Assessing the effect of time, thixotropy, curing, and environmental conditions on interlayer bond strength in 3D concrete printing.

Abstract

The technology of 3D concrete printing has globally gained the interest of a large number of researchers. Yet despite the fact that the formation of poor bonds is highly probable during the construction of a layered system as compared to a monolithically cast structure, factors that affect bond strength have not been sufficiently investigated. Poorly developed interlayer bonds reduce the printed structure’s tensile resistance leaving it susceptible to many interfacial stresses and, possibly, leading to structural failure. Hence, the aim of this study is to assess the impact of mortar thixotropy, the time gap between printing successive layers, curing (i.e. air vs. wet curing), and environmental (i.e. heat ageing vs. freeze-thaw) conditions on bond strength development. This was achieved by designing and conducting an experimental protocol to evaluate interfacial bond strength. The samples were made using mortars covering low to high thixotropy levels while considering different time gaps. They were then exposed to different curing regimes and environmental conditions that mimic actual service conditions. The collected data were validated using a laboratory 3D printing machine. The results have shown that the lowest percent drop in bond strength relative to the splitting tensile strength of the mortars themselves (10.87percent) corresponded to the moderate thixotropic mixture indicating that medium structuration rates are best suited for proper bond development. As for time, the percent drop generally increased regardless of the thixotropy level or the type of exposure as the gap increased, and the variation in the drop reached 34.7percent as the time gap was increased from 5 to 15 min. Wet curing has proved efficient for preserving bond strength, while strength was exacerbated in air-cured specimens. The results have also revealed that high temperatures are detrimental to interlayer bonding as the drops in heat aged samples hovered around 80percent for all thixotropy levels and time gaps. Finally, the specimens showed b