Deformability of concrete beams with unbonded FRP tendons

As fibre-reinforced polymer (FRP) behaves linearly until it ruptures without any yielding, the ductility and deformability of prestressed concrete beams with FRP tendons are of prime concern. This paper describes a comparative study of five existing deformability indices for unbonded partially prestressed concrete (UPPC) beams with FRP tendons. A numerical method has been developed to predict the full-range response of prestressed concrete beams with bonded and/or unbonded FRP tendons under loading, and the results agree well with experimental results reported in the technical literature. The results show that the deformability index defined as the ratio of the product of moment and deflection at ultimate to the corresponding value at cracking is consistently decreasing with the increase of combined reinforcement ratio. In addition, this index is sensitive to changes of factors that may influence the deformability of UPPC beams with FRP tendons. The present investigation also indicates that, when the ratio of neutral axis depth to the depth to FRP tendons at the critical section at ultimate is greater than 0.3, the beam would fail by crushing of concrete, which is the more favourable failure mode compared to the rupture of tendons.