Comparison of Properties of 3D-Printed Mortar in Air vs. Underwater

Abstract

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.

Effects of Nozzle Details on Print Quality and Hardened Properties of Underwater 3D Printed Concrete

“…Overall, dimensional consistency was found to be good. In the case of WP, surface cracking occurred from the side and the surface quality was worse than that of air printing in the same HSRS [15]. Presumably, the cohesion on the surface of the printout, which is the contact surface with water, was reduced because the mortar that passed through the nozzle came into contact with water.…”

“…The mortar mix for the WP test of the 3DCP system was derived based on multiple mix tests performed several times in advance, and the mixture proportions are summarized in Table 3 [15]. The specified compressive strength was 50 MPa and the water-tobinder ratio was 38.4%.…”

“…3, 6, and 7 silica sands with sizes of 1.2-2.4 mm, 0.25-0.70 mm, and 0.17 to 0.25 mm, respectively, were used. To improve the printability and buildability of the 3D printed concrete and provide an anti-washout performance underwater, a viscosity-modifying agent (VMA) produced by Dongnam was used with 2.0% water content [15]. The VMA dosage was determined through the anti-washout test and buildability test on the printing concrete with the VMA dosage variable.…”

“…Based on Table 3, the slump flow at the start of printing increases by approximately 4-8 mm compared to that immediately after mixing. This is probably because high pressure and heat are generated as the mortar passes through the pump, hose, and hopper, causing the HRWRA to react more actively [15].…”

“…Underwater 3DCP can be used in the maintenance of damaged underwater structures. In addition, although the production of formworks for underwater structures with complex shapes is limited depending on their use, such as breakwater and reef blocks, manufacturing optimal underwater structures using underwater 3DCP technology is possible [15]. This paper presents one step in the technology development process that can use an extrusion-based 3DCP technology underwater.…”