Quantcast Use of Conventional Building Materials

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8.7 USE OF CONVENTIONAL BUILDING MATERIALS. Conventional building materials are not normally
selected on the basis of their electromagnetic shielding properties however most materials do provide some
limited degree of shielding.  Some documented evidence of the shielding provided by common construction
materials is available (8-12). Though the data is sketchy, enough does exist to give a preliminary indication of
what can be expected from a building made of various materials.
8.7.1 Concrete. Figure 8-31 shows that the shielding effectiveness of ordinary concrete is very low. (It may
be assumed that the properties of brick are similar to concrete.) The addition of coke and other forms of
carbon to concrete can greatly enhance shielding properties. Approximately 30 dB shielding effectiveness from
1 GHz to 10 GHz can be achieved by using concrete and carbon.  A concrete-coke aggregate apparently can
provide shielding in excess of 30 dB above about 20 MHz and can offer more than 100 dB above 300 MHz.
8.7.2 Reinforcing Steel (Rebar).
Limited shielding to low frequency fields can be provided by the reinforcing steel or wire mesh in concrete. For
maximum shielding, the conductors must be welded at all joints and intersections to form many continuous
conducting loops or paths around the volume to be shielded.  The degree of shielding will depend on the
following parameters
The size and shape of the volume to be shielded.
The diameter of the bars and spacing (the distance between bar centers).
c .  The electrical and magnetic characteristics of the reinforcement steel materials (conductivity and
relative permeability).
The frequency of the incident wave.
The family of curves shown in Figure 8-32 describes the attenuation at approximately 10 kHz for an enclosure
whose height is 4.5 meters (15 feet), and other dimensions vary over a 5 to 1 range. Bar diameters are 4.30 cm
(1.692 inches) with a spacing of 35.56 cm (14 inches) on centers. The room dimensions, bar spacing, and
diameters shown in Figure 8-32 are typical and cover most situations encountered in practice. The values of
attenuation indicated are those obtainable at the center of the room. There will be less shielding near the
edges of the room. For more detailed design information on the use of reinforcing bars as shields, consult
Reference 8-13.
Welded wire fabric imbedded in the walls of a room or building can provide effective shielding if the individual
wires of the fabric are joined to form a continuous electrical loop around the perimeter of the area to be
shielded. At each seam where the mesh meets, each wire must be welded or brazed to the corresponding wire,
or the meshes may be connected by a continuous strap. Additional attenuation may be obtained by use of a
double layer of welded wire fabric separated by the thickness of a regular wall.


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