Bond Governed Interactions Between Helical Reinforcement and 3D Printed Concrete (2024-12)¶

A range of novel reinforcement technologies attuned to the specific needs of 3D Concrete Printing has been introduced in recent years. However, their characteristics can deviate significantly from conventional reinforcement bars as applied in cast concrete. Their behaviour, therefore, deserves study. Particular attention is required regarding the applicability of existing structural engineering approaches to these novel reinforcement technologies. The current study uses tension chord specimens, direct pull-out tests, and 3-point bending tests, to explore how automatically inserted helical reinforcement in 3D printed concrete affects crack spacing, anchorage strength, and failure modes in singly reinforced beams. Helical reinforcement demonstrated markedly higher stiffness and bond strength than conventional ribbed bars, achieving peak bond stresses of 23.84 MPa compared to 11.54 MPa, as confirmed by confined pull-out tests. Tension chord specimens under uniaxial tension showed that helical reinforcement mirrors the stress-strain behavior of conventional reinforcement, with both exhibiting similar phases of crack development and tension stiffening, but the crack spacing reduced significantly from 123.02 ± 19.24 mm to 67.25 ± 7.71 mm due to the high bond strength. The adoption of an analytical model using bond-stress displacement provided crack spacing predictions that matched experimental outcomes, validating its effectiveness for helical reinforcement in printed concrete. Unconfined direct pull-out tests revealed the dominance of concrete splitting at low concrete volumes and short embedment lengths, rather than pull-out slipping or reinforcement yielding. For specimens with 40 mm embedment length, failure loads increased from 1.28 ± 0.08 kN at 60 mm width to 5.54 ± 0.36 kN at 150 mm width. While the thick-walled cylinder model correctly predicted this trend of increased capacity with wider specimens, it consistently overestimated maximum loads at splitting failure for short embedment lengths. Correlations to changes in concrete volume and embedment length are presented. Material based solutions to provide confinement, such as Strain Hardening Cementitious Composites (SHCC), are introduced as potential solution, increasing beam failure loads from 3.45 kN to 6.63 kN. Singly reinforced beams with SHCC and helical reinforcement adhered to predicted ultimate bending loads based on Eurocode 2, successfully avoiding brittle failures that occurred in plain mortar beams. Despite these advances, the experiments show the need for improved shear reinforcement and anchorage strategies. Overall, the results suggest that existing analytical models for the structural analysis of conventional reinforced concrete are suitable to predict failure behaviour and loads of 3D printed concrete with helical reinforcement, but some adjustments to the specifics of this type of elements are required, depending on the type of analysis.