Slipform-based vertical precast concrete printing with embedded reinforcement and hollow cross sections

The construction industry is seeking automation-based solutions to overcome labor shortages, improve construction quality, and reduce waste in production. Although 3D concrete printing (3DCP) has shown potential for automated on-site fabrication, the conventional layer-deposition approach faces limitations in reinforcement integration, pumping requirements, size constraints, and surface quality. This study, as an alternative, proposes a slipform-based vertical printing method specifically designed for fabricating precast concrete elements in a single process. A prototype vertical printing device was developed at the lab scale, and the printability of cementitious materials, produced with various water-to-binder ratios and high-range water-reducing admixture dosages, was evaluated through the channel flow tests and printing experiments. Consequently, yield stress criteria were established: An initial yield stress of 85 Pa or lower is required to ensure gravity-driven flow of material into the nozzle, whereas an increase in yield stress to 240 Pa after a 20 min waiting time is necessary to satisfy adequate shape stability for vertical precast printing. The printing materials satisfying both criteria were successfully printed at 30 mm/min. In addition to the printability, surface wrinkles and air voids observed during printing were effectively handled by applying localized vibration (4 V, 60 s), which improved filling ability of the material around the embedded rebar substitute. These results demonstrate the feasibility of the proposed non-layered vertical printing process and provide quantitative rheological criteria for material design in the proposed vertical precast printing method.