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In this paper, a system for the broadband microwave nondestructive testing of steel fiber reinforced concrete (SFRC) is presented. The system is able to produce a pixeled image of the fiber dosage in the concrete. The SFRC material is seen as a dielectric mixture formed by thin-wire metallic inclusions and a host medium. An electromagnetic model based on the Maxwell-Garnett approach for dielectric mixtures is used to relate the electromagnetic properties of the SFRC to the fiber dosage. A criterion to determine the practical upper frequency limit for the measurements is formulated. A measurement system based on two wideband waveguide antennas is designed for the analysis of two SFRC multiblock test structures. The electric capacitive host medium calibration is proposed in the cases where the concrete structures have parts fabricated with different processes. The laboratory measurements on the individual SFRC elements and on the SFRC multiblock structures are conducted to obtain the images of the fiber dosage in the SFRC. Good agreement between the real, simulated, and measured values is achieved.