Pulsed Eddy Current Response to Liftoff in Different Sizes of Concrete Embedded Rebar

Corrosion of carbon steel rebar in concrete structures, such as highways, bridges, and buildings have a direct impact on their structural integrity, since the rebar provides the tensile strength within the structure. Rebar strength depends on the remaining effective cross-section of a given rod. Presence of water and migration of corrosive elements, such as chloride, may result in rebar corrosion with subsequent loss of overall structural strength. In order to adequately quantify the cross-section of the ferromagnetic rod, it is necessary that the transient response is strong enough to be detected. In addition, rebar used in different applications will be placed at different depths from the outermost concrete wall. Examination of long-time decay, on five rebar samples of diameters ranging from 19 mm (0.75 in) to 57 mm (2.25 in), in the transient response of Pulsed Eddy Current (PEC) was examined as a potential method to quantify a maximum liftoff-to-size relationship. The transient response of a coaxial solenoidal drive-receive coil pair, oriented parallel to the rebar axis, was analyzed over a range of liftoffs (distance within the concrete) up to 150 mm. At times greater than ~30 ms, a single exponential decay constant appeared as a constant slope, independent of liftoff in a semi-log plot. The intercept of the linear long-time decay, in semi-log space, decreased exponentially with liftoff, and maintained a viable window for analysis up to 85 mm, 110 mm, 125 mm, 138 mm and 141 mm distances for rebar diameters of 18 mm (0.75 in), 25 mm (1 in), 38 mm (1.5 in), 51 mm (2 in) and 57 mm (2.25 in), respectively. Stated data demonstrate a power law dependence on rebar radius of limits on signal analysis due to liftoff. This report shows the potential for PEC analysis to quantify the cross-sectional area of rebar in concrete structures up to size-dependent threshold depths.