Capabilities of diagonally-cracked girders repaired with CFRP.

Capabilities of diagonally-cracked girders repaired with CFRP.

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Capabilities of diagonally-cracked girders repaired with CFRP.
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    The technique of using carbon fiber reinforced polymer (CFRP) for strengthening conventionally reinforced concrete (CRC) girders in flexure is well understood, but strengthening girders for shear is a newer application and less data are available. A literature review and survey were conducted to document the state of understanding and experience with respect to using CFRP for shear reinforcement of CRC girders. A laboratory investigation using full-size T- and inverted T-girder specimens with diagonal (shear) cracks was conducted to investigate the shear capacity improvement due to CFRP reinforcement and the effect of cyclic loading on CFRP shear strengthening. National and international code provisions for the design of CFRP in shear were used to compare the predicted shear capacity with the experimental results. An example application was included for using external CFRP strips to strengthen a bridge girder. Strengthening with carbon CFRP strips provided a significant increase in load capacity and stiffness compared to unrepaired beams, and the improved capacity was maintained even after being exposed to the equivalent of twenty years of traffic-induced fatigue. At high load levels prior to failure, progressive debonding of multiple strips provided a clear, visual warning of distress. The ACI 440 methodology provided a reasonably simple approach for shear capacity prediction of RC girders with externally bonded CFRP shear reinforcing for T-beams and is recommended for design. However, the ACI 440 method was unconservative where the CFRP strips terminate in the flexural tension zone. To provide a consistent level of reliability between T- and inverted T- conditions, the CFRP stress should be reduced by a factor of 2 for conditions when the CFRP strip is terminated in the flexural tension zone. Placement of at least one CFRP strip across the diagonal crack with an anchorage length of at least one-half the height of the web is critical. An equation for strip layout is presented. In addition, girder shear strength can be increased using a targeted repair approach, applying CFRP material only to key critical sections rather than over the entire member. Thus, better economy may be achieved by judiciously applying CFRP materials just to those sections that are understrength for shear.
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