Particulate and fiber reinforcements are being increasingly employed in cement-based materials to improve mechanical properties. Recently it has been suggested that conductive fiber-reinforced concrete (c-FRC) could act as its own sensor by using the conductivity of c-FRC for stress-strain monitoring [1]. Furthermore, damage associated with permanent deformation may be detectable via deformation-induced electrical changes [2-6]. Both the DC and AC (impedance) properties of c-FRC have received attention in recent years [1-10]. To the authors' knowledge, however, a comprehensive microstructural/mechanistic model has yet to be established for the electrical properties of c-FRC, especially the frequency-dependent AC behavior.
Since the middle of the 1980's, impedance spectroscopy (IS) has been employed to investigate transport-microstructure relationships in cement-based materials [11-14] without fibers. Results have been presented in Nyquist plots, where the y-axis is -Im Z, the x-axis is Re Z, and Z is the impedance. The Nyquist plot is parameterized by frequency, which decreases from left (high frequency) to right (low frequency). There are typically two obvious features or arcs in the impedance spectra of ordinary portland cement paste (OPC) measured with steel electrodes. The "neat paste" curve in Fig. 1 shows a typical example. The first arc on the left, at high frequency, is that of the bulk cement paste. The second arc, found at lower frequencies on the right of Fig. 1, is associated with polarization/double layers plus any oxide film (passivation) on the electrode surfaces. This arc is usually much larger than the bulk arc, but only a small portion of the electrode arc is typically observed, as can be seen in Fig. 1. Sub-mHz measurements are required to resolve a reasonable portion of this feature, which were not made in Fig.1. When observed at all in the present work, the electrode feature is apparent only as an upturn at the lowest frequencies (right side) of experimental Nyquist plots. For the sake of simplicity, a third feature, occasionally observed at intermediate frequencies in OPC/steel specimens [10], will be ignored.
When conducting fibers are added to neat cement paste, the single bulk arc is sub-divided into two arcs, as can be seen in the 1 wt% of fibers curve in Fig. 1. The origin of this feature must come from some combination of individual fiber geometry, fiber properties, and fiber arrangement and orientation in the cement paste matrix. The present work was undertaken to understand this effect and to better understand the electrical property-microstructure relationships in conductive fiber-reinforced cement-based materials. In this paper, experiments are described that were carried out on model systems consisting of physical simulations consisting of conducting needles in aqueous solutions. This was done to elucidate the principal elements of the behavior of c-FRC, without the complications of the random fiber dispersion in real c-FRC materials. In essence, the focus was on individual fiber geometry and properties, rather on the random arrangement of the fibers. The results of these experiments compared well with actual impedance spectra of conductive fiber-reinforced cement pastes, and were explained using pixel-based computer simulations, which help in establishing a model equivalent circuit for the frequency-dependent electrical properties of conductive fiber-reinforced cement-based materials.
Fig. 1. Experimental Nyquist plots for 28-day old 0.4 w/c cement paste with and without 1 wt.% of steel fibers.