Re: [TowerTalk] Conductivity of a reinforced concrete rooftop

[Grant Saviers <grants2@pacbell.net>]

Another way to think about this is that  rebar is spaced more closely 
than most any HF radial wires.
Which raises questions for my site:  what to do when a buried radial 
field encounters a concrete pad short of the desired radial length? In 
one situation it is a driveway 15' x 150' and the other case is a 56' 
footing of a steel building with a Ufer ground inside.  It is easy to 
connect to the steel building sides to extend the radial field.  What 
should I do at the edge of the concrete driveway?  (no rebar access 
without  concrete sawing).  I was thinking of burying a Cu strip flat 
against the concrete for maximum mutual surface area and silver 
soldering the radial wires to it.
Grant  KZ1W


On 4/4/2013 6:41 AM, Jim Lux wrote:
> On 4/4/13 4:25 AM, David Robbins wrote:
> > better than dirt, worse than solid metal.
> >
> >
> > Apr 4, 2013 12:27:03 AM, hk1kxa@hotmail.com wrote:
> >
> > Any hint on the values of conductivity and relative dielectric 
> > constant of a reinforced concrete rooftop?
> > David
> > HK1A
> > EC5KXA
> > AE5XQ
>
> At what frequency?  I have a lot of data on concrete and other 
> building materials.
> At 1 GHz, a typical number is epsilon =6, sigma = 20-40 mS/m
> Brick is around 4-5, 17.5 mS/m
>
> That is for the concrete, without rebar.
>
> If you add rebar, it becomes a very good conductor at frequencies 
> where the wavelength is > spacing between the bars.  A decent 
> approximation would be to take the conductivity of iron and the 
> conductivity of concrete, and combine them with the relative cross 
> sectional area.
> Take a slab that is 4" thick with 1/2" bars on 6" centers.  For every 
> foot, the total cross sectional area is 48 square inches. There's two 
> bars in that foot with cross sectional area 0.39 square inches (or 
> about 1%).
> So you would approximate conductivity as 0.99*20E-3 + 0.01*1E7
>
> I would just ignore the concrete.. the conductivity is 1E5 S/m..
>
>
> (this is why a Ufer ground works so well..it doesn't much metal in the 
> concrete to make it a "really good" conductor compared to soil)
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