Microwave Time-Domain Reflection Imaging of Steel Fiber Distribution on Reinforced Concrete

This paper presents a time-domain reflection electromagnetic analysis for nondestructive testing of the fiber dosage inside steel-fiber-reinforced concrete (SFRC). With the fibers treated as electrical dipoles, the Maxwell-Garnett mixing rule for dielectric mixtures is used to obtain an estimation of the density of fibers in the composite material from the time-domain measurement of the effective permittivity of the sample. First, a numerical electromagnetic modeling of the fiber polarizability is performed using a Method-of-Moments-based code and a simple frequency-dependent polarizability is obtained. Second, a robust and novel relationship between the mean propagation delay and the total stored energy with the effective permittivity of the medium is obtained. A wideband measurement system based on a time-domain setup in a reflection arrangement is used for the analysis of SFRC test structures. The fiber dosage in the SFRC structures is reconstructed, showing good agreement with the expected dosage figures.