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Measuring the polarisation resistance of reinforcing steel is one of the few methods of assessing corrosion rate, with the current density being related to the rate of loss of metal. Although well tried and tested in the laboratory the technique is not as frequently used in the field as half-cell potential and concrete resistivity.
Polarisation resistance and corrosion rate can be measured at selected key locations either with a movable sensor on the surface or with embedded retrofitted probes. If the corrosion is uniform over the surface of the reinforcing bar, the measured corrosion current density can be converted into an equivalent loss in metal section in mm/year. However, where pitting corrosion is occurring, usually due to chloride contamination, corrosion may be concentrated in small, localised areas. Local loss of section may be significantly greater than indicated by the uniform corrosion rate. It is possible to underestimate with a margin of error of five to ten times. Any chloride contamination must therefore be identified before evaluating reinforcing bar section loss.
Corrosion rate measurements are affected by various factors such the temperature and humidity. Thus measurements should be taken over time to understand trends for a basis for discussion, rather than relying on a single reading.
Acknowledgement: The Concrete Society
Other references:Concrete Society, Current Practice Sheet 132, ‘Measuring the corrosion rate of reinforced concrete using linear polarisation resistance’, CONCRETE, March 203, pp 36-38.
(Note that copies of Concrete Society Current Practice Sheets can be downloaded from the Members Area of the Concrete Society web site.)
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