Analysis of the electrochemical phenomenon at the rebar–concrete interface using the electrochemical impedance spectroscopic technique

The corrosion rate of rebar during the various stages where it occurs, such as passivation, initiation of corrosion and severe corrosion, needs to be determined non-destructively for the maintenance, restoration and replacement of concrete structures. The double layer capacitance (C dl ) and the charge transfer resistance or polarisation resistance (R p ) of the corrosion processes have been associated with the slope of the low-frequency arc in the Nyquist plot, and this can be related to the electrochemical phenomenon that occurs at the steel–concrete interface. The present studies, based on electrochemical impedance spectroscopy (EIS) conducted on three different densities of concrete with addition of 0·5 and 1% chloride over a period of 1765 days, reveal that the capacitive behaviour of a low-frequency arc with a slope more than −1 indicates the passive condition of rebar. Warburg diffusion behaviour with a slope exactly equal to −1 denotes the initiation of corrosion on the rebar. A slope of less than −1 is obtained when corrosion spreads uniformly on the rebar. Other electrochemical parameters such as R p , C dl and phase angle are correlated with the phenomenon occurring at the steel–concrete interface. An R p value greater than 250 kΩ cm 2 indicates the passive condition of rebar, whereas values of less than 230 and 14 kΩ cm 2 indicate initiation and severe corrosion of the rebar respectively. Similarly, a C dl value greater than 1000 μF/cm 2 indicates the severe corrosion of rebar, whereas less than 100 μF/cm 2 denotes the passive condition of rebar. If the rebar is in the passive condition, the phase angle is more than 30°, whereas it is less than 20° under severe corrosion. The reduction of intrinsic chloride diffusivity owing to pore restructuring by pozzolanic reaction and adsorption of a greater amount of chloride ions into the interlayer of additional calcium silicate hydrate content are responsible for delayed initiation of corrosion in Portland pozzolana cement (PPC) and Portland slag cement (PSC) concretes, when compared with ordinary Portland cement (OPC) concrete.