3D printed concrete (3DPC) is rapidly developing and promisingly expected to contribute to free modeling of structural member, un-manned construction method and so forth. On the other hand, it is known that the anisotropic property of its layered structure is influenced by the existence of interface between the extruded materials. Since the heterogeneity of aggregate distribution affects the mechanical properties, it can have effects on the interfacial bonding and adhesion between layered filaments of the extruded materials of 3D printed object. This paper aims to clarify the mechanism of aggregate heterogeneity formation in the printing nozzle and the printed filament with evaluating the influence of material extrusion speed. The sequential cross-sectional images are acquired with X-ray CT method and analyzed three-dimensionally to quantify the distribution of aggregates. The result indicates that the aggregate heterogeneity shows the characteristic distribution of aggregate, which is influenced by material extrusion speed. Specially, it is found that the area with large size aggregate moves from the outside toward the center of the cross section, as the extrusion speed increases, inside the nozzle. Also, as the extrusion speed increases, large size aggregate inside the nozzle moves from the center of the cross section toward the bottom. Meanwhile, the lubrication layer between the internal surface of nozzle and the extruded material, which is formed by increment of pressure inside nozzle determined by extrusion speed, alters the flow velocity distribution inside the nozzle from laminate flow to plug flow and the aggregate distribution before and after extrusion.
