Alkali resistance of flax Textile Reinforced Mortar composites under accelerated environmental exposure

Natural fibre Textile Reinforced Mortar systems are emerging as sustainable alternatives for the retrofitting of masonry structures due to their favourable mechanical properties and reduced environmental impact. However, the susceptibility of plant fibres to degradation in alkaline and moisture-rich environments remains a critical barrier to their wider adoption in novel bio-based composite systems. This study presents a systematic multi-scale experimental investigation into the alkali resistance of flax TRM composites exposed to different accelerated ageing environments. In order to induce accelerated ageing, individual yarns and textiles were conditioned in an aqueous alkaline environment, while flax-TRM composites, comprising two and three reinforcement layers embedded in lime-based mortar, were immersed in water. Specimens were conditioned at 23°C and 40°C for up to 3000 h and subsequently subjected to uniaxial tensile tests. Performance was assessed through retention rates of mechanical properties and exploitation ratios across the examined scales. Digital Image Correlation (DIC) was used to monitor crack initiation and evolution. The results reveal a significant difference in the degradation kinetics of the textiles between direct exposure and mortar environment. While yarns and textiles retained less than 60% of their strength after 1000 h in alkaline solution, flax-TRMs maintained over 80% of their strength during water immersion for the same duration. However, prolonged exposure at 40°C resulted in significant degradation, with strength retention values decreasing to 60% after 2000 h. The change in cracking behaviour observed in specimens subjected to prolonged exposure, particularly at elevated temperatures, suggests that textile degradation can lead to a reduction in the effective residual reinforcement ratio below critical levels, ultimately compromising stress transfer mechanisms within the composite.