Impedance spectra of fiber-reinforced cement-based composites: A modeling approach

The impedance response of conductive fiber-reinforced cement-based materials was investigated using model systems (physical simulations) consisting of conducting needles in aqueous solutions. Two discrete bulk arcs are observed in the Nyquist plots for fiber-reinforced samples, while only a single bulk arc was observed for samples without fibers. This difference is attributable to thin, resistive, and highly capacitive layers (e.g., oxide film or polarization/double layers) which reside on the surfaces of the conducting fibers. These layers fully insulate the fibers at low frequencies (near DC), so that the highly conducting fibers behave as if they were insulating fibers. At intermediate frequencies, these layers short out, so that the fibers act as highly conducting elements in the microstructure. Spreading resistance effects from the ends of the fibers play an important role in the high frequency behavior. Pixel-based computer modeling results are presented to rationalize both DC and high frequency behavior. Experimental and modeling results are used to develop an equivalent circuit model containing a frequency-switchable fiber coating element.