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TECHNICAL PAPER

100 Electrodes modulating Reference electrode
the confinement
OPC 300-w/c 0.38
FAC 300-w/c 0.5 Concrete surface
OPC 400 w/c 0.5
Electrochemical weight loss (g) 10 Current Auxiliary
Guard
OPC 400 w/c 0.6
ring
counter
electrode
lines

1
1 10 100 Area where the Current is confined
Gravimetric loss (g)
Figure 10: Modulated confinement of the current (guard ring) method
Figure 9: Comparison of gravimetric (weight decrease) and electrochemical
(from the integration of the corrosion rate-time values) steel losses
– The guard ring is not modulated through the “ring
By means of this electrical delimitation to a small zone of the controllers” (electrodes between the central counter and
polarized area, any localised spot or pit can be first localised, ring electrode)
and second its measurement can be made by minimising the
inherent error of R p . Not all guarded techniques are efficient. – The concrete may be too dry and the ohmic drop is too
The use of guard rings without the ring-controllers can lead to high.
too high values of the I corr for moderate and low values, and the – The surface moistening is not enough and there is a bad
error introduced in the case of very localised pits, is very high. contact between the probe and the concrete.

2.4 Potential attenuation method 2.5 On-site measuring possibilities
The other method consists of measuring the attenuation of When assessing the corrosion level in a structure there are three
potential from the counter electrode border distance . The possibilities of measurements :
[11]
[15]
probe has a small disc acting as the only counter electrode, and
a reference electrode is placed at the centre for the recording 1. Single visits with isolated measurements in time
of E corr . Other three reference electrodes are placed aligned 2. Periodic visits
with the E corr one at fixed distances. For the measurement, a 3. Permanent sensors
potentiostatic step of around 100 mV, lasting between 15 - 60 s,
is applied to the bar. This applied potential step attenuates with In the case with single visit, the inspector has a single
the distance as observed in Figure 11. From the distance (L crit ) opportunity for measuring. Peridodic visits can collect several
reached by the signal and certain geometrical considerations of data along a period of time. This is an essential point because,
the bar diameter, it is possible to calculate the True-R p (referred as previously commented, the differences of temperature and
to a particular steel area). humidity during the seasons may induce to have completely
different corrosion conditions. Then, the climatic considerations
This method is recommended for very wet structures, where should be taken into account. In the case of single visits, it is
the confinement is very difficult to achieve because of the low recommended to drill cores from the site and measure the
electric resistance. It is not recommended where localized
corrosion is occurring, because it assumes a large area having resistivity in the laboratory by previous conditioning them
homogeneous corrosion. If the corrosion is localized, then the at different degrees of saturation. These values will help to
error in the corrosion rate value may be very high. know the maximum and minimum expected values during the
evolution of the climate seasons.
Sources of error in on site measurement of the corrosion rate.
In the case of periodic visits, the optimal is to select the extreme
There are several sources of error that may come from, among weather conditions in order to register the extreme values which
others: would produce dry and wet concrete. These extreme values
– The equipment may use incorrect method for the will be averaged considering a “time of Wetness” to calculate
measurement or too short waiting period not allowing an average or “annual representative value” of the three
the capacitor effect to dissipate or not having the correct electrochemical techniques: potential, resistivity and corrosion
compensation of the concrete ohmic drop. rate.

12 THE INDIAN CONCRETE JOURNAL | NOVEMBER 2020