Concrete walls reinforced with rebars have poor shielding efiectiveness for telecommunication frequencies (frequencies above 0.5GHz). An efiective method to increase the shielding efiectiveness of the walls is to increase the complex permittivity of the concrete. This can be done by mixing in thin fllaments of a material with high conductivity. One such material is carbon. In this paper the Maxwell Garnett mixing rule is used to model a concrete material with carbon fllaments. The shielding efiectiveness computed with the mixing rule is found to agree with previously published measurement results.