Towards scalable underwater concrete additive manufacturing for marine and aquatic settings

Underwater concrete underpins marine and aquatic infrastructure, yet its placement remains constrained by limited access and reliance on specialized, diver-dependent methods. Underwater Concrete Additive Manufacturing (UCAM) offers automated, formwork-free placement and access to geometries that conventional underwater concreting cannot reach. Moving extrusion from air to water, however, disrupts the rheology, early-age stability, and interlayer bonding on which printing depends. This mini-review consolidates the small but rapidly expanding UCAM literature into a single framework linking mixture design, process and equipment adaptation, and material characterization. It compares the binder systems, anti-washout and rheology-control admixtures, and aggregate and reinforcement strategies reported to date; maps the printing-process envelope of nozzle geometry, print speed, and flow rate across gantry and robotic systems; and benchmarks fresh- and hardened-state performance against cast and air-printed concrete. Three coupled barriers emerge as specific to UCAM: washout of fresh material, buoyancy acting on deposited filaments, and the influence of hydrostatic pressure on hydration. Interlayer bonding, weakened by water entrapped at layer interfaces, is identified as the principal limit on mechanical reliability. The review contributes a consolidated, comparative basis for mixture and process selection, together with a defined set of standardization and durability gaps that must close before field deployment—structure that the current case-specific literature lacks.