Structural and Microstructural Behavior of Novel 3DP-UHPFRC Beam with Discrete Steel-Fibers and Steel-FRP Composite Bar (2025-01)¶

This study proposed a novel flexural design for 3D-printed ultra high-performance fiber reinforced concrete (3DP-UHPFRC) beam with steel-FRP composite bars (SFCBs) in the tension zone and discrete steel fiber. A total of 10 beam specimens with different preparation methods, UHPFRC layer thickness (Lt from 40 to 100 mm), and secondary stiffness ratios (rs from 0 to 0.49) were systematically designed and analyzed under a three-point bending test. The damage evolution of tested beam specimens was evaluated by the digital image correlation method, and the internal structural characteristics of the interface between 3DP-UHPFRC and SFCB were quantitatively unveiled through micro X-ray Computed Tomography (CT) scanned image. The experimental results demonstrate that the increase in Lt and rs directly correlates with an enhancement of flexural load capacity. The flexural strength of 3DP beam specimens increased by 8–34 % with UHPFRC layer thickness of 50–100 mm, and secondary stiffness ratio increases from 0.14 to 0.49 led to a 66–168 % improvement. The energy absorption of 3DP beam specimens with SFCBs increased by 47–150 % and 53–131 %, respectively, with increases in UHPFRC layer thickness and secondary stiffness ratios. Additionally, ductility indices showed a modest increase of 10–17 % with UHPFRC thickness, while enhancing the secondary stiffness ratio improved ductility by 42–106 % compared to beam with steel bars. The distribution of fibers in the tension zone, as identified by CT images, may greatly contribute to the structural performance of the proposed flexural design of 3DP beams with SFCBs. The pore defect was notably prominent in the bonding interface area, which may have resulted in the pull-out failure of SFCB. This study laid the foundation for the forward design approach in constructing large-scale structures using 3D printing and composite FRP composite materials.