High-frequency impedance measurements on cement paste loaded with steel fibres

Conducting fibre-reinforced cement-based materials is of great technological interest for their mechanical performance and multifunctional character, from structural to self-sensing applications. The composite's properties depend mainly on the fibre's concentration. However, for a given nominal concentration, the critical practical aspect is the degree of dispersibility achieved during the mixing process. This unsolved problem that greatly conditions the reproducibility of mixes. The present research addresses this problem using the electrochemical impedance spectroscopy technique. Different cement paste model samples were loaded with stainless steel wires acting as reinforcing fibres. The system's impedance was recorded mainly from 50 MHz to 20 Hz, but lower frequencies were also explored. The results show that the presence of fibres modifies the impedance spectra, decreasing the apparent resistivity measured. This result was already reported in the literature. What is new in this research is the fact that the orientation of the fibres with respect to the electric field used for testing significantly affects the impedance spectra. The numerical simulations show that the presence of fibres modifies the current lines in a manner largely dependent on the ratio of the interfacial impedance to the cementitious impedance, which defines the minimal size for resolving orientation effects.