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• An extrusion-based 3D printer for concrete with the largest aggregate size of 20 mm was introduced. • The range of ratios of cement and aggregate of printable concrete with coarse aggregate was recommended. • The direction-dependent mechanical properties of 3D printed concrete were investigated. • The voids distribution and the microstructure of interlayer area in 3D printed concrete were studied through X-CT and SEM. Concrete with coarse aggregate in the 3D concrete printing (3DCP) has broad prospects for high strength, low cost and shrinkage. In this study, an extrusion-based 3D printer was designed and utilized to print concrete with the largest aggregate size of 20 mm. Printable concrete with coarse aggregate was designed by different volume ratios of cement to aggregate (C/A). Then the effect of C/A on the printability and direction-dependent mechanical performance was investigated. X-ray micro-computed tomography (X-CT) was used to detect and analyze the voids distribution characteristics of printed specimens. Scanning electron microscope (SEM) investigations of microstructure at the interlayer area were conducted. Results indicated that the initial flowability of printable concrete should be within 178–200 mm, and the recommended printable C/A was within 0.35–0.60. The decrease of C/A improved the maximum printing height and mechanical performance but weakened the shape-stability of the multi-layer structure. X-CT results indicated that reducing excess slurry content caused by the decline of C/A decreased the compactness of the printed structure. The compressive strength and flexural strength of 3D printed specimens showed a direction-dependent characteristic, mainly related to the non-uniform distribution of voids revealed by X-CT. SEM images revealed the "micro-bridging" morphology in the interlayer area and proved that there were carbonation and structural weakening problems at the surface and surrounding of this area. [ABSTRACT FROM AUTHOR]