Temperature-dependent rheology and mechanical performance of underwater 3D printed ultra-high performance concrete: Insights from microstructure

Ultra-high performance concrete (UHPC) features a dense, cohesive matrix and superior durability, making it a promising candidate for underwater 3D concrete printing in the automated construction and repair of marine infrastructure. However, the application of UHPC in underwater concrete additive manufacturing remains largely unexplored, particularly regarding how aquatic thermal conditions govern material behavior. This research investigates the influence of low ambient underwater temperatures (10, 15, and 20 °C) on rheology, setting time, and early-age mechanical performance of 3D-printed UHPC mortar. Rheological evolution was characterized using modified tests protocols designed for submerged conditions, while compressive strength and interfacial bonding were evaluated for specimens printed in both air and simulated underwater environments. The results indicate that lower underwater temperatures increased the stiffness of the fresh material, as evidenced by the higher plastic viscosity and shear stress, but delayed the initial setting time. This rheological stiffening critically restricted "post-extrusion relaxation"—the viscoplastic spreading necessary for filaments to conform to adjacent layers. Consequently, lower temperatures limited self-adjustment at the interface, weakened inter-filament contact zones, and increased micro-entrapped porosity at lower temperatures, as revealed in micro-computed tomography results. In addition, underwater printed samples exhibited a more refined pore structure than their in-air counterparts. This was driven by a "controlled washout" mechanism, where fine particles settled into and self-leveled large process-induced voids during the manufacturing process. These findings establish a framework for balancing rheological stability with microstructure observation, providing essential guidelines for robust underwater additive manufacturing. Funding:This research was supported by the National Science Foundation Future Manufacturing program (Project No. 2328188) and Louisiana Transportation Research Center (LTRC) (Project LTRC/LADOTD 24-1ST), whose support for interdisciplinary research is gratefully acknowledged.