The use of fiber reinforced polymers (FRP) to reinforce and retrofit reinforced concrete (RC) structures has become one of the main applications of composites in civil engineering. This paper describes a procedure, based on a finite element formulation, that can be used to perform numerical simulations of RC structures reinforced with FRP. Composites are treated using the serial/parallel mixing theory, which deduces the composite behavior from the constitutive equations of its components. A new construction-stages algorithm is developed for simulating retrofitted structures. The performance of the proposed formulation is proved comparing numerical and experimental results. Finally, the developed code is used to obtain the optimum FRP reinforcement configuration for a RC frame structure.