Response of Concrete Beams Partially Prestressed with External Unbonded Carbon Fiber-Reinforced Polymer Tendons

To assess the behavior of concrete beams partially prestressed with external unbonded carbon fiber-reinforced polymer (CFRP) tendons, three specimens prestressed with CFRP tendons and one reference specimen prestressed with external steel tendons were tested to failure under third point loading. It was found that the load-deflection curve of all specimens exhibited three stages, namely, elastic, cracked-elastic and plastic. The transition from the elastic to the cracked-elastic stage was caused by the development of cracks at the bottom of the beam, while the transition from the cracked-elastic to the plastic stage was caused by yielding of the non-prestressed steel. For beams prestressed with external CFRP tendons and that with external steel tendons, the relationships between the stress increment in external tendons and the mid-span deflection of specimens were linear or approximately linear. Although CFRP materials are brittle in nature and behave linearly elastically until failure, proper combination of external CFRP prestress tendons with internal steel reinforcing bars can impart ductility to the beams.