Effects of nozzle height on the layer deposition process and forming performance of 3D printing concrete: Evidence from experiments and CFD simulations

In extrusion-based 3D printing concrete, nozzle height directly governs filament shape, internal stress distribution, and stacking stability. This study examines how varying nozzle height affects filament cross-section, deposition dynamics, and interlayer bonding. Combining experiments with computational fluid dynamics, a set of descriptors including cross-sectional area, aspect ratio, settlement ratio, coefficient of variation, and the height-to-nozzle-diameter ratio are introduced to systematically characterize filament cross-sections formed during extrusion layer deposition from single to multilayer stacking. The results show that increasing nozzle height alters filament shape, modifies the magnitude and duration of transient impact forces during deposition, reshapes the stress distribution at the interlayer interface, and reduces splitting tensile strength, following a Logistic relationship with a dimensionless impact indicator. Mechanistically, a higher nozzle reduces peak transient forces but prolongs their duration, causing irreversible settlement of underlying layers. These findings reveal the critical mechanistic role of nozzle height in controlling deposition behavior and interlayer bonding, and suggest a height-to-diameter ratio of 0.5–0.7 for circular nozzles to optimize filament shape consistency, stress symmetry, and bonding quality. These findings provide a basis for developing a parameter ranges and quality control methods that are independent of nozzle size.