Effect of thermo-hygrometric exposure on FRP, natural stone and their adhesive interface

As well known, the performance of Fiber Reinforced Polymer (FRP) materials as external strengthening technique is strongly dependent on the bond behavior between FRP and substrate. Several experimental studies have been performed on this topic, however limited attention has still focused on the bond durability. In this paper, the effect of a thermo-hygrometric environment on the interface behavior FRP-calcareous natural stones is investigated. Each utilized materials (natural stone, adhesive, FRP sheets) was firstly exposed to the same thermo-hygrometric atmosphere; a relevant decay of mechanical properties has been found for the analyzed substrates (Lecce stone and Neapolitan tuff) while a negligible influence of the exposure has been observed for the composite reinforcements (CFRP and GFRP). The results regarding the variation of mechanical properties of the resins evidenced that the effect of the performed exposure is strictly correlated to the specific materials properties: a relevant degradation or even an improvement of mechanical performances has been,in fact, registered. The bond strength and the kind of failure were both analyzed as a function of the treatment used, as well as the strain and stress distribution at the interface. The kind of failure changed in some cases when passing from unconditioned to conditioned specimens; the bond strength, the maximum bond stress and the interface stiffness were affected by the treatment, manly depending on the adhesive resin deterioration. Finally, on the basis of the provisions given by the CNR-DT 200 R1/2013 document, the possibility of defining design relationships, able to take into account also durability aspects, is discussed.