Concrete Beams Strengthened with Fiber Reinforced Polymer (FRP) for Nuclear Power Plant

This project involved strengthneing reinforced concrete beams with glass FRP for increased flexural and shear strength. A major Nuclear Power Plant in the U.S. consists of nine cooling towers similar to the one shown on this slide. The vapors from the cooling tower fans continuously spray moisture on the roof. Over a relatively short service life (about 10 years) the roof structure had suffered major corrosion damage.

As shown on the close-up view, the roof structure consists of a large number of prestressed concrete double-tee beams supported on 3-ft deep reinforced concrete beams. Both the double tee beams and the supporting concrete girders were damaged by corrosion of reinforcement. As a part of evaluation, three of the concrete girders were removed and shipped to the University of Arizona for repair with Fiber Reinforced Polymer (FRP) and subsequent testing.

The three 22-ft long concrete beams are shown on the left; the beams were reinforced with mild steel and the bottom of the beams were curved, making it difficult to strengthen them by conventional techniques, such as bonding steel plates to them.

Flexural strengthening of the beams was achieved by bonding three layers of glass fabric FRP (2-ft wide x 20-ft long) along the bottom face of the beams; the unidirectional glass fibers were in the same direction as the longitudinal steel in the beam to achieve optimum efficiency. To prevent premature failure in shear, the beam was also strengthened for shear with glass FRP prior to testing. Shear strengthening was achieved by bonding 3-ft wide strips of the same glass fabric FRP to the sides of the beam; as can be seen in the slide, the flexibility of the fabrics allows them to conform to the semicircular shape of the bottom of the beam and can be easily bent over the ledge portion of the beam.

The beam strengthened with FRP was tested as shown in the slide. The flexural capacity of the FRP-strengthened beam (shown in red color) was nearly doubled compared to the companion unretrofitted beam (shown in black). Note that although FRP has a linear stress-strain behavior with no yielding, the strengthened beam behaved in a very ductile manner since the design ensured that yielding of the longitudinal steel would be achieved.

Similar studies have been carried out in the laboratory and analytical models have been developed that accurately predict the behavior of beams strengthened with FRP.