Comparative Study into the Interlayer Bond Strength of 3D Printed Mortar : Underwater vs. Air Environments

For 3D printable concrete, interlayer bond strength is an important consideration. Several studies have confirmed the lack of bond strength attributed to air entrapment, lack of surface roughness, thixotropy of mix, water evapo-
ration and printing parameters. However, the same conclusion may not be extended to understand the layer bonding behavior for underwater printed specimens. The present study utilizing anti-washout admixture (AWA) shows that AWA-mixed concrete (H7) when printed underwater has 13.9% and 18.2% lower bond strength in the Y and Z directions, respectively compared to the specimens printed in air. Notably, the absence of AWA in the control mix (C) resulted in further reduction of bond strength (up to maximum 41.4%) in the underwater environment with 5-minute interlayer time gap. Micro-CT analysis of pore structure revealed that the volumetric pore size distribution in the interlayers significantly influences the bond strength behavior of specimens printed in both air and underwater. In-line monitoring of the printing process detected the settlement of washed binder near the layer interface, leaving a distinct white line mark in the hardened printed specimen. Optimal interlayer time gaps were found to be beneficial for enhancing bond strength of mixtures containing AWA, attributed to pore structure refinement due to settled binder and increased extrusion pressures during underwater printing.