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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]