Influential factors on mechanical properties and microscopic characteristics of underwater 3D printing concrete

Underwater 3D Printing Concrete (U3DPC) technology can be a promising approach to address the challenges of renewable marine energy and coastal protection. Conducting extensive research on the mechanical properties of U3DPC is crucial for its successful implementation. This study investigates various factors, including printing environment, construction methods (mold-cast and printing), curing age, and material composition, regarding their significance on the mechanical properties of U3DPC. The compressive strength, anisotropy, interlayer bonding, and interface microstructure are evaluated. The research findings reveal that compared to casting specimens in air, the printing method and water environmental factors result in approximately 20% and 15.1% reduction in the compressive strength of U3DPC, respectively. U3DPC exhibits different anisotropic variations compared to specimens printed in air, attributed to weak interlayer bonding interfaces caused by external water. The influence of material components on the interlayer bonding strength of U3DPC has a critical threshold that should not be exceeded; otherwise, it weakens the interlayer bonding. The variation in fine aggregate types and fibers leads to changes in interlayer interface roughness, where excessive roughness captures more ambient water due to the “dog-tooth overlapping” structure, thereby reducing the adhesive capacity. The pore distribution reveals the underlying mechanism of how interface roughness affects interlayer bonding.