Electrochemical measurements and direct observations of corrosion rate

The purpose of the research was to study the corrosion rate of steel reinforcement by the linear polarization technique and to compare the results with direct observations of corrosion rate. Other electrochemical measurements such as corrosion potential and concrete resistivity were also studied. Six reinforced concrete test slabs were prepared in the laboratory. Corrosion of the reinforcement was initiated by adding chloride to the concrete mix. The slabs were stored in conditioned climate rooms. The rate of corrosion, corrosion potential and concrete resistivity were monitored by periodical measurements of the slabs over 420 days. The scatter of the measurement results was studied. To determine the real rate of corrosion some of the concrete slabs were broken after the measurement period. The steel reinforcement was exposed and the amount of corrosion was evaluated both visually and by the gravimetric method. The observed amount of corrosion was compared with the theoretical amount of corrosion calculated on the basis of corrosion rate measurements. The depths of corrosion determined by the gravimetric method and the electrochemical method correlate well with each other but the depth of corrosion based on the gravimetric method was on an average six times greater than that based on corrosion rate measurements. The reason for this is not fully known to the author. It seems that of all the measurement results, those that indicate the maximum values of corrosion rate are nearest to the correct level and some of the measurement results show too low a rate of corrosion. Thus a single measurement may not be adequate to obtain a reliable value of corrosion rate.