FRP rod reinforced concrete structures fail in a brittle manner at low axial forces either due to crushing of the concrete (due to the low stiffness of the FRP bars) or due to tensile fracture of the FRP bars. This is different behaviour to steel rod reinforcement which is very ductile once the yield stress has been reached. As such where axial forces and bending moments develop at different rates it is feasible for fracture of the FRP bars to occur prior to full load development and hence the capacity at full load development can be overestimated.
The following image illustrates some possible load paths for the load development in a member. The blue line indicates a load development that is typically assumed when no other information is available and indicates a safe load development. The yellow line shows a situation where the bending moment develops before the axial force develops. In this case whilst the intermediate case is outside the capacity of the member, the final case is inside the capacity. Although the intermediate point is outside capacity, for steel reinforcement, the final case would still be acceptable as the bars capacity would remain even when the applied load is outside the envelope. For FRP reinforcement the reinforcement is assumed to have fractured at the intermediate point and therefore the full capacity can not be assumed in the final condition. Once fracture has occured then the unreinforced capacity curve may be more appropriate than a reinforced curve.
Of course there are other subtle issues to take into account. FRP has a higher factor of safety than steel bars and FRP typically has a much higher short term strength in the short term than the long term. As such only in extreme cases might it be expected that this stress history dependancy would be critical, however in situations where the load development is complex it is appropriate for the designer to be aware of this issue.