concrete encased electrodes

[don_resqcapt19]

Is there any documented evidence that using the re-bar as a concrete encased electrode will cause concrete failure under high current conditions

I was told that this a is an issue and that I could solve the problem by adding a couple of ground rods. I told him that one, the grounding electrode should not be carrying very much current in the event of a fault, and two, even if this is an issue, the relative impedance of the ground rods would be so much greater than that of the CEC that it wouldn't make any difference.

He responded that he was talking about lighting strikes. He cited some issues with concrete bases for high voltage tranmission towers where the re-bar in the bases was used as a grounding electrode and it was causing the concrete to be damaged when the tower was hit by lighting. He said adding the ground rods prevented this damage. I seem to recall reading a bit about this, but that the fix double tying or welding the re-bar, not ground rods.

He also said that the use of the re-bar as a grounding electrode causes the re-bar to corrode and as it corrodes, it gets expands and caused the concrete to crack or break.

I have not seen any papers discussing these issues, and wonder if they are real world issues, or what?

 

[Dennis Alwon]

I too have heard about lightning strikes blowing hole in the footer when a ufer is involved. I have seen a ground rod that was driven inside a utility room that had a concrete slab. Lightning hit at the pool about 200 feet away traveled somehow to the panel area and blew out a small chunk of slab under the panel. Nothing in the panel was destroyed- lots of light bulbs to replace

 

[jumper]

I did a quick search on the subject and found references that alluded to the concrete being damaged by super heating the water/moisture in the foundation when lightning is passed though a CEE during a strike.

However, I found no hard core documentation or studies other than a IEEE paper abstract and I could not access the whole document to see if it addressed the subject completely.

 

[don_resqcapt19]

I seem to recall reading somewhere, that if the re-bar was joined by using double the normal tie wire, or by welding the issue went away. The heat build up was at the points of the connections between the re-bar. I just can't see an addition ground rod external to the concrete and bonded to the re-bar as being able to make much difference.

 

[jumper]

As I stated I could not find any credentialed sources that support the super heated claim of CEE damage caused by a lightning strike.

Everything was basically hearsay/anecdotal.

I found this statement, which I would say sums in up IMO. I assume you recognize the author.

I read a document a few years back that made the claim that moisture within the concrete becomes super heated, turns to steam, expands, and thus cracks, breaks, and/or blows out the concrete along the path of surge current flow from the direct lighting strike through the concrete.

However, the document had no substantiation or other evidence such as pictures, reports, or engineered testing. I have seen several light poles / signs struck by lighting that incorporated concrete bases. Some had augmented grounding electrodes, other did not. NONE every showed sings of concrete damge as a result of the lightning strike.

I have seen pictures of damaged concrete walls and other concrete structures posted on websites that claimed were from lighting, but again, no real evidence or proof.
_________________________
Bryan P. Holland, MCP

http://www.iaeifl.org/forums/ubbthreads.php/topics/4287/Re_Concrete_Encased_Electrode_.html

 

[don_resqcapt19]

Thanks, I have looked too and found nothing solid to support this idea.

 

[mwm1752]

Is there any documented evidence that using the re-bar as a concrete encased electrode will cause concrete failure under high current conditions

I was told that this a is an issue and that I could solve the problem by adding a couple of ground rods. I told him that one, the grounding electrode should not be carrying very much current in the event of a fault, and two, even if this is an issue, the relative impedance of the ground rods would be so much greater than that of the CEC that it wouldn't make any difference. Current will take all paths and adding a couple or rods may help I find it hard to believe it would solve the problem of concrete failure

He responded that he was talking about lighting strikes. He cited some issues with concrete bases for high voltage tranmission towers where the re-bar in the bases was used as a grounding electrode and it was causing the concrete to be damaged when the tower was hit by lighting. He said adding the ground rods prevented this damage. I seem to recall reading a bit about this, but that the fix double tying or welding the re-bar, not ground rods.

He also said that the use of the re-bar as a grounding electrode causes the re-bar to corrode and as it corrodes, it gets expands and caused the concrete to crack or break. I've seen rebar the has been in concrete for decades being remove/demo'd most of the time it appears to be intact -- without oxygen rust will not continue.
I have not seen any papers discussing these issues, and wonder if they are real world issues, or what?

Is he comparing the validity of building decay against saving a wiring system or equipment from being fried due to lighting? Are we talking a single strike in the area? direct hits? multiple hits?

A few years back a lighting strike was found to be the cause for a residence being burned down. The electrode system included a UFER, water pipe & additional rod electrode. I was told that the fire was started underneath the service meter/disconnect. This was a wood sided residence and apparently the staples securing the rod electrode conductor was loose enough to cause extereme arcing & heat setting the wood a blaze. The rod certainly was the weak link among all of the electrode and was not required. In a way a freak situation which is probably in the same catagory as concrete damage. IMO

 

[don_resqcapt19]

Is he comparing the validity of building decay against saving a wiring system or equipment from being fried due to lighting? Are we talking a single strike in the area? direct hits? multiple hits?
...

He is just saying that any time you use a concrete encased electrode you must directly connect it to ground rods. He says if you don't and there is heavy fault current or a lighting strike, it is likely that the concrete will be damaged.

 

[jxofaltrds]

http://forums.mikeholt.com/showthread.php?t=111891&highlight=lightning+concrete

 

[John120/240]

He is just saying that any time you use a concrete encased electrode you must directly connect it to ground rods. He says if you don't and there is heavy fault current or a lighting strike, it is likely that the concrete will be damaged.

Doesn't this contradict the point of using a Ufer in the first place ?

 

[mwm1752]

http://forums.mikeholt.com/showthread.php?t=111891&highlight=lightning+concrete

do you know what the electrode system consisted of ? -- melting insulation and phase crossing can cause quite an explosion.