  An electrical impedance spectroscopy-based nondestructive testing (NDT)
method is proposed to image both cracks and reinforcing bars in concrete
structures. The method utilizes the frequency-dependent behavior of thin
insulating cracks: low-frequency electrical currents are blocked by insulating
cracks, whereas high-frequency currents can pass through the conducting bars
without being blocked by thin cracks. Rigorous mathematical analysis relates
the geometric structures of the cracks and bars to the frequency-dependent
Neumann-to-Dirichlet data. Various numerical simulations support the
feasibility of the proposed method.
