Shear behavior of prestressed concrete beams using fiber-reinforced plastic tendons.

Abstract

Despite their excellent properties, such as high corrosion resistance and high strength, FRP reinforcements have a number of technical drawbacks for use in concrete structures, particularly their lack of ductility and low transverse strength. These properties are likely to lead to premature tendon rupture, particularly when combined effects are present, such as at shear-cracking planes where dowel action exists in prestressed concrete beams. This study deals with the shear behavior of concrete beams prestressed with CFRP tendons. It comprises an experimental and an analytical program. In the experimental program, the shear-tendon rupture failure mode was investigated in detail and experimentally confirmed. Also, the dowel behavior of tensioned FRP tendons at shear-cracking planes was investigated and modeled. The analytical program focused on developing a failure analysis model for FRP tendons subjected to tension and dowel shear forces. The shear test results confirmed that premature shear-tendon rupture failure is prevalent in (and is characteristic of) concrete beams prestressed with FRP tendons, resulting in reduced shear resisting capacity. The premature failure is due to tendon rupture by dowel shear at the shear-cracking plane and is attributed to the brittle behavior and low transverse resistance of FRP tendons. The shear-tendon rupture failure occurred in beams even with low prestressing ratio and steel stirrups provided according to the ACI code. A dowel study of FRP tendons was carried out and included first the development of a realistic test set up and procedure. The test results showed that FRP tendons fail due to interaction of tensile and shear forces following the Tsai-Hill failure criterion, that is, the ultimate shear force decreases elliptically as the tensile force in the tendon increases. On the other hand, the ultimate shear displacement varied linearly with the inverse of the tension ratio. A mathematical failure analysis model was developed to predict the behavior of FRP tendons subjected to tensile and dowel shear forces. The model was based on two sub-models, the BEF (beam on an elastic foundation) model and the Tsai-Hill failure criterion, which were modified and combined. Results predicted by the model for the ultimate dowel shear force and corresponding displacement are shown to be in very good agreement with experimental observations. From the experimental and analytical programs, it was suggested for shear design that the ultimate shear resisting capacity of concrete beams prestressed with FRP tendons should be taken as about 80% of that of beams with steel tendons.