Innovative Sustainable Strengthening Systems for Unreinforced Masonry using Natural Fibre Textiles embedded in Inorganic Matrices

The unacceptable number of casualties and substantial economic losses caused by past earthquakes highlights the high seismic vulnerability of unreinforced masonry structures and the urgent need for resilient and cost-effective retrofitting solutions. In parallel, pressing sustainability requirements foster the need for alternative materials with minimum environmental impact. This study aims at developing an innovative composite retrofitting solution comprising natural fibre textiles embedded in a lime-based mortar (NTRM). Natural fibres have good mechanical properties and excellent environmental credentials, while lime mortars can ensure physical and mechanical compatibility with masonry. Both fibres and mortars are cost-effective and widely available, thus making NTRM systems easily applicable in both developed and developing countries. However, their implementation is hindered by the lack of a comprehensive understanding of their composite behaviour and the very limited experience in structural applications. A systematic and holistic multi-scale experimental programme was undertaken to investigate the performance of NTRM and identify key performance parameters to enable the development of design guidelines. The proposed system was examined at composite level through detailed tensile and bond characterisation, and its effectiveness as in-plane seismic retrofitting solution was assessed through structural tests on medium scale unreinforced masonry walls. The parameters under investigation included: fibre type, textile architecture, mortar overlay thickness, number of NTRM layers, bonded area (length and width), and retrofitting configuration. The results of this study confirm the potential of Flax-TRM as a seismic strengthening solution for unreinforced masonry structures and highlight the importance of yarn and textile architecture on the overall composite performance. Smaller diameter low linear density yarns with a higher level of twist develop a better composite action with the mortar and result in composites with good mechanical properties and high utilisation of the textile tensile strength. The use of mechanical reinforcement ratios greater than 3% were found to spread the cracking well and result in highly ductile behaviour. Flax-TRM was shown to provide significant in-plane strength and ultimate drift enhancement and promote the development of energy dissipation mechanisms, while ensuring structural integrity and delaying the development of brittle failure modes. Based on the experimental evidence and a detailed analysis of shear resisting mechanisms, it is recommended that the shear contribution mechanisms are further investigated, as they were found not to be additive. A simplified design model that accounts for the contribution of the mortar and adopts a more rational effective strain limit that can be developed in the textile is proposed. Finally, design recommendations are provided to inform the selection of suitable reinforcing materials and the design of optimal NTRM strengthening systems.