Recent Advancements in Polypropylene Fibre-Reinforced 3D-Printed Concrete: Insights into Mix Ratios, Testing Procedures, and Material Behaviour

This review presents a comprehensive analysis of polypropylene (PP) fibre incorporation in three-dimensional printed concrete (3DPC), focusing on the material behaviour in both fresh and hardened states. PP fibres play a critical role in improving rheological properties such as buildability, flowability, and extrudability. While increased fibre content enhances interlayer bonding and shape retention through the fibre bridging mechanism, it also raises yield stress and viscosity, which may compromise extrudability. In the hardened state, PP fibres contribute to improvements in compressive and flexural strength up to an optimal dosage, beyond which performance may decline due to fibre clustering and reduced packing density. When aligned with the printing direction, fibres are particularly effective in mitigating shrinkage-induced cracking by redistributing internal tensile stress. However, their inclusion can lead to a slight increase in porosity and promote mechanical anisotropy. This review also discusses mix design parameters, fibre characteristics, and experimental protocols, while identifying key research gaps including the lack of standardized testing methods, limited understanding of fibre orientation effects, and insufficient exploration of hybrid fibre systems. Based on the synthesis of reported studies, optimal print quality and structural consistency have been associated with the use of 6 mm long fibres, nozzle diameters of 4 to 6 mm, and printing speeds ranging from 40 to 60 mm/s. Overall, PP fibre reinforcement shows strong potential for enhancing the structural integrity and dimensional stability of 3D-printed concrete, while emphasizing the need for further studies to optimize its use in practice.