Experimental investigation of the mechanical properties of recycled aggregate concrete reinforced with water hyacinth fibres for sustainable construction.

The construction sector requires sustainable concrete that reduces natural aggregate consumption and valorises waste-derived resources. This study investigates recycled aggregate concrete (RAC) incorporating 100% recycled concrete aggregate (RCA) and water hyacinth fibres (WHFs), an invasive aquatic biomass. Natural coarse aggregate (NCA) was replaced by RCA at 0% and 100% by volume, while WHFs were added at 0.5% and 1.0% by cement weight. Four fibre categories were examined: treated bark, untreated bark, treated core, and untreated core fibres. The experimental programme included fibre tensile characterisation, compressive, splitting tensile, and flexural tests, microstructural observations, empirical strength prediction, and nonlinear finite element modelling. Based on the adopted mix design, 100% RCA avoided approximately 906.91 kg/m3 of NCA, while WHFs utilised 1.67-3.33 kg/m3 of invasive biomass as bio-based reinforcement. RCA replacement reduced 28-day compressive strength by 12.81-25.56% owing to weaker and more heterogeneous interfacial transition zones. WHF addition further reduced compressive strength by 20.94-48.58%, mainly due to fibre-induced voids, reduced compaction, and local matrix discontinuities. Conversely, WHFs increased splitting tensile strength by 35.76-64.24% through crack bridging and stress transfer. Treated bark fibres at 0.5% provided the best flexural response, achieving 5.39 MPa and increasing flexural toughness by 56.97%, attributed to higher fibre tensile strength, fibrillated surfaces, and improved fibre-matrix bonding. The predictive equations are limited to the investigated range, while the finite element model reproduced peak loads with errors of 2.90-5.74%. These findings demonstrate the potential of treated bark WHFs for enhancing tensile resistance and toughness in 100% RAC.