Design of RC Beams Strengthened with Hybrid FRP Reinforcement

The idea of combining different kinds of fibers for making FRP reinforcements manly arises from the necessity to overcome the brittle behaviour of composite materials. Particularly, Carbon fibers are characterized by high strength and a high modulus, but at the same time are expensive and fragile; Glass fibers, instead, are cheaper and have greater ultimate elongation but a lower strength. Consequently, some of the above-mentioned mechanical and economical characteristics reduce the use of FRP materials for strengthening Reinforced Concrete (RC) structures. The mechanical behaviour of FRP materials, in fact, collides with the necessity to realize structural elements with a high level of ductility and low cost, especially in seismic area. Many studies, carried on recently, aimed at modifying the constitutive law of FRPs for obtaining reinforcements with high stiffness at serviceability and an adequate ductility at ultimate conditions, as well as lower costs. One of the most attractive solutions is achieved embedding carbon fibers and glass fibers together in a polymeric resin using different volume fractions. This technique allows to obtain FRP materials with a pseudo-ductile behaviour due to the so called hybrid effect: when the high stiffness fibers reach the ultimate strain, the load redistributes within the remained fibers, resulting in a decrease of the elastic modulus and an increase of the ultimate strain. This paper starts discussing the results, in terms of stress-strain curves, of an experimental analysis on the mechanical properties of hybrid carbon-glass FRP sheets. Then, as result of the experimental research, it has been possible to carry out a theoretical study, of parametric type, from which interesting design indications have been point out about strengthening of RC beams with Hybrid FRP.