In this paper we have considered both frequency-domain and time-domain shielding provided by reinforced concrete walls and floors in planar geometries, against electromagnetic fields due to electromagnetic pulses. The concrete composites have been modeled as isotropic, homogeneous lossy materials and the metal bars of the reinforcement have been modeled as a wire-mesh grid under the assumptions that the wire junctions are connected and that thin-wire approximation applies. The presence of imperfect wire junctions due to oxidation of overlapping or poor welding is taken into account by introducing an equivalent distributed junction impedance. We have used the formalism of the transmission matrix of network theory. Such a formalism is useful in dealing with shielding problems involving bodies with non-separable layers, such as the reinforcement and the concrete. The behavior of a reinforced concrete wall with respect to an incident electromagnetic field is a function both of the low-pass behavior of the concrete, and of the inductive effects of the metal frame impedance. The electric field inside a reinforced concrete structure shows an attenuation, a time delay when it reaches its maximum value, and an increase in its temporal width with respect to the electric field outside. The proposed model is validated by comparison with numerical and measured results.