UFER grounding - rebar vs. copper 1

[jimg964]

The grounding method devised by Herbert Ufer, bearing his name, originally consisted of a connection of a grounding conductor to the rebar encased in a concrete foundation. Through the years this has evolved to include copper conductors encased in the concrete with the rebar or in concrete without rebar.
A few questions come to mind:
First, is there any evidence, empirical or otherwise, to suggest that copper or iron (rebar) is better suited to conduct current to the concrete and the earth? Some articles I have read indicate a problem in the copper to concrete junction, just what this is I don't know.
Second, the NEC states that the electrode(rebar) must be encased by "at least 2" of concrete". Would the impedance of the system be affected by the rebar protruding through the upper surface of the concrete?
Third, if Ufer grounds are as good as some believe, why are they not replacing the all powerful ground rod?
Any other comments are appreciated.

 

[cuky2000]

1- Both steel and copper is expose to corrosion in embedded installation. Copper won't corrode in concrete unless soluble chlorides are present. Steel can begin to corrode due to chlorination, carbonation and/or air. Good engineering practice should provide sufficient corrosion allowance to compensate for the inevitable effects of material degradation during the design life of buried or embedded electrode in concrete.

2- The resistivity and resistance of grounding electrode is reduced with increase in diameter but more dramatically reduced with increase in depth. The total impedance should be close to the total resistance (I could not found any evidence of significant value to calculate electrode reactance).

3- The following are few reasons that I can see why the late acceptance of concrete electrode:

a) Ufer ground was recognized on the NEC as a good grounding method many years after ground rod been used.
b) Some designer do not have any motivation for new challenger or incentive to reduce cost.
c) Many civil eng. may object grounding rebars for concern of accelerating corrosion or creating crack on foundation during lightning discharge.
d) Presently with the availability of new computer programs is easy to model foundation and recognize the benefits of the concrete as supplemental ground electrode.
e) Substations today are designed in a competitive environment. There are many private owner including generating plan, less man power in the utility and new available grounding investigation that force engineers to find cost effective alternatives while mainain an acceptable safety level.

COMMENTS: Few mitigation techniques to reduce corrosive conditions are as follow:
1. Increased depth of cover over the rebar.
2. Avoid galvanic action separating copper from steel.
3. Use additives in concrete such as fly ash and silica fume.
4. Increase the Water-cement ratio around 0.4.
5. Avoid crack in concrete with proper design strength.

See the following discussion: thread238-28182 thread242-21381 thread256-32160

 

[peebee]

I've heard a rumor that Ufers can accelerate the deterioration of the concrete. I forget exactly why, perhaps due to lightning.