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© 2012 by Proceq SA
Typical orders of magnitude (for information only) for the half-cell potential of steel in concrete meas-
ured against a Cu/CuSO4- reference electrode are in the following range (RILEM TC 154-EMC):
•
water saturated concrete without O2:
-1000 to -900 mV
•
moist, chloride contaminated concrete:
-600 to -400 mV
•
moist, chloride free concrete:
-200 to +100 mV
•
moist, carbonated concrete:
-400 to +100 mV
•
dry, carbonated concrete:
0 to +200 mV
•
dry, non-carbonated concrete:
0 to +200 mV
2.2
Factors affecting the Potential Measurement
Provided that the corrosion conditions are equal (chloride content or carbonation of the concrete at
the steel surface) the main influences upon the half-cell potentials are:
Moisture
See the figures above for moist, carbonated concrete and dry, carbonated concrete. Moisture has a
large effect on the measured potential leading to more negative values.
Temperature
In order to measure the potential there must be a contact between the probe and the electrolytes in
the pore system of the concrete. Therefore a measurement below the freezing point is not recom-
mended and can lead to incorrect readings.
Concrete cover thickness (Measure with Profometer / Profoscope)
The potential that can be measured at the surface becomes more positive with increasing concrete
cover. Variations in the concrete cover can cause deviations in the measurements. Very low concrete
cover can lead to more negative potentials which would seem to indicate high levels of corrosion.
Therefore it is advisable to make concrete cover measurements along with the half-cell measure-
ments.
Electrical resistivity of the concrete cover (Measure with the Wenner Probe)
This topic is dealt with in detail in section 2.4
Oxygen content at the reinforcement
With decreasing oxygen concentration and increasing pH-value at a steel surface its potential
becomes more negative. In certain cases of concrete components with a high degree of water satu-
ration, low porosity and/or very high concrete cover and thus low oxygen supply, the potential at the
steel surface may be very negative even though no active corrosion is taking place. Without checking
the actual corrosion state this may lead to misinterpretation of the potential data.
The air permeability of the concrete may be tested with Proceq’s Torrent instrument.
2.3
Application Limits of the Potential Measurement Technique
The potential field measurement even with a coarse grid delivers good results for chloride induced
corrosion. This type of corrosion is typified by pitting developing into troughs. This strongly affects
the diameter of the reinforcement and thereby greatly affects the load bearing capacity. Corrosion
due to carbonation is typified by the development of smaller macro-elements and is only possible to
determine by using a very fine grid if at all.
Corrosion of pre-stressed steel reinforcement is not possible to detect if it is located within a protec-
tive tube.
The potential field measurement alone offers no quantitative conclusions about the rate of corrosion.
Empirical studies have shown that there is a direct relationship between the corrosion rate and the
electrical resisitivity. However corrosion rate readings are of limited value as the rebar corrosion rate
varies considerably with time. It is more reliable to work with corrosion readings taken over a period
of time.
