Evaluation of ductility in prestressed concrete beams using fiber-reinforced plastic tendons.

Abstract

Despite many advantages of FRP (Fiber Reinforced Plastic) reinforcement, such as corrosion resistance, their linear elastic behavior up to rupture is likely to result in a lack of structural ductility. This research deals with the ductility of concrete members prestressed with FRP tendons. It consists of an experimental program, an analytical program, and synthesis of the experimental and analytical results leading to new design guidelines as well as methods for defining, predicting, and improving the ductility of such members. The experimental program comprised tests on four under-reinforced T beams and four over-reinforced rectangular beams, fully or partially prestressed with carbon fiber tendons, trade-named CFCC. As expected, the fully prestressed T beam showed a very brittle behavior; indeed, the rupture of each FRP tendon layer releases a large fracture energy leading to serious longitudinal cracks in the concrete. Partially prestressed T beams showed step-like decreases in the load carrying capacity following their peak resistance. The step-like behavior can be explored to simulate a ductile response. Tests on the rectangular beams included three beams which used a special type of fiber reinforced concrete called SIFCON (Slurry Infiltrate Fiber Concrete) in order to improve structural ductility; results indicated that an over-reinforced beam could achieve substantial ductility, provided the ductility of the matrix is improved, such as by using SIFCON. The analytical program investigated several schemes to improve structural ductility through an extensive computerized parametric study. The methods considered include: optimization of sectional ductility through proper reinforcement, matrix reinforcement with fibers, controlled bond slip failure, and prestressing with unbonded tendons. A new definition of the ductility index is proposed, which is expressed in terms of the ratio of energies. It is applicable to beams with steel as well as FRP reinforcements, thus providing a common basis for comparison. New design guidelines are recommended to estimate the nominal moment, M$\sb{\rm n}$, and the resistance factor, $\phi$, for the design of concrete beams prestressed with FRP tendons; however, the value of the $\phi$ factor needs further research and refinement, which depends on a closer assessment of the consequences of failure.