Experimental Study on Void Reduction and Bonding Behavior in 3D Concrete Printing with a Trowel-Integrated Nozzle (2026-03)¶

Three-dimensional concrete printing (3DCP) is a digitally driven construction process that enables automated, formwork-free fabrication of intricate concrete geometries with high precision and minimal material waste. However, inline reinforcement integration remains a major challenge, often limiting bond strength and overall structural performance. This study introduces a novel nozzle design equipped with trowels that actively compact the concrete matrix during inline cable reinforcement, effectively minimizing voids and improving mechanical behavior. A dual-channel coextrusion system was developed to deposit cementitious material and steel cable simultaneously, while the trowels ensure radial compaction around the embedded reinforcement. Experimental investigations were conducted using three nozzle diameters (D1: 20mm, D2: 22.5mm, and D3: 25mm) and three cable sizes (C1: 1.0mm, C2: 1.2mm, and C3: 1.5mm). Results from pullout, split tensile, and three-point bending tests showed that the smallest nozzle (D1) consistently provided enhanced cable bond strength and flexural performance across all cables. Cable C3 (1.5mm diameter), along with Nozzle D1, achieved bond strengths up to 17MPa (73% of cast specimen); flexural moment was enhanced by about 40% for Cable C3. It also improved the interlayer bond strength (90% of cast specimen) for D1. Digital image correlation analysis confirmed uniform strain distribution and controlled crack propagation for Nozzle D1. This work establishes the effectiveness of a nozzle attached to a trowel in mitigating voids around the cable and improving interfacial adhesion between the cable and concrete in 3DCP. It lays the foundation for future parametric optimization of trowel geometry and extrusion parameters and opens avenues for microstructural investigations at the concrete–steel interface to further advance digitally fabricated structural systems.