[TowerTalk] Conductivity of cement

[David Robbins K1TTT]

Yes, there is a bit of a difference.  Conduction in concrete is usually
ionic rather than electronic.  Much like a beaker full of salt water with
two electrodes, part of the conduction is from moving the heavy ions through
the solution.  This causes frequency dependent effects such that low
frequencies conduct much better than high frequencies, fortunately for ufer
it is good enough a conductor for power line and the 'relatively' low
frequency lightning components.  Capacitance between bonded rebar and the
soil, ionization of the small air gaps in the structure, and inductance of
the rebar and bonding conductors make the whole thing much more complicated
than you may expect on the surface.  Having participated in the setup and
analysis of large scale man made lightning experiments on ground rods and
radials I can attest to the complexity of the whole problem.  Even lightning
in soil is a complex phenomenon, including conduction, ionization of air
gaps, and even streamers that follow relatively random paths for some
distance... to add to the confusion the ionization and deionization time
constants are different, and effects of the surface of the soil make it even
stranger by distorting the fields in the soil and above it... one of our
first full size tests on a ground rod was early in the morning, we quickly
learned that we had to wait for the grass to dry out or we lost lots of our
current to visible arcs through the wet grass.  Later in the day after the
sun dried it out and the grass was trimmed back things were a bit more
controllable. 


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of K4SAV
> Sent: Saturday, September 06, 2008 16:18
> To: towertalk at contesting.com
> Subject: [TowerTalk] Conductivity of cement
> 
> It just occurred to me that I have two pieces of data that seem
> contradictory.  Most people know that you can build a Ufer ground for a
> tower using concrete and rebar.  The concrete is said by most people to
> be more conductive than the surrounding soil.  Several years ago before
> GFIs, if you were ever unfortunate enough to touch a lamp that had a
> short, while standing barefoot on a concrete floor, you can attest to
> the conductivity of concrete.
> 
> On the other hand..... There have been studies to look at the
> attenuation of RF signals by cement.  This is important because it
> affects signal transmission thru building materials.  The results of
> these studies show that cement is virtually transparent to RF.  A
> summary of that data is listed below.
> 
> So how do you reconcile these differences?  If cement is really
> conductive, it seems like if you totally enclose an RF source inside it,
> that there wouldn't be much RF transmitted thru it.  It it's not
> conductive a ufer ground wouldn't work.  Is there some unusually
> differences here between wave transmission and conducted emissions?  It
> would have to be something really unusual because wave transmission is
> usually blocked by conductors.
> 
> Jerry, K4SAV
> 
> ---------------
> This data is for cured Portland cement, moisture included.  Here is a
> brief summary of the data.
> 
> The report is from Rome Air Development Center RADC-TR-67-446, Feb. 1968
> by C.M. Brennan et al titled EM shielding of building materials. It is
> referenced in a more recent Sept. 1984 report by J.C. Wyss titled
> Building Penetration Project that is available through NTIS, number
> PB85-126001. The loss through Portland cement concrete, aggregate to
> cement ratio = 3:1, that was saturated with moisture is reported from
> the former in the latter, with a 10% "assigned uncertainty", to be:
> 
> Frequency  Attenuation
>  Hz  ______ dB/cm
>  1.0 E4 ___ 2.98 E-7
>  1.0 E5___ 4.41 E-7
>  1.0 E6 ___1.50 E-6
>  1.0 E7 ___2.58 E-6
>  1.0 E8 ___4.84 E-5
>  1.0 E9___ 7.6 E-4
>  1.0 E10 __7.6 E-3*
> 
> * denotes extrapolation.
> ----------------
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