The origin of nonlinear current-voltage behavior in fiber-reinforced cement composites

Two distinct slopes (resistances) are obtained in current-voltage (I-V) plots of discontinuous, conductive fiber-reinforced cement composites. The low-field resistance correlates with the DC resistance (R-DC) of each composite. The high-field resistance correlates with the intermediate frequency cusp resistance (R-cusp) in Nyquist (-Z(imag) vs. Z(real)) plots obtained using impedance spectroscopy (IS). A model is developed that is based on passive oxide film formation on copper or steel fiber surfaces at low fields (I-V) or low frequencies (IS) due to the high pH pore solution of cement paste. With increase of field, leading to film breakdown (active or transpassive corrosion behavior), or increase of frequency, leading to short-circuiting of the passive layer, the fibers act as short-circuiting elements in the composite microstructure, resulting in a decrease in overall resistance. (C) 2002 Elsevier Science Ltd. All rights reserved.