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The Response of Galvanic Protection Current to Environmental Changes

1Steven Holmes

1

Department of Civil & Building Engineering, Loughborough University, Leicestershire,LE11 3TU, UK, S.P.Holmes@lboro.ac.uk

ABSTRACT

An observed favourable feature of galvanic anodes used in concrete repair is the variation of their

current output in response to changing environmental conditions, such as concrete moisture content,

chloride content and temperature. This means that the anodes provide more protective current to the

steel reinforcement when the environment is aggressive and less current when it is more benign, thus

conserving sacrificial anode life.

Over the last two years, the current outputs of several hybrid anode systems installed in lab samples

and on a bridge in a concrete repair application have been monitored with respect to changes in

temperature, moisture and chloride content (variable concrete resistivity). The aim was to investigate

the relative effects of these parameters on the protective current output.

The results confirmed that both the laboratory and site applied anode systems are responsive to

changes in concrete resistivity brought about by variations in temperature, moisture and chloride, with

the latter two having the greater effect. On site, this is a distinct advantage for both conservation of

anode life and protection in aggressive conditions.

Keywords: Responsive behaviour; hybrid anode; variable resistivity.

1.0 INTRODUCTION

Galvanic cathodic protection has recently received much attention as a method of protecting

steel in concrete. When attached to the steel, these anodes effectively suppress the corrosion

reaction at the surface, with anodic dissolution being shifted to the installed anode.

Traditional galvanic anodes are used in topical applications following patch repairs, where

they are attached to the steel and immersed in the repair mortar or concrete. The aim of these

anodes is to counteract the incipient anode effect which can see reinforcement corrosion in

areas adjacent to the patch due to the residual chloride contained within it and the relocation

of the corrosion reaction [1]. Problems arise when the use of high quality, low conductivity

mortars limits the current thrown by the anodes, or gradual loss of anode material over time

results in loss of protective current and new corrosion.

A hybrid anode system [2,3] has been introduced which combines elements of both chlorideextraction/re-alkalisation and galvanic cathodic protection, in that, for a short period, a power

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supply is used to drive a high current from the discrete sacrificial anode to re-passivate the

steel and draw chloride to the anode. The same anode is then connected directly to the steel to

provide maintenance free cathodic protection by means of a galvanic current.

The impressed current (re-alkalisation) phase halts the corrosion at the steel surface by

generating hydroxyl and neutralising the acid in the pit thus increasing the pH; whilst the

galvanic phase sustains hydroxide production and maintains the high alkalinity (see Figure 1).

Figure 1. The two stages of the hybrid anode system.

When applied to patch repairs, this system, instead of being immersed in the repair mortar is

placed in adjacent un-repaired concrete to counteract the anodic behaviour of the steel, with

the number of anodes being tailored depending on amount of steel and size of the patch.

As well as patch and topical repairs, the hybrid system can be applied to whole structures

(bridges, car-parks, piers and industrial structures) to repair and protect against rebarcorrosion, using the same treatment methodology [4,5].

1.1 Responsive Behaviour

The current passed between an installed galvanic anode and the steel it is protecting very

much depends on the resistivity of the concrete which separates them. The resistivity of the

concrete can be influenced by the quality of the mix on casting (water to cement ratios,

aeration, amount and size of aggregates, mixed in chlorides etc), features induced during

service (cracking, and chloride diffusion during service) and environmental conditions

(moisture content and temperature). Also, the distance between the anode and the steel will

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determine the resistance to current flow due to the volume of concrete through which it must

pass. Varying any of these conditions will have an effect on the resistance of the cell and in

accordance with Ohms law the current passed will also change

I = E/R

Where I = Current (Amps); E = Potential (Volts); and R = Resistance (Ohms).

This investigation has looked at the responsive behaviour of current output brought about by

different temperature and moisture conditions as well as varying mixed-in chloride levels.

Taking an electronic circuit approach, each of these factors can be seen as a variable resistor

in an electrical circuit (see Figure 2), with the 3 variable resistances making up the total

resistance of the concrete.

Figure 2. The resistance of concrete in a galvanic cell, represented by the variable resistances

of the contributing concrete conditions.

Resistivity has been used by several authors to evaluate the risk of steel reinforcement

corrosion [6,7,8,9] with these authors proposing relationships between measured concrete

resistivity and active corrosion, as well as the chloride concentration threshold for concrete of

different resistivities.

These studies have focussed on the influence of concrete resistivity on steel corrosion rates,

with respect to humidity/moisture, concrete structure and chloride concentrations. None of

these papers discuss responsive current behaviour when considering galvanic cathodic

protection, where concrete resistivity has a huge part to play in the effectiveness and

longevity of the system. The benefits of responsive current behaviour include:

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1. Increased anode lifetime in less aggressive environments (i.e., dry, cold, low chloride),meaning replacement of sacrificial systems can be delayed due to reduced metal

dissolution.

2. Increased protection in aggressive environments (i.e., hot, wet, high chloride),meaning that the protective current offered will increase where corrosion is more

likely.

The work presented charts the authors experience of responsive behaviour in the galvanic

phase of the anode system over the last 2 years; taking results from both laboratory and site

studies.

2.0 EXPERIMENTAL

2.1 Whiteadder Bridge Application

773 days of current and temperature data has been recorded from a bridge over the

Whiteadder River in Northumberland, UK. The bridge spans the river estuary and although it

is fresh water, is affected by tidal fluctuations.

The anodes were installed in the concrete cover in February 2007 during bridge maintenance

and a variety of measurements have been taken over the subsequent 2 years. A data logger

was used to collect galvanic current data at noon each day. This was calculated from the

voltage drop measured across a 1 Ohm resistor. The temperature was measured using a Ttype thermocouple. Data was frequently downloaded from the logger using a GSM modem

and software configuration.

Galvanic current data was taken from 2 zones, each housing 25 anodes (5 rows of 5 anodes at

400 mm centres) connected together with a wire, which was riveted to the steel in a break-out

zone. These zones were located on a west-facing pier section, as can we seen in Figure 3.

Following installation a waterproof coating was applied to pier.

The galvanic current data presented follows a one week impressed current phase whereby a

high current density is driven from the anode to re-passivate the corroding steel (Figure 1).

Current, charge and residual life data can be found in Table 2.

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Figure 3. Plan of the west-facing pier section housing the two anode zones

2.2 Varying chloride blocks and environmental testing

In a separate lab-based study, concrete samples were prepared in late December 2007. These

were made with an 8:1 (all-in 20 mm aggregate:opc) mix, at a water to cement (w/c) ratio of

0.6.

The samples contained 0, 1, 2.5 or 5% chloride by weight of cement. NaCl (Sodium chloride)

was used as the source of chloride and was hand-mixed into the water prior to its addition,

ensuring complete dissolution. Mix details can be seen in Table 1.

Upper Zone

Lower Zone

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Block

Letter

Cl % New/old Cement

A 0 Old

B 0 Old

C 1 OldD 1 New

E 2.5 Old

F 2.5 New

G 2.5 Old

H 5 New

I 5 New

Table 1. Block concrete composition data.

The concrete was cast in re-useable wooden moulds (dimensions 600 x 120 x 120 mm) with

the 15 mm diameter mild steel reinforcing bar (rebar) being fed in from the centre at one end

(Figure 4). The approximate surface area of the steel was 230 cm2.

The samples were removed from the moulds between one and two days after casting and left

to cure in the dry laboratory air (12-20C, ~50-70% Relative Humidity). After a period of six

months had passed, a hole 30 mm diameter and ~90 mm deep was drilled into the block to

house the anode which was 14.5 mm high and 17 mm in diameter (surface area 12.3 cm2).

The steel had a surface area of 230 cm2.The anode was held in place with a lime mortar

which was allowed to dry before treatment began.

For 1 week a 12V power supply was used to deliver the impressed current phase of the

treatment before the anode was connected to the steel galvanically by a rivet. Block F did not

receive the impressed current treatment and the anode was connected to the steel galvanically

throughout. Current readings were facilitated by recording the voltage drop across resistors of

various sizes (depending on the chloride content of the block).

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Figure 4. Test block schematic.

Following the short term experiments detailed above, all of the blocks were placed outdoors

throughout the winter/spring of 2008/2009. In May 2009, blocks B, E, D and H were placed

indoors, leaving blocks A, C, F, G and I outdoors (i.e., one of each chloride concentration in

either of the regimes). The blocks were thought to be fairly wet when this was done due to

recent rainfall.

Identical logger equipment was used to record the galvanic current by measuring the voltage

drop across a resistor placed between the anode and steel on each block. The indoor and

outdoor temperatures were also logged and rainfall/meteorological events noted so that the

causes of major fluctuations could be identified. From these readings, the charge passed could

be calculated and the residual life estimated (Table 2).

2.2 Short-term wetting experiment

Prior to the test, all nine blocks had been in laboratory air (~19C/~65% Relative humidity)

for around 10 weeks, but were not thought to be excessively dry.

A data-logger was used to record the voltage drop across a resistor connected between the

anode and steel. The resistors varied between blocks due to the different amounts of current

being passed. The temperature was also recorded using a thermocouple.

40 ml of tap water was poured into the anode hole of each of the blocks and the current

response measured over a ten day period. Only blocks B, D, G and H were logged to see the

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effect of chloride content on the response, although all nine blocks were watered in this

manner.

3.0 RESULTS & DISCUSSION

3.1 Whiteadder Bridge installation

Upper and lower zone current data along with temperature readings for a 733 day period can

be seen in Figure 5.

Figure 5. Current and temperature data for the Whiteadder river bridge, beginning in late

March 2007.

0

5

10

15

20

25

30

35

40

45

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400 500 600 700 800

Airtemp(C)

Current(mA)

Time (days)

Upper zone

Lower zoneTemp

River in flood

causes highcurrent to protect

the steel

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Figure 6. Current and temperature data from a 5 day period.

Figure 5 clearly shows the fluctuation of temperature on a monthly/seasonal basis. The

current response to the changes in temperature can also be seen, but is better highlighted inFigure 6. The effect of different moisture contents on galvanic current can also be seen clearly

in Figure 5; the bottom row of anodes on the lower zone were frequently under water and

upward diffusion of water from the river ensured that the lower zone concrete had a

consistently higher moisture content (and therefore lower resistance to current flow)

throughout the year. The decay in both the upper and lower zone currents in the first ~200

days can be explained by gradual drying after hydro-demolition and pressure washing during

the repairs to the bridge deck.

Figure 6 gives more detailed information about the effect of temperature on the resistance of

the concrete. The secondary current peak seen on the lower zone, and less so on the upper

zone is though to be a result of sunlight/shade effects due to the bridge geometry and the

direction it faces.

During the summer months the difference in current between the upper and lower zones is

more pronounced, as the upper dries out more quickly due to the lesser influence of the river.

This theory is backed up by the data from the winter months, which brings the current

readings from the two zones much closer together, seemingly due to the upper zone becoming

wetter.

5

10

15

20

25

30

0

0.5

1

1.5

2

2.5

3

3.5

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Temperature(C)

Current(mA)

Date

Upper Zone Lower zone Temperature

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On September 9th

2008, the river flooded. This can be seen by the large spike in protective

galvanic current and the gradual drop as the flood subsided. The water rose high enough to

significantly wet the concrete of the upper zone, reducing its resistance and increasing the

current. It can be seen from Figure 7 that the lower zone recovered in a shorter period than the

upper zone, probably due to the fact that the concrete held significant moisture prior to the

flood. The upper zone concrete, as can be seen from the current prior to the flood, was drier

than the lower at the time of the flood, so both the effect on its galvanic current and

subsequent recovery after the flood subsided were more significant.

Figure 7. Current and temperature data taken from the upper and lower zones around the time

of a flood.

From the logged currents, the charge passed per anode was calculated for both the upper and

lower zones. From this data, residual anode life was calculated. This data can be found in

Table 2.

-10

-5

0

5

10

15

20

0

2

4

6

8

10

12

14

16

18

03/09/2008 13/09/2008 23/09/2008 03/10/2008

Temperature(C)

Current(mA)

Date

Upper zone

Lower zone

Temperature

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Calculation/Measurement Upper

Zone

Lower

Zone

Units

Time 773 days

Average Temperature 12.1 C

Average current 0.62 2.71 mAAverage current per anode 0.03 0.11 mA

Average charge per anode 1.17 7.71 kC

Average charge per anode for 1 year 1.06 6.97 kC

Charge left per anode after initial treatment 362.3 361.1 kC

Calculated average anode life 340 50 years

Steel surface area 2.05 1.92 m2

Current density applied to steel 0.30 1.42 mA/m2

Table 2. Relevant current and temperature data for the upper and lower zones

Due to responsive behaviour, the residual life of the anodes in the lower zone is more than 6.5

times less than that of the upper as the charge delivered is representative of metal dissolution.

This shows that in aggressive environments the anodes will pass more protective current to

the steel at the cost of their effective life, whereas in less aggressive environments, metal

dissolution is minimal and life conserved. The results take into account an efficiency and

utilisation factor of 0.85.

3.2 Blocks with varying chloride content and environmental testing

The current and temperature from the indoor and outdoor blocks was logged for a 29 day

period in May and June 2009. The anode current density was then calculated using the

original anode surface area and was, as a result, a conservative estimate. The differences

between the indoor and outdoor temperatures can be seen in Figure 8 and the current densities

with respect to temperature over the four weeks can be seen in Figures 9 and 10.

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Figure 8. Temperatures indoors/outdoors.

Figure 9. Current density/temperature plot for the outdoor blocks.

0

5

10

15

20

25

30

35

40

45

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Temperature(C)

Date

Indoor

Outdoor

0

5

10

15

20

25

30

35

40

0

50

100

150

200

250

300

350

400

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Temperature(C)

A

nodeCurrentDensity(mA/m2)

Date

Block A

Block F

Block GBlock I

Block C

Temperature

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Figure 10. Current density/temperature plot for the indoor blocks.

It is clear from the results of the outdoor blocks in Figure 9 that the resistance of the concrete

and therefore current delivered to the steel is heavily dependent on the amount of chloride inthe concrete mix. The purely galvanic 2.5% chloride Block F had the lowest current followed

by the blocks with 1, 2.5 and 5% chloride that had received the impressed current phase.

It is also clear from Figure 9 that the amount of chloride in the concrete determined the degree

to which the current was affected by temperature fluctuations. Block F showed variations of

up to ~20 mA/m2

and Block I up to ~250 mA/m2.

The moisture content of the blocks can be seen to have an effect between 6th

and 9th

June,

where the current increases and then reaches a plateau despite a significant drop in

temperature. The effect of moisture on concrete resistance is also highlighted in Figures 11 &

12.

From Figure 9 it is also apparent that the current generally peaks 1-3 hours after the peak

temperature has been recorded. This effect is due to the anode being mortared into concrete

which takes time to heat up in contrast to the thermocouple which reads an air temperature.

Interestingly the peak currents in the indoor blocks did not generally occur until 3-5 hours

after the maximum temperature was reached.

The blocks kept indoors showed somewhat anomalous results, with the currents measured forBlock D being especially erratic. Blocks H and E produced similar current outputs throughout

-60

-40

-20

0

20

40

60

0

10

20

30

40

50

60

70

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Temperature(C)

AnodeCurrentDensity(mA/m

2)

Date

Block H

Block D

Block E

Block B

Temperature

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the test period, despite having different chloride levels. The currents measured generally

related well to the temperature indoors, although this would be expected without the influence

of precipitation.

What is not plainly evident from Figures 9 & 10 is the difference in magnitude of the current

fluctuation, which is seemingly the result of increased moisture content in the outdoor blocks.

Figures 11 & 12 represent two blocks with the same chloride content, one of which has been

left outdoors. Assuming similar temperature trends (Figure 8), the very different current

profile between 5th

and 7th

of June demonstrates the effect of rain and the resulting drop in

concrete resistivity.

Figure 11. Current of indoor/outdoor blocks, both containing 2.5% chloride by weight of

cement.

0

20

40

60

80

100

22/05/2009 01/06/2009 11/06/2009 21/06/2009

AnodeCurrentDensity(,A/m2)

Date

Block E

Block G

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Figure 12. Current of indoor/outdoor blocks, both containing 0% chloride by weight of

cement.

3.3 Lab based moisture test

The current response to the water addition for blocks with different chloride levels can be

seen in Figure 13.

0

5

10

15

20

25

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AnodeCurrentDensity(mA/m2)

Date

Block B

Block A

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Figure 13. Current response to the addition of 40 ml tap water to the anode.

All four blocks responded extremely quickly to the addition of the water, with the currentsspiking immediately. The nature of the rise in current and the time taken to reach a peak after

the addition varied considerably with the chloride contents of the blocks.

In general it can be seen that irrespective of chloride levels the anode responds to the

increased moisture in the concrete, producing more protective current as the environment

becomes more aggressive. As the water diffuses through the concrete/drains through the

bottom of the block, the current drops off and eventually reaches a relatively stable state

which continues to respond to changes in temperature.

CONCLUSIONS

The experiences gained over the last two years are summarised below:

1. The hybrid anodes installed on the Whiteadder Bridge remain active after more than 2years, responding to changes in concrete resistivity brought about by temperature and

moisture fluctuations as well as geometric effects.

0

5

10

15

20

25

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

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Temperature(C)

Current(mA)

Date

Block B

Block D

Block G

Block H

Temperature

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2. The differences in concrete resistivity between the upper and lower zones haveresulted in a large difference in the charge passed. The result is that the anodes in the

more aggressive lower zone have a predicted lifetime of 50 years and the upper zone

340 years, which highlights the ability of the anode to preserve its life in benign

environments and offer extra current when the environment demands it.

3. Data from the indoor/outdoor samples highlights the responsive behaviour of theanodes to differing chloride levels in the concrete, with the anodes in the more

aggressive environment (wet, high chloride, high temperature) delivering more current

to the steel than the drier low chloride blocks. Increasing chloride also increased the

degree to which the current was affected by temperature fluctuations

4. This study has shown that the varying influences of moisture, temperature andchloride content on the resistivity of concrete result in a dynamic current response

which can be measured over the course of a few seconds or a few years.

REFERENCES

[1] Broomfield, J.P., 1997, Corrosion of steel in concrete - Understanding, investigation

and repair London; E & FN SPON.

[2] Glass, G.K., et al, Hybrid Electrochemical Treatment in the Repair of Corrosion

Damaged Concrete, Proceedings from: Concrete Platform 2007 Queens University,Belfast, 19

th& 20

thApril 2007.

[3] Glass, G.K., Roberts, A.C., Davison, N., Hybrid corrosion protection for chloride

contaminated concrete Proceedings of the Institute of Civil Engineers, Construction

Materials, 161 (2008) p. 163-172.

[4] Glass, G.K., Roberts, A.C., Davison, N., Hybrid Electrochemical Treatment in the

Repair of Corrosion Damaged Concrete, Proceedings from: Concrete Platform

2007 Queens University, Belfast, 19th

& 20th

April 2007.

[5] Holmes, S.P., Repair of Corrosion Damaged Concrete Using a Two-Stage

Electrochemical Treatment, Proceedings of Structural Faults and Repair 2008,

Edinburgh.

[6] Morris, W., Vico, A., Vazquez, Chloride induced corrosion of reinforcing steel

evaluated by concrete resistivity measurements, Electrochimica Acta 49 (2004) p.

4447-4453.

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[7] Morris, W., Vico, A., Vazquez, M., de Sanchez, S.R., Corrosion of reinforcing steel

evaluated by means of concrete resistivity measurements, Corrosion Science 44

(2002) p. 81-99.

[8] Hunkeler, F., The resistivity of pore water solution a decisive parameter of rebar

corrosion and repair methods, Construction and Building Materials 10 No. 5 (1996)

p. 381-389.

[9] Gonzalez, J.A., Lopez, W., Rodriguez., Effects of Moisture Availability on Corrosion

Kinetics of Steel Embedded in Concrete Corrosion 49 No. 12 p 1004-1010.