A fresh-state tensile test for in-situ quality control of 3D-printed FRCC linking filament failure, printability, and hardened performance

Preventing filament tearing and fracture during high-efficiency 3D concrete printing remains a significant challenge, given their potential to compromise structural integrity and manufacturing efficiency. This study proposes a novel gravity-driven fresh-state tensile test, adapted from the “slug test”, for in-situ quality control of 3D-printed fiber-reinforced cementitious composites (FRCC). By analyzing the fracture response of extruded filaments, two quantitative indicators, fresh-state tensile strength ( σ f r e s h ) and strain ( ε f r e s h ), were established, potentially linking to the resistance against filament tearing during high-speed printing. The influences of processing method (casting vs. extrusion) and nozzle geometry (area, shape, and thickness) were investigated. Results show that optimizing nozzle geometry significantly enhanced filament integrity. Compared with casting, extrusion and further reducing nozzle area (380 to 150 mm 2 ) and thickness (10 to 3 mm) increased fresh-state tensile strength by 62.1% (6.52 to 10.57 kPa) and strain by 60.7% (12.2% to 19.6%). These improvements effectively suppressed filament fracture during high-speed printing, increasing critical travel speed from 105.8 to 139.2 mm/s and reducing defect index from 0.16 to 0.02. The advantages established in the fresh state were ultimately preserved after hardening, yielding a 59.1% increase in ultimate tensile strength (to 9.45 MPa) and a 117.8% increase in strain (to 11.04%). The improvements might be attributed to enhanced fiber alignment (47.7° to 11.6°) induced by nozzle extrusion, which improves reinforcement efficiency, especially fresh-state resistance against filament fracture. This study establishes a potential link among fresh-state tensile response, printability, and hardened performance, providing an in-situ quality control protocol for 3D printing of FRCC.