taken in the deposited filament and the material extrusion nozzle for printing. Results show that large aggregate moves from the outside of the printing nozzle toward the center with increasing extrusion speed from 1.8 to 7.1 mm/s. As extrusion speed increases, it is inferred that a lubrication layer forms on the inner surface of the nozzle, causing the transition of material extrusion behavior from laminate flow to plug flow. Thus, the aggregate distribution appears differently inside the filament. This finding indicates that the magnitude of friction against the nozzle wall alters the die swell during discharge as well as the aggregate distribution before and after extrusion.

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