|To:||Tom Rauch <email@example.com>|
|Subject:||[TowerTalk] Dielectric properties of Concrete|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Wed, 26 Jan 2005 15:53:14 -0800|
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"
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...
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.
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