Grounding: rebars, concrete and buildings

[radug]

Hello,

I have been searching IEEE 80-2000 and 142-2007 for references about construction metallic parts treatment as grounding electrodes and the information is few.

I am assuming that every metallic part must be joined to the grounding electrode grid (in bare copper) and so it becomes part of the electrode. But, if you have rebars in concrete, as they form a very dense grid (separated about 20 cm or less), do you still need to bury a copper electrode below the concrete? I am considering and external copper ring and connections to rebars, footings, piles, from that ring.

Returning to the rebar topic, one could determine that at such dense grids, you will not have any problems with touch and step voltages at all. I would only bury a copper conductor if I needed to connect some equipment grounding to it and I am very far from the outer copper ring.

And last, I have a question about concrete resistance. I have read in previous threads about it. I read it also in the above standards but they are not consistent with regulations in Spain, where concrete resistivity is given a unique value of 3000 ohms*m, and asphalt for example, 10000 ohms*m (this one coherent with ieee std 80 and 142). Has any of you any explanation for this?


Thanks.

 

[MikeDB]

Because concrete is porous to some extent, its resistivity when wet can be much lower than asphalt which is not porous. Seems like I have seen those values in IEEE 80.

You don't say what kind of structure you are building or why you are bonding to the rebar, so I don't understand your application.

In our substation work, we typically bond all anchor bolts and rebar in the foundations to the ground grid. I don't think that it is necessary to provide direct bonds to all rebar.

Unless you have deep foundations, you may need to drive 8' ground rods.

Personally, I would not want to rely soley on reinforced concrete for my ground grid.

 

[advidana]

Look for grounding under UFER, THe last name of the UL guy in USA that came up with concrete encase grounding.

 

[dpc]

As advidana mentions, you should find a lot of available information under the name of "Ufer" grounding. Bonding of each rebar is not required, however the steel must be in contact with the concrete, so coated rebar is not helpful.

Resistance of concrete varies dramatically with moisture content. When underground in normal soil, reinforced concrete is actually a reasonably good conductor.

Ufer grounding is a requirement of the National Electrical Code in the US.

Concerns are sometimes raised regarding the effect of lightning strikes and also corrosion of the rebar. But it is very widely used, at least in the US.

 

[peebee]

Regarding corrosion and lightning issues, see:

http://eng-tips.com/viewthread.cfm?qid=193951

 

[peebee]

Here's what 2005 NEC says about Ufer grounds, in 250.52(A)(3):

Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located horizontally near the bottom or vertically, and within that portion 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. Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.