Effect of interlayer reinforcement on the axial cyclic behaviour of RC prisms with 3D-printed formwork: experimental testing and analytical model

3D concrete printing (3DCP) has emerged as a transformative technology with the potential to modernise traditional construction practices. However, its adoption remains constrained by several challenges, notably the integration of conventional steel reinforcement into printed elements. 3DCP formwork may be used as a permanent structural formwork, yet its structural contribution remains insufficiently understood. The goal of this research is to assess the effectiveness of 3D-printed shells, both with and without a novel interlayer steel reinforcement, in contributing to the structural capacity and ductility of short prisms. A total of nine prism specimens were tested under uniaxial tension-compression cyclic loading: three traditional cast (CC), three with basic printed formwork (PBC), and three with printed formwork incorporating interlayer stirrups (PSC), placed every five layers during the printing process. The printed shells were produced using limestone calcined clay cement (LC 3 )-based mortar, while the inner core and cast prisms utilised a conventional concrete mix. Experimental results revealed enhanced peak load-bearing capacities in PBC and PSC, on average 47% and 36% higher, respectively, compared to CC prisms. PBC exhibited the greatest compressive strength, being influenced by the boundary conditions of the shell. PSC demonstrated significantly improved ductility, deformation capacity, 35% larger than both CC and PBC, and a shift in failure mode from shear to compression-dominated behaviour. The proposed interlayer reinforcement detailing is presented as a cost-effective method to improve the structural response of 3D-printed structures. The findings highlight the reliability of 3D-printed formwork to be used as a structural component in future construction systems. • 3D-printed shells (PBC specimens) increased axial strength up to 47% over cast prisms • Interlayer stirrups improved ductility and shifted failure to compression mode • OOP deformation analysis showed enhanced shell restraint with interlayer reinforcement • Analytical model accurately predicted cyclic response of prisms with printed-formwork