A common source of error encountered when making potential measurements in or through concrete is junction potential; this topic has been discussed in Technical Note 19. There are additional sources of error in concrete potential measurements. The mobility of all ions in concrete is retarded due to the material’s cellular microstructure. Consequently, concrete has high electrolyte resistivity so the presence of any internal current flowing through it will be marked by relatively high IR drops. Sources of these internal currents are often corrosion currents from corrosion of rebars. The associated IR drops become incorporated into corrosion potential measurements and can result in a several hundred millivolt error.
Errors of the same magnitude have been documented for measurements made through concrete, such as to a structure buried beneath a concrete slab. In a detailed study on this topic1, potential measurements were made on buried tanks at nine different service stations in the northeast. When the measurement was made with the reference electrode contacting the concrete slab, the potential was from 20 to 260 mV more negative than when the reference electrode was directly contacting the soil through an access hole. The rectifier was off during these measurements to eliminate CP currents as a possible error source. In the same study, potential measurements were made on a pipe located beneath an airport runway. Measurements made through the concrete near the edge of the runway were about 200 mV more negative than the same measurement made through the grass adjacent to the slab. Wetting both the concrete and the grass did not significantly change the measured values. Clearly, measurements of buried structures should never be made through concrete without using a soil contact access port.
- B. Husock, “Techniques for Cathodic Protection Testing Over Airfield Pavements,” US Air Force Report CEEDO-TR-78-31, Tyndall AFB, FL, July 1978.
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