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TECHNICAL PAPER

120
Potentiostat 100
Polarization (mV) 80

60
40
20
0
0 5 10 15 20 25
Distance from border central counter (cm)
Active corrosion Passive

Figure 11: Potential attenuation with the distance in active and passive concrete structures: left arrangement and right decay of
potential giving the critical length.

The selection for the measuring zones will depend on the The whole monitoring system illustrated should consist of the
particular case, but, at least, two readings or cores should be following elements:
made/drilled to represent a “lot” or structural element. Three, – The probes embedded
if possible, is better for the averaging and obtention of a – The unit making the measurement with a microcomputer
representative value.
to measure and record the data
In the case with permanent sensors, the use of embedded – The wireless transmitter to the computer
sensors is one of the most promising developments in order to – The data representation and analysis
monitor the long term behaviour of the structures (measuring
– The energy source, either a connection to the mains or a
the evolution of corrosion parameters with time and the
influence of the environment [5,16-17] ). battery.

2.5.1 Types of sensors 3. APPLICATION OF CORROSION RATE DATA:
MODELLING THE PROPAGATION PERIOD
The most common method is to embed reference electrodes or
resistivity electrodes. They give information about the presence If the steel remains passive, then there will be negligible loss of
of moisture and on the evolution of corrosion potential (Ti, Mn/ metal. However, if the corrosion is in active state, it produces
a progressive loss of the cross-section (P corr ) of the bar and
MnO 2 , Ag and Ag/AgCl). Other events that can be monitored it usually starts from the outer zone of the bar as shown in
are the advancement of the carbonation or chloride fronts, the Figure 8.
oxygen availability, temperature, concrete deformations, and the
corrosion rate. It has to be remarked the need to have always There are two main parameters of the model:
a temperature sensor due to all parameters evolve with the – the “corrosion rate” V corr in mm/y or µm/y (or I corr if
temperature and it is necessary to establish the corresponding expressed in µA/cm ) which evolves all the time due to
2 [18]
trends. the evolving local conditions and
– the loss in diameter (P corr in mm or µm) or in steel section at
Regarding the corrosion sensors, the modulated confinement a particular time
method is not feasible and it would need a guard ring
permanently embedded, Then, either- If the annually averaged corrosion rate V corr; remains constant,
then the model is that of Figure 1. The propagation model can
– small probes are used and not the main rebar
be expressed as [19] :
– if the main rebar is measured, the attenuation of potential
method should be used. = ( 0 ) = . (10)
The sensors can be either installed The t p is the corrosion propagation time, φ 0 is the initial diameter
of the bar and φ t is the diameter after corrosion, P corr (mm) is the
– during construction
accumulated corrosion or penetration of attack after a certain
– attached to the surface of an existing structure. In this period and V corr (mm/y) is the corrosion rate. For example, if the
case, care on the interface has to be taken due to the average annual corrosion rate is of 5 µm/y, then after 10 y, the
mortar to attach the sensor will give values of resistivity corrosion penetration would be of P corr = 50 µm [and the pitting
that are not those of the original concrete. depth would be of 500 µm following equation (8)].

THE INDIAN CONCRETE JOURNAL | NOVEMBER 2020 13