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TECHNICAL PAPER

Connectionwi re
B

Rebar with
AnodeA1

Rebar with
Alkali activated AnodeA2
‘rod’ anodes
A A
Rebar with
AnodeA3

B

Figure 7: Alkali activated ‘rod’ galvanic anodes positioned along
steel reinforcement in an expansion joint prior to the placing of the
repair mortar Figure 8: Schematic of concrete slabs showing the position of the
18, 32mm dia steel bars and of the 9 galvanic anodes. Steel bars and
configuration and some were cast with no anodes (Figure 8). All anodes were connected externally via a junction box which allowed
monitoring of the current and potential of the steel bars 24 hours after
had 2% chloride by weight of cement additions in the mix water, disconnection of the anodes (depolarised potential)
a level that would normally cause significant corrosion of the
steel reinforcement. After 10 years of exposure in the outdoor prevention current density range of 0.2-2.0 mA/m suggested in
2
warm and humid conditions of Mumbai, the slabs containing the ISO 12696: 2016 [10] .
anodes remained intact (Figure 9) while the slabs with no anodes
had suffered from severe corrosion and cracking of the concrete Owing to the relatively low current delivery of galvanic anodes
cover (Figure 10). compared to impressed current systems, depolarisation
values do not always exceed or reach 100 mV as indicated in
The mean 24-hour depolarised potential of the protected steel Figure 11 [14] . In the case of ICCP systems, the current output
measured from the top surface of the concrete using a saturated can be adjusted so that it can achieve 100 mV depolarisation,
calomel reference electrode (SCE) remained passive throughout as required by standards [10] , and appears to be a limitation
its life (mean of around -150 mV) wheras that in the non- with galvanic CP. What is apparent, however, is that the level
protected slab was of the order of -400 mV to -600 mV indicating of polarisation is directly related to the logarithm of current
significant corrosion. Corrosion current density values of the (Figure 11). This is described by the Butler Volmer equation
steel determined by linear polarisation after around 9 years (Equation 2) which relates applied current density to level of
confirmed the large disparity of the corrosion rate between the polarisation of the steel.
two slabs, the protected steel showing insignificant corrosion
i appl
(<<0.5 mA/m ) wheras the corrosion current density of the non- i corr(app) = 2.3η -2.3η (2)
2
protected steel was a very high 6 mA/m . Anodes were still exp ( βa ) –exp ( βc )
2
delivering a protective current density of the order of 0.4 mA/m
2
by area of steel at the later years, confortably within the cathodic

Figure 10: Corroded slab without anode protection showing extent of
Figure 9: Slab protected by galvanic anodes (lines are wires connecting cracking of the concrete caused by steel corrosion after exposure to
individual anodes and/or steel bars) outdoor warm and humid conditions for 10 years [13]

THE INDIAN CONCRETE JOURNAL | APRIL 2021 65