Capacitance-based defect detection and defect location determination for cement-based material

This paper provides the first report of capacitance-based nondestructive cement-based material defect detection. It is based on the use of the fringing electric field (2 kHz) of a capacitor that comprises the cement-based material (cement paste slab, 250 × 250 × 9.31 mm³) and two relatively small copper electrodes separated from the slab by an electrically insulating polymer film. Due to the fringing field through the slab, the apparent permittivity is high, thus enabling the measured capacitance to be sensitive to defects. The through-thickness capacitance, which relates to the apparent relative permittivity of the cement-based material, is measured using electrodes that sandwich the slab. The in-plane capacitance is measured using closely spaced parallel co-planar electrodes. A region (150 × 150 × 9.31 mm³) of the slab is rendered defective by containing gypsum pellets, the porosity of which causes the apparent relative permittivity of this region (142) to be lower than that of the adjacent perfect region (199). To determine the position of the boundary of the defective region, the capacitance is measured using a series of electrodes, with each electrode geometry consisting of N squares linearly aligned perpendicular to the boundary. The capacitance is measured in order of increasing N, which is sufficient for the electrode to cover regions on both sides of the boundary. The capacitance increases linearly with N in each region, with the slope being different for the two regions. The intersection of the extrapolated linear curves of the two regions gives the position of the boundary.