Source: Transportation Research Board, Washington, D.C.

University of Florida Civil and Coastal Engineering investigators report that carbon fiber-reinforced plastic can mitigate performance problems in prestressed concrete bridge girders exhibiting strand debonding. A bonded CFRP reinforcement scheme applied to specimens increased capacity 9 and 21 percent, respectively, for shear span-to-depth ratios of one and three. The university’s Shear Performance of Existing Prestressed Concrete Bridge Girders report was prepared for Florida Department of Transportation and released by TRB. It explores the results of shear capacity testing conducted on three types of concrete bridge girders: AASHTO Type IV and III, and 1950s-era post-tensioned.   

AASHTO Type IV test girders were built to replicate those on existing FDOT structures. Investigators found that specimens’ compromised capacity was not controlled by the typical shear failure mechanisms, but rather was due to cracking and separation of the bottom bulb flange. This was a result of the unusual debonding pattern that placed the fully bonded strands out in the bulb flange and the debonded strands under the web. Salvaged from an existing bridge, the AASHTO Type III test girder specimens were tested at a/d ratios ranging from one to five. For a/d ratios of three or less, the failure mode was strand slip, which was precipitated by formation of cracks in the strand development length zone. While these cracks resulted in strand slip, transverse and longitudinal mild steel reinforcement at the girder end were engaged, which improved the capacity and ductility beyond first strand slip.

Post-tensioned test girders were constructed to replicate a circa 1950s bridge design. Unique features included the presence of both straight and parabolic PT bars and lack of shear reinforcement away from the end block. The girder tested with a direct bearing on concrete displayed a 7 percent larger capacity and nearly half the displacement capacity of the girder tested on neoprene.