Durability of dry and impregnated reinforcement fibres exposed to different alkaline environments

The durability of fibrous composite materials used as strengthening systems in civil engineering is of primary importance in order to calibrate design guidelines and to assess the design life of the strengthened structure. Recent applications of Fibre Reinforced Mortar (FRM) systems, used as external strengthening in masonry and concrete buildings, have shown that dry fibres may be used as long reinforcement within a cementitious or lime-based mortar. This situation is quite different from FRP (Fiber Reinforced Polymers) composites in which, typically, an epoxy resin acts as a protection for the fibres, which are not in direct contact with the pore solutions of the substrate. In this study an extensive campaign was conducted with the aim of evaluating possible methods for the assessment of the durability of fibrous reinforcement in alkaline environment. Two different configuration of reinforcement were studied: dry fibrous reinforcement and industrially pre-cured fibrous reinforcement (with different polymer resins). This choice was done since at the moment different reinforcement systems are used in FRM systems, which can be in forms of dry fibres or FRP grids. Glass fibres having different chemical composition (E-glass, AR-glass), basalt fibres, steel fibres, aramid fibres and carbon fibres were tested and used as benchmark. Four different alkaline environments were chosen for the accelerated exposure, according to the existing technical recommendation and scientific literature. In this perspective the presence of different ions and pH levels were accounted. Different exposure times were considered, from 168 up to 4320 hours; the wet environments were maintained at different temperatures in order to see possible acceleration effects. The comparison was made in terms of residual tensile strength, since all unexposed and conditioned specimens were tested up to failure under uniaxial tensile forces. SEM microscopy images are presented to show the damage at fibre level which was observed in vulnerable fibres.