Research and technological advancements in 3D concrete printing (3DCP) have led to the idea of applying it to offshore construction. The effect of gravity is reduced underwater, which can have a positive effect on 3DCP. For basic verification of this idea, this study printed and additively manufactured specimens with the same mortar mixture in air and underwater and evaluated properties in the fresh state and the hardened state. The mechanical properties were evaluated using the specimens produced by direct casting to the mold and specimens produced by extracting from the additive part through coring and cutting. The results of the experiment show that underwater 3D printing required a greater amount of printing output than in-air 3D printing for a good print quality, and buildability was improved underwater compared to that in air. In the case of the specimen layered underwater, the density and compressive strength decreased compared to the specimen layered in air. Because there are almost no effects of moisture evaporation and bleeding in water, the interlayer bond strength of the specimen printed underwater was somewhat larger than that printed in air, while there was no effect of the deposition time interval underwater.

Recently, the three-dimensional concrete printing (3DCP) method has been garnering considerable interest owing to its ability to significantly reduce the construction time. In this study, 3D printing or additive manufacturing was applied to mortar using a small gantry type equipment and the performance of the method was evaluated. The mixture proportioning for good mortar printing and deposition was derived. The parameters of printability, buildability, compressive strength, flexural tensile strength, and anti-washout were considered for the performance evaluation. The results showed good printability with a constant width and no surface defects. In the buildability test, the rate of yield stress development increased, and the rate of change in the layer height decreased as the interlayer time interval increased during underwater printing. The flexural tensile strength of the specimen cast into the mold was lower than that of the specimen extracted from the additive parts owing to the longitudinal confinement during printing. The compressive strength in the lateral direction was slightly higher than that in the perpendicular direction, whereas the compressive strength of the specimen extracted from the part printed underwater was higher than that of the specimen cast into the mold.