Impact of Multilayer Coatings on the Mechanical and Durability Performance of FRCM Composites

Fabric-reinforced cementitious matrix (FRCM) composites are strengthening systems composed of a technical textile embedded in a cementitious or lime-based matrix and are increasingly used for strengthening existing masonry and concrete structures due to their compatibility with traditional substrates. The mechanical behavior of FRCM composites is controlled by the combined contribution of the textile reinforcement, the matrix, and the interface developed between them, with the textile-matrix interface playing a critical role in stress transfer, crack development, and post-cracking response. Since this interface is primarily defined by the coating applied to the textile, coating configuration represents a key parameter influencing both the mechanical and durability performance of the composite. In this study, carbon textile-reinforced FRCM systems incorporating a lime-based matrix and different coating strategies, including single-layer SBR coatings and multilayer SBR-epoxy coatings, were experimentally investigated. Tensile tests were conducted on unconditioned specimens as well as after exposure to water and alkaline environments to assess the evolution of tensile behavior and damage mechanisms under durability-related conditioning. The results indicated that the influence of coating configuration is slightly detectable in the pre-cracking elastic stage but becomes significant in the post-cracking stages, where load transfer and damage evolution are predominantly governed by the textile-matrix interface. Scanning electron microscopy (SEM) observations supported the mechanical findings by revealing distinct differences in coating, interfacial continuity, and fiber-matrix bonding, particularly after environmental exposure. Overall, the multilayer coating configuration, consisting of the factory SBR-coated carbon textile further modified with epoxy, resulted in higher maximum tensile strength (reaching up to 1958 MPa compared with 1531-1780 MPa for the single SBR-coated configuration), greater strain capacity (ε_max up to 0.01244 mm/mm compared with 0.00925-0.01066 mm/mm), and higher energy absorption under prolonged water and alkaline conditioning up to 3000 h. In quantitative terms, the multilayer SBR-epoxy coating improved the maximum tensile stress by approximately 10-15% and the total energy absorption capacity by 25-35%, depending on the conditioning regime. These findings demonstrate the effectiveness of multilayer coating architecture in improving long-term tensile retention, interfacial stress transfer, and post-cracking deformation capacity of lime-based carbon FRCM systems.