Testing of bond solutions for UHTS steel strand composites applied to extruded bricks

Fibre-Reinforced Polymers (FRPs) are currently a viable and widespread solution for retrofitting masonry buildings, including those of the architectural heritage where interventions need to meet strict requirements. Nevertheless, continuous investigations on possible fibre-matrix combinations and lay-up methods are needed to find further FRP bonding solutions that are possibly more resilient and efficient. In this study an Ultra-High Tensile Strength Steel (UHTSS) fabric was considered as the reinforcement for the FRPs, while either Polyurethane (PU) or epoxy polymers were considered as the matrix. UHTSS fabrics have been introduced in the last decade to overcome some of the limits of common reinforcing fabrics (e.g. carbon, glass, etc.) like, for example, the limited shear resistance that complicates the use of mechanical anchoring for improving the bond of FRPs, especially under peeling stresses. The suitability of PU flexible polymers (Young's modulus lower than 30 MPa) as matrices for FRPs, in place of the most commonly used epoxy resins, has been investigated in the past few years. Their key characteristic is the diffusion of bond stresses over longer adhesion lengths, with the consequent reduction of peak values. This paper presents and discusses an experimental investigation that compares the bond strength of two Steel-FRP (S-FRP) reinforcements applied to extruded solid clay bricks, made of UHTSS fabric embedded in either a PU or an epoxy matrix. Lastly, two possible lay-up solutions, with one aimed at improving the bond behaviour by inserting an additional interface, are also discussed.