Ductility evaluation of prestressed concrete beams with CFRP tendons

Conventional definition of ductility index which are based on yielding of the reinforcement, are inappropriate for the evaluation of structural ductility of concrete beams reinforced or prestressed with FRP tendons. In this paper, a new definition of ductility index is expressed as the ratio of the total energy to the elastic energy at the failure state of a beam. It is applicable to beams with steel as well as brittle FRP reinforcements, thus providing a common basis for comparison. From the experimental results of prestressed concrete beams, the beams with a carbon fiber reinforcement plastic (CFRP) tendon have higher flexural cracking load, flexural yielding load, and flexural fracture load. While the displacement at the fracture stage was lower compared to steel tendons, excessive steel reinforcement lead to a lower ductility index. Prestressed concrete beam with a CFRP tendon have sufficient ductility when ruptured by crushing of concrete or used with unbonded tendon. Accordingly, ductility of structure could be secured by overcoming reinforcement failure as inducing compress-controlled failure based on overreinforcement designs in the beam which is used in CFRP tendons. Therefore, the practical design method for prestressed concrete beams with CFRP tendons turn out to be overreinforcement, and use of unbonded tendon.