Question How can I model reinforced concrete foundations?
Answer The concrete in a reinforced concrete foundation placed into the ground will assume roughly the same resistivity as that of the surrounding soil, since the resistivity and permittivity of both media is mainly influenced by their moisture content.
Furthermore, the resistance of the embedded re-bar foundation will not be affected greatly by the presence of the concrete, as long as the concrete covers approximately the same area as the re-bar. Also, if the rebar is connected to the grounding grid the touch and step voltages above the reinforced concrete foundation are normally not a concern, since the foundation forms (almost) an equipotential.
As a result, the presence of the concrete will have no practical impact on the results. To simplify the modelling, you can usually ignore it and consider that the volume occupied by the concrete is actually filled with soil with the same electrical characteristics as the surrounding soil.
More considerations about concrete foundations can be found in Section 14.6 of IEEE Std 80 (2000 Edition).