Is concrete conductive?

[copper123]

I have a question about UFER grounds and the concrete in general. Is the concrete itself conductive to Earth? It must be, right? If you are bonding to the rebar, and the rebar is incased in the concrete, and the concrete is in contact with the earth. Then it would seem that the concrete itself must be part of the path to ground. Now to my dilemma. Yesterday I arrived on a job to put a stub into a basement wall. It?s the concrete insulated forms so I just need to stub a conduit through the block wall. At this time I was going to bond a piece of copper to the horizontal rebar and bring it through the wall for a UFER Ground. As I looked down into the wall, I noticed that the rebar that was stubbed up during the footer pour was not tied to any of the verticals in the wall. I asked the builder if any of the rebar in the wall itself was tied to any of the rebar stubs that came out of the footer. He said, it was not needed, and called it proximity placement. Evidently from what he told me, you don?t actually need to tie the vertical run going up the wall to the vertical coming out of the footer as long as they are so close. Anyhow, I don?t have any actual rebar bonding in my wall that bonds to the bar in the footer. And to make it a little more complicated, the horizontals around the walls are not tied to the verts going down.
So if my reinforcing bars are not bonded at all but are 20? long, but not near the bottom of concrete foundation, would this still be a good ground? I am sorry this is so long winded, but the real question is this. If you bond to a horizontal piece of bar that is up 8? on a concrete wall, and that bar is only using the concrete as its grounding path, it surely can?t be a good ground, correct? But, in the same line of thinking, your rebar that you bond in a footer is not in direct contact either, it using the concrete as a path, so it would be great to understand completely what is going on.

Thanks!

 

[jwelectric]

Re: Is concrete conductive?

Is the concrete itself conductive to Earth?

Yes

 

[tom baker]

Re: Is concrete conductive?

Yes concrete is conductive, it absorbs and holds water.
The requirements for a 250.52(A)(3) electrode are 20 ft of 1/2" bar or 4 AWG in or near the footing. 20 ft in the wall is not the footing. While it may work it does not meet the literal intent of the NEC.

 

[iwire]

Re: Is concrete conductive?

No IMO your out of luck.

Your supposed to be using the bottom of the footer.

I think you could tie in at the wall if the wall rebar was tied direct to the footing rebar.

 

[George Stolz]

Re: Is concrete conductive?

Tom Baker wrote:
...in or near the footing.

Bob wrote:
...the bottom of the footer.

I don't think that's entirely correct.

NEC-2002 250.52(A)(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, [located within] and [near the bottom] of a [concrete foundation] or [footing] that is in direct contact with the earth...

When I think of a foundation I think of a basement, or a slab. The NEC appears to draw a distinction between "footing" and "foundation", and accepts both as the "husk" for a Ufer. I think what the NEC is driving at, is there is more moisture, the lower you go. If you can get the footer, it's ideal. If you can get the bottom of a foundation wall, that's acceptable.

In the case originally laid out, you'd need to go after the rebar that is sticking out of the footer or at the bottom of the foundation wall. Why don't they tie the verticals you can access, to the horizontals in the same wall space?

 

[iwire]

Re: Is concrete conductive?

Slabs are not acceptable.


It will really be up to the inspector but my read on this is that the 'electrode' must be located in the concrete that is poured in direct contact with the earth, if the wall sits on a footing IMO you must be in the footing.

 

[infinity]

Re: Is concrete conductive?

I'm with Bob, it has to be at or near the bottom of the footing or the wall rebar would need to be tied to the footing rebar.

 

[pierre]

Re: Is concrete conductive?

Concrete is a good conductor. Next time you are at a service, take one prod of your tester and touch the concrete floor that is in contact with the earth. Take the other prod and touch a phase in the panel. I will bet that your reading will be very close to the actual voltage that is present at the panel.


The reason for requiring the use of the rebar in the footing or foundation wall, is the weight of the building will be pressing down on these and help make the contact resistance of the wall or footing lower in value.

 

[George Stolz]

Re: Is concrete conductive?

Originally posted by iwire:
...if the wall sits on a footing IMO you must be in the footing.

Then why does 250.52(A)(3) state "concrete foundation or footing"? It doesn't lay out a hierarchy of preference. Either term has equal weight in this sentence.

 

[iwire]

Re: Is concrete conductive?

In some areas there is no footing required, there is only foundation wall.

 

[George Stolz]

Re: Is concrete conductive?

In those cases, there's only one option: There's only 1 of 2 physically present. When there are 2 of 2 present you have a choice. I don't see where it mandates use of one over the other.

Is there another reference that you have in mind, or is this based on your read of (A)(3)?

If I had a foundation wall exposed to more soil with more rebar in it than the floating footer it rested on, I would be inclined to use the choice with more soil exposed to it.

 

[copper123]

Re: Is concrete conductive?

That sounds pretty good George, but in my area,the only portion of the concrete that is actually exposed to the earth is the footer. The slab, concrete walls ect. all have foam or some insulating medium on them or under them. And if not some sort of foam, they they?have a rubber membrane or material that produces a watertight seal. No ground contact. It is the same with the water line. The water lines are usually spray foamed in the ground with out any ground contact and the copper at the well is attached to pitless adapter that is rubber O ring. This prevents the copper line from bonding directly to the well case.

 

[iwire]

Re: Is concrete conductive?

Originally posted by georgestolz:
If I had a foundation wall exposed to more soil with more rebar in it than the floating footer it rested on, I would be inclined to use the choice with more soil exposed to it.

We will have to agree to disagree on this one.

IMO you are overlooking "near the bottom"

(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (? in.) in diameter, or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means.

Near the bottom is not half way up the wall.

Do I think it makes a bit of difference as to performance?

No.

However IMO the words of the NEC require the electrode to be at the bottom.

If the rods in the wall where tied to the footer rods I would say tyeing at the wall would meet the NEC.

 

[sandsnow]

Re: Is concrete conductive?

This came up in a related post pre fab concrete walls. I can't find it though.

If you have concrete in some configuration that acts to support the building and is resting on or in the earth, then you have a concrete foundation. I have never seen any concrete used in this way (in my small world) without reinforcing steel.

Slabs can, but rarely act as a foundation. We had one building with a slab that varied from 24 to 36 inches thick over the entire footprint of the building. A copper UFER was installed near the bottom and I approved it. If I remember, it was installed near the outside edge.

Common 4" slabs, as iwire pointed out, are never acceptable in any condition I can envision.

 

[copper123]

Re: Is concrete conductive?

Hi Larry,
I see slabs used as the foundation all the time. We call them monolithic slabs. The outer portion of the slab is thickened and then tappers up to 6" at the interior portion. Maybe I just misunderstood what you were saying. However, i just read an article about the PSI strength of Foam board. They were actually incasing the entire monolithic slab with 2" foam board. This was on the bottom and also the sides. Basically you had a floating foundation. The numbers worked for what the PSI rating of the foam board was at. It could take the entire weight of the dwelling when you broke it down to the square inch scale. Nothing to bond to there!

 

[sandsnow]

Re: Is concrete conductive?

Right, we have monolithic slabs also.
Technically it is a slab and a footing. The footing is the thickened edge.

A typical 4" or even 6" slab cannot be a footing since it will crack under the weight of the structure. Standard, if there is such a thing would be a minimum of 12" deep for a footing.

The foamboard condition is a perfect example of a footing not in contact with the earth.

 

[George Stolz]

Re: Is concrete conductive?

Originally posted by iwire:
If the rods in the wall where tied to the footer rods I would say tyeing at the wall would meet the NEC.

Agreed.

IMO you are overlooking "near the bottom"
...

However IMO the words of the NEC require the electrode to be at the bottom.

Yes, but at the bottom of what?

located within and near the bottom of a concrete foundation or footing

IMO, you're overlooking "foundation or footing".

 

[sandsnow]

Re: Is concrete conductive?

George,
Your picture shows a wall sitting on top of a footing. The electrode would be in the bottom of the footing 20' horizontal of rebar.

 

[George Stolz]

Re: Is concrete conductive?

I guess it's all in how you read it.

I'd say the rebar in the bottom of the wall is near the bottom of the foundation. But I can see the other side of the argument too.

It's weird having an electrode inside an electrode. It makes everything a little hard to discern.