Effects of Indigenously Sourced Natural and Hybrid Fibres on Fresh and Hardened Properties of Ultra-High-Performance Concrete

The effect of indigenously sourced natural fibres jute, sisal, basalt, and nylon and their binary combinations with high-carbon steel microfibres on the fresh and hardened properties of commercial ultra-high-performance concrete (UHPC) was investigated in the Indian construction context. Six fibre types, added at 1% and 2% by volume, and five binary combinations at a total of 2% fibre content, were evaluated for flowability, compressive strength, flexural performance, toughness, bulk resistivity, and density. Jute (JF) and sisal (SF) fibres, treated with 5% NaOH solution to improve fibre-matrix adhesion, were the primary focus as cost-effective and domestically available alternatives to high-carbon steel. Results indicate that NaOH-treated jute fibres at 2% addition achieved flexural strength of 20.1 MPa, comparable to the 20.8 MPa of the steel control mixture (CS2). Binary combinations of steel-jute (CS1JF1) and steel-sisal (CS1SF1) demonstrated synergistic flexural behaviour, achieving 24.1 MPa and 23.5 MPa respectively, representing increases of 9–16% over single-fibre equivalents at the same total dosage. Sisal fibres significantly reduced flowability due to their high aspect ratio and hydrophilic surface chemistry, analogous to polyethylene fibres in prior studies. Jute-basalt binary mixes exhibited a cost per flexural unit 26% lower than the steel control, making them particularly relevant for bridge repair and pavement overlay applications in Indian infrastructure. Clearly, there are trade-offs among cost, flowability, flexural strength, and compressive strength that can be leveraged through informed fibre selection.