Impedance spectra of fiber-reinforced cement-based composites - A modelingapproach

The impedance response of conductive fiber-reinforced cement-based material s was investigated using model systems physical simulations) consisting of conducting needles in aqueous solutions. Two discrete bulk arcs are observe d in the Nyquist plots for fiber-reinforced samples, while only a single bu lk are was observed for samples without fibers. This difference is attribut able 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 fi bers. At intermediate frequencies, these layers short out, so that the fibe rs act as highly conducting elements in the microstructure. Spreading resis tance effects from the ends of the fibers play an important role in the hig h frequency behavior. Pixel-based computer modeling results are presented t o rationalize both DC and high frequency behavior. Experimental and modelin g results are used to develop an equivalent circuit model containing a freq uency-switchable fiber coating element. (C) 2000 Elsevier Science Ltd. All rights reserved.