Elucidating the Behaviour of Hybrid Fibre-Reinforced Concrete Using Post-Consumer Carpet Fibres

Abstract Carpet waste degrades slowly, posing an environmental burden. Incorporating this waste as fibre reinforcement in concrete helps to reduce accumulation and improve the properties of concrete. However, varying fibre characteristics and their interaction in blends are not well established. Hence, understanding hybrid fibre effects is crucial for construction adoption. This study evaluates the mechanical performance, shrinkage, microstructure, pore structure, and interfacial characteristics of hybrid carpet fibre-reinforced concrete. Four fibres, nylon, polypropylene (PP), polytrimethylene terephthalate (PTT), and polyester were analysed using grey relation analysis in six hybrid combinations with three mix ratios at a 0.3% volume fraction. At 28 days, flexural and splitting tensile strengths increased by 4–11% and 8–26%, respectively. All mixtures exhibited reduced shrinkage compared to the control. The nylon/PP 2:1 mix showed the best overall performance. Microstructural analysis revealed pore refinement, reduced porosity, and improved fibre–matrix bonding influenced by fibre hydrophilicity or hydrophobicity. Nanoindentation indicated hydrophilic fibres minimized the interfacial transition zone thickness. The findings indicate fibre combinations exhibit additive effects. This suggests performance can be predicted for blended fibre mixes, potentially allowing the design of mixes using fibre combinations to compensate for poorer performing fibres, enabling their reuse and avoiding landfill disposal.