Hybrid RC columns with 3D printed formworks: A case study on process-informed structural integrity

3D printed permanent formworks offer an efficient means of fabricating reinforced concrete (RC) members. However, their structural contribution under axial compression remains uncertain due to variations in interface quality. This study investigated the structural roles of printed formworks and established an Optimal Casting Window (OCW) to ensure reproducible interface integrity. The printed formwork and post-cast concrete were proportioned to reach a nominal 28-day strength of 21 MPa to eliminate strength-gradient bias. The casting timing was governed by the stiffness evolution of the formwork, monitored via electrical resistivity and ultrasonic pulse velocity. Two hybrid configurations (OCW and 72 h casting) were evaluated alongside a conventional RC control. Axial compression tests were conducted under full-section and core-focused section loading to decouple direct load sharing from passive confinement effects. The results indicated that under full-section loading, the HRC-QC samples displayed higher peak loads and energy absorption than the conventional RC. Crucially, a normalized stress-based interpretation suggested that these differences resulted from a combination of geometric expansion and process-driven interface quality. Under core-focused section, the HRC-QC samples exhibited superior load capacity and peak displacement, suggesting effective mobilization of passive confinement. These findings suggest that casting within the process-verified OCW is essential for attaining consistent structural integrity, establishing a process-informed framework for hybrid RC members with 3D printed permanent formworks.