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• Using concrete with coarse aggregate for 3D printing concrete permanent formwork. • Interface splitting strength can be improved with the presence of coarse aggregate. • The reason for weakened interface bonding was investigated by stereomicroscope and X-CT. • The proportion of smaller stones in postcast concrete should be raised for better bonding. Using 3D concrete printing technology to produce permanent formwork for building components is a promising method that maximizes design freedom and material savings. In this study, 3D-printed permanent formwork made of concrete with coarse aggregate is manufactured and post cast with concrete containing different coarse aggregate particle size ratios. The results indicate that the splitting tensile strength of the concrete interface reaches 2.3 ∼ 4.4 MPa, of which the reduction rate range from 12.8%∼35.7% with higher bonding positions. Mesoscopic observation reveals the voids concentration and a lack of coarse aggregate phenomenon in the interface area. Increasing the proportion of small-sized coarse aggregates in the pouring concrete can better fill and improve the splitting strength of the interface area. [ABSTRACT FROM AUTHOR]

• 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]