Concrete Encased Electrode within footer poured on gravel

[cdcengineer]

We are working on a job where a UFER or concrete encased electrode was going to be used in addition to structural steel and the H2o pipe to make the grounding electrode system (GES). However this building is different than many I have been involved with in the past. The footers where the CEE was going to be located are poured on top of compacted gravel. I always thought that the UFER must be in direct contact with "earth" meaning dirt. The vertical foundation is made up of precast walls which just appear to set on top of footers and the exterior will be back-filled with gravel (not dirt). So it's my understanding that we can't use these walls as CCE. Perhaps this is just my own preconceived notion, but can someone set me straight? Are dirt, gravel and earth all the same in the eyes of grounding?

Next, the structural steel is attached with hold-down bolts, but these bolts do not tie to any rebar in the concrete encased electrode (CEE) so it's my thought that building structural steel is not "present" as an available electrode per 250.50. It seems as though the miles of structural steel would need to be part of the GES, but how can the steel connect to the CEE without tie between the hold-down bolts and rebar?

Thanks for the feedback

 

[ActionDave]

The gravel will probably reduce the effectiveness of the footer used as a CCE but I would still tie to it. It still has to be better than a ground rod.

 

[GoldDigger]

Dirt will eventually, if not sooner, infiltrate into the gravel bed, unless the soil at that level is always dry.
And unless plastic coating or insulation is used, there will be dirt contact on the sides at almost the same depth.

 

[Dennis Alwon]

IMO you don't have a ufer and the building steel is not an electrode. Two rods or perhaps you can do a ground ring. Chemical ground rods are good but expensive and requires maintenance

 

[cdcengineer]

We have H2O pipe, and there is a UFER which they built outside the foundation (per the AHJ's request). But I was hoping to get building steel since I felt that the AHJ's recommendation for a constructed UFER did not meet the intent of the NEC because it wasn't part of the footing or foundation. Am I right that gravel is not really earth?

On a separate but related note, why does the NEC dictate that a UFER have 2" of concrete separating the electrode (copper or rebar) from the actual earth? Wouldn't it be better if the metal touched the earth?

Thanks

 

[ActionDave]

We have H2O pipe, and there is a UFER which they built outside the foundation (per the AHJ's request). But I was hoping to get building steel since I felt that the AHJ's recommendation for a constructed UFER did not meet the intent of the NEC because it wasn't part of the footing or foundation. Am I right that gravel is not really earth?

I don't think there is a problem with making your own Ufer. The intent of the code is to have a chunk of concrete with 20' of steel or copper in it buried in the dirt. Since you only need a 4AWG wire attached to it I imagine the physics are the same as a ground rod only needing 6AWG....bigger wire or more concrete and steel isn't making any thing better. That is also why I don't think there is a problem with the footer sitting on gravel. No matter how ideal your grounding electrode is there is only so much current it is going to conduct.

Build your own ufer, use the footer steel, either, or, or both... sure, why not. It's like giving a dead man an enema. It may or may not do any good but it can't do any harm.

On a separate but related note, why does the NEC dictate that a UFER have 2" of concrete separating the electrode (copper or rebar) from the actual earth? Wouldn't it be better if the metal touched the earth?

Thanks

I used to work on a concrete crew. I was told that any rebar that pokes outside of its concrete and comes into contact with any dirt will corrode and decompose and that corrosion/decompossion, over time, will continue and spread to the rebar inside the concrete.

 

[Smart $]

We have H2O pipe, and there is a UFER which they built outside the foundation (per the AHJ's request). But I was hoping to get building steel since I felt that the AHJ's recommendation for a constructed UFER did not meet the intent of the NEC because it wasn't part of the footing or foundation. Am I right that gravel is not really earth?

...

Well gravel is earth in its original undisturbed location.

However, a quarried and hauled in gravel bed is not considered 'earth' for any NEC compliant electrode IMO. But when you think about it, add a little sand, cement, and water, and what do you have?

Ultimately, it'll always come down to the AHJ's interpretation, because the NEC does not have a refined definition of 'earth'.

 

[kwired]

However, a quarried and hauled in gravel bed is not considered 'earth' for any NEC compliant electrode IMO. But when you think about it, add a little sand, cement, and water, and what do you have?

Many sites have sand, clay, or any other natural material hauled in before construction of a building that was not native to the site location.


I still like the chances of having lower resistance from this CEE sitting on gravel then from most any 8 foot rod.

 

[starbolin]

We have H2O pipe, and there is a UFER which they built outside the foundation (per the AHJ's request). But I was hoping to get building steel since I felt that the AHJ's recommendation for a constructed UFER did not meet the intent of the NEC because it wasn't part of the footing or foundation. Am I right that gravel is not really earth?

On a separate but related note, why does the NEC dictate that a UFER have 2" of concrete separating the electrode (copper or rebar) from the actual earth? Wouldn't it be better if the metal touched the earth?

Thanks

The core of Mister Ufer's idea is to significantly increase the contact patch of the grounding electrode by surrounding the electrode with a conductive material, i.e. concrete. This was found to be effective in mitigating the deficiencies of many types of poor soils, i.e. gravel.

If the grounding electrode is sufficiently large the low conductivity of the gravel should not matter as there are many parallel paths through the gravel. What you have is a thin, poorly conductive layer, sandwiched between your large area grounding conductor and the much larger earth around and under the structure. My 20 years of engineering design has shown me time after time that wide thin layers such as that do not significantly change overall conductance. I also suspect that gravel, which would appear to be a poor conductor at DC, turns out to be a much better conductor at high voltages such as in a lightning strike.

 

[cdcengineer]

Hey gang, thanks again for all the input. It is greatly appreciated.

Last two questions, does anyone use and like the Mike Holt's reference book on grounding? I was thinking of getting this as an illustrated supplement for reference purposes.

And lastly, per NEC 250.52(A)(2) - does structural steel have be be connected to the UFER via hold-down bolts? What I mean is, if there is a piece of exposed rebar (which is inside the structure) larger than 1/2", can this rebar be bonded to the structural steel with a properly size bonding jumper (similar to what is shown in NEC Exhibit 250.29) - see the right hand structural steel bonded to the CEE? This does not seem to meet the intent outlined in 250.52(A)(2) (1) or (2) because (1) the steel is not in direct contact with earth and (2), the hold-down bolts would not be connected directly to the rebar (CEE). However the steel itself is connected to the CEE.

Thanks again.

 

[Smart $]

Many sites have sand, clay, or any other natural material hauled in before construction of a building that was not native to the site location.


I still like the chances of having lower resistance from this CEE sitting on gravel then from most any 8 foot rod.

I'm not saying relocated 'earth' is insufficient as 'earth'. That'd be quite illogical... :happyyes:

A bed strictly composed of gravel is rarely, if ever a natural occurrence and has substantial amount of gaps as an 'earth' substrate... at least until sediment from soil erosion fills in the gaps. So I would not consider such as the required 'earth' for a qualifying CEE.

 

[Smart $]

....

And lastly, per NEC 250.52(A)(2) - does structural steel have be be connected to the UFER via hold-down bolts? What I mean is, if there is a piece of exposed rebar (which is inside the structure) larger than 1/2", can this rebar be bonded to the structural steel with a properly size bonding jumper (similar to what is shown in NEC Exhibit 250.29) - see the right hand structural steel bonded to the CEE? This does not seem to meet the intent outlined in 250.52(A)(2) (1) or (2) because (1) the steel is not in direct contact with earth and (2), the hold-down bolts would not be connected directly to the rebar (CEE). However the steel itself is connected to the CEE...

I would consider structural steel that had an exothermic-welded bonding jumper to a CEE as a grounding electrode, regardless of not being constructed exactly per the requirement of 250.52(A)(2). JMO, though. Your AHJ's will supersede mine... :happyyes:

 

[jxofaltrds]

IMO you don't have a ufer and the building steel is not an electrode. Two rods or perhaps you can do a ground ring. Chemical ground rods are good but expensive and requires maintenance

Agreed. Compact fill is not the earth.

Metallic components shall be encased by at least
50 mm (2 in.) of concrete and shall be located horizontally
within that portion of a concrete foundation or footing
that is in direct contact with the earth or within
vertical foundations or structural components or members
that are in direct contact with the earth. If multiple
concrete-encased electrodes are present at a building or
structure, it shall be permissible to bond only one into
the grounding electrode system.

 

[mwm1752]

Even though the footer is bedded on gravel there is a great chance the footer or wall will come into contact with the earth -- most of the verticle rebar I see for the walls are tied to the footer steel. lots of earth contact around walls. I do not allow a UFER to be in contact by earth as it will corrode

 

[kwired]

Agreed. Compact fill is not the earth.

Metallic components shall be encased by at least
50 mm (2 in.) of concrete and shall be located horizontally
within that portion of a concrete foundation or footing
that is in direct contact with the earth or within
vertical foundations or structural components or members
that are in direct contact with the earth. If multiple
concrete-encased electrodes are present at a building or
structure, it shall be permissible to bond only one into
the grounding electrode system.

Is compacted clay "earth"?

 

[jxofaltrds]

Even though the footer is bedded on gravel there is a great chance the footer or wall will come into contact with the earth -- most of the verticle rebar I see for the walls are tied to the footer steel. lots of earth contact around walls. I do not allow a UFER to be in contact by earth as it will corrode

What about?

(4) Ground Ring. A ground ring encircling the building or
structure, in direct contact with the earth, consisting of at
least 6.0 m (20 ft) of bare copper conductor not smaller
than 2 AWG.

 

[kwired]

I do not allow a UFER to be in contact by earth as it will corrode

Not allowing the rebar within to find it's way outside the concrete is one thing, but if the CEE is not in contact with earth you don't have a CEE.

 

[jxofaltrds]

Is compacted clay "earth"?

Good point but to 'me' earth is undisturbed soil.

 

[kwired]

Good point but to 'me' earth is undisturbed soil.

So no need for a CEE on any site that has had fill brought in then? That would apply to a lot of places as mother nature tends to not leave things perfectly level.

 

[jxofaltrds]

So no need for a CEE on any site that has had fill brought in then? That would apply to a lot of places as mother nature tends to not leave things perfectly level.

Nope. Case by case. 90.4 Enforcement.