Durability of FRP-confined concrete subjected to severe conditions

Confinement of compressed reinforced concrete (RC) columns by using Fibre Reinforced Polymer (FRP) composites has been considered an innovative and effective technique since load-carrying capacity and ductility can be easily increased also respect to seismic forces. Laboratory tests supported by analytical models, and in-situ applications demonstrated that lateral confinement of circular columns by using FRPs is effective and cheap at the same time. Hundreds of applications were developed all over the world, using different wrapping techniques, for seismic retrofit or repair of bridge piers and columns inside reinforced concrete or masonry buildings. Respect to these encourageing information there is some evidence of accelerated degradation of specific types of FRP composites exposed to certain environmental conditions that may be encountered for infrastructure applications. This degradation emphasizes the need for design guidance specific to the use of FRPs with concrete in the long term. At the moment a limited number of durability studies [1, 2, 3, 4, 5, 6, 7] is available; further research is needed to translate experimental or empirical information into design recommendations, that should take into account the potential decay of mechanical properties due to ageing or possible chemical attack of the FRP systems. The results of this research offer a contribution for evaluating reduction factors that should be introduced for design of FRP-strengthened columns exposed to harsh environment., which is necessary for scientists and practitioners. A large program was conducted to study the behaviour of FRP-confined concrete under severe conditions that include saline environment and extreme temperature cycles. Experimental variables included different FRP systems, different wrapping techniques and simulation of possible cure at low temperature. One hundred-four concrete cylinders were prepared and tested under uniaxial compression after accelerated ageing; FRP-wrapping of cylinders was conducted by using manual wet lay-up (on existing elements) or by directly casting concrete into FRP structural formworks fabricated by automated filament winding (fibres at 54° with respect to the vertical axis of the cylinder). The study focused also on the possible damage caused by aggressive agents to FRP system itself and to polymeric adhesive. Results that are discussed in the paper show that FRP-confined concrete cylinders prepared by manual wet lay-up are prone to mechanical decay more than concrete confined by structural GFRP formworks . Different FRP systems had different decrease of mechanical properties after accelerate ageing. Retained strength and strain capacity will be shown in the paper; different mechanical behaviour due to detrimental action of harsh environment will be described herein.