i

Draft Report on

Basic Understanding of Various Deterioration

Mechanisms in Concrete Structures: With

Emphasis on Rebar Corrosion

Prof. B. Bhattacharjee & Kamal Kant Jain

Department of Civil Engineering, IIT Delhi

Hauz Khas, New Delhi, India - 110016

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Table of Contents Table of Contents .........................................................................................................ii

List of Tables .............................................................................................................. iii

List of Figures............................................................................................................. iii

Chapter 1: Introduction ................................................................................................1

Chapter 2: Literature Review .......................................................................................6

2.1 Carbonation.........................................................................................................7

2.1.1 Effect of carbonation of concrete...............................................................8

2.1.2 Primary factors influencing carbonation..................................................9

2.2 Chloride Ingress ................................................................................................11

2.2.1 Chloride penetration in concrete .............................................................11

2.2.2 Primary factors affecting the chloride ingress .......................................13

2.3 Corrosion ...........................................................................................................16

2.3.1 Free Energy consideration .......................................................................18

2.3.2 Corrosion Kinetics ....................................................................................20

2.3.3 Primary factors affecting steel corrosion in concrete ............................23

2.3.4 Corrosion diagnosis and measurement methods ...................................25

Open circuit potential (OCP) measurements: .................................................25

Concrete resistivity measurement:...................................................................26

Linear polarization resistance (LPR) measurement:.......................................28

Tafel Extrapolation:.........................................................................................30

Electrochemical impedance spectroscopy (EIS): ............................................31

2.3.5 Damages due to corrosion ........................................................................33

Common damages in RC Structures: ...............................................................33

Stress corrosion cracking (SCC): ....................................................................34

Hydrogen embrittlement (HE): ........................................................................35

2.3.6 Corrosion control techniques...................................................................36

Patch Repair: ...................................................................................................36

Cathodic Protection:........................................................................................37

Electrochemical chloride extraction (ECE) and Realkalization: ....................40

Inhibitor Application: ......................................................................................41

Coatings:..........................................................................................................41

Design Considerations: ...................................................................................42

iii

2.4 Alkali aggregate reactions (AAR).....................................................................46

2.5 Acid Attack ........................................................................................................49

2.6 Sulphate attack..................................................................................................51

2.7 Freeze - Thaw damage ......................................................................................53

2.8 Fatigue, Shrinkage and Creep..........................................................................57

2.8.1 Fatigue........................................................................................................57

2.8.2 Shrinkage ...................................................................................................58

2.8.3 Creep...........................................................................................................59

List of Tables Table 2.2.1: Primary factors affecting chloride ingress...............................................14

Table 2.3.1: Electrode potentials for anodic and cathodic corrosion reactions ...........20

Table 2.3.2: Primary factors affecting steel corrosion in concrete ..............................25

Table 2.3.3: Corrosion condition related with HCP measurements [7].......................26

Table 2.3.4: Corrosion risk from resistivity.................................................................28

Table 2.3.5: Guidelines for rebar condition assessment using icorr ..............................31

Table 2.3.6: Relative volume ratios for various iron oxides in comparison to iron ....33

List of Figures Fig. 2.3.1: Reinforced concrete performance against corrosion deterioration.............16

Fig. 2.3.2: Schematic presentation of corrosion process in reinforced concrete .........17

Fig. 2.3.3: Cathodic and anodic over potentials due to polarization [5]......................21

Fig. 2.3.4: Open circuit potential (OCP) measurement [6]..........................................26

Fig. 2.3.5: Circuit for electrical resistance measurements ...........................................27

Fig. 2.3.6: Guard ring test set-up .................................................................................29

Fig. 2.3.7: Schematic polarization curves showing Tafel extrapolation [14] ..............30

Fig. 2.3.8: Equivalent circuit diagram for simple corrosion system [18] ....................32

Fig. 2.3.9: Schematic Nyquist Plot ..............................................................................32

Fig. 2.3.10: Schematic application diagram for CP using impressed current..............38

Fig. 2.3.11: Schematic application diagram for CP using sacrificial anodes ..............40

Fig. 2.3.12: Schematic diagram for application of ECE..............................................40

Fig. 2.4.1: Effect of AAR on concrete [2] ...................................................................47

Fig. 2.5.1: Effect of Acid Attack on concrete [3] ........................................................49

iv

Fig. 2.7.1: Types of freeze - thaw damage [5] .............................................................55

1

Chapter 1: Introduction Concrete is the most versatile material used in the construction industry worldwide.

Benefits such as mould ability, robustness, fire resistance, readily availability and

durability along with allowed variations in mix design to obtain desirable physical

properties suitable to various applications have made concrete ideal for use in bridge

construction. Concrete in bridges is used mainly in the form of reinforced or

prestressed concrete, mainly using steel for reinforcement purposes. This steel-

concrete system in general is highly durable and is used extensively.

But because of severe environment, exposure to harmful chemicals, unplanned higher

loads, inadequate materials selection and poor workmanship etc., Concretes ability to

perform well above its serviceability limits over planned design period i.e. durability

of concrete is adversely affected.

Under action of one or more of above mentioned detrimental factors, various

damaging mechanisms may occur in concrete bridges, of which the commonly

detected are:

Carbonation Chloride Ingress Corrosion of reinforcement/prestressing steel Alkali aggregate reactions Acid Attack Sulphate attack Freeze - Thaw damage Shrinkage, Fatigue and Creep

Carbonation of concrete occurs when carbon dioxide (CO2) from the atmosphere

reacts with calcium hydroxide (Ca(OH)2) in hydrated concrete in the presence of

moisture(H2O) and alkalis (Na+, K+ etc.) forming calcium carbonate (CaCO3). This

reaction causes reductions in the alkalinity of the concrete due to acidic nature of CO2 gas. Reduction in alkalinity impairs the natural passivating nature of concrete i.e.

capability of forming a protective oxide layer around steel reinforcement. Carbonation

starts on the surface and gradually penetrates the concrete, when carbonation front

2

reaches reinforcing or prestressing steel depassivation occurs and thereafter allows

corrosion to take place under presence of corroding substances.

Chloride ingress can be termed as a process in which chlorides from surroundings,

permeates through the porous concrete mainly by the means of diffusion and/or

absorption processes. The chloride ions attack the passive layer and therefore lead to

depassivation and corrosion initiation when present in sufficient concentration at rebar

level; this concentration level is commonly known as chloride threshold. Furthermore

chlorides available act as catalyst for corrosion process as they are not consumed in

the process and allow the corrosion process to proceed quickly. This is a great

concern in marine splash zones.

Corrosion in concrete occurs mainly because of chlorides and/or CO2 ingress, as they

depassivate the protective oxide layer formed around steel embedded in naturally

alkaline concrete. Corrosion can be defined concisely as loss of steel due to oxidation

of iron (electrochemical process) followed by reactions producing iron oxides,

resulting in production of high voluminous of rust.

Corrosion causes several problems and deteriorations in bridges which ultimately lead

to the premature failure with respect to designed service life. Most commonly

detected deteriorations are:

Reduction in effective diameter of reinforcement steel due to consumption of iron, leading to reduction in load carrying capacity

Rust stains on concrete surface impairing the aesthetics Production of high voluminous rust products which cause tension cracks or

spalling in surrounding concrete

In case of prestressed concrete bridges corrosion can lead to stress corrosion cracking as strands are typically under high tension

Hydrogen embrittlement in prestressing steel may take place which further can lead to brittle failure of system

Alkali- aggregate reaction (AAR) also called sometimes as alkali- silica reaction

(ASR), is chemical reaction that takes place between the alkali hydroxides in the

hydrated concrete and certain siliceous aggregates. The reaction product absorbs

3

water and in some situations expansion stresses sufficient to crack the concrete can be

developed in the concrete. The reaction is usually slow and it can take many years for

visual damage to develop under normal conditions.

Acid attack on concrete is promoted because of concretes alkaline nature and

reaction with acids may occur instantaneously at surface if concrete is exposed to acid

smoke, acid rain and aqueous solutions containing acids such as sulphuric, nitric,

hydrochloric and acetic acid, etc. This reaction mainly causes the dissolution of

Ca(OH)2 in presence of acids forming calcium salts. Therefore acid attack damages

concrete as components of the cement paste break down due to consumption of

Ca(OH)2 during contact with acids, leading to leaching and reduction in strength.

Presence of sulphuric acid is a highly damaging as it results in a combination of acid

attack as well as sulphate attack.

Sulphate attack on concrete can be defined as chemical reaction between the sulphate

(SO4-2) ions and hydrated calcium aluminates and/or the Ca(OH)2 components of

hardened concrete in the presence of water. The products resulting from these

reactions are calcium sulphoaluminate hydrate (Ettringite), and calcium sulphate

hydrate (Gypsum). These solids have a very high volume than the solid reactants and

as a consequence cause expansion, tensile stresses are produced that may result in

breakdown of the hardened paste and ultimately in breakdown of the concrete.

Freeze Thaw damage occurs in bridges when wet concrete is exposed to cycles of

freezing and thawing. The principle mechanism is a consequence of the expansiveness

of water as it freezes within the confined concrete pores. This is due to the significant

(around 9%) increase in volume as water turns to ice. This generates hydraulic

pressure in the pores, exposing the pore walls to stress that can cause severe

disruption to the concrete pore structure, ultimately leading to loss of durability due to

reduction in strength and development of cracks in concrete.

Cracking of concrete may also take place in bridges because of following factors:

Shrinkage (Volumetric decrease with time due to drying process of concrete) Fatigue (Material deterioration due to repeated variations of stress or strain) Creep (Time dependent deformation due to sustained load)

4

If concrete is subjected to one or more of the abovementioned damaging mechanisms;

their adverse affects make bridges more prone to development of various defects, of

which commonly detected are:

Change in microstructure Strength reduction Leaching/Staining Debonding of reinforcement Loss of prestress Crack development Spalling

So it can be commented and reasoned that synergetic effect of various damaging

mechanisms is responsible for occurrence of various defects as reported. And it can

therefore be reasoned that presence of the various deteriorating factors, will impair the

performance of bridge and so the ability to be functional without risk of failure till

end of the planned service life of the concrete bridge i.e. reduces age of the bridges.

But corrosion of steel reinforcement/prestressing is a single major concern that

primarily needs to be taken care of in concrete bridges as corrosion is almost

inevitable and has several severe consequences as mentioned earlier. Therefore to say

that even corrosion alone will extensively impair durability of concrete bridges would

not be an overstatement.

Damages caused due to corrosion call for unplanned repair works for further use of

the affected bridges for risk free functioning. This is a major economic issue

worldwide because of high repair costs. A study by FHWA/NACE in US estimated

direct cost of corrosion as $8.3 billion per year for bridges, over the next 10 years

from which $3.8 billion is for replacement purposes, $2.0 billion for maintenance of

existing decks, $2.0 billion for maintenance of existing substructure components and

$0.5 billion for painting bridges [1].

If the time at which the serviceability limit will be reached in a given RC structure

subjected to corrosion and other simultaneous damaging mechanisms, is reasonably

5

known then effective monitoring and maintenance systems can be used in time to

avoid sudden need for repair. It will help in cutting down the high costs for unplanned

repairs and will cause fewer disruptions to proper functioning of bridge over their

design life.

But to model durability and hence serviceability analytically in a deterministic

manner is a difficult task as durability is affected by various mechanisms discussed.

Therefore it will not only require good knowledge and understanding of various

streams of science and engineering such as: Concrete engineering, corrosion

engineering, fracture mechanics, solid mechanics etc.; but also tools capable to

combine these effects reasonably accurately. Likewise it will not be reasonable if

experimental results alone as obtained in laboratory or at some site are taken as

standards because of temporal and spatial variations in various parameters involved

that will alter the degradation mechanisms significantly.

Therefore it is much necessitated that, a model that includes knowledge of experts

from various fields having significant exposure to problem of durability of bridges

combined with experimental investigation, be used to provide a better insight about

the serviceability status of a given bridge structure. For this purpose, diagnosis of

extent of degradations can be made possible with judicious combination of fuzzy

mathematics which can transform experts knowledge from qualitative to quantitative

and models for reinforced/prestressed concrete behavior from experimental data

obtained using non destructive tests (NDT) and/or semi destructive tests (SDT). And

furthermore modeling prognosis will allow us to estimate the remaining service life of

a bridge structure.

Once the extent of various degradations and remaining service life of a bridge are

known appropriate measures can be implemented to improve or sustain the health of

the structure and a maintenance methodology can be developed.

References

1. Virmani Y. P., Payer J.H. and Koch G.H. et.al., Corrosion costs and preventive

strategies in the United States, FHWA Publication, No. FHWA-RD-01-156,

(2002).

6

Chapter 2: Literature Review As mentioned in introduction, concrete structures are usually subjected to various

combinations of deteriorating mechanisms which lead to degradation in performance

of structure. These degradations are inevitable in concrete structures and therefore

must be accounted for during design phase for realistic assessment of service life of

structure.

But various limitations such as, incomplete understanding on spatial and temporal

variations of degradation mechanisms, synergetic effects of various degradations etc.,

make service life assessment task complex to undertake at design stage. Same

concerns needs to be taken care of even if the service life assessment is carried out

after the signs of damage in the structure have been observed.

For Bridges the structural service life thus assessed is mandatory for development of

management systems. These systems will allow the stakeholders to optimally utilize

the resources and funds available, to minimize the maintenance and repair costs.

So there is a need to develop the basic understanding of various damage mechanisms

involved, existing service life assessment techniques and bridge management systems

already in use, to arrive at a realistic bridge management system. This then can

logically be adopted for the abovementioned purpose.

Literature review reported in following sections will helps us to develop the

understanding and will provide essential information on:

Commonly observed degradation mechanisms in concrete Service life assessment models in existence Existing Management systems

And furthermore shall provide with the course for the actions to be taken for

development of an efficient management system.

7

2.1 Carbonation

In concrete, under normal conditions, reinforcing steel does not corrode. As in the

highly alkaline environment of concrete, iron is oxidized to Fe2O3 / Fe3O4 / -FeOOH.

These oxides provide a tightly adhering passive oxide film (passive layer) on the

surface of the steel reinforcement, which limits the access of oxygen and moisture to

rebar and thus arrests corrosion. Drop in pH might cause depassivation of this film,

thus allowing corrosion to proceed [1, 2].

In moist environments, carbon dioxide CO2 present in the air, diffuses through pores

of concrete and forms an acid aqueous solution that can react with alkaline

constituents of concrete, mainly calcium hydroxide Ca(OH)2 formed during hydration

process of cement paste and tends to neutralize the alkalinity (Ca(OH)2 in the pore-

water of the concrete is the main reason for its alkalinity) of concrete, this

phenomenon commonly is termed as carbonation of concrete and is commonly

detected in concrete. The reactions in presence of alkalis and moisture, involved in

carbonation process in concrete can be written as [1, 3]:

CO2 + H2O H2CO3 (2.1.1)

H2CO3 + Ca(OH)2 CaCO3 + 2H2O (2.1.2)

This can be summed up as:

CO2 + Ca(OH) 2 CaCO3 + H2O (2.1.3)

In this process CO2 diffuses through the concrete pores and the rate of movement of

the carbonation front can be approximated reasonably using laws of diffusion, which

states that the rate of movement is inversely proportional to the distance from the

surface as presented in equation below [3]:

0Ddxdt x

= (2.1.4) Where D0 = CO2 diffusion coefficient

x = Distance from surface of concrete

This gives variation of CO2 penetration depth called commonly as carbonation depth

with respect to time. And the relation which is obtained by integration of Eqn 2.1.4

NaOHH2O

8

under the assumption that diffusion coefficient is constant in space (homogenous

concrete) and time can be written as following:

d = Kt (2.1.5)

Where d = Depth of carbonation (mm)

t = Time (yr)

K=Carbonation coefficient (mm/yr0.5)

K can be assumed as a measure of the rate of penetration of carbonation for given

concrete and environmental conditions. This relationship (Eqn 2.1.5) is widely

accepted for depth of CO2 penetration in concrete [1, 3-4].

2.1.1 Effect of carbonation of concrete

Carbonation does not cause any damage to the concrete itself, although it may cause

the concrete to shrink. Indeed, in the case of concrete obtained with Portland cement,

it may even reduce the porosity and lead to an increased strength [1]. However,

carbonation has important effects on corrosion of embedded steel. As main

consequence of carbonation is the drop in pH of the concrete pore solution to the pH

range of 8-9, hence approaching neutrality, which otherwise is alkaline normally

having the pH greater than 12.5 [1, 3]. This leads to depassivation of protective oxide

layer around steel reinforcement formed due to alkaline nature of concrete and thus

impairs the electrochemical protection of rebars against corrosion, and allow

reinforcement corrosion initiation, which is a big problem in bridges as will be

discussed in detail in following sections.

If chlorides are not present in concrete, the pore solution following carbonation is

composed of almost pure water. This means that the steel in humid carbonated

concrete corrodes as if it was in contact with water. Presence of chlorides in the

concrete, even in such small quantities that in themselves they would not give rise to

corrosion leads to a more damaging situation. As depassivation of reinforcement by

carbonation will allow the present chlorides to induce corrosion and thus damage the

structure. The presence of a small amount of chlorides in concrete may be due to the

use of raw materials (water, aggregates) containing these ions or to the penetration of

chlorides from the external environment (seawater, de-icing salts). The complete

corrosion process will be discussed in detail in following sections [1].

9

The carbonation reaction starts at the external surface and penetrates into the concrete

producing a low pH front. The measurement of the depth of carbonation is normally

carried out by spraying an alcoholic solution of phenolphthalein on a freshly broken

face. The areas where pH is greater than 9 take on a pinkish color typical of

phenolphthalein in basic environment, while the color in carbonated areas remains

unchanged [3-5].

2.1.2 Primary factors influencing carbonation

Relation obtained using Ficks law is an approximation as due to the carbonation

process, concrete pore structure is modified as carbonation proceeds. Deviations

prevail due to cracks, changes in concrete composition and moisture levels with

depth. Therefore rate of carbonation depends mainly on environmental factors and

properties of the concrete. Following factors have significant influence on rate at

which carbonation will take place in concrete and thus govern the depth of carbonated

concrete as presented in table 2.1.1:

Factor Effect on carbonation of concrete

CO2 Concentration

in surrounding air

As the CO2 content in the surrounding air increases, the carbonation rate increases

Permeability of

concrete

The diffusion process takes place through permeable pores in the concrete, therefore higher permeability leads to high

rate of carbonation

Degree of concrete

pore saturation

The rate of diffusion of CO2 consequently decreases with an increase in water content of the concrete until it becomes

zero in water-saturated concrete. As diffusion of CO2

within concrete is facilitated only through the aerated pores

The carbonation reaction occurs only in the presence of water therefore it becomes negligible in dry concrete

Wetting & drying

cycles

Carbonation rate and depth of carbonation in concrete may be variable even in different parts of a single structure. If

the structure is subjected to periodic cycles of wetting and

drying as it will affect the saturation level of concrete

significantly

10

Wetting of concrete occurs more rapidly than drying, leading to water saturation of concrete, so more frequent

although shorter periods of wetting cause more reduction in

the rate and depth of CO2 penetration than less frequent and

longer periods. This means that the wetting time, as well as

the frequency and duration of wettingdrying cycles are

important parameters

Cracks in concrete Cracks in concrete increase the carbonation rate manifolds

and it may provide direct access of CO2 at rebar level

Table 2.1.1: Primary factors influencing carbonation of concrete

References

1. Bertolini L., Elsener B., Pedeferri P. and Polder R., Corrosion of Steel in

Concrete: Prevention, Diagnosis, Repair, Wiley-VCH, Weinheim Publication

(2004).

2. Montemor M.F., Simoes A.M.P. and Ferreira M.G.S. Chloride-induced corrosion

on reinforcing steel: from the fundamentals to the monitoring techniques Cement

& Concrete Composites, Vol. 25, pp. 491502 (2003).

3. Broomfield John P., Corrosion of Steel in Concrete: Understanding, Investigation

and Repair, E&FN Spon. London (1997).

4. Pade C. and Guimaraes M. The CO2 uptake of concrete in a 100 year

perspective, Cement and Concrete Research, Vol. 37, pp. 13481356 (2007).

5. Jung W.Y., Yoon Y.S. and Sohn Y.M., Predicting the remaining service life of

land concrete by steel corrosion Cement and Concrete Research, Vol. 33, pp.

663677 (2003).

6. Apostolopoulos C.A. and Papadakis V.G., Consequences of steel corrosion on

the ductility properties of reinforcement bar Construction and Building

Materials, Article in press (2007).

11

2.2 Chloride Ingress

Chloride contamination of concrete is a frequent cause of corrosion of reinforcing

steel and leads to extensive corrosion damage after depassivation has taken place,

which can occur either due to carbonation and/or due to chlorides having sufficient

concentration at rebar level to cause depassivation. Chloride content in concrete that

alone might lead to depassivation of protective oxide layer formed around

reinforcement and further allowing corrosion to initiate, is commonly termed as

Threshold chloride content [1]. Depassivation in case of chloride ingress is assumed

to occur through formation of hydrochloric (HCl) acid which leads to reduction in

alkalinity of concrete. As chlorides draw the iron ions through the passive layer,

create rust, produce HCl acid and release the chloride ions again in the solution to

continue the attack and therefore chlorides act only as a catalyst for the corrosion

process to start [2,3].

2.2.1 Chloride penetration in concrete

The main reason for chloride contamination of concrete is penetration from the

environment. Chloride penetration in concrete from the environment is mainly a

diffusion process, which produces varying chloride content in the concrete

characterized by high chloride content near the external surface and decreasing

contents at greater depths. The studies worldwide have shown that, in general chloride

profile can be approximated using Ficks second law of diffusion under the following

assumptions [1, 4-6]:

Concentration of the Cl- ions at concrete surface, does not change with time Diffusion coefficient is independent of time and concrete is homogeneous, so

that diffusion coefficient does not vary through the thickness of the concrete

Concrete does not initially contain chlorides i.e. (C = 0 for x > 0 at t = 0).

These assumptions are rarely met in real structures. As surface concentration might be

subjected to changes based on exposure environment and concrete is not

homogeneous. Also chlorides may already be present in structures due to [7-8]:

Use contaminated mixing water and aggregates (for instance by using water, sand and gravel extracted from sea)

Addition of chloride based admixtures such as calcium chloride. Application of chloride based coatings or salts

12

Only in concrete completely and permanently saturated with water can chloride ions

penetrate by pure diffusion. In most situations, other transport mechanisms such as

permeation and convection due to difference in hydraulic pressure (moisture content)

and migration due to difference in electrical potential also contribute to chloride

penetration. For instance, when dry concrete comes into contact with salt water,

initially capillary suction of the chloride solution occurs. Furthermore, binding due to

adsorption or chemical reaction with constituents of the cement paste alters the

concentration of chloride ions in the pore solution [1]. Nevertheless Ficks second law

can be used for a good approximation, considering the total content of chlorides

usually, hence including bound chlorides and can be written as [4-6]:

2

. 2AppC CD

xt = (2.2.1)

This is an initial value boundary problem which on integration under assumed initial

value and boundary conditions gives us:

( ).

, 12s App

xC x t C erfD t

= (2.2.2)

Where C(x,t) = Total chloride content at time t at depth x from surface of the concrete

DApp. = Apparent diffusion coefficient for chloride (m2/s)

Cs = Surface chloride content (% by mass of cement or concrete

erf = Error Function: 2

0

2( )x terf x e dt

=

The apparent diffusion coefficient and the surface chloride content are calculated by

fitting the experimental data to Eqn. (2.2.2) and are often used to describe chloride

profile measured on real structures. DApp. is often used as the parameter that describes

the resistance of concrete to chloride penetration, when performances of different

materials are compared. The lower DApp is, the higher the resistance to chloride

penetration is. It should, however, be observed that, while the diffusion coefficient

obtained from pure (steady-state) diffusion tests can be considered as a property of the

concrete, the apparent diffusion coefficient obtained from real structures also depends

on other factors (such as the exposure conditions or the time of exposure). Therefore,

results obtained under particular conditions, especially during short-term laboratory

tests, may not be applicable to other exposure conditions [1].

13

In principle, if DApp and Cs are known and can be assumed to be constant in time, it is

possible to evaluate the evolution with time of the chloride profile in the concrete and

then to estimate the time t at which chloride threshold will be reached and corrosion

will initiate. Though in general DApp and Cs, cannot be assumed as constants in the

case of real structures where binding as well as processes other than diffusion take

place.

2.2.2 Primary factors affecting the chloride ingress

As the chloride penetration in concrete is controlled by various properties of concrete

and environment, we can say that chloride induced corrosion in RC structures will

also be significantly influenced by these parameters. Most of the concrete parameters

that affect carbonation process will in general also have the similar affect on the

chloride ingress process as both are diffusion process through pores of concrete. The

parameters that have significant effect are presented in table 2.2.1

Factor Effect on chloride ingress and induced corrosion

Threshold chloride

content

Lower threshold chloride content will lead to depassivation and initiation of steel corrosion in comparatively shorter time

The chloride threshold depends primarily on: pH of concrete, i. e. the concentration of OH- in the pores:

(Controls alkalinity of concrete)

Potential of the steel: (O2 availability on steel surface - High availability lowers the threshold)

Presence of voids at the steel/concrete interface: (Promotes local acidification, leading to pitting corrosion)

Permeability of

concrete

High permeability of concrete promotes chloride ingress by providing chlorides an easy access to rebar

Surface chloride

concentration

High surface chloride concentration will lead to high rates of ingress leading to an early depassivation, this controls the

chloride ingress in splash zones and in marine environments

Chloride binding

capacity of concrete

Presence of tricalcium-aluminate (C3A) governs the chloride binding capacity of concrete, as chlorides form tricalcium-

chloro-aluminate (Fridels salt) on reaction with C3A

14

So only a fraction of the total chlorides are available for corrosion initiation as bound chlorides can not participate

under normal conditions.

Thus chloride binding can delay the corrosion initiation due to higher threshold chloride content, as it will have to take into

account the effect of bound chlorides

Cracks in concrete Cracks in concrete will allow an easy access to chlorides at

rebar level and may lead to rapid depassivation

Wetting and drying

cycles

Ingress of chlorides in concrete takes place through chloride bearing solution. When wetting takes place, chlorides enter

inside the concrete through pores in the solution phase. When

drying takes place, chlorides remain deposited in concrete

pores. On re-wetting, these deposited chlorides further move

inside the concrete

Therefore chloride ingress proceeds in steps, when the structure is subjected to repeated cycles of wetting and drying.

Table 2.2.1: Primary factors affecting chloride ingress

References

1. Bertolini L., Elsener B., Pedeferri P. and Polder R., Corrosion of Steel in

Concrete: Prevention, Diagnosis, Repair, Wiley-VCH, Weinheim Publication

(2004).

2. West J. S., Larosche C. J., Koester B. D., Breen J. E. and Kreger M. E., State-of-

the-Art Report About Durability of Post-Tensioned Bridge Substructures,

Research Report 1405-1, Center for Transportation Research, Austin (1999).

3. Glass G. K., B. Reddy, and Clark L. A., Making reinforced concrete immune

from chloride corrosion, Proceedings of the Institution of Civil Engineers,

Construction Materials, Vol. 160(CM4), pp. 155164 (2007).

4. Chatterji, S., Bernhardsvej C., Frederiksberg C., On the applicability of Fick's

second law to chloride ion migration through portland cement concrete, Cement

and Concrete Research, Vol. 25(2), pp. 229-303 (1995).

15

5. Costa A. and Appleton J., Chloride penetration into concrete in marine

environment-Part I: Main parameters affecting chloride penetration, Materials

and Structures, Vol. 32, pp 252-259 (1999).

6. Costa A. and Appleton J., Chloride penetration into concrete in marine

environment- Part II: Prediction of long term chloride penetration, Materials and

Structures, Vol. 32, pp 354-359 (1999).

7. Broomfield John P., Corrosion of Steel in Concrete: Understanding, Investigation

and Repair, E&FN Spon. London (1997).

8. Glass G. K. and Buenfeld N. R., Chloride-induced corrosion of steel in

concrete, Prog. Struct. Engng. Mater., John Wiley & Sons, pp. 448 -458 (2000).

9. Siegwart M., Lyness J. F. and Cousins W., Advanced analysis of published data

on chloride binding in concrete, Magazine of Concrete Research, Vol. 55(1), pp.

4152 (2003).

16

2.3 Corrosion

Corrosion of reinforcement steel bars (rebars) in RC structure has drawn attention of

several researchers over the world because of the interesting inter disciplinary

mechanisms involved and magnitude of problems it causes.

Concrete is an alkaline material having its pH in range > 12.5 normally. Because of its

alkaline nature concrete provides as an excellent material for rebar protection against

corrosion through formation of an adhering passive oxide layer around rebar in

reinforced concrete. But under the action of depassivating agents mainly CO2 and/or

chloride ions, this protecting layer starts deteriorating. Consequent exposure to these

elements ultimately breaks the passive layer and allows corrosion initiation to take

place. Time taken for corrosion to initiate is called as initiation time [1, 2]. Once

initiated, corrosion propagates under supportive conditions i.e. availability of

chlorides, moisture and oxygen at rebar level and in process produces rust as end

product. Surrounding concrete not having enough pore space to occupy rust produced

is subjected to tensile stresses and this leads to crack development in concrete which

with time appear at surface, as concrete is a poor material in tension.

Crakes developed catalyze the corrosion process by providing easy access to

chlorides, CO2, O2 and moisture to rebar level. This process leads to spalling of

concrete and thus making structure susceptible to untimely failure earlier than its

design life as structure attains the serviceability limit earlier than planned. Time taken

after corrosion initiation to reach the defined serviceability limit is commonly termed

as propagation time [1].

Fig. 2.3.1: Reinforced concrete performance against corrosion deterioration

Failure

Time

Det

erio

ratio

n

Initiation Time

Serviceability Limit

Propagation Time

17

In simple words rebar corrosion can be defined as, An electrochemical process in

which iron (Fe) is oxidized to ferrous ions (Fe++), these ions react with hydroxyl ions

(OH-) available because of oxygen (O2) reduction in presence of water (H2O), to

produce ferrous hydroxide (Fe(OH)2). Thus formed Fe(OH)2 reacts with available O2

and H2O to produce ferric hydroxide (Fe(OH)3), which constitutes high voluminous,

granular hydrated red rust (FeO(OH)+H2O). Reactions involved are as followed [3]:

Fe Fe++ + 2 e - (2.3.1)

O2 + 2 H2O + 4 e - 4 OH - (2.3.2) Fe++ + 2 OH - Fe(OH)2 (2.3.3) 4 Fe(OH)2 + O2 + 2 H2O 4 Fe(OH)3 (2.3.4)

In this process oxidation of metal [Eqn. 2.3.1] occurs at anode (Negative pole) while

reduction [Eqn. 2.3.2] takes place at cathode. Electrons flow from anode to cathode in

metal and thus current flows from cathode to anode in metal. Negative OH- ions flow

from cathode to anode in concrete pore water (electrolyte) and so current flows from

anodic area to cathodic area in concrete. Similarly positive metal ions diffuse from

metal to concrete at anode while electron flows from metal to concrete at cathode.

Fig. 2.3.2: Schematic presentation of corrosion process in reinforced concrete

Now as mentioned that corrosion is an electrochemical process represented by

reactions [2.3.12.3.4], and therefore the question whether corrosion will take place or

not, is governed by thermodynamics of the reactions involved as it does in case of any

other chemical reaction. Corrosion thermodynamics can be understood by considering

the change in free energy of the corrosion system [4].

O2 2 H2O 4 e 4 OH

Fe Fe++ + 2 e

Rebar Iron dissolution after depassivation

High Voluminous Red Rust - Fe(OH)3

Cl - & CO2 Anode Passive layer CathodeConcrete with pore water as electrolyte

18

2.3.1 Free Energy consideration

Free energy change (G) of any reaction is a measure of driving force for a reaction

and is represented by difference in free energies of formation of products and

reactants i.e.

G = Gproducts - Greactants (2.3.5)

For electrochemical (cell) reactions change in free energy can be related with the

electrode (cell) potential in following manner [3-5]:

G = - nFE0 (2.3.6)

Where G =Change in free energy

n = No. of electrons involved in reaction

E0 = Standard electrode (cell) potential with reference to S.H.E.

If value of G is negative (or Positive E0) for a reaction then reaction will take place

spontaneously as written, otherwise may take place in reversible direction if G is

positive (or negative E0) and reaction equilibrium is represented by G or E0 equals

zero [4, 5]. For corrosion cell; electrode (cell) potential is the difference between

cathodic and anodic half cell potentials, which consequently is equal to the difference

in electrode potentials for oxygen reduction at cathode (Eqn. [2.3.2]) and iron

oxidation at anode (Eqn. [2.3.1]) i.e.

E0Corr = E0Cathode - E0Anode (2.3.7)

Also we can say from discussion above on corrosion process that:

Esta < Estc (Electrons move from anode to cathode in steel)

Econa > Econc (OH- ions move from anode to cathode in concrete)

Esta - Econa < Estc - Econc (Considering both above inequalities together)

Also we can write for anodic and cathodic half cells that:

Anode Half Cell Potential = Ea = Esta - Econa (2.3.8)

Cathode Half Cell Potential = Ec = Estc - Econc (2.3.9)

Where Esta = Potential of steel at anode

Estc = Potential of steel at cathode

Econa = Potential of concrete at anode

Econc = Potential of concrete at cathode

19

Values for these half cell potentials are available in literature and are equal to 1.229

volts and -0.440 volts respectively for cathodic and anodic cells at 25 oC with respect

to Standard Hydrogen Electrode (SHE) [3]. It explains that standard cell potential is

positive for corrosion cell. Therefore corrosion reactions will occur under conducive

environment i.e. after depassivation of protective oxide layer under standard

conditions, because corrosion cell potential E0 is positive and hence value of G is

negative, leading to a spontaneous reaction. Another tool for predicting the corrosion

reactions is the Pourbaix diagram, which deals with the thermodynamic equilibrium

of reactions involved and demonstrates a qualitative picture of what can happen at a

given pH and potential. The Pourbaix diagram illustrate the relations of the potential

established between a pure metal (electrode) and its ions in solution (electrolyte), and

the pH of the electrolyte; indicating which pH-potential conditions might lead to

corrosion. In reinforced concrete, water acts as electrolyte while iron rebars serve as

electrode [4].

Cell (Electrode) Potentials for various reactions involved in corrosion process can be

obtained using Nernst equation [3, 5]. It represents variation of cell potential from

standard potential values depending on absolute temperature and rate of electrode

reactions involved and can be written as:

0 ln oxidred

QRTE EzF Q

= + (2.3.10)

Where E = Electrode potential

E0 = Standard electrode potential

R = Gas constant = 8.314 J/oK

T = Absolute temperature (oK)

z = No. of electrons involved in reaction at electrode

F = Faradays Const. = 96500 (Coulombs) Qoxid / Qred = (Rate of oxidation reaction) / (Rate of reduction reaction)

Rate of electrode reactions i.e. of oxidation and reduction depends on activity (a) of

various species participating in reactions. But activity for solvent can be written as

mole fraction of solvent in the liquid which in corrosion reactions is water having

mole fraction equals unity. Activity for dissolved matters in diluted solutions can be

20

represented using molar concentration of dissolved species in mol/l and for solid

matter and pure liquids varies very little and can be taken as unity [5]. Using these

concepts electrode potentials for reactions [2.3.1 2.3.2] can be obtained as presented

in table 2.3.1.

Reaction E0(Volts) Nernst Equation for Elect. Poten. (E)

Fe Fe++ + 2e - - 0.440 E0 + (RT/2F)*ln([Fe++] / [Fe]) O2 + 2H2O + 4e - 4OH - 1.229 E0 + (RT/4F)*ln([O2][H2O]2 / [OH-])

Table 2.3.1: Electrode potentials for anodic and cathodic corrosion reactions

Rebar corrosion in concrete once initiated is then propagated, in process deteriorating

the RC structure. Question on what controls the rate of corrosion reaction can be

answered with the help of theory of kinetics, which explains rate of reactions.

2.3.2 Corrosion Kinetics

For corrosion to occur, two electrochemical reactions, an oxidation and a reduction

reaction, must proceed at the same time and at the same rate to preserve the electro-

neutrality of the system, leading to equilibrium. But potential of half cell being

dependent on the standard redox potential, concentration of reactants and temperature;

will change as reactions take place on the electrode. So usually, the electrode

potential for half cell reactions shifts away from equilibrium, as a result of the net cell

reaction occurring, i.e. a net electric current flowing through the interface between

metal and electrolyte, and retards reactions at electrode [5].

The deviation from equilibrium is called polarization, and we say that the electrode is

polarized. A simple measure of polarization is the overvoltage (), i.e. the difference

between the equilibrium electrode potential and the cell potential. When corrosion

process takes place on a surface, the electrode potential adopts the cell potential value,

which is somewhere between the equilibrium potential of the cathodic and anodic

reactions leading to polarization, which slows down the reaction at electrode. The

polarization can briefly be categorized as following, based on the source of origin:

21

Ea

Ecell

Ec

Cathode Over potential (c) < 0

Anode Over potential (a)> 0

Activation polarization: Polarization due to kinetic hindrance of the rate controlling step of electrode reaction

Concentration polarization: Polarization due to change in concentration of electrolyte in vicinity of electrode

Ohmic (Resistance) polarization: Polarization due to resistance of the electrolyte and of any films/material surrounding the electrode surface

Fig. 2.3.3: Cathodic and anodic over potentials due to polarization [5]

There exists an energy barrier that the species involved in a reaction (atoms/ ions)

have to overcome to be transferred to a new state. The rate-determining step may be

ion or electron transfer across the interface or some kind of conversion of a species

involved in the reaction. Activation polarization is caused by the resistance against

these reactions itself at the metalelectrolyte interface. For activation polarization the

relationship between current density, i and overvoltage, is given by the Tafel

equation, which states that in absence of concentration and ohmic polarization, the

activation polarization is proportional to logarithm of current density [5]. Tafels

equation for activation over potential is given as:

2.303* * lncorr

RT izF i

=

(2.3.11)

Where, = Over voltage

= 2.303/z = Transfer coefficient

icor = Corrosion current density

This can further be classified as:

a = a*ln(ia/icor) = Ea Ecell

c = c*ln(ic/icor) = Ecell - Ec

22

In cases, where activation polarization dominates the mass or charge transfer across

the interface is rate determining. On the other hand, the mass transport within the

solution may be rate determining, due to dominance of concentration polarization.

This implies either there is a shortage of reactants at the electrode surface or that an

accumulation of reaction products occurs. Effect on over potential due to

concentration polarization can be obtained using the Nernst equation, which takes in

to account the effect of concentration of participating species in potential calculation.

This can be explained using the effect of oxygen availability during corrosion process,

which is a major rate determining factor as will be discussed later. Where cathode

over potential due to change in concentration of oxygen (conc.) can be calculated as

mentioned below [5]:

. * lnf

conci

CRTzF C

= (2.3.12)

Where Cf - Ci = Change in oxygen concentration

Also the resistance of system (R) to current flow due to corrosion process (I);

significantly affects the over potential in some cases. This resistance may be due to

surface layers on metals (passive layer) and materials surrounding (concrete) and it

leads to change in over potential. This phenomenon is termed as resistance

polarization and the change in over potential due to resistance (res.) is given by:

res. = RI (2.3.13)

Now rate of any reaction can be defined as rate of consumption of involved reactant

species or production rate of end products. Which for electrochemical (ionic)

reactions involved in corrosion process can be obtained using Faradays law which

gives us:

r = I / (z*F) (2.3.14)

Where: r = rate at which reactants are consumed [mol/s]

z = Valancy of participant in reaction

I = Current

F = Faradays Const. = 96500 C/moles

So we can say the rate of corrosion process will depend on the equilibrium cell

potential (corrosion potential Ecorr) and corresponding current flowing in the system

(corrosion current Icorr) in case of steel corrosion in concrete. This rate i.e. corrosion

23

current is a direct and simple measure for two main deteriorating effects of corrosion

i.e.; Amount of iron consumed in the corrosion reaction and corresponding amount of

rust produced. Commonly the rate is expressed in corrosion current per unit area i.e.

current density. Higher the rate is more damaging it will be to reinforced concrete

structures as it will cause more damage to reinforcement by consuming more iron in

even smaller time. But due to various controlling parameters the overall process rate

is not as simple to model as it looks. Some parameters that have significant influence

on corrosion rate are briefly discussed in following section along with their influence

on overall corrosion process.

2.3.3 Primary factors affecting steel corrosion in concrete

It is well accepted that if mixed properly, concrete provides as an excellent material

for construction and also protects steel from corrosion attack as long as passivation is

not destroyed due to action of harmful agents such as CO2 and chlorides. But poor

construction practices, exposure to severe environments and harsh working conditions

cause several damages in concrete. Steel corrosion is the most common and

extensively studied problem that occurs in reinforced/prestressed concrete structures

including bridges, due to the scale of problems it causes. Steel corrosion in concrete is

controlled by various environmental and concrete parameters that will alter the rate of

corrosion reactions significantly. Table 2.3.2 presents primary factors that control

corrosion process in concrete.

Factor Effect on Corrosion in Concrete

Cover Thickness

Increase in cover thickness improves performance of concrete against corrosion as longer time is required for

Chlorides / CO2 to reach at rebar level

Permeability of concrete Less permeable concrete reduces rate of diffusion of Cl-/

CO2 in concrete, thus delaying the corrosion initiation time

Resistivity of concrete

High resistivity of concrete impedes the flow of charge through concrete, therefore causing significant reduction in

corrosion current and hence rate of corrosion is controlled

Oxygen availability As can be observed that O2 is required both at anode and

cathode for corrosion reactions to take place and production

24

of rust [Eqn 2.3.2-2.3.4]. Therefore oxygen availability may

significantly alter the rate and end products in corrosion

reactions and therefore the affect on the structure will vary

Black rust (Fe3O4) forms in case of restricted O2 supply while red rust (Fe(OH)3) is commonly produced otherwise

Degree of pore

saturation in concrete

Concrete pore saturation plays an important role as it controls various factors affecting corrosion such as:

Diffusion rates of Cl- and CO2 ingress Resistance of concrete: Increase in saturation leads to

decrease in resistivity

Corrosion can occur only in partially saturated conditions as H2O and O2 both are must for corrosion

Chloride ion content

Chloride ion content depends on concrete constituents, exposure conditions and controls the corrosion initiation

time, as depassivation will occur when concrete Cl- content

exceeds the threshold chloride content.

pH of concrete pore

water

pH of concrete pore water is critical as the depassivation can only take place when pH falls in range of 9. Therefore

it will require more time and more amount of CO2 and/or

Cl- to depassivate the oxide layer, in a concrete having high

pH pore water, thus delaying the initiation of corrosion

Carbonation

Affects pH of the pore water significantly following the acidic nature of CO2, which neutralizes alkalinity of

concrete and thus plays an important role in depassivation

and corrosion initiation time.

Cracks in concrete

More the cracks more the corrosion and vice versa, as cracks will allow an easy access to CO2 and/or chlorides

which consequently will accelerate the corrosion process

and therefore rust production, which further will cause

formation of more cracks due to high volume of rust.

Stray currents

Stray current often arising in railway structures where electric lines are used, can induces pitting corrosion on

buried metal structures, leading to severe localized damage

25

Wetting and drying

cycles

Corrosion is dependent on moisture content (pore saturation) in concrete and so is significantly influenced by

the duration and frequency of wetting and drying cycles

Table 2.3.2: Primary factors affecting steel corrosion in concrete

2.3.4 Corrosion diagnosis and measurement methods

The detection and measurement of corrosion in concrete structures is essential.

Although there are several methods for the diagnosis, detection and measurement of

corrosion in reinforcing steel, there is no consensus regarding which method assesses

corrosion levels in reinforced concrete structures most accurately. Various techniques

used for detection and/or measurement of corrosion provide important information on

the causes, detection or rate of reinforcement corrosion in concrete. These techniques

are mainly based on the electrochemical aspects of corrosion process. Following are

the commonly used techniques for corrosion detection or measurement.

Open circuit potential (OCP) measurements:

The main method of detection of corrosion is the half-cell potential (HCP)

measurements and it can easily be obtained using open circuit potential (OCP)

measurement techniques. This is the most typical procedure to the routine inspection

of reinforced concrete structures.

The tendency of any metal to react with an environment is indicated by the potential it

develops in contact with the environment. In reinforced concrete structures, concrete

pore water acts as an electrolyte (environment) and the reinforcement will develop a

potential depending on the rate at which iron dissolves (controls corrosion rate) and it

may vary from place to place. This potential can be obtained using OCP measurement

technique, the schematic diagram for which is shown in Fig. 2.3.4 [6]. The principle

involved in this technique is essentially measurement of corrosion potential of rebar

with respect to a standard reference electrode, such as saturated calomel electrode

(SCE), copper/copper sulfate electrode (CSE) etc.. As per ASTM standard C876 [7],

the probability of reinforcement corrosion is as follows in Table 2.3.3.

26

Fig. 2.3.4: Open circuit potential (OCP) measurement [6]

Open circuit potential (OCP) values (mV)

w.r.t. SCE w.r.t. CSE

Probability of Reinforcement

Corrosion

OCP < - 276 OCP < - 350 High (> 90%) risk of corrosion

-275 < OCP < - 126 -350 < OCP < - 200 Uncertain (May be taken as 50%)

OCP > -125 OCP > -200 Low (

27

compared to concrete with low resistivity in which the current can easily pass through

concrete between anodic and cathodic areas.

Two different techniques, namely AC and DC measurements are used for

determination of electrical resistivity. In these measurements both surface and

embedded probes are applied. Applying a constant electric field between the two

embedded electrodes and measuring the resulting current as a voltage drop over a

small resistance accomplish the DC measurements.

The AC measurements can be conducted both by means of two and four-pin methods.

The most common surface mounted probe is known as the Wenner array, which uses

four probes. An alternating current (AC) is passed between the outer electrodes and

the potential between the inner electrodes is measured. Concrete resistivity is usually

measured by using the Wenner four probe method [6]. Schematic setup for the

resistivity measurement is shown in Fig 2.3.5.

Fig. 2.3.5: Circuit for electrical resistance measurements

A known current `I' is impressed on the outer probes and the resulting potential drop

`V' between the inner probes is measured and resistance `R' is given by V/I. and then

resistivity of concrete () can be calculated as following:

= 2aR (2.3.15)

Where a = inner electrode distance in cm

R = measured resistance in ohm

The resistivity measurement is a useful additional measurement to aid in identifying

problem areas or confirming concerns about poor quality concrete in context of

Probes Sponges Concrete Rebar

aaa

V

R = V/I = 2aR

A

28

corrosion and is being increasingly used to evaluate concretes corrosion

characteristics. Measurements can only be considered along side other measurements

as reinforcing bars will interfere with resistivity measurements. It has been established

that the likelihood of corrosion in concrete is inversely proportional to the electrical

resistivity of concrete [8]. It has also been shown that for concrete, corrosion

performance and resistivity can be related as follows in Table 2.3.4 [6, 9, 10].

Resistivity (.cm) Corrosion risk

> 20,000 Negligible

10,000 - 20,000 Low

5,000 - 10,000 High

0 < 5,000 Very high

Table 2.3.4: Corrosion risk from resistivity

Linear polarization resistance (LPR) measurement:

LPR monitoring has been developed to address the need of a non-destructive

technique, which enables accurate assessment of the condition of corroded reinforced

concrete structures and has become a well-established method of determining the

instantaneous corrosion rate measurement of reinforcing steel in concrete. The

technique is rapid and non-intrusive, requiring only localized damage to the concrete

cover to enable an electrical connection to be made to the reinforcing steel. In LPR

measurements the reinforcing steel is perturbed by a small amount from its

equilibrium potential. This can be accomplished potentiostatically by changing the

potential of the reinforcing steel by a fixed amount E, and monitoring the current

decay I, after a fixed time. Alternatively it can be done galvanostatically by applying

a small fixed current I, to the reinforcing steel and monitoring the potential change

E, after a fixed time period. In each case the conditions are selected such that the

change in potential E, falls within the linear SternGeary range of 1030 mV [6].

The polarization resistance Rp, of the steel is then calculated using following equation:

Rp = E / I (2.3.16)

From which the corrosion rate Icorr, can then be calculated using

Icorr = B / Rp (2.3.17)

29

Where, B is the SternGeary constant. For which a value of 26 mV is adopted in case

of active steel and 52 mV in case of passive steel [11-13]. Also B can be obtained

using Tafels slopes for anodic and cathodic polarization curves i.e. a and c

respectively using the following equation:

2.303( )a c

a c

B

= + (2.3.18)

In order to determine the corrosion current density, icorr, the surface area A, of

polarized steel needs to be accurately known and then:

icorr =Icorr / A (2.3.19)

In a conventional LPR test the perturbation is applied from an auxiliary counter

electrode on the concrete surface. The surface area of steel assumed to be polarized is

that lying directly beneath the auxiliary electrode. However the current flowing from

the auxiliary electrode is unconfined and can spread laterally over an unknown, larger

area of steel. This can lead to inaccurate knowledge of the surface area of steel

polarized and will therefore result in an error in the calculation of the corrosion

current density. In order to overcome the problem of confining the current to a

predetermined area, a secondary auxiliary guard ring electrode surrounding the inner

auxiliary electrode is commonly used. The principle is that the confinement current is

maintained by the outer guard ring electrode during LPR measurement. This

confinement current prevents the perturbation current from central auxiliary electrode

spreading beyond a known area [6]. Fig 2.3.6 shows the schematic set-up arrangement

of LPR measurement using guard ring technique.

Fig. 2.3.6: Guard ring test set-up

Auxiliary electrode Reference electrode

Guard ring electrode

Sensor electrodes

Working electrode

30

Tafel Extrapolation:

The Tafel extrapolation technique (TP) is another electrochemical method for

calculating corrosion rate based on the intensity of the corrosion current (Icorr) and the

Tafel slopes. Tafel slopes also could be used to calculate corrosion rate with LPR as

mentioned earlier. This technique is also based upon application of either steady fixed

levels of current, followed by monitoring of the potential (galvanostatic) or

application of specific potential followed by monitoring of the current (potentiostatic).

The main difference between these two methods is that the change in potential must

be kept within Stern- Geary range i.e. less than 30 mV for the LPR technique, while

the change of potential can go up to 250 mV for the Tafel extrapolation technique.

Fig. 2.3.7: Schematic polarization curves showing Tafel extrapolation [14]

In TP, corrosion rate can be calculated using straightforward substitution of Tafel

slope values (a and c) to get the corrosion current then, by calculating corrosion rate

using Eq.2.3.20 as following [6, 14]:

{ } { }1 2exp ( ) exp ( )corr corr corrI I S E E S E E = (2.3.20) Where S1 = 2.303 / a ; S2 = 2.303 / c

E = Potential at timet

I = Current at timet

Icorr = Corrosion current (A)

Ecorr = Corrosion potential (V)

31

The corrosion current density can then be obtained by using the exposed surface area

(A) of steel, using Eq. 2.3.19 as mentioned earlier. Main disadvantage of using Tafels

extrapolation technique is that it is an electrochemically destructive technique, as

application of external activation (voltage or current) may alter the electrochemical

behavior of the system significantly.

Rebar condition can be interpreted on the basis of corrosion current density, and

following guidelines have been suggested and are widely accepted for the rebar

condition assessment [15, 16].

Corrosion Current Density (icorr)

A/cm2 Rebar Corrosion Condition

icorr < 0.1 Negligible

0.1 < icorr < 0.5 Low to moderate corrosion

0.5 < icorr < 1.0 Moderate to high corrosion

icorr > 1.0 High corrosion rate

Table 2.3.5: Guidelines for rebar condition assessment using icorr

Electrochemical impedance spectroscopy (EIS):

Electrochemical impedance spectroscopy (EIS) is a technique that works in the

frequency domain. The basic concept involved in EIS is that an electrochemical

interface can be viewed as a combination of passive electrical circuit elements, i.e.,

resistance, capacitance and inductance [17]. This technique is commonly known by

the name AC impedance technique and is a useful non-destructive technique for

quantifying corrosion of steel rebars embedded in concrete. In this technique an

alternating voltage of about 10-20 mV is applied to the rebar and the resultant current

and phase angle are measured for various frequencies. The response to an A.C. input

is a complex impedance Z, which is the ratio of A.C. voltage to A.C current and has

both real (resistive) and imaginary (capacitive or inductive) components Z and Z

respectively. For a simple corrosion system equivalent circuit can be drawn as

presented in Fig. 2.3.8 and then equivalent complex impedance of the system can be

written as:

32

. 1p

seqp

RR

CRZ = + + (2.3.21)

Where Rs = Ohmic resistance of the cover concrete

Rp = Polarization resistance of system

C = Capacitance of the system

= Operating frequency

1/C = Impedance of capacitor

Fig. 2.3.8: Equivalent circuit diagram for simple corrosion system [18]

Eq. 2.3.21 on simplification gives real and imaginary parts of impedance i.e.

( )21p

s

p

RZ R

CR= + + (2.3.22)

( )2

2"1

p

p

CRZ

CR

= + (2.3.23)

On simplification (elimination of ) above relations give us equation of a circle.

Plotting the imaginary impedance against the real impedance gives a semicircle, with

a diameter equal to Rp with center at Rs + 0.5*Rp, these plots are commonly known as

Nyquist plots and can be presented as shown in Fig.2.3.9.

Fig. 2.3.9: Schematic Nyquist Plot

Rs

C

Rp

max

Z Rs

Z

Rp

Simple Corrosion System

33

At the highest point on the semicircle the frequency can be found by interpolation

and then double-layer capacitance value is then given by Eq. 2.3.24 as following:

max

1CpR= (2.3.24)

In practice, an AC Impedance response will often be a combination of several

different semicircles, due to different RC parallel components, which could arise from

film resistance, diffusional impedance etc.[17]. The A.C. impedance technique has the

advantage that it can give more information than DC LPR measurements, but it can be

very time-consuming due to wide range of frequencies it needs to be performed upon

to get the Nyquist plot. Therefore its use has been generally confined to the laboratory

rather than on structures in the field [6].

2.3.5 Damages due to corrosion

Corrosion in RC structure is most commonly observed damaging mechanism. As in

case of corrosion rebar is oxidized to produce high voluminous rust. The volume of

rust produced depends on the level of oxidation (thus on corrosion products formed)

and can be 2-6 times more than that of steel depending on the constituents. Typical

oxides formed during oxidation have the following volume ratios relative to the

volume of the iron have been reported by various researchers and are presented in

table 2.3.6 [19-22].

Iron Oxide VOxide / VIronFeO 1.7 - 1.8

Fe3O4 2.0 - 2.2

Fe2O3 2.1 - 2.2

Fe(OH)2 3.6 - 3.8

Fe(OH)3 4.0 - 4.2

Fe(OH)33H2O 6.2 - 6.4

Table 2.3.6: Relative volume ratios for various iron oxides in comparison to iron

Common damages in RC Structures:

The rust produced initially occupies the pore space surrounding the rebar, once all the

pore space available is used then; concrete is subjected to tensile stresses due to

34

excess volume of remaining rust. This ultimately causes cracks in concrete which is a

poor material in tension. This will lead to spalling of cover concrete in area

surrounding the corroding rebar and hence is a major concern as it will affect the

serviceability of the structure significantly by making it accident prone. In general

following deteriorating consequences of corrosion are common in RC structures:

Rust stains on concrete surface due to leaching of corrosion products, this impairs the aesthetics

Reduction in effective cross sectional area of rebar as iron is consumed in the process, leading to significant reduction in load carrying capacity of structure.

Impairs the bonding of rebar with surrounding concrete, and thus adversely affects load transfer between concrete and rebar

Cracking and spalling of the cover concrete due to tensile forces arising from excessive volume of corrosion products, reducing structural serviceability

Rebar becomes more accessible to the aggressive agents due to cracking, leading to further corrosion at an accelerated rate

Pitting (localized) corrosion of the rebar may also occur and is more dangerous than uniform corrosion because it reduces the cross-section area of

rebar to a point, leading to a catastrophic failure of the structure.

In concrete bridge structures use of prestressed steel strands is common, and in this

case corrosion is of more concern. As corrosion can lead to catastrophic brittle failure

prestressed steel strands due to any of the following mechanism:

Stress corrosion cracking (SCC):

SCC can be defined as highly localized corrosion that produces cracking in the

simultaneous presence of corrosion conditions and externally applied or residual

tensile stresses. SCC induces sudden, brittle, unexpected failure as SCC-initiated

cracks propagate across the components load-carrying section i.e. in planes

perpendicular to tensile stresses. In simple words it can be termed as anodic

corrosion under high sustained tensile stresses in case of prestressing steel in

concrete [5, 15, 23-25]. Most significantly, the process progresses at a far faster rate

than the more commonly recognized corrosion mechanisms, and shows almost no

signs of initiating and progress on the visible surfaces of the structure, such as

concrete cracking, spalling, and debonding. Existence of surface flaws including

35

longitudinal wire splits, wire seams created by overused/faulty drawing dies, and pits

in wire from atmospheric corrosion created during storage and prior to grouting, are

known to have provided crack initiation sites. Other locations susceptible to the

phenomenon include locations where moisture penetrates to the steel in areas where

steel is not passivated, e.g., with carbonated concrete, or steel not in direct contact

with concrete or grout.

Hydrogen embrittlement (HE):

Hydrogen embrittlement is a related phenomenon, which causes liberated atomic

hydrogen molecules to penetrate (through absorption) the steel lattice and

consequently leads to reduction in ductility of strands. This process can be termed as

Cathodic corrosion under tensile stress and will lead to brittle cracking of

prestressing strands due to embrittlement of hydrogen. As hydrogen can be created by

the electrochemical cathodic corrosion process itself if not available otherwise, for

example at a pit, or corrosion at manufacturing and wire drawing flaws, and from

cathodic protection systems which produce excessive electrical currents. Rupture is

sudden and brittle [5, 23-25]. Hydrogen embrittlement can also take place even in

absence of applied tensile stresses [25].

In case of SCC or HE the failure process develops such that initially, one or several

micro cracks are generated at the surface of the steel and then, these cracks grow and

propagate very quickly until they reach a certain depth, leading to the unexpected

brittle failure of the strand wire.

Main concern with SCC or HE is that, they cant be noticed by means of measuring

the corrosion rate, as the loss of metal is negligible and the corrosion potential

measured at the concrete surface may not indicate the development of the process.

Therefore, this type of corrosion cannot be electrochemically measured during its

occurrence. Only the risk of its appearance may be approached by the traditional

electrochemical techniques.

Even after the failure the identification of the nature of the failure is not an easy task.

It can only be detected by microscopic observation of the fractured surface when it is

fresh, and not corroded or contaminated. SHE or HE shall not to be mistaken with the

36

failure induced by chlorides in prestressed wires. The ions inducing localized

corrosion may also aim into a failure, but of simple reduction (localized attack) of the

cross section produced by the electrochemical process.

2.3.6 Corrosion control techniques

Corrosion is the most prominent among the factors that govern the performance of a

concrete structure during the service life of the structure. Therefore it is mandatory to

control the corrosion process in existing as well as in new structures if required, to

maintain the minimum required performance levels over the service life period of the

structure. In present scenario corrosion process can be controlled by various

techniques, which have established their application in field over the period of time.

The commonly utilized techniques and materials for corrosion control are:

Patch Repair Cathodic protection (CP) using impressed current Cathodic protection using sacrificial anode Electrochemical chloride extraction (ECE) Realkalization Inhibitor application Coatings Use of corrosion resistant rebars Design considerations

Patch Repair:

Patch repair is a traditional approach used for repair of corroded concrete structures;

where spalling has taken place or cracks appear on the concrete surface or structure

show signs of rebar corrosion over an area. And therefore areas having rebar

corrosion is usually identified by means of visual inspection, though sometimes

electrochemical methods may also be used. In this method basically the contaminated

concrete is replaced with fresh mortar.

In this method the concrete placed over the corroded rebars is removed completely by

breaking out the concrete behind the steel reinforcement. At all locations, the steel

must then be at very least wire brushed to remove all loose deposits or preferably grit

blasted or water jetted back to bright metal. Once the area to be repaired has been cut

37

out and cleaned thoroughly of all debris and dust, the repair can begin. Usually

designed repair mortars usually contain one or more of acrylic or other polymers,

shrinkage compensating additives, silica fume etc. to improve workability,

permeability, or ease of application. Mortar for repairs is then usually hand placed

packing the mortar on in layers. The repair must then be cured in properly. Once

finished and cured, there may be surface irregularities and will almost certainly be

color variation between the patch and the surrounding parent concrete. It is usual

therefore to apply a thin skim coat of a proprietary material, usually known as a

"fairing coat" to fill in any surface blemishes and to mask the patches themselves.

This may then be followed by the application of a final coating of anti-carbonation

paint which has the dual purpose of stopping further carbonation in unrepaired areas

and providing a pleasing and even color finish [10].

Patch repair is very handful in case of global corrosion where corrosion takes place

over a large area, that is in case of carbonated induced corrosion. But the patch repair

of chloride-induced corrosion damage presents problems because of the localized

nature of the corrosion. Active corrosion sites provide a form of electrochemical

protection (Cathodic protection due to incipient anode effect) for adjacent passive

steel that may also be exposed to chloride-contaminated concrete. The traditional

patch repair of corroding areas therefore triggers corrosion initiation in these passive

areas as the local protection is removed. Thus, when conventional patch repairs are

used to inhibit chloride-induced corrosion deterioration, all contaminated concrete

should be removed if further deterioration is to be avoided. One method of

overcoming this problem is to provide a sacrificial anode in the area of the patch

repair to continue the provision of local electrochemical protection. Placing small

sacrificial anodes such as Zinc covered with an appropriate mortar in the patch are

used commonly [10, 26-27].

Cathodic Protection:

Cathodic protection (CP) is a widely used and effective method of corrosion control.

Cathodic protection suppresses the corrosion current that causes damage in a

corrosion cell and forces the current to flow to the metal that is to be protected. The

basic fundamental is that cathodic areas in an electrochemical cell do not corrode.

Therefore if all the anode sites on rebar were forced to function as cathodes, then the

38

rebar would be a cathode and corrosion would be eliminated. To achieve this potential

of the rebar is artificially shifted so that it acts as cathode and hence becomes either

immune or passive. This phenomenon is appreciably used by converting active anodic

area of rebar in concrete to the cathodic ones, by the means of either an impressed

direct current (DC), or by connecting it to a sacrificial or galvanic anode [26-29].

Cathodic protection using impressed current:

In this technique the steel is connected to the negative terminal of an external

electrical power supply forcing it to undergo cathodic reaction as it receives current.

The anode, connected to the positive terminal of the power supply, is usually chosen

to be a relatively non-reactive conductor such as carbon or titanium so that its

corrosion rate is low. Fig. 2.3.10 provides the schematic application diagram for CP

using impressed current.

Fig. 2.3.10: Schematic application diagram for CP using impressed current

This technique for corrosion control using CP of concrete structures requires the

following basic components:

DC power supply (Rectifier): A rectifier is used to convert alternating current (AC) to direct current and provides required current to the CP system and

must be capable of providing either constant current or constant voltage to the

anode system.

Relatively inert anode material: It is used to distribute protective current to the reinforcing steel and provides locations for anodic reactions to take place in

Less reactive anode + __

External power source

Concrete & Rebar Protection currents

39

lieu of the reinforcing steel. By using relatively inert materials, such as

catalyzed titanium, anode consumption is minimized.

Instrumentation: such as embedded reference electrodes etc. for monitoring purposes.

Though used extensively this technique has certain limitations that need to be taken

care of for an efficient protection system. Some of the limitations of this technique are

as followed:

Requires skilled labor and routine monitoring and maintenance of voltage, current and anode etc.

It is difficult to assure a uniform current density in the high resistivity concrete pore water environment

Cathodic protection is not necessarily effective in protecting steel exposed at concrete cracks and at spalls in atmospheric applications

While using with prestressed system, one must be very careful as in presence of cathodic reaction at steel atomic hydrogen may be produced and that may

lead to brittle failure due to hydrogen embrittlement

Cathodic protection using sacrificial (galvanic) anode:

Sacrificial anode systems for CP are based on the principle of dissimilar metal

corrosion and the relative position of different metals in the galvanic series. In this

procedure a sacrificial anode is connected with the rebars. and hence a galvanic cell is

set up by connecting the steel to a more reactive metal, usually zinc. The zinc then

undergoes the anodic reaction and corrodes whilst the steel is rendered entirely

inactive because the whole surface undergoes the cathodic reaction and the iron no

longer dissolves. This may also be thought of as the anodic sites on the steel being

shifted to the sacrificial anode material.The direct current is generated by the potential

difference between the sacrificial anode and reinforcing steel when connected and

thus doesnt require external power source. The sacrificial anode will corrode during

the process and is consumed. Current will flow from the anode, through the concrete,

to the corroding rebar. Because current is to travel through concrete therefore this

method is not very effective in high resistance concretes, which impedes the flow of

current [26-29]. Fig. 2.3.11 represents schematic application diagram for CP using

sacrificial anodes.

40

Fig. 2.3.11: Schematic application diagram for CP using sacrificial anodes

Electrochemical chloride extraction (ECE) and Realkalization:

Chlorides (Cl-) are negatively charged, so we can use an electrochemical process to

repel the chloride ion from the steel surface and move it towards an external anode,

So that the passivity of rebar system is restored as alkalinity surrounding the rebar wil

be restored. For removal of chlorides a process similar to cathodic protection is used

but with both external power source as well as an external sacrificial anode (used

temporarily) [10].

In this process the external power source supplies current to rebars, forcing them to

act as cathodes and the used external anode material attracts chlorides present, away

from the rebars. Selected anode material must be able to stand the procedure till

desired reduction in chloride reduction at rebar level has been achieved. Once the

desired Cl- concentration has been achieved at rebar level, the external anode material

may be removed. Fig. 2.3.12 presents schematic diagram for application of ECE.

Fig. 2.3.12: Schematic diagram for application of ECE

+ __

External power source

Concrete & Rebar

Anode material

Protection currents

Cl- Cl- Cl-

Connection

Concrete & Rebar

Sacrificial galvanic metal (Anode)