Influence of carbonation
Carbonated concrete has a higher resistivity than concrete without carbonation, however pro-
vided the depth of the carbonated layer is significantly smaller than the probe spacing, the effect
of this layer is small. Consequently if the carbonated layer is thick, it may be necessary to increase
the probe spacing to obtain good results.
4.2 Applications
Empirical tests and theory have shown that resistivity is directly linked to both the likelihood of
corrosion due to chloride diffusion and to the corrosion rate once depassivation of the steel has
taken place.
Estimation of the likelihood of corrosion
Resistivity measurements can be used to estimate the likelihood of corrosion. When the electrical
resistivity (ρ) of the concrete is low, the likelihood of corrosion increases. When the electrical resis-
tivity is high (e.g. in case of dry and carbonated concrete), the likelihood of corrosion decreases.
Empirical tests have arrived at the following typical values for the measured resistivity which can
be used to determine the likelihood of corrosion. These figures are for Ordinary Portland Cement
at 20°C.
When ≥ 100 kΩcm Negligible risk of corrosion
When = 50 to 100 kΩcm Low risk of corrosion
When = 10 to 50 kΩcm Moderate risk of corrosion
When ≤ 10 kΩcm High risk of corrosion
Indication of corrosion rate
The following interpretation of resistivity measurements from the Wenner four-probe system has
been cited when referring to depassivated steel (Langford and Broomfield, 1987).
> 20 kΩ cm Low corrosion rate
10-20 kΩ cm Low to moderate corrosion rate
5-10 kΩ cm High corrosion rate
< 5 kΩ cm Very high corrosion rate
Empirical reference values
A large body of empirical resistivity data has been collected by many studies over a number of
years. The data presented here is taken from “Test methods for on-site measurement of resistiv-
ity of concrete – a RILEM TC-154 technical recommendation” by Rob B. Polder. The values have
been converted to kΩcm to match the display of the Resipod. More details on the interpretation
of results may be found in that document.
To Next Page
Loading...
Need help?
General
