Long Term Field Tests of Reference Electrodes For Concrete

ABSTRACT

This paper presents the results from ten years of stability testing of commercial silver/silver chloride, manganese oxide and graphite embedded reference electrodes used in reinforced concrete structures. Tests were performed in chloride-free and chloride-contaminated slabs exposed to the weather in the northeastern United States. Also included is the effect on potential readings of IR drops caused by corrosion of an embedded rebar.

CONCLUSIONS

1. Embedded silver/silver chloride reference electrodes which are designed to be compatible with a concrete environment show excellent long term stability in both chloride-free and chloride-contaminated slabs

2. The reference potential of the graphite cells remained stable for about three months after initial installation after which they began to deviate. Because of this behavior, these cells are only suitable for short term relative measurements. Graphite cells should not be used for long term or absolute measurements.

3. Manganese oxide reference electrodes show good stability in chloride-free concrete but demonstrate long term deviations in chloride contaminated concrete.

4. Differential thermal expansion between the electrode body and the adjacent concrete can lead to an open circuit condition at cooler temperatures. To avoid this, it is necessary to use reference electrodes specifically designed for use in concrete.

5. Corrosion currents resulting from corrosion of embedded rebars can set up a complex IR drop field within concrete which can affect potential readings. When calibrating embedded reference electrodes, this effect makes it appear that the reference potential has shifted when, in fact, the shift is simply a measurement error.

6. There is no correlation between the resistance of an embedded reference electrode and whether or not it is providing reliable potential readings.