Reinforcing 3D Printed Concrete with Sprayed SHCC Overlay

Plain 3D printed concrete (3DPC) suffers from inherent brittleness and anisotropic behavior that limit its structural applications. This study aims to investigate sprayed strain-hardening cementitious composite (SHCC) as a thin overlay to reinforce 3DPC. In particular, this paper investigated the effects of sprayed SHCC overlay thickness (10, 20, and 30 mm) on the flexural performance of reinforced 3DPC. Four-point bending tests were conducted on specimens with two cutting orientations, namely parallel (H) and perpendicular (V) to printing direction, and three loading directions (X, Y, and Z). The digital image correlation (DIC) technique was used to monitor strain distributions and interfacial behavior. The 3DPC reinforced with the 10 mm-thick overlay still showed brittle behavior but with reduced anisotropy. For reinforced 3DPC with a 20 mm-thick overlay, the flexural capacity increased by 35%–970% depending on loading direction, with the ultimate deflection enhanced by over 400% in all directions. The 30 mm-thick overlay further improved flexural strength by 155%–1200% and ultimate deflection by 350%–533% relative to plain 3DPC. DIC analysis revealed brittle fracture via a single crack in the composite with the 10mm-thick overlay, while the specimens with thicker overlays exhibited multiple cracks, and no shear slip was observed at the interface between the 3DPC substrate and S-SHCC overlay. An anisotropy-modified plane-section model was developed to predict structural behavior, with elastic stage deviations within 28% in the X-direction and 30% in the Y- and Z-directions. The peak load predictions showed 30% deviation in the X-direction and less than 15% spread in the Y- and Z-directions. These findings provide guidelines for designing 3DPC structures reinforced with sprayed SHCC overlay.