[TowerTalk] exploding foundations

[K4SAV]

The article does say what the cause of this crack was.  Here is what 
they said:

"A word of caution: A Ufer ground consisting solely of the tower 
foundation is a bad idea. Lightning surges passing through the 
foundation can vaporize water in the concrete and damage the foundation 
through rapid expansion of steam. An example of this unfortunate event 
is shown in Figure 4. This particular tower had no earth electrode 
system, other than the leg foundations."

Jerry, K4SAV


Jim Lux wrote:

> Paul Christensen wrote:
>
>>> What exploding concrete??  Back up your claim with real life validated
>>> proof.  My experience has been the opposite.
>>>
>>> Doug
>>
>>
>> Want evidence?  See Figure 4 (and the text below it) in the link below:
>>
>> http://www.mikeholt.com/newsletters.php?action=display&letterID=407
>
>
> The text says the only grounding was through the foundation and makes 
> no mention of whether it was a properly constructed CEGR (i.e. 20 feet 
> of conductor, etc.)
>
> Also, the picture does not show whether a structural failure occurred 
> as a result of the lightning damage (the tower is still standing, 
> after all), or whether that's just surface damage.  If it's a 1/8" 
> deep crack in the surface and doesn't penetrate, who cares?  For all 
> we know from the limited data in the article there was a prexisting 
> crack in the concrete that was enlarged as a result of steam induced 
> spalling.
>
>
> Considering that Ufer did an awful lot of tests over the years, and 
> subsequent researchers have also done a lot of tests, I think that if 
> there actually was a realistic concern about spalling and exploding 
> foundations it would have shown up in the reviewed literature (as 
> opposed to in anecdotal reports or ground rod manufacturer sales 
> literature).  I have no doubt that there are cases where there has 
> been lightning damage, but I would suspect that those don't fit in the 
> category of a proper concrete encased grounding electrode. For 
> instance, the "3 2 foot J bolts in the top of the footing" used to 
> bolt the tower mounting plate is probably NOT a suitable grounding 
> connection.
>
> The article also sort of contradicts itself.  On the one hand it 
> advocates external grounding systems, but then, it says that the 
> external 20 rod grounding system showed an impedance of 150 ohms, but 
> after bonding to the structural steel, the impedance dropped to 1 ohm. 
> Sounds to me like the steel was a better ground than the rods.
>
> The article also mentions the "conduit as choke" thing, which I used 
> to believe in, but subsequently have not seen a good analysis to show 
> that it actually works, particularly for large surge currents, taking 
> into account the magnetic saturation of the steel.
>
>>
>> According to NFPA 780, Ufer grounding is generally satisfactory 
>> provided that it is not the only earth grounding mechanism and that 
>> the ground is brought out beyond the concrete encasement to make the 
>> earth connection.
>
>
> Which section? (just curious.. I don't have my copy of 780 here)
>
>>
>> Paul, W9AC
>>
>>
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