[TowerTalk] Ground rod

Jim Lux jimlux at earthlink.net
Wed Jul 17 20:48:32 EDT 2013


On 7/17/13 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.

That could be.  I was thinking more along the lines that the microcracks 
within the concrete (in contact with the rebar) don't have any 
connection with the outside world (except diffusion through the 
concrete) and so, the liquid in the gap is basically a saturated 
solution of the residual alkaline stuff (although eventually, it would 
leach out, but that might be a centuries time scale thing).

A crack that is at the outside world, though, has an easier time 
leaching out.

I have no actual basis for this statement, though.  Just speculation.
I'd be real interested to know.


>
> 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.


I would agree in general. I'm just thinking of things that might cause a 
small crack to grow to a large one, where the difference is "passing 
through the interface" vs "entirely contained within the mass".





>
> 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.


I also don't think that steam is the cause of spalling. Although there 
are some pictures of (badly rusted) anchor bolts with spalled concrete 
after a lightning strike.  I don't recall whether there were pictures of 
the "before", but the spall looked fresh (different color than the rest 
of the concrete).  I think that can fairly be called a sort of 
pathological case.

I have seen actual spalling at the site of the lightning strike touching 
the concrete, but it was pretty minor.  I think hitting the concrete 
with a hammer might actually have done more damage. It was maybe 1/8" 
deep and 1" in diameter. For all I know, it was something like the 
concrete topcoat layer flaking off due to the localized heating.



>
> 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.

Perhaps it's easier to have a blanket rule than to have to separately 
analyze each situation?


>
> 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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