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The dipolar relaxation phenomena and attendant absorption displayed by water in the microwave region of the spectrum has been used as a basis for the nondestructive testing of the moisture content of building materials. Because of this relaxation effect, the dielectric properties of moist materials at these frequencies depend sensitively on the quantity of moisture present, and thus the macroscopic electromagnetic parameters, and in particular the transmission attenuation, associated with a building structure at a particular frequency are characteristic of the water content. A practical method of assessing the moisture phase of materials is thus available. Further, observation of the dielectric behavior of the water-material mixture can yield information about the distribution of moisture in porous media. The theory of these new methods of testing, based on electromagnetic theory and dielectric mixture studies, is presented in some detail, followed by a description of the apparatus and experimental procedure. Problems of calibration and accuracy are discussed and results compared with other work in the field.
Boot, A. R.
Senior scientific officer, Building Research Station, Garston, Watford, Herts
Senior experimental officer, Building Research Station, Garston, Watford, Herts