Bond characterization between historical concrete substrate and SRG/SRP strengthening systems

Strengthening of existing reinforced concrete structures by using composite materials has been growing in popularity, since they are characterized by high strength-to-weight ratios and good versatility. Recently, beside common Fibre-Reinforced Polymers (FRP) that consist in various types of fibres (e.g. carbon, glass, aramid, basalt, etc.) embedded in a plastic matrix, mostly epoxy, new composites have been introduced; they are characterized by the use of small high-strength twisted steel wires as resisting elements, combined either with cementitious grouts (SRG) or epoxy (SRP) as matrix. Epoxy resins are widespread as composites matrix and adhesive, however they require special attentions like fireproof and UV protections; moreover, their substantial impermeability might cause compatibility problems, especially in the case of historical RC structures. Based on these observations, SRG have been developed to combine traditional materials, i.e. cementitious mortars and steel, in order to reach structural performances comparable to FRP materials, and meet additional requirements such as a lower cost, a simpler application and an improved compatibility with the original substrates. However, the structural performance of such strengthening systems strongly relies on the bond between reinforcement and substrate. The present work deals with the bonding performance of both SRP and SRG composites applied to a historical concrete substrate, investigated through pull-off and tangential shear tests. The influence of steel fibres and matrix (organic or inorganic) has been studied by combining two types of steel wires with either epoxy or cementitious mortar. Concrete samples were removed from the decommissioned St. Francesco bridge in Veneto (Italy), built in the Fifties: this allows testing the reinforcement bond performances on an historical concrete characterized by a mix design completely different from current ones. Pull-off and shear tests were carried out after a preliminary phase of material characterization, aimed at identifying materials mechanical properties, namely substrate, cementitious mortar and steel fibres.