Effects of Nozzle Thickness on the Mechanical Properties of 3D Printable Ultra-High Performance Strain-Hardening Cementitious Composites (UHP-SHCC) (2025-06)¶

The extrusion nozzle design in 3D concrete printing has attracted widespread attention in industrial innovation and scientific research. This study investigates the effects of rectangular nozzles (3 mm to 10 mm in thickness) on the mechanical properties of 3D-printed Ultra-High Performance Strain-Hardening Cementitious Composites (UHP-SHCC). Extrudability, compressive, uniaxial tensile, and four-point bending tests were conducted. Results revealed that nozzle thickness significantly influenced filament extrudability and mechanical performance. As nozzle thickness decreased and fiber content exceeded 2.0 %, clogging occurred and worsened. The compressive strength in perpendicular loading direction increased with decreasing nozzle thickness. The compressive strength in longitudinal and lateral loading direction was enhanced under the combined failure of interface and filament. With thinner nozzle, the tensile strength reached 9.45 MPa, the tensile strain reached 11.04 %, and the flexural strength reached 24.69 MPa, with toughness increasing to 7117.5 kJ/m³ . These values were 1.59, 2.17, 1.63, and 3.83 times higher, respectively, than those of the mold-cast specimens. Moreover, the crack pattern transitioned from localized to saturated. Crack deflection and twisting enhanced flexural properties and toughness. In addition, the tensile failure mode perpendicular to printing direction shifted from quasi-brittle to ductile with multiple cracking. μCT analysis revealed improved fiber alignment and reduced porosity in filaments extruded through thinner nozzles, contributing to better strength and ductility.