Abstract
Extrusion-based 3D concrete printing is a new technology under development for the construction of buildings and structures of complex geometries without the use of expensive formwork. The weak interlayer strength between printed layers is one of the limitations of this technology when compared to cast-in-the-mold concrete. Part one of this study investigates effects of the print-time interval on the interlayer strength along with the compressive and flexural strengths of extrusion-based 3D printed concrete in different directions. Specimens were printed with 10, 20, and 30minutes delay times (print-time intervals). Compressive, flexural, and interlayer strengths of the 3D printed concrete samples were measured. The interlayer strengths of the specimens printed with 10 and 30minutes delay times were comparable, but higher than that of the specimens printed with a 20minutes delay time. A correlation was found between the results of interlayer strength and the surface moisture content at the interface of the layers. The surface moisture content, in turn, depends on the bleed rate of the concrete and the rate of drying moisture from the surface, among other factors. The results also indicated that the compressive and flexural strengths of 3D printed concrete depended on the testing direction. The orthotropic phenomenon was more pronounced in compressive than flexural strength. Part two of this study investigates a method of strengthening the interlayer bond through the application of a cementitious paste to the top of an extruded substrate layer before the secondary layer is deposited. It was demonstrated that the application of a paste at the interlayer shows an increase in bond strength. The greatest bond strength was found in pastes mixed with additives to sustain flow characteristics, over a time gap. The increase in contact area on both layers was verified to be a crucial factor in bond strength development.
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