Underwater 3D printing of concrete: tailoring mix-design to tackle washout

Abstract Underwater 3D concrete printing (UW3DCP) relocates early-age evolution into a saturated, low-effective-stress regime where washout and seam weakening are controlled by the time-dependent mobility of pore water. This study investigates whether the time-dependent fresh-state hydraulic conductivity, K ( t ), can serve as a practical metric for UW3DCP materials. A framework for UW3DCP is formulated by coupling hydraulic conductivity $$K(t)$$ K ( t ) in the fresh state (proxy for transient preferential path formation) with interlayer integrity measured in air-printed and submerged-printed elements, while also examining the influence of aggregate size. Biopolymer admixtures are examined qualitatively at comparable extrusion and buildability conditions. Cellulose ethers (HPMC) cause only short-lived reductions in $$K(t)$$ K ( t ) ; their apparent benefits in shape retention are not sufficient to prevent the early development of washout transport paths. In contrast, a Sphingan gum forms a more stable and cohesive gel network that suppress both level and drift of $$K(t)$$ K ( t ) within the deposition-to-setting time window. Within the scope of the present permeability test, K ( t ) discriminates mixture stability and captures the influence of polymer chemistry and granular packing on washout susceptibility. This result therefore supports the use of fresh-state K ( t ) as a promising acceptance metric for UW3DCP mixture qualification.