Impact of FRP and FRCM on the ductility of strengthened masonry members

Strengthening strategies have become extremely efficient as demonstrated in recent scientific works and real field applications. The recent calamitous events focused the interest on existing masonry structures. In the common practice the strengthening strategies are performed improving the load capacity of existing structural elements. However, especially for slender masonry elements like as arches and barrel vaults, the increase of load capacity may not be the only and optimal approach. The ductility capacity represents an important aspect that should be taken into account in a strengthening strategy. A great number of applications is performed without relying on and assessing the ductility of the strengthened elements. This approach could promote deleterious effects on the structural behavior due to the brittle behavior provided by excessive amount of strengthening. Furthermore, a strengthening strategy must respect the compatibility with the masonry substrate especially for heritage applications. The present paper focuses on the out-of-plane behavior of slender masonry elements strengthened with FRP or FRCM systems. A parametrical analysis was performed and results, in terms of bending moment–curvature diagrams and ultimate curvatures were analyzed in dimensionless form. The behavior of strengthened masonry is independent on the type of stress–strain constitutive relationship of composite, depending on mechanical fiber reinforcement ratio, ω. It is very important for practical applications when an overestimation of the effective amount of reinforcement is designed. In this sense the type of composite can be optimized, especially for poor masonry, where higher ω could promote brittle behaviors. At low ω, the type of the stress–strain constitutive relationship becomes a key aspect (e.g. linear or bi-linear strongly correlated to the type of composite: FRP or FRCM). Furthermore, the impact of the axial load both on the ductility capacity and on the load capacity becomes negligible at high values of ω. The results, provided in a dimensionless form, constitute the basis for a valid support to the design of interventions using composites on masonry structures. This work represents a preliminary study to highlight some issues often overlooked in the strengthening of masonry structures, based on direct application of available design guidelines. It is the basis for future development of normalized approaches to perform targeted experimental validations and optimize the design of strengthening systems.