Does Grounding Rebar (Ufer) Weaken Concrete?

[nchrista]

Hello All,
Does anyone know where I can find information on this topic. Particularly helpful would be a study that provides ground fault current vs structural concrete strength and/or life expectancy.

 

[peebee]

I recall reading somewhere that lightning dissipating through a footing could crack the concrete due to the water instantly vaporizing. But I also perhaps heard a counter-argument that the damage could be even worse if the copper is not there to assist the lightning make its way through the footing.

Our structural engineers are always worried about corrosion due to dissimilar metals (when you wire-tie the copper to the rebar) but IEEE Green Book (or Emerald Book?) says that won't happen when you're embedded in concrete.

You should be able to find lots of information on the Internet. In addition to Google, I'd check ecmmag.com and other trade journals, as well as erico.com and other grounding equipment vendor websites.

 

[RRaghunath]

As I understand from IEEE80 the use of Concrete rebars can affect the concrete work due to water evaporation and thus is not recommended.
The rebars need to be bonded, however, to earth mat nearby to avoid any undue sparking due to induced voltages or static voltage buildup.

I have come across some engineers counting te rebar metal in the earthing system design but I am not sure of the basis.

 

[jghrist]

NEC Rule 250.50 requires that reinforced concrete foundations (above a certain size) be a part of the building or structure grounding electrode system. The requirement does not apply to foundations of existing buildings and structures if the rebar is not accessible.

 

[nchrista]

jghrist,
I know about the NEC requirement but I'm being asked to verify that grounding the rebar does not weaken the concrete. Neither IEEE-80 nor 142 shed any light on the issue that I've been able to find. I think common sense should dictate this one. Not only would not grounding the rebar be in violation of the NEC but the damage to the foundation would be potentially far greater from a direct lightning strike or fault.

Simply quoting NEC scripture would likely invite more scrutiny and potentially a redesign of the system. One of our structural guys thinks that as long as the grounds are bonded to the top layer of the rebar (using a 2 row embedded cage) that any weakening that might occur would be minimal in the overall strength of the pad.

 

[itsmoked]

Can you just project the rebar out of the concrete somewhere in a civil/acceptable manner so you can not worry about the wire/rebar bonding?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

See Concrete-encased Electrodes and the Grounding Electrode System in IAEI News Magazine at

http://www.iaei.org/subscriber/magazine/01_d/johnston.htm

The IEEE papers written by H.G. Ufer confirm the validity and reliability of concrete-encased electrodes. History and data have proven the worthiness of the concrete-encased electrode.

There are also those who contend that lightning strikes can have a destructive effect on concrete-encased electrodes. IAEI is unaware of data that supports not using the concrete-encased electrode because lighting has a destructive effect on the concrete in some conditions or because of frost or frozen earth.

 

[groundperfect]

I am a slight bit biased on our method of grounding but it makes simple sense and may be a round about answer to your question. We have been intalling deep earth (100 foot plus)electrodes in all earth types including rock for over 20 years now. We usally shoot for 3 ohms or less resistance which is probably less than the common Ufer ground. Common sense says that lightning is going to follow the path of least resistance to ground. Bonding the Ufer ground would be done for life saftey practice in combination with the deep earth ground which should prtect your concrete from any lightning damage in the event of a direct strike.

 

[dpc]

Ufer grounds on large buildings are often 3 ohms or less.

But this dc resistance measurement of resistance to ground is not a good predictor of performance on lightning surges because of the high frequency nature of the event. Usually, shorter and straighter provides the best surge grounding.

As far as the OP question - the two main issues usually raised with Ufer grounds are corrosion of the rebar and damage from lighting.

When I worked for a large consulting firm, our corrosion engineers looked at the corrosion issue from time to time and always concluded that there should little if any galvanic current between copper and reinforced concrete.

As far as the surge current, anything is possible, but maybe the structurals don't realize that the steel columns are already tied to the concrete so a Ufer ground is not necessary to have concrete damage from lightning strikes.

 

[peebee]

From 1986 IEEE Std. 141 (Red Book), para. 7.5.6 Concrete-Encased Grounding Electrodes -- this is NOT the latest revision, not by a long shot -- "The concrete encasement of steel...serves to immunize the steel against corrosive disintegration.... Even though copper and steel are in contact with each other within the bed of moist concrete, destructive disintegration of teh steel member does not take place."

Maybe someone has a newer revision at hand and can check if this still reads the same.

The IEEE Green & Emerald books also both discuss concrete-encased electrodes -- but neither speak specifically to corrosion or galvanic action. All IEE color series books seem to be unanimous in stating that it's a good, cost-effective idea -- and I could find no stated disadvantages. One of them (green or emerald) warned that NOT connecting anchor bolts to rebars can cause concrete to dry out & weaken, and to suffer from both continuous low-level ground currents and high-magnitude fault currents.

 

[rbulsara]

As pee-bee and others indicated IEEE standarads and NEC are unanimous on effectiveness of the ufer grounding and I have not read any offical statements of its disadvantages.

I routinely specify them and request structrual engineers to include the grounding detail of bonding ground conductors to rebars in footings and not single strucrual engineer has ever objected to it.

 

[stevenal]

The '93 red book, reaffirmed in '99 has the same wording. This one is the latest revision.

 

[nchrista]

The Green book has a section that actually provides formulae and a brief but succinct description on ufer grounding and when it may weaken the concrete. It appears that concrete is impervious to AC current but DC presents a problem when it breaches 60A. The concrete has a rectifyng effect on AC injected into it and this would add to any DC levels already present. The calculations are not trivial and you need extra field data to fully solve the problem. Suffice it to say, it would take a large AC fault to raise the DC level (through rectification) to more than 60A.

One of the other problems is rapid evaporation of concrete moisture. This also weakens the pad.

 

[ELEP]

In power plant grounding design, we are grounding concrete rebars as per standard IEEE665, check this standard if you can get desired information.