Effect of the fringing electric field on the apparent electric permittivity of cement-based materials

The fringing electric field increases the apparent relative permittivity of cement when the electrodes do not cover the entire specimen area. The apparent permittivity increases with increasing thickness, decreases with increasing area, and is much higher when the permittivity is obtained from the slope (P) of 1/C versus thickness than the slope (Q) of C versus area (C = measured capacitance). Using P, the value (2 kHz) for various areas is 830-1760 and 810-1750 for plain and silica-fume cements, respectively. Using Q, the value for various thicknesses is only 150-375 and 144-354 for plain and silica-fume cements, respectively. When the electrodes cover the entire area, the fringing field effect is weaker, with lower relative permittivity 24-38 and 23-36 for plain and silica-fume cements, respectively.