Evaluation of the performance of reinforced concrete beams with 3D-printed permanent formwork

Understanding the advantages of combining traditional construction methods and 3D concrete printing is the gateway to the general structural applicability of three-dimensional concrete printing (3DCP) technology. To further this objective, in this study, 3D-printed concrete is used as permanent formwork for the manufacture of reinforced concrete beams. Different treatment methods for the interfaces between the printed formwork and the cast core concrete, including embedding of shear connectors, entraining of micro-cables, and inclusion of ribs in the 3D printing of the formwork, are explored to improve the integral mechanical capacities of so-called 3D-printed and cast-in-place composite beams. Specifically, flexural behaviors are studied experimentally through observation of damage and failure processes via visualization with the digital image correlation (DIC) method. The mesoscale architecture at the interface between 3D-printed permanent formwork and cast-in-place concrete is investigated through computed tomography (CT) scanning to infer the cooperative bonding mechanisms of different bonding treatment methods. The stress-transfer mechanism is thus elucidated. The applicability of 3D-printed concrete as permanent formwork is validated in view of a 14.3% increase in the bending capacity from a composite beam with bottom ribs compared to that of a completely cast counterpart. From the experiments, incorporation of shear connectors contributes most to the bonding performance of the formwork–concrete interfaces. Meanwhile, interfacial bonding can be enhanced by increasing the roughness of the 3D-printed formwork, interlocking of the formwork with the inner aggregated concrete, or improving the consistency of the elastic and shear moduli of the formwork and cast concrete. In particular, the appropriate thickness of both the formwork and the aggregated concrete as a cover to yield the optimal integrated bending capacity of composite beams is derived from the current study.