Effect of fibre surface coating on the mechanical properties of natural fibre-reinforced soil

The application of natural fibres in soil reinforcement is cost-effective and environmentally friendly. However, natural fibres exhibit dimensional instability when exposed to moisture. Their hydrophilic characteristics affect mechanical performance and durability of natural fibre-reinforced composites. In this study, the effects of fibre surface coating on the mechanical properties of soil–fibre composite were investigated. Natural-based water repellent was coated onto sisal fibres to prevent water absorption. The single-fibre pull-out tests were conducted with varying soil matrix moisture contents and densities. Unconfined compression tests were performed on the soil composite containing 25 mm coated fibres at 0.75% fibre content. The results showed that interfacial shear strength (IFSS) of coated fibres was lower than uncoated fibres at the matrix optimum moisture content. This was attributed to the presence of thin film of moisture at fibre–matrix interface, which suppressed adhesion and mechanical interlock between fibre and soil matrix. Increasing matrix density led to high IFSS of both fibre types due to the enhanced interfacial frictional resistance. The composite with coated fibres exhibited higher rate of moisture loss due to the hydrophobic characteristics of fibres. Shear lag model was used to predict axial load transfer of fibres. The model prediction accuracy was dependent on the material properties, geometry and friction at fibre–matrix interface. Rosin can effectively be used to prevent water absorption of natural fibres and enhance long-term strength properties of natural fibre-reinforced soil.