© 2017 Proceq SA
32
Concrete cover thickness (Measure with Profometer / Profoscope)
The potential that can be measured at the surface becomes more posi-
tive 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 measure-
ments along with the half-cell measurements.
Electrical resistivity of the concrete
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 undetected anodic areas with flatter gradients be-
comes lower. However, as the resolution between corroding and passive
areas is reduced, this may lead to an overestimation of the actively cor-
roding 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 mere potentials
may be misinterpreted as passive areas when only considering the abso-
lute value of the potential.
Temperature
The main effect of temperature on potential measurements is given by
its influence on the electrical resistivity. High temperature will cause
concrete resistivity to be lower and low temperature will cause concrete
resistivity to be higher; the resulting effect on potential value is as de-
scribed in the previous paragraph “Electrical resistivity of the concrete”.
In order to measure the potential there must be a contact between the
probe and the ectrolytes in the pore system of the concrete. Therefore a
measurement below the freezing point is not recommended and can lead
to incorrect readings.
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 saturation, low porosity and/or
very high concrete cover and thus low oxygen supply, the potential at the
steel surface may be highly 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.
4.1.4. 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. 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 protective 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 resisitiv-
ity. However corrosion rate readings are of limited value as the rebar cor-
rosion rate varies considerably with time. It is more reliable to work with
corrosion readings taken over a period of time.
