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Thread: Concrete conductivity

  1. #1
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    Concrete conductivity

    How conductive is concrete? For some reason I always thought that concrete was non-conductive, but apparently I am wrong. My thought is that 20 ft or 1000 ft of ufer or rebar perfectly encased in 2 inches of concrete should by thoery never touch the ground/earth.

  2. #2
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    Conductive enough to make Table 110.26(A)(1).:grin:

    Condition 2 — Exposed live parts on one side of the working space and grounded parts on the other side of the working space. Concrete, brick, or tile walls shall be considered as grounded.

  3. #3
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    In this link, Zog posted some great info.

    Dry concrete above grade 1-5 M ohms,
    Dry concrete on grade 0.2-1 M ohms,
    Wet concrete on grade 1-5 k ohms.

    Of course resistance means nothing without an area / cross-section. In these examples I assume that the area is about equal to the contact area of an average boot sole.

  4. #4
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    Concrete is definitely conductive. One time I was sitting in a crawl space making a splice and I got blasted from a conductor that I thought was dead. Shock went from my hand right down through my butt and this place didn't even have a CEE.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  5. #5
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    Quote Originally Posted by Volta View Post
    In this link, Zog posted some great info.

    Dry concrete above grade 1-5 M ohms,
    Dry concrete on grade 0.2-1 M ohms,
    Wet concrete on grade 1-5 k ohms.

    Of course resistance means nothing without an area / cross-section. In these examples I assume that the area is about equal to the contact area of an average boot sole.
    Hey, thanks for saving me the work Yes, it is based off the area of a boot sole. I forget the sq-cm basis but I can look it up quickly if anyone cares.

  6. #6
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    If you have a panel in a room with the concrete on grade, take volt meter. Put one prod on the concrete floor, and the other prod on the bus of the panel. You should read very close to the actual reading of voltage at the panel.

    Concrete that is in contact with the earth is a very good conductor.
    Instructor, Industry Advocate

  7. #7
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    Concrete is such a good conductor, it is the only acceptable ground electrode method the military will allow in bomb storage vaults and aircraft hangers.

    Herbert G. Ufer invented what is known as A Ufer Ground during WWII. Today the NEC accepts a variant of the Ufer ground we call concrete encased ground electrode.

    I have designed built quite a few of them in data centers and telephone switching offices, and in most cases the impedance is so low (less than 1 ohm) it cannot be accurately measured.

  8. #8
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    Concrete conductivity

    Concrete is slightly alkaline (base) which is defined as OH- in solution. Anything acid (H+) or alkaline (OH-) or ionized has charge carriers in solution (in the solid).

    Having an unbalanced charge, ions are hungry to give or receive another charge carrier making them conductive. Insulators have their outer valence shells completely filled and are very difficult to make share or give up a charge carrier.

    This property of concrete being slightly alkaline causes steel to become passive with regard to corrosion when steel is embedded in concrete.

  9. #9
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    Quote Originally Posted by dereckbc View Post
    Concrete is such a good conductor, it is the only acceptable ground electrode method the military will allow in bomb storage vaults and aircraft hangers.

    Herbert G. Ufer invented what is known as A Ufer Ground during WWII. Today the NEC accepts a variant of the Ufer ground we call concrete encased ground electrode.

    I have designed built quite a few of them in data centers and telephone switching offices, and in most cases the impedance is so low (less than 1 ohm) it cannot be accurately measured.
    I read that old Herbert spent years testing his uffer ground theory in the desert were he was doing electrical work they had just sand 40 feet deep grounds rods did not work and the deeper they went with the rods was so hard they could not drive them into the rock hard bed of the desert floor so thats when he came up with the concrete encasement its always cool and damp and it takes 100 years to cure its the best ground your footer gets wet when it rains so its always a conductor . some say its better than a rod .

  10. #10
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    Thanks

    Thanks to everyone for replying. The reason I asked is that i took and ohm meter to concrete at work (inside in a dry area) and it would not register on the 200 ohm scale. It makes sense that the concrete closer to the earth and in direct contact with the earth will have a much higher moisture content and therefore, more conductive.

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