Would it be possible to create a Faraday cage (an electromagnetic shield which prevents transmissions in or out) around a building by electrifying the metal reinforcing bars (“REBAR”) embedded in the concrete?
WAG:
I doubt it as I do not think the bars embedded in concrete form a circuit. I’m not sure how you can electrify something if the electricity has nowhere to go.
In any case a Faraday cage isn’t electrified. It need not be electrically connected to anything to be effective, in fact. However, the rebar in a typical building is far too widely spaced to make an effective cage, even if they were electrically connected together, which they aren’t.
So for standard reinforced concrete, the answer appears to be “no.” Could you space the rebar closer together and connect them with cables or something before you pour the concrete? Would this even begin to have the desired effect?
Background: The tech guy at work and I were talking about this. He said he heard that the NSA headquarters was electromagnetically shielded like this to prevent bugging.
Possibly, but then there’s openings like doors and windows to consider. I doubt you could completely enclose a building effectively, especially nowadays, with the use of microwave transmitters so common.
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- Yea but they could be. I was surprised to read somewhere that when they build bridges, they arrange the rebars as they want them so they are touching/“interlinked”, then they pour the cement, then they hook high-amp power up to the ends {?} of the rebars, welding them together inside the concrete. So they aren’t just floating in the concrete, both “ends” are accessible somehow. It’s possible they could do this with buiildings too, I guess…
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- Yea but they could be. I was surprised to read somewhere that when they build bridges, they arrange the rebars as they want them so they are touching/“interlinked”, then they pour the cement, then they hook high-amp power up to the ends {?} of the rebars, welding them together inside the concrete. So they aren’t just floating in the concrete, both “ends” are accessible somehow. It’s possible they could do this with buiildings too, I guess…
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I don’t work in the field myself, but if you’re really interested, Architectural Electromagnetic Shielding Handbook: A Design and Specification Guide would be a start (published in 1988). Possibly you can find it at a library near you.
The shielding is most likely to be a conductive material attached to the walls (similar to a vapor-barrier material like Tyvek) but it could even be sheets of metal in the walls. Obviously an open door or window is a problem, but most secret buildings don’t have windows, and some sort of “air-lock” could be set up with doors. Then again, conductive window material could be used, in a place that wants light. It’s not just a matter of privacy - laboratories or even recording studios might want to prevent unwanted signals from getting in (or out).
To correct myself, it’s probably not something thin and flexible but might be full metal sheets or mesh in the walls, or conductive tile (in the floors).
More to the OP’s interest, I found out there’s actually Conductive concrete, which in some form might be used for shielding.
Well they are electrically connected. If you’ve ever prepped for concrete you know that you have to wire all the joints together. Not soldered, of course, but probably nice contact patches nonetheless.
And is the effectiveness of a cage related to opening size? That is, a cage with a 6" average opening only keeps out 6" wavelength EM waves? Do tell…
I doubt if you’d find cood continuity, however. I’ve seen raw rebar, and it’s usually pretty brown with a coating of oxide. Unless they polished it up before wiring and pouring the connection will abe less thaqn satisfactory.
As far as the spacing, it must be less than a half-wavelength to be at all effective. Of course, any gaps at all will allow some radiation to leak out, and the smaller the gaps, the less leakage there is. To determine the shielding effectiveness for a given wavelength of RF radiation use E[sub]s[/sub] = 20log(l / 2L) where l is the wavelength of the radiation, and L is the aperture width.
Slight correction: L is the largest dimension of the aperture.
You will have to have a cite for this because I don’t think this is true nor do I think you can weld this way. Electrical connections would probably be to stop galvanic corrosion, but I don’t think that would work.
What I’m seeing used is an epoxy coated rebar and they only wire it so provide a mechanical connection.
Thanks, QED. I thought it might have something to do with half wavelengths but was not so bold as to say so.
As re the rebar (I’ve been dying to say that) rebar is often clean and iron oxide, if thin, is fairly conductive.
If the rebar overlaps by 24" or more, wiring is not required. At least that’s what the folks who designed my foundation said.