[TowerTalk] Dielectric properties of Concrete

[Jim Lux]

>
>
>Anyone know what the resistivity per unit volume concrete
>is?

There is  concrete that has enhanced conductivity (so that would probably 
represent an upper bound on conductivity).. given as having resistivities 
of 1-40 Ohm-cm

Another reference gives 30-90 Ohm-m resistivity, which is roughly 
consistent with what  IEEE Std 80-2000 (Grounding for AC substations) uses. 
That standard gives Oven Dried concrete as 1E8 to 1E11 Ohm-cm and wet 
(damp, not uncured) concrete as 21-100 Ohm-m (2100 to 10k Ohm-cm) (that's 
10-50 mS/meter).  Page 68 of that spec says "Buried in soil a concrete 
block behaves as a semiconducting medium with a resistivity of 30-90 
ohm-meter." and then goes on to talk about how, particularly in resistive 
soils, concrete encased grounding is a good idea, because the concrete will 
tend to have lower resistivity than the surrounding soil, which from a 
fault current dissipation standpoint, is a good thing.

IEEE Std 80-2000 does give the short time current loading capacity for 
concrete encased elctrodes based on multiplying the result from 
Ollendorff's formula by 1.4 (Sure.. Ollendorff.. Yep, that'll be in the 
technician question pool next year).  But, they do give the formula in the 
spec, as well as one from Fagan and Lee.

Here's something from the "Portland Cement Association"
http://www.cement.org/pdf_files/SN2457.pdf
58 pages of excruciating detail on the resistivity of concrete, with 
details on corrosion, etc.  mostly aimed at low frequency (as in electric 
trains)
Ball park summary... concrete is about the same as soil, maybe a bit lower 
conductivity (more akin to rock, which after all, is what it is). Depends a 
lot on what aggregate was used (sandstone has high conductivity, fly ash low)

But, of course, for lightning discharges, what you really want is RF 
properties...


http://www.ndt.net/article/ndtce03/papers/v078/v078.htm

gives some data that looks like epsilon is about 3-10 from 500 MHz to 2.5 
GHz (and pretty flat).  Conductivity is steadily decreasing with decreasing 
frequency.. Call it 40 mS/meter at 500 MHz, 150 mS/meter at 2.5G

Then, there's this posting on a list in 1998 
http://www.sowacs.com/archives/98-03/msg00023.html with the interesting 
observation:

Hydrating concrete is shown to simulates the dielectric behaviour of soils of
different textures. Its dielectric spectrum from 10 MHz to 1 GHz illustrates
the effect of water binding (> 100 MHz) and the Maxwell-Wagner effect (< 100
MHz). Around 100 MHz concrete exhibits only small changes of the dielectric
properties; this is known to occur also for soils of different textures. The
compressive strength of concrete appears to be predictable from the electrical
permittivity at 20 MHz, due to the Maxwell-Wagner effect.



  The following might be useful...especially since they're looking at 100 
kHz to 40 MHz

Al-Qadi, et al.,"Dielectric Properties of Portland Cement Concrete at Low 
Radio Frequencies" Journal of Materials in Civil Engineering, V7, #3, Aug 
1995, p192-198

Abstract:This paper presents a nondestructive evaluation method to 
characterize portland cement concrete (PCC) based on its dielectric 
properties. The work reported is part of an ongoing study to develop a 
nondestructive evaluation method for PCC structures using electromagnetic 
waves. A custom-designed parallel setup with an HP 4195A Network/Spectrum 
Analyzer was used to form the measurement system. A model was established 
for the measurement system and a set of calibration standards was developed 
for the system calibration. Six different PCC batches were cast and their 
complex permittivity was measured over 28 days of moist curing, with an 
average of four specimens per batch. The selected batches covered a 
representative variation of water to cement ratio and two aggregate types 
using Type I portland cement. The complex permittivity of PCC was evaluated 
over a frequency range of 0.1–40 MHz. Preliminary measurements showed 
significant changes in the real part of the dielectric constant versus the 
curing time. However, the change in the loss tangent was less pronounced. 
Significant differences were also observed in the dielectric properties of 
PCC specimens due to mix-design parameters’ variations and curing state.