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• 3DPC with 4.75–20 mm coarse aggregate size in continuous/discontinuous grading was designed. • The effect of coarse aggregate gradation on the fresh and hardened properties of 3DPC was investigated. • A correction coefficient for the existing compression stress–strain model was proposed to and fit 3DPC well. • Relationship between the coarse aggregate spacing and the correction coefficient was built and discussed. Coarse aggregate is an indispensable component of concrete, the gradation of which plays an important role in the properties of concrete due to its effect on particle distribution. The extrusion-based 3D concrete printing (3DCP) has become a promising technology while the influence of coarse aggregate gradation on the material design and performance of 3D printed concrete still lack understanding. This study experimentally investigates the fresh and hardened properties of 3D printed concrete with 4.75–20 mm continuous and discontinuous grading coarse aggregates, including flowability, buildability, axial compression, and drying shrinkage. The results reveal that 3D printed concrete with continuous grading performs better on flowability retaining and multilayer structure stacking than that with discontinuous grading. The evaluation coefficient of aggregate particle size is proposed to characterize the average particle size of coarse aggregates with different gradations, which shows linear relationship with coarse aggregate 3D spacing in 3D printed concrete. The current axial compressive stress–strain model for the conventional concrete prediction curve is modified to predict the effect of coarse aggregate gradation on constitutive relationship of 3D printed concrete. The increasing proportion of large-sized coarse aggregates can lead to a reduction in long-term drying shrinkage of 3D printed concrete. [ABSTRACT FROM AUTHOR]

• 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]