Abstract The advancement of underwater concrete additive manufacturing (UCAM) is currently hindered by the lack of specialized characterization techniques that accurately reflect the challenges of submerged marine environments, such as material washout and temperature-induced rheological changes. This study introduces a comprehensive experimental framework for evaluating and optimizing ultra-high-performance concrete (UHPC) mortar designed for underwater applications. A printable UHPC mortar was first developed by incorporation of high-range water-reducing (HRWR), slump retaining agent (SRA) and anti-washout admixtures (AWA) to achieve target consistency and cohesion. To simulate realistic field conditions, all fresh-state tests were conducted in 35 ppt seawater at controlled temperatures of 10, 15, and 20°C using a chiller-assisted setup. The framework includes three modified testing protocols: a submerged flow table test, an adapted rheometer sample preparation for viscosity and yield stress assessment, and an underwater Vicat setting time test. The experimental results demonstrated that environmental conditions significantly alter the behavior of the material. The reduction in seawater temperature from 20°C to 10°C reduced the flowability from approximately 114 mm to 103 mm. Although submersion in 20°C seawater initially accelerated the setting time to 170 minutes compared to 200 minutes in air, the thermal effect dominated at lower temperatures, extending the initial set to 220 minutes at 10°C. The rheological profiles confirmed that viscosity and shear stress increase as temperatures drop, emphasizing the need for temperature-adjusted pumping parameters. This investigation establishes a robust and realistic approach for the characterization of underwater-printable UHPC, providing the necessary data to improve the structural integrity and efficiency of autonomous marine construction and repair.

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.