INDIA - Influence of Fly Ash on the Corrosion...Jan2004

8/6/2019 INDIA - Influence of Fly Ash on the Corrosion...Jan2004

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Influence of Fly Ash on theCorrosion of Steel

Reinforcement in Concrete

A Review

by

Theodore (Ted) BremnerUniversity of New Brunswick, Canada


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Corrosion occurs when two different metals,

or metals in different environments, areelectrically connected in a moist or dampconcrete. Rust forms with a volume larger

than the metal consumed and in reinforcedconcrete the concrete cover spalls off,exposing the metal directly to theaggressive environment.


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This will occur when:

1. Steel reinforcement is in contact with analuminium conduit.

2. Concrete pore water composition varies betweenadjacent or along reinforcing bars.

3. Where there is a variation in alloy compositionbetween or along reinforcing bars.

4. Where there is a variation in residual or applied

stress along or between reinforcing bars.5. Where there are imposed stray electrical currents.


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Corrosion in Reinforced Concrete

O2H2O

Fe2O3H2O (rust)

--- -

electron transfer

anodicreaction

2Fe++

2Fe(OH)2

secondaryreaction

O2

H2O

4(OH-)cathodicreaction

O2

H2O

4e-

anodic dissolution of iron cathodic region


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Can corrosion be avoided in reinforced concrete?

Yes if:(a) Concrete is always dry, then there is no H2Oto form rust. Also aggressive agents cannot easilydiffuse into dry concrete.

(b) Concrete is always wet, then there is nooxygen to form rust.

(c) Cathodic protection is used to convert all thereinforcement into a cathode using a battery. This

is not easy to implement because anodic mesh isexpensive, and this technology is not easy toinstall and maintain.


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(d) A polymeric coating is applied to the concrete

member to keep out aggressive agents. These areexpensive and not easy to apply and maintain.

(e) A polymeric coating is applied to thereinforcing bars to protect them from moisture and

aggressive agents. This is expensive and there issome debate as to its long- term effectiveness.

(f) Stainless steel or cladded stainless steel is usedin lieu of conventional black bars. This is muchmore expensive than black bars.


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Can we avoid corrosion?

No, not entirely:

Concrete is not usually under water orcontinuously dry. Aggressive agents such

as carbon dioxide, de-icing agents and/orsea water can diffuse into the best of moistconcrete, and corrosion will eventually

result.


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If corrosion cannot always be avoided and

economical solutions are required, theeffects of corrosion can be minimized bymaking a better concrete.

As will be shown later, Fly Ash added to alow w/c concrete produces a muchenhanced corrosion resisting structure withno significant increase in cost.


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The intrinsic nature of concrete is to be very

protective of embedded steel. As soon assteel is placed in the high pH concrete(>12), a thin dense passive layer forms thatis virtually continuous and the subsequentrate of attack is so low as to beinsignificant.


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Unfortunately when the carbonation front

reaches the steel or when chlorides diffuseinto the steel and reach a threshold level,this coherent protective layer is replaced by

a porous incoherent expansive coating.The formerly protective oxide layer becomes

an expansive porous oxide layer whichcauses cracking and eventually spalling ofthe concrete cover layer.


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The Economical Solution:

We must make concrete more protective ofthe steel reinforcement so that it will protectthe passivating oxide layer.

Making better concrete, using only Portlandcement, will not make a substantialimprovement.

Fortunately Fly Ash added to a properlydesigned and cured concrete mixture

will.


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The key to protecting the protective passive

layer is to make a much more impermeableconcrete than we have made in the past.

This can be done using a Fly Ash concrete

with very low permeability, which willdelay the arrival of carbonation andchlorides at the level of the steelreinforcement.


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Fly Ash makes concrete almost impermeable

Fly Ash is a finely divided silica rich powder that,in itself, gives no benefit when added to a concretemixture, unless it can react with the calciumhydroxide formed in the first few days of

hydration. Together they form a calcium silicahydrate (CSH) compound that over timeeffectively reduces concrete diffusivity to oxygen,carbon dioxide, water and chloride ions. By

reducing ion diffusion, the electrical resistance ofthe concrete also increases as Datta, Garg andRehsi have shown (1).


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How does Fly Ash reacting with Ca(OH)2

reduce permeability?The CSH compound formed in the presenceof moisture in the void space within the

hydration products effectively reduces thenumber and continuity of the capillarypores. This reaction, because it entailsdiffusion, takes place in a moist

environment at a decreasing rate that cantake a decade or more to reach completion.


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The Role of Water in Concrete

Water in concrete exhibits a Dr. Jekyll andMr. Hyde behaviour. Water is essential forthe proper hydration of a Portland cement

pozzolan combination such as onecontaining Fly Ash. Also fully saturatedcement paste is almost impermeable to boththe oxygen needed for corrosion, and to

carbon dioxide that destroys the passivatinglayer.


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On the other hand, too much water leaves the

concrete highly permeable. Also the excesswater in the pores contributes to thecorrosion process. In addition, it provides a

medium whereby chloride ions can diffuseeasily toward the reinforcement and destroythe passive layer.


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An antidote for excess water

in concrete is usually in theform of a superplasticizer.


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Factors other than permeability influencing

corrosion(i) A portion of the chloride ions diffusing

through the concrete can be sequestered inthe concrete by combining them with thetricalcium aluminate to form a calciumchloro-aluminate (Friedels salt). Dass andRajj (2) have shown that this can be a

significant effect in reducing the amount ofavailable chlorides thereby reducingcorrosion.


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(ii) In low-strength concrete and in hot dry

climates, carbon dioxide can diffuse intothe concrete, react with calcium, sodiumand potassium to form carbonates and in

the process lower the pH to a level wherethe passivating oxide layer is no longerstable.

(iii) The chloride levels needed to initiatecorrosion decrease as the level of cementreplacement increases (3).


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Fortunately increasedimpermeability of concrete madewith Fly Ash more than

compensates for the above threenegative effects when Fly Ash isused to replace cement as the

following experiments will show.


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Test #1 - Uncracked Concrete

Malhotra et al. (4) carried out tests on aseries of slabs 150-mm thick made with 20mm maximum size aggregates and with

reinforcing bars 20, 40, 60 and 80 mm fromthe top surface. After moist curing forseven days and air drying for 21 days, theslabs were continuously ponded for eight

years with a 4% calcium chloride solution.


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Test # 1 Uncracked Concrete (contd)

The effect of fly ash in reducing chloride ionsingress measured at 8 years at 18 mm (and 30mm) below the surface.

Mixture w/(c + fa) fa/(c + fa) % Cl, % by mass of concrete

Cl, % by mass of

cementitiousmaterials

1

2

3

45

6

0.28

0.33

0.30

0.390.57

0.57

55

0

56

025

0

0.01 (-)

0.18 (.02)

0.02 (-)

0.17 (0.12)0.29 (0.01)

0.31 (0.24)

0.06 (-)

1.07 (0.12)

0.14 (-)

1.19 (0.84)2.43 (0.08)

2.60 (2.02)

The very low Cl levels in Mixtures 1 and 3 clearly indicate the very long servicelife that can be expected with high volume fly ash concrete.


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The steel reinforcing bars in concreteincorporating moderate (25%) and high(~55%) volumes of fly ash and ponded with

4% calcium chloride solution for 8 yearsshowed superior performance in regard tothe corrosion activity compared with thosein the corresponding control Portlandcement concrete.


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Of the various mixtures tested, only the W/C = 0.57concretes showed evidence of corrosion of theembedded reinforcement when the slabs were

broken open at eight years. The reinforcement

embedded in the concrete with 25% replacementof cement with fly ash that had a cover of 20 mmshowed signs of corrosion whereas the controlspecimens with only Portland cement showed

signs of corrosion with both 20 and 40 mm ofcover.


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Test #2 Uncracked Concrete

Gu et al. (5) in 1999 and Quian (6) in 2003reported on a series of concrete slabs 153mm thick made with 19 mm maximum size

aggregates that had been moist cured for 7days followed by exposure to laboratory airfor approximately 50 days. The top surfacewas ponded with 3.4% sodium chloridesolution in the laboratory.


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Test # 2 Uncracked Concrete (contd)After 5.3 years the following results were obtained for barswith 25 mm cover.

Min.

No.

Type of SCM SCM % W/C Corrosion

Rate A cm2

N1

N2

N7

N8

N3

N5N6

Fly Ash Type F

Fly Ash Type C

Silica Fume

Slag

-

--

58

58

10

55

-

--

.32

.32

.32

.32

.32

.43

.55


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Test # 3 Uncracked Concrete

Thomas et al. (3) in 1986 installed a series of steel-reinforcedconcrete prisms at Shoeburyness in the Thames estuary,England and after ten years the prisms were evaluated. Thereinforcement steel mass loss in % is given below forreinforcement with 20 mm cover.

Mass Loss

Fly Ash %

W/C + Fly Ash

.54 - .68 .45 - .57 .39 - .49

0

15

30

50

3.0

1.3

0.4

0.8

1.8

0.2

0.2

0.2

0.4

0.2

0.2

0.2


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Test # 3 Uncracked Concrete

Conclusions:Fly Ash concretes showed substantial

increased resistance to the penetration ofchlorides which resulted in reducedcorrosion of steel bars embedded in theconcrete.

Chloride ion threshold levels were 0.70, 0.65,

0.50 and 0.2 (% by mass of cement and FlyAsh) for Fly Ash contents of 0, 15, 30 and50% respectively.


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Uncracked Concrete Conclusion

Aggressive agents diffuse into the concreteand when they reach the embeddedreinforcement at a threshold level, corrosion

takes place. Eventually this results incracking in the plane of the bar leading tospalling of the concrete cover.

Fly Ash, when used properly in a concrete

mixture, will substantially delay this

distress


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Like Death and Taxes Concrete will crack

Beeby (7) quotes results from exposure tests donemainly in Germany to arrive at the conclusion thatthe crack width has no significant influence onthe amounts of corrosion that will occur during the

life of the structures.Beeby goes on to state:

a crack perpendicular to the line of the bar does not

constitute a corrosion risk in practice, while theexistence of longitudinal crack may.


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Cracking of reinforced concrete members

Step 1. Flexural cracks form in properly designedmembers.

Step 2. Cracks act as portals for aggressive agentsto reach the reinforcement.

Step 3. Micro corrosion takes place on the exposedsteel at the crack and more damaginglyalong the steel/concrete interface adjacent tothe crack faces.

Step 4. Corrosion products that form on thisinterface cause cracking to form in the planeof the reinforcing bars and spalling ensues.


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Francois and Aaliguie (8) mapped the progressive

carbonation and chloride ingress in a simplysupported beam subjected to a load at mid-span.The aggressive agents can be seen to move alongthe crack and along the reinforcement where the

crack intersected a bar. The following figuresshow the appearance of the carbonated area andthe area contaminated with chlorides that is atright angles to the crack with carbonation

spreading in the concrete adjacent to the steel fromthe point of convergence of the crack and alongthe longitudinal reinforcement under stress.


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Carbonated concrete zone in front

of a crack Ref. 8


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Shape of the chloride contaminated

concrete zone in front of crack (Ref. 8)


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Francois and Aaliguie (8) also showed that

there is about a 30% increase in chloridecontent on the tensile surface as comparedto the compressive surface of a beam and

this increases to1

00% at a depth of 35 mmfrom the surface of a flexural member.Carbonation depth is influenced in a similarmanner, as can be seen in the following

figure.


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Extent of carbonation at support (end ofbeam section) and at mid-span (middlesection (Ref. 8)


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Test # 1 Cracked Concrete

Montes (9) tested a series of 27 slabs thatwere cast with and without fly ash and withtwo 15 x 260- mm long reinforcing barsintersecting either a construction joint or

preformed cracks that were 0.25 and 0.50mm wide. The reinforcing bars had a coverof 20 mm, and a 12.5 mm maximum size

coarse aggregate was used. The cementwas Type I normal portland with 8% silicafume.


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Test # 1 Cracked Concrete (contd)

After 7 days moist curing and 21 days dryingin laboratory air the concrete was subjectedto two cycles per day at 26C in the wet

cycle and 55C in the dry cycles. Also aseries of 54 slabs were cast and placedslightly below high tide level at TreatIsland, Maine. The results are as follows:


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Corrosion current density at 12 months in A/cm2

- LaboratoryTests (averaged over the bar)

Fly AshReplacement %

w/c + Fly Ash0.29 0.37 0.45

With 0.0 mm Preformed Crack Construction Joint

0

20

40

0.5

0.1

0.1

0.1

0.4

0.1

1.2

0.8

0.3With 0.25 mm Preformed crack

0

20

40

1.3

0.8

*

1.0

1.2

0.6

1.8

1.7

1.4

With 0.50 mm Preformed crack

0

20

40

2.4

*

*

2.1

1.7

1.0

3.3

2.8

2.4

* Improperly Consolidated Concrete


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Corrosion current density at 2 years at Treat Island

A/cm2

Fly Ash

Replacement %

w/c + Fly Ash

0.29 0.37 0.45

With 0.0 mm Preformed Crack Construction Joint

0

2040

0.2

0.20.1

0.2

0.30.2

0.4

0.30.3

With 0.25 mm Preformed Crack

0

20

40

0.5

0.4

0.4

0.6

0.5

0.5

0.7

0.7

0.9

With 0.50 mm Preformed Crack

0

20

40

1.0

1.1

0.8

1.5

0.4

1.4

1.1

1.8

1.8


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Number of bars with longitudinal cracks caused by

corrosion at transverse cracks in 0.45 W/C + Fly Ash

specimens Laboratory testing

Fly Ash % Months AcceleratedTesting

8 10 12

0

20

40

1

1

0

2

1

0

2

2

1

Note:

Specimens with w/c + Fly Ash of 0.29 and 0.37 had no longitudinalcracks.


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At 12 months, all of the specimens

subjected to accelerated testing werebroken open to measure areas ofcorrosion and amount and depth of

corrosion pitting. Fly Ash appears tohave no influence on these parameters.Pitting depths ranged from 0.2 to 1.7

mm and areas of corrosion ranged from0.4 to 91.1 cm2.


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Specimens with deep pitting had

generally smaller areas of corrosion.Testing of specimens at Treat IslandMarine Exposure Site will continue,

and in time should provide morespecific information.


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Corrosion spreading along the reinforcing bar at

either side of the crack appears to be caused bycrevice corrosion. Corrosion occurs at creviceswhere oxygen and chlorine ion concentration islowest. Perhaps this type of concentration cell

arises because the mobility of iron (ionic radius of0.74 ) is greater than that of the oxygen (1.40 )and chlorine (1.81 ). Fly Ash is very effective inclosing the coarser pores but may not be as

effective in closing the finer pores that provide aconduit for iron in ionic form.


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Conclusion

Fly Ash concrete, if properly proportioned,placed and cured, makes a substantialimprovement in enhancing the protection ofembedded reinforcing steel from corrosive

agents.

It is also effective in reducing the longitudinal

cracks that form as a result of crevicecorrosion that develops at transvers flexuralcracks.


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Crevice Corrosion

Oxygen in the liquid which is deep in thecrevice is consumed by reaction with themetal. Oxygen content of liquid at the

mouth of the crevice which is exposed tothe air is greater, so a local cell develops inwhich the anode, or area being attacked, isthe surface in contact with the oxygen-

depleted liquid (from Corrosion BasicsNACE).


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Crevice Corrosion of Rebar Has Some

Similarities with Filiform Corrosion

The head of the advancing filament becomesanodic, with a low pH and a lack of oxygen,

as compared with the cathodic areaimmediately behind the head where oxygenis available through the semipermeablefilm. Corrosion proceeds as the cathodefollows behind the anodic head (fromCorrosion Basics NACE).


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Documents

INDIA - Influence of Fly Ash on the Corrosion...Jan2004