Mechanical and chemical effects of wet-dry cycling and UV exposure on flax textile reinforced recycled aggregate concrete: role of rice husk ash

This study examines the effects of wet-dry cycling combined with ultraviolet (UV) irradiation (at 20 °C, 30 weeks) on the mechanical properties and chemical compositions of reinforced recycled aggregate concrete (RAC) reinforced with epoxy-coated flax textile and its individual components, including flax yarn, epoxy, epoxy-coated flax rod, and RAC. One group of reinforced RAC samples incorporated rice husk ash into the concrete, and the influence of rice husk ash on resistance to weathering-induced ageing was also investigated. The mechanical and chemical degradation mechanisms induced by these environmental factors were evaluated by tensile or compression tests and Fourier-transform infrared spectroscopy analysis. The results showed that flax yarns alone underwent severe deterioration, including compound leaching, fibrillation, and discoloration, leading to reduced cellulose and hemicellulose content as well as decreased cellulose crystallinity, along with the formation of chromophoric compounds and microbial activity. The degradation resulted in a significant reduction in tensile loading capacity, with only 7.4 % of residual load capacity remaining. Degradation of epoxy was also observed, with a 20 % reduction in tensile strength due to the breakdown of amide groups and aliphatic structures. In the epoxy-coated flax rod, wet-dry cycling caused flax fibre/epoxy interfacial debonding. Although photo-oxidation of the fibres noticeably reduced the cellulose crystallinity and content, the overall rod showed no significant tensile deterioration, with only a 9 % decrease in average tensile strength and a 10 % decrease in average E-modulus. Both reinforced RAC samples, with or without rice husk ash, retained their tensile properties after exposure, with no significant changes detected in tensile strength, toughness or ultimate strain; however, the flax fibre extracted from the sample without rice husk ash exhibited more severe alkaline degradation in hemicellulose and cellulose crystallinity compared to that from the sample containing rice husk ash. This indicated that incorporating rice husk ash into the RAC helped to mitigate flax fibre degradation and sustain the tensile load capacity of reinforced RAC, due to its pozzolanic reaction which consumed Ca(OH)2, as confirmed by Fourier-transform infrared spectroscopy analysis. In all, this study advances the understanding of the durability of recycled aggregate concrete reinforced with plant-based textiles and proposes an improvement strategy using agricultural waste. The findings highlight the potential of integrating recycled and bio-based materials to enhance sustainability in construction.