Flexural Behavior of Unbonded Posttensioned Concrete Members Strengthened Using External FRP Composites

Abstract

This paper presents the results of an experimental study for evaluating the use of external fiber-reinforced polymer (FRP) laminates for strengthening unbonded posttensioned concrete members. Twenty-four full-scale simply supported beam and slab specimens reinforced with an internal unbonded tendon system and strengthened using external FRP composites were tested. An additional 12 companion bonded prestressed concrete (PC) and reinforced concrete (RC) specimens were also tested for comparison. The test parameters included area of internal tension reinforcement, area of external FRP reinforcement, span-to-depth ratio of the member (slab, beam), and profile of the unbonded tendons. It was found that the use of FRP laminates increases the load capacity and postcracking stiffness of unbonded members. The increase in load capacity was accompanied by a reduction in the deformation capacity. Failure of the specimens occurred either by concrete crushing or by FRP debonding or FRP fracture. No distinct difference beyond expectation was observed between the flexural responses of FRP-strengthened unbonded PC and those of bonded PC or RC systems. Provided a method is available for calculating the strains or stresses in the unbonded tendons at ultimate flexural strength, the same standard guidelines for designing the FRP system for flexural strengthening of RC and bonded PC members can be applied to unbonded members.