Electromagnetic skin depth of cement paste and its thickness dependence

Abstract

The skin depth delta describes the ability of electromagnetic radiation (particularly microwave) to penetrate a material. This ability is relevant to radar-related applications, electromagnetic interference shielding and telecommunication. However, the values of delta have not been adequately reported for cement-based materials and the effect of the absorption distance (thickness) on delta has not been previously addressed. This work reports and clarifies delta (3.5 mm-12 mm) of cured cement paste. The previously and scantily reported delta values of cement-based materials are mostly incorrect, with discrepancy as large as three orders of magnitude, due to the flaws in the method of delta determination. In addition, this work reports the thickness dependence of delta of a cement-based material for the first time. Due to the Skin effect, delta increases with increasing thickness, with the fractional increase in delta per unit thickness 1.15, which is high because delta (3.5-12 mm) exceeds the thickness (2.61-8.26 mm), so that the electric field decays precipitously as it traverses through the thickness. In contrast, for aluminum, the fractional increase in delta per unit thickness is 0.080, which is low because the thickness exceeds delta, so that the electric field decays gradually. The delta of cement paste tends to decrease slightly with increasing frequency (8.5-11.5 GHz).