Experimental Assessment on Printing Performance and Mechanical Properties of Underwater Self-Protecting 3D Printing Concrete

3D printing concrete technology has gained significant attention for its mold-free construction and precision molding capabilities, yet its application in underwater environments remains underexplored. This study evaluates the feasibility of underwater self-protecting 3D printing concrete (USP-3DPC) and investigates the effect of varying underwater protective agent (UPA) ratios on key performance metrics, including underwater anti-dispersion, extrudability, and buildability. Compressive strength, splitting tensile strength, and porosity characteristics of USP-3DPC were also analyzed. The results indicate that incorporating UPA enhances underwater anti-dispersion, with an optimal ratio of 6% providing the best balance of extrudability and buildability. Although USP-3DPC specimens exhibit lower compressive and splitting tensile strengths than air-printed 3DPC, both demonstrate similar mechanical anisotropy. Analyses of porosity using mercury intrusion porosimetry reveal distinct differences in pore structure evolution and strength development between specimens printed in air and underwater environments. Strength comparisons at different curing ages confirm that USP-3DPC retains substantial mechanical integrity, supporting its potential for underwater construction applications.