Moisture measurements in concrete and characterization using impedance spectroscopy and RC network circuits
The importance of moisture in concrete is unquestionable. However, quantifying the moisture in concrete is very difficult as concrete microstructure water interactions are not well understood. Concrete is a very complex material spanning the range from the atom to the civil infrastructure. It is the medium that controls moisture at the FRP/concrete interface. Concrete is also a composite material at the level of concrete/rebar, aggregate/sand/cement paste and at the hydration product level.
Water is vital in concrete microstructure development, properties and concrete durability. A moisture sensor based on the dielectric and resistive properties of cement paste was developed. Impedance spectroscopy techniques are used to explore the moisture behavior in relation to dielectric and resistive properties of the sensors. The sensor capacitive response is frequency dependent and it has been described with a multi-linear curve. Resistance values are related to capacitance through a power Law. Both the capacitance/moisture and capacitance/resistance behaviors were observed in all four cement/sand/aggregate mixtures considered.
Although the dielectric constants of water and dry cement paste are not frequency dependent with in the 400 kHz and 10 MHz frequencies considered, the effective dielectric constant of the mixture is frequency dependent
Mixing rules cannot predict the effective dielectric constant of the dielectric medium used in the sensors. Impedance analysis indicated also multiple time constants exist within the cement paste. Using the observation from the experimental results in conjunction to the high conductivity of cement pore solution a random R-C network model was developed to explore the impedance behavior of cement paste.