Multi-Parameter Evaluation of Steel Fibre-Reinforced Cementitious Materials for Extrusion-Based 3D Concrete Printing

Extrusion-based three-dimensional concrete printing (3DCP) has emerged as a promising digital construction technology that requires precise control of material rheology and structural performance. This study investigates the influence of steel fibre dosage on the rheological behaviour, mechanical performance, and potential printability of cement-based materials. Mortar mixtures incorporating straight copper-coated steel fibres at dosages from 0 to 1.0% were evaluated. Rheological properties, including yield stress, plastic viscosity, thixotropy, structuration rate, re-flocculation rate, viscosity recovery, and flow index were characterised using rotational rheometry, while compressive and flexural strength were measured at 14 and 28 days. Results show that steel fibres significantly enhance structural build-up and recovery, with both static and dynamic yield stress increasing markedly with fibre dosage. Plastic viscosity and flow behaviour indicate increased resistance to flow at higher contents. Compressive strength exhibits a non-monotonic trend, reaching a maximum increase of approximately 40% at 0.2%, whereas flexural strength is improved at higher fibre dosages due to crack-bridging effects. A normalisation-based composite index was applied to integrate performance indicators. Based on rheology-derived printability implications, a practical working window was proposed, including static yield stress of approximately 280 to 400 Pa, dynamic yield stress of 70 to 160 Pa, plastic viscosity of 7 to 15 Pa·s, re-flocculation rate of 40 to 55 Pa/min and flow index of 0.6 to 1.0. Although higher fibre dosages produced higher composite index values due to dominant rheological enhancement, excessive rheological resistance may reduce practical processability. Based on the proposed working window, 0.2% steel fibre provides the most balanced performance for potential large-scale 3DCP applications.