10.2.10. The additional strength reduction factor, ψf, is used to improve the reliability of strength prediction and accounts for the different failure modes observed for FRP-strengthened members (delamination of FRP reinforcement).
The nominal flexural strength of FRP-strengthened concrete members with mild steel reinforcement and with bonded prestressing steel can be determined based on strain compatibility, internal force equilibrium, and the controlling mode of failure. For members with unbonded prestressed steel, strain compatibility does not apply and the stress in the unbonded tendons at failure depends on the overall deformation of the member and is assumed to be approximately the same at all sections. No specific guidelines on FRP strengthening of concrete members with unbonded prestressing steel are provided at this time.