Modulus Effect of Bonded CFRP Laminates Used for Repairing Preyield and Postyield Cracked Concrete Beams

Two different prerepair loading histories were simulated in 3,000×300×150 mm reinforced concrete beams, namely cracking within the elastic range, and overloading in the plastic range. After unloading, the beams were repaired with either high or ultrahigh modulus (210 or 400 GPa) carbon fiber–reinforced polymer (CFRP) plates, or a hybrid system, and then reloaded to failure. It was shown that the level of preexisting damage has insignificant effect on strengthening effectiveness and failure mode at ultimate. The 210 and 400 GPa CFRP of reinforcement ratio ρf=0.17% increased the yielding load by 33 and 60%, respectively, whereas the ultimate strength was increased by up to 29 and 51%, respectively. Doubling ρf of the 400 GPa CFRP to 0.34% resulted in a 63% overall gain in flexural strength, which is only an 8% increase over ρf=0.17%, because of change in failure mode from rupture to concrete cover delamination. The beam retrofitted by hybrid CFRP showed remarkable pseudoductility and warning signs. The parametric study revealed a critical balance in proportioning the areas of hybrid CFRP to achieve reliable pseudoductility. A maximum of 30% ρf of 400 GPa CFRP is recommended. A higher percentage would lead to a final ultimate load lower than the first failure peak load.