Impedance spectroscopy of fiber-reinforced cement composites

The addition of chopped conductive fibers to cement matrices results in a characteristic "dual-arc" electrical impedance spectrum below the percolation threshold. This behavior can be explained on the basis of a "frequency-switchable fiber coating" model, in which a "coating" (e.g., passive oxide film on steel or charge transfer resistance/double layer on other conductors) insulates the fibers at DC and low AC frequencies, but is shorted out at higher frequencies, where the fibers become short-circuit paths in the composite microstructure. The present work investigates various factors governing the impedance spectra of fiber-reinforced cement composites - fiber aspect ratio, fiber volume fraction, fiber orientation (relative to field direction), and fiber shape. The "gamma" factor (ratio of the low frequency arc diameter to DC resistance) is a useful parameter to characterize the microstructure-property relationships of fiber-reinforced composites.