Stiffness and ductility of continuous reinforced concrete beams strengthened with externally bonded FRP sheets

The application of FRP Externally Bonded Reinforcements (FRP EBR) in form of sheets and bars for flexural strengthening of reinforced concrete (RC) members is becoming a very widespread technology. A large amount of experimental and theoretical research has been devoted to the analysis of FRP EBR concrete structures especially to quantify the ultimate strength and the failure conditions. Less attention has been paid to stiffness and ductility aspects even if they are essential for a correct design. Due to high strengthening ratios and premature failure debonding, in fact,the ductility of externally bonded FRP strengthened members decreases respect to the unstrengthened members . The analysis of FRP EBR reinforced concrete structures, besides, imposes more reliable and accurate design methodologies than traditional ones adopted for steel reinforced concrete structures. Serviceabilty conditions rather than ultimate become critical for the design; consequently stiffness and ductility are fundamental design parameters. This paper is devoted to the analysis, both theoretical and experimental, of the influence of the most relevant parameters on stiffness and ductility of FRP EBR reinforced concrete beams. The theoretical analysis is carried out by means of a non linear model derived from a cracking analysis founded on slip and bond stresses; the experimental analysis refers to continuous reinforced concrete beams strengthened with externally bonded FRP sheets tested up to failure. Using the moment-curvature law, determined by means of the non linear model, it is possibile to evaluate load-deflection diagrams, rotational capacity and the ductility, varying all geometrical and mechanical parameters involved in the structural behaviour of strengthened beams. Obtained results, presented and discussed in the paper, furnish useful informations for design purposes.