Sustainable reinforcement of dry ultra-high performance concrete using renewable sisal fibres with surface treatment and hybridisation

Dry ultra-high performance concrete (DUHPC) is an emerging cementitious material that integrates several benefits of dry concrete and traditional self-compacting UHPC, offering superior mechanical properties, markedly reduced cement consumption, increased use of recycled materials, and practical construction advantages such as rapid setting and efficient placement. This study investigates the mechanical performance of DUHPC reinforced with untreated sisal fibres (UTSF), alkali-treated sisal fibres (TSF), and hybrid steel-treated sisal fibres. The study focuses on strength, crack mouth opening displacement, fracture toughness and energy, strain mapping, microstructural behaviour, and water absorption rate. The mono fibre mixes contained 10 mm-length sisal fibres at dosages of 0.5-1.5%, while the hybrid fibre mixes maintained total fibre contents of 1.0% and 1.5%. Test results indicate that after 28 days of curing, the compressive strength of UTSF and TSF specimens reached its optimum at 1.0% fibre dosage, while the flexural strength of UTSF and TSF specimens increased by 35.2% and 42.1% at 1.5% fibre dosage, respectively, compared to the fibre-free counterpart. Hybrid-reinforced DUHPC exhibited significant enhancements in mechanical performance, attaining a compressive strength of 116.7 MPa, flexural strength of 19.1 MPa, initial fracture toughness of 2.28 MPa·√m, and fracture energy of 11.03 N/mm. Strain mapping revealed superior crack-restraining and more uniform strain fields in hybrid specimens, while the microstructural analysis confirmed a synergistic toughening mechanism involving fibre rupture and pull-out of treated fibres, and plastic deformation of steel fibres. Moreover, alkali treatment mitigated fibre swelling, while the hybrid fibre system showed the lowest absorption rate (<3.5%). The combined use of steel/sisal fibres offers well-balanced trade-offs between composite performance and sustainability, demonstrating the feasibility of incorporating sustainable plant-based fibres into DUHPC systems and providing a valuable basis for future evaluations of its performance in thin-walled structural applications.