Mechanistic insights into microstructural changes caused by stapling in extrusion-based 3D printed concrete (3DPC)

Stapling is an effective post-implantation method for reinforcing three-dimensional (3D) printed concrete (3DPC); however, it may alter the microstructure of the adjacent concrete slurry. To uncover the mechanisms of microstructural alterations induced by stapling, remaining inadequately understood, we comprehensively characterized the microstructure of extrusion-based 3DPC with two viscosity levels, stapled using steels of five different diameters. The results suggest that the anchoring effect of the staples enhanced the interlayer bonding, while the formation of the staple-matrix interfacial defects decreased the compressive strength. These defects were non-uniformly distributed along the staples, and the 3DPC with higher viscosity exhibited more severe defects. The contribution of staple-matrix defects to mechanical strength was assessed based on defect ratio and connected length. Finally, a model was developed to capture the deformation of concrete slurry during the stapling process. These findings deepen the mechanistic understanding of the conflict between stapling reinforcement and interfacial weakness in 3DPC.