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TECHNICAL PAPER

Table 4: Ranges of corrosion rate in concrete exposed to different condition
CORROSION LEVEL µA/cm 2 µm/Y REBAR CONDITION CONCRETE CONDITION

negligeable < 0.1 < 1 Passive state - Non-carbonated without chlorides
- Very dry concrete carbonated or with chlorides
low 0.1 – 0.5 1 – 5 Depasivated in low humidity - with chlorides or carbonated at RH < 90%
concrete
Moderate 0.5 – 1 5 – 10 Active corrosion in medium level - with chlorides or carbonated at RH > 90%
humidity

High > 1 > 10 Large areas corroding in wet - saturated concrete heavily contaminated with chlorides
concrete

systems. However, its application in real size elements is 2.3.2 Modulated confinement of the current
not straight forward due to the need for the geometrical (guard ring) method
standardization of the area. A quasi-infinite area cannot be well
polarized, because it will not be feasible to use the counter This is the most efficient and simple way of accounting for a True
electrode with the same area as that of the working area. Also, R p value, although electronically it is the most complex . As
[13]
the estimation of R p values from ∆E/∆I measurements in large shown in Figure 10, there is a ring which surrounds the central
structures is not feasible unless certain mathematical treatments circular counter electrode, and it applies an independent
are applied [11,13] . This is because the counter electrode area is electrical field in such a manner that the effect of central
much smaller than the working area to be measured. In concrete electrode is obliged to remain inside the ring. Hence, the ring is
structures the electric signal is not concentrated just below the named as ‘guard’ due to its effect of keeping the signal inside .
[8]
small counter electrode, but its intensity decreases from the The electronic operation is complex because, in order to confine
border of the counter electrode until a distance (named the the current to a particular rebar area, the guard ring has to
critical length, L crit ) which depends on the square root of the apply a counter current changed at each step throughout the
ratio between the actual concrete ohmic resistance (moisture polarization time. To control the guard ring in the “modulated”
level) and the polarization resistance, confinement method, two small reference electrodes are placed
between the central counter and the guard ring called “ring
(9)
controllers”. It is essential to have such “modulation” in order to
L crit ~ √R e /R p
Several methods can be used to overcome the problem of avoid under - or over confinement of the current which ends in
delimiting the area reached by the applied current. Here, two wrong results.
methods are commented: The measurement has to be made by applying a pulse (more
– confining the current by means of a so named “guard easily galvanostatic) lasting from 30 s (corroding) until 100 s
ring” (passive), from the central counter. Then, another counter
current is applied from the external ring. As said, this external
– measuring and calculating the potential attenuation with current is accounted by means of the reading of the voltage
the distance
drop between the two ring controllers, in order to achieve
the required counterbalancing electrical field. The method
10
then makes an electrical delimitation of the area instead of
determining it. Additionally, the equipment has to be able
(µm) 1 to make automatically the removal (compensation) of the
P corr potential drop due to the concrete electrical resistance (IR) when
(µA/cm ) and
determining the the R p .
2 0.1

I corr 0.01

0.001
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Time (y) Pcorr Pcorr
I corr P corr
Figure 7: Evolution of the instantaneous corrosion rate (I corr ) and of the Figure 8: Progressive loss of section (assuming homogeneous corrosion)
corresponding loss in diameter (P corr ) on carbonated specimen produced when the aggressive front advances with age

THE INDIAN CONCRETE JOURNAL | NOVEMBER 2020 11