I did a seach on Yahoo for "dielectric constant concrete" and got this research paper from the University of Vermont (my alma matter, coincidently) in the #1 position. I did not do a lot of scanning through it, but it looks like a lot of useful info in there. You need to make sure steel reinforcement bar matrices are included in your attenuation calculation.
EFFECT OF ADMIXTURES, CHLORIDES, AND MOISTURE ON
DIELECTRIC PROPERTIES OF PORTLAND CEMENT CONCRETE IN
THE LOW MICROWAVE FREQUENCY RANGEhttp://scholar.lib.vt.edu/theses/availa ... 0concrete'
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