[TowerTalk] re; exploding concrete

[Jim Lux]

jeremy-ca wrote:
> He is not talking about gaps. The water used to form concrete is 
> absorbed by the cement and turns into a crystaline structure. It is that 
> structure that is conductive to some degree that is affected by the heat 
> generated by a stroke.

Indeed.. but water molecules that have been dissociated from the 
concrete being heated ( a process that I believe is negligible in a 
lightning heating context) or are just there because the concrete is 
"damp" aren't going to turn into steam.  Even if the concrete is damp, 
there's no spaces for liquid water to form, and hence, no large 
expansion on the phase change to gas.  There's no phase change occurring 
at all.. it's just water molecules distributed in the pores of the 
material (adsorbed, if you will).  No different, really, than damp 
clothes in a dryer.  The water molecules are in the interstices of the 
fabric, and as you heat it, they come out and are carried away.






> 
> Exploding is too strong a term, it should be "fractured".

Yep. I'll go for that.. or even shattered.. but I would contend that any 
such damage is due to thermal stresses e.g. putting a cold ceramic or 
glass dish into the oven or on a stove... the thermal conductivity is 
low enough that the expansion of the hot side relative to the cold side 
creates forces greater than the intermolecular forces that hold it 
together, so it spalls, or forms a surface, and the crack then 
propagates all the way through.


BUT, I don't think lightning can cause this level of heating in 
concrete. It's just not good enough of a conductor, nor is the 
conductivity so inhomogenous that you'd get significant differential 
heating to create the thermal stresses needed for failure.
> 
> How many years or decades does it take a certain size mass of concrete 
> to completely cure and become a non conductor?

Concrete is always a conductor.. generally better than the surrounding 
soil. It's hygroscopic and tends to take up moisture from the soil. 
Soil can dry out and actually get air pockets between the soil grains, 
except, perhaps in clay, or marshy areas below the water table.. 
concrete has no little gaps between the grains, assuming it was properly 
mixed and poured.

There are insulating concretes made with very high resistivities where 
the aggregate is something with very low affinity for moisture (I think 
fly ash is one thing used for this), but even there, it's still a 
conductor, just not a very good one.  They use the stuff for making 
concrete railroad ties, because some "train detector" systems rely on 
measuring the resistance between the rails to tell if the train is there.
> 
> After so many decades of strikes, the base encased in snow and ice in the
> winter and all manner of rain and temperature cycling it is impossible to
> determine the cause of any of the many cracks. However Im prety sure that
> the lightning didnt help the matter.
> 
> Who here can predict what todays uber grounds will look like decades down
> the road, especially in locations subject to all matter of weather and
> temperature extremes as well as soil conditions. What works in a Southwest
> desert may very well fail in Florida.

Herb Ufer had just such a question and actually did long term aging 
studies in a variety of places around the country in connection with his 
work for UL. He wrote a paper in 1963 that looked at 20 year old 
grounding systems he did in 1942 at Davis-Monthan AFB (Arizona) as well 
as a number of installations  around California. Subsequent authors have 
continued the longitudinal study, and all have reported that they work 
well for a long time.

An Example: Ufer, H.G., "Investigation and Testing of Footing-Type 
Grounding Electrodes for Electrical Installations", Paper 63-1505, IEEE 
Western Appliance Technical Conference, Los Angeles, CA, Nov 1963.


Jim