[TowerTalk] Ground rod

Thu Jul 18 09:32:32 EDT 2013

"extrapolation" may have been a poor word choice, but my reasoning is 
that copper pipes are not buried without special precautions for 
domestic water service because of corrosion.  Another reason is that the 
galvanic cells and/or stray currents that exist in soils can cause 
electrolytic erosion.  Copper clad ground rods are known to have finite 
life, sometimes quite short in certain soils.  Some concrete mixtures 
were a disaster with buried radiant heating copper tubes and that 
application now very rarely uses copper in concrete. There are 
discussions on this reflector that AM broadcast station copper ground 
screens disappear over time. Copper in air lasts a long time, not so in 
certain soils or electrolytic environments.

So, why expose a Ufer ground lead to what might be an environment that 
significantly shortens its life, particularly if it isn't possible to 
inspect it?  As I have read the Ufer literature and codes, steel rebar 
is the conductor, not buried copper wire, and the connection is to a 
protected rebar exit point from the concrete.

wikipedia re Ufer: "Ufer's original grounding scheme used copper encased 
in concrete. However, the high pH of concrete often causes the copper to 
chip and flake. For this reason, steel is often used instead of copper."

see http://www.psihq.com/iread/ufergrnd.htm and 
http://en.wikipedia.org/wiki/Ufer_ground

Grant KZ1W

On 7/17/2013 10:36 AM, David Gilbert wrote:
>
> Sorry, but I don't buy either of those viewpoints.
>
> Concrete in the ground is typically loaded with moisture ... 
> practically saturated with it in some parts of the country. Concrete 
> also typically has lots of micro (and some not so micro) shrinkage 
> cracks in it, which is why we have to put rebar in it to hold it 
> together in the first place.  There is therefore ample opportunity for 
> moisture to reach the rebar no matter what we do. The reason rebar 
> doesn't rust is because the alkali in the concrete inhibits it ... 
> nothing more.  The reason that we need to keep the rebar from poking 
> out through the concrete is that the CORE of the rebar would then be 
> able to rust from the exposed end, since the interior of the rebar 
> isn't protected by the alkali. The rust can progress up the interior 
> of the rebar into the concrete and essentially hollow out the rebar.  
> I've seen that happen, and you can find pictures of it on the internet 
> if you look.  Copper wire is a totally different story since it 
> doesn't rust.
>
> I also don't buy the idea that the difference in thermal expansion 
> between concrete and a cooper wire is going to cause problems, since 
> such crevices are going to be minor and likely no larger than 
> naturally occurring cracks and crevices in the concrete itself.  In 
> fact, I'd be willing to bet $100 that the average concrete slab has 
> FAR larger voids in it (where moisture can condense into pockets of 
> water) due to inadequate rodding of the concrete when it was poured.  
> Rodding is the vibratory action that is used to make sure that the 
> cement and various sizes of aggregate (sand, gravel) flow completely 
> around each other, but it is also the action that brings excess water 
> to the surface so that it is not trapped inside ... later generating 
> interstitial voids (which weakens the concrete) as the excess water 
> not needed for the chemical reaction eventually dries out.  Unless 
> time is spent rodding throughout the pour, pausing periodically to 
> remove the excess water, significant water is almost guaranteed to 
> remain trapped inside ... some of it most likely in "clumps" as fresh 
> concrete was poured over surface water that had accumulated from early 
> rodding.  Whatever gaps form around the copper wire would be trivial 
> by comparison.
>
> Lastly, the "steam" hypothesis for blasted concrete doesn't make sense 
> to me either.  Any reference I've ever seen to it has been pure 
> speculation, and doesn't fit the fact that pockets of moisture almost 
> certainly exist in any concrete structure that isn't dry. And as far 
> as I know dry concrete explodes just as easily due to a direct 
> lightning hit as does wet concrete ... probably more so. Localized 
> thermal shock makes far more sense to me.  In any case, the moisture 
> surrounding a copper wire isn't going to explode anything, and the 
> pictures I've seen of exploding concrete didn't involve any protruding 
> conductor at all.
>
> Lots of this stuff is not fully understood, and the many variables 
> involved make cookie-cutter generalizations highly questionable. The 
> comment that we should under no circumstances allow any conductor to 
> breach the concrete-soil interface seems totally unwarranted to me ... 
> it just doesn't fit the physical model as I picture it.
>
> I have nine different #4 solid copper wires protruding from my tower 
> foundation below grade ... three from the bottom to buried ground rods 
> (the ground rods were driven below the surface before the concrete was 
> poured) and six from the sides each running to 30 foot long runs of #4 
> solid copper wire with ground rods every ten feet. I live on a 
> hillside in a high lightning area and if my foundation ever explodes 
> I'll be the first to tell everyone.
>
> 73,
> Dave   AB7E
>
>
>
> It allows moisture to migrate along the wire/concrete interface 
> reducing the effectiveness if it doesn't connect to the rebar.  If 
> connected to the rebar, moisture will also migrate along it causing 
> corrosion.
>
> I have seen photos on the net of broken concrete , claiming the 
> moisture caused steam to form from the strike.  I do not know if that 
> was the case or not.  It'll take someone a bit more knowledgeable than 
> me to answer that which is a bit controversial.
>
> 73
>
> Roger (K8RI)
>
>
>
> On 7/16/2013 9:56 PM, Jim Lux wrote:
>> small crack with capillary action. The thermal coefficient of 
>> expansion is also different, so you have the possibility of the 
>> crevice getting bigger and smaller with every thermal cycle.  Once 
>> you get liquid water in there, then you have all sorts of potential 
>> problems. Above grade, you don't get water standing at the join, so 
>> the problem is less than below grade.
>>
>> I suppose that a AWG4 copper wire sticking out is probably less of a 
>> problem than a 1/2" steel rebar. Smaller area to worry about, copper 
>> is less corrodable than iron, etc.
>>
>> And you see small steel wires and nails sticking out (e.g. from forms 
>> that have been stripped off, etc.) so it's probably not a killer.
>>
>
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