Strength and Ductility Trends for Concrete Members Strengthened in Flexure with Carbon Fiber-Reinforced Polymer Reinforcement

For reinforced concrete (RC) beams and slabs under-reinforced with steel and strengthened in flexure with carbon fiber-reinforced polymers (CFRPs), the yield plateau is eliminated and post-yield flexural capacity increases linearly by elastic straining in the CFRP. At the ultimate flexural limit state, failure occurs by either concrete crushing or CFRP rupture. The consequence is an increase in strength and decrease in ductility. In this study, a parametric analysis is presented where strength and energy ratios are investigated as a function of existing steel and strengthening CFRP reinforcements. Both normal-strength and high-strength concretes are considered. A graphical representation showing the controlling strength limit state is developed. Strength and energy ratios for various service, yield, and ultimate limit state combinations show that strength increase and ductility loss are significantly affected by the amounts of steel and CFRP reinforcements. The ultimate-to-yield strength ratio increases rapidly with decreasing steel and CFRP reinforcements; consequently, the service strength approaches the yield strength. Ductility loss is nonlinear and inversely related to the amounts of steel and CFRP reinforcements. For the parameters considered, ductility relative to the unstrengthened condition was reduced between 35 and 93%.