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© 2012 by Proceq SA
2.4
The Electrical Resistivity Measurement Principle
As we have seen above, corrosion is an electro-chemical process. The flow of the ions between the
anodic and cathodic areas and therefore the rate at which corrosion can occur is affected by the
resistivity of the concrete.
The Wenner Probe is used to measure the electrical resistivity of the concrete. A current is applied to
the two outer probes and the potential difference is measured between the two inner probes.
I
V
a
a
a
Resistivity
ρ
= 2
π
aV/l [k
Ω
cm]
Empirical tests have arrived at the following thresh-
old values which can be used to determine the likeli-
hood of corrosion.
When
ρ
≥
12 k
Ω
cm
corrosion is unlikely
When
ρ
= 8 to 12 k
Ω
cm corrosion is possible
When
ρ
≤
8 k
Ω
cm
corrosion is fairly certain
The electrical resistivity of the concrete cover layer decreases due to:
•
increasing concrete water content
•
increasing concrete porosity
•
increasing temperature
•
increasing chloride content
•
decreasing carbonation depth
When the electrical resistivity of the concrete is low, the rate of corrosion increases.
When the electrical resisitivity is high, e.g. in case of dry and carbonated concrete, the rate of cor-
rosion decreases.
2.5
Influence of Rebars on Electrical Resisitivity Measurements
The presence of rebars disturbs electrical resisitivity measurements as they conduct current much
better than the surrounding concrete. This is particularly the case when the cover depth is less than
30mm. In order to minimize the effect, none of the electrodes should be placed above a rebar when
measuring, or if this is unavoidable, then perpendicular to the rebar.
2.6
Effect of Electrical Resistivity on Half-cell Potential Measure-
ments
Low electrical resistivity leads to more negative potentials that can be measured on the surface and
the potential gradients become flatter.
In this case the measurement grid for potential measurements may be coarser, as the risk of unde-
tected anodic areas with flatter gradients becomes lower. However, as the resolution between cor-
roding and passive areas is reduced, this may lead to an overestimation of the actively corroding
surface area.
High electrical resisitivity leads to more positive potentials that may be measured on the surface and
potential gradients become steeper.
In this case the measuring grid must be made finer, in order to be able to locate an anode with a
very steep gradient. However, the more potentials may be misinterpreted as passive areas when only
considering the absolute value of the potential.
