[TowerTalk] Conductivity of a reinforced concrete rooftop

[Jim Lux]

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 at 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)