Flexural Strength and Deformation Capacity of PC Members with Combined Bonded and Unbonded Steel or CFRP Tendons

Some potential applications where bonded steel or carbon fiber–reinforced polymer (CFRP) tendons are combined with unbonded tendons are in the staged construction of prestressed concrete (PC) members and when strengthening existing PC members using external prestressing. CFRP tendons offer advantages over steel tendons in that they are light, making them easier to handle during construction, but more importantly, they are corrosion-resistant, thereby increasing the life span of the structures and reducing their cost of maintenance. However, since CFRP is a perfectly elastic material, CFRP prestressed concrete members are perceived as nonductile compared to traditional steel prestressed members, which may hinder their application in the construction industry. This is exacerbated by the fact that no guidelines are provided in the codes of practice for designing or analyzing concrete members prestressed using a combination of bonded and unbonded tendons, be it steel or CFRP, and no widely accepted measures are currently available for evaluating their ductility. In this paper, in analogy with the approach adopted in the American codes of practice for traditional PC members, a simple and computationally efficient design-oriented method for evaluating the flexural behavior and nominal moment capacity of combined bonded–unbonded PC members utilizing steel or CFRP tendons (referred to as hybrid members) is presented and generalized expressions for quantifying their ductility or deformation capacity are generated.