The effectiveness of externally bonded carbon fiber-reinforced polymer (CFRP) systems in strengthening pre-cracked concrete beams post-tensioned with unbonded tendons is not yet fully understood. Therefore, the objective of this experimental study is to partially fill this knowledge gap. The experimental program was comprised of four large-scale unbonded post-tensioned concrete (UPC) beams. One of the beams was loaded continuously to failure while the other three beams were first pre-loaded to simulate pre-damage. One of the pre-damaged beams was then loaded to failure while the remaining two beams were strengthened with four and six layers of CFRP composite sheets before being loaded to failure. The CFRP strengthening sheets has demonstrated its capability in restoring the functionality of the heavily pre-damaged UPC beam as well as enhancing the performance of the beam. By controlling cracks, carrying tensile stress and reducing beam stiffness degradation, the CFRP sheets significantly improved the behavior of precracked UPC beams in terms of crack, displacement and loading capacity as compared to the unstrengthened precracked beam. The performance of the FRP-strengthened precracked beams was even better than that of the unstrengthened beam without pre-cracks in the ultimate phase.
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