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A microwave nondestructive testing technique to measure the steel fiber content in hardened concrete slabs is presented. The technique is based on an open-ended coaxial probe reflectometry method for measuring the effective permittivity of the steel fiber reinforced concrete and on a classical homogenization approach for determining the fiber content. Two different full-wave models for the probe-aperture admittance as a function of the permittivity are applied: a spectral domain model for the forward problem and a rational function approximation for the inverse problem. A Maxwell-Garnett type of mixing rule is derived for randomly oriented conducting prolate spheroids in a dielectric host medium. A coaxial probe has been constructed, calibration aspects are discussed, and experimental results are presented for concrete slabs with various fiber contents. It can be concluded that the presented technique is a promising one.