Performance of preloaded CFRP-strengthened fiber reinforced concrete beams

Extensive experimental investigation was conducted to evaluate the performance of fiber reinforced concrete (FRC) beams that have been externally strengthened with and without preloading in flexure using carbon fiber reinforced polymer sheets. Steel, synthetic and hybrid fibers (mix of steel and synthetic fibers) with a volume fraction of 0.5% were added to the concrete matrix to prepare 15 beams. Test results show that the addition of all fibers types improved the flexural capacity, crack initiation and propagation, stiffness, post cracking behavior, deflection and ductility of the beams. Hybrid fibers was found to improve the ductility of the FRC beams by 4 times when compared to the control specimen without fibers. Furthermore, preloading before strengthening of concrete beams without fibers led to increase of the ultimate capacity by about 120%. Preloading caused improvement in elastic stiffness of the beams by 33.4%, 23.5% 17.6 for steel, synthetic and hybrid fibers, respectively. While strengthening increased the elastic stiffness for all beams and up to 65.3%. However, for the FRC beams there was no significant improvement in the ultimate capacity from those strengthened without preloading. Applicability of ACI 440 equations to predict the capacity of fiber reinforced concrete beams was also investigated.