Failure mechanisms of 3D printed formwork arches: The critical role of interfacial bond and manufacturing parameters

While 3D printed formwork (3DPF) offers unprecedented geometrical freedom and construction efficiency for concrete structures, existing structural design frameworks present a critical deficiency: they predominantly assume a “perfect bond,” thereby dangerously ignoring the complex inter-layer weaknesses inherent to 3D printing. To bridge this gap, this study investigates the complex failure mechanisms of reinforced 3DPF arches through comprehensive static experiments and numerical simulations. The results reveal a counter-intuitive "anomalous reinforcement effect": increasing the number of reinforcement bars from two to three significantly decreased the ultimate load capacity by 8.77%. Strain monitoring and ductility analysis (yielding extremely low ductility coefficients of 1.01–1.02) confirmed that at ultimate load, the rebars were approaching yield, but a brittle bond failure occurred prematurely, preventing the full utilization of the steel's strength. This demonstrates that heavily reinforced 3DPF structures are “interface-controlled” rather than “material-controlled.” Furthermore, the study quantifies a “dual effect” of internal topology: the unreinforced arch with no ribs exhibited a higher load capacity (148.57 kN) compared to heavily ribbed specimens (120.05 kN) due to the detrimental increase in weak interface density. Concurrently, increasing the printed strip width fundamentally altered the macroscopic failure mode from interfacial debonding to brittle shear. Crucially, analytical comparisons reveal that conventional RC codes can dangerously overestimate the ultimate capacity of heavily reinforced 3DPF structures by up to 46.6%. These findings underscore that the traditional RC design concept of indiscriminately adding reinforcement is inapplicable for bond-critical 3DPF structures, necessitating the development of novel, interfacial-shear-governed design formulations.