The numerical simulation of composite materials is a field under continuous development. Its application to structural design has shown important improvements, leading to a better simulation of strengthened structures. This paper uses the serial/parallel mixing theory to predict the mechanical behavior of composites. This theory uses the constitutive equations of the component materials to obtain the composite performance. The advantages provided by this formulation to simulate concrete structures reinforced or retrofitted with Fiber Reinforced Polymers (FRP) will be shown from the analysis of several finite element models, consisting of a set of concrete frames with different strengthening configurations. The results obtained will show that the use of this theory to simulate FRP reinforced structures improves their analysis and design.