direct studies report (aynsley griffin) 2016

Aynsley Griffin, BEng, EIT Reinforcement Steel Corrosion

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CVG 6108 – Direct Studies Report

Professor B. Martin-Perez

Reinforcement Steel Corrosion

Aynsley Griffin (MASc. Candidate)

Student No. 6527163

University of Ottawa

Department of Civil Engineering

April 24, 2016


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Abstract

As required by the uOttawa Direct Studies course, this report displays the knowledge acquired from an

accumulation of textbooks, journal articles, and reports on the study of corrosion of steel reinforcement

in concrete. While this report is not a complete representation of information found in the study of

corrosion of steel in concrete, this report makes an effort to discuss the basic concepts.

This report starts with a basic discussion on the transport processes of corrosives in concrete. The

transport processes discussed are diffusion, capillary action, permeation, and migration. Following that,

an extensive discussion on the corrosion process and two of the most common corrosion mechanisms,

carbonation and chloride attack are discussed.

The second half of this report focuses on two common electrochemical inspection techniques, reference

electrode potential and electrical resistivity; followed by two common corrosion measurement

techniques, linear polarization resistance and alternating current impedance. For the purpose of this

Direct Studies course, the relationship between the electrical resistivity and rate of corrosion was

discussed along with affecting factors.

Key Words: Corrosion; reinforced concrete; transport processes; electrochemical; carbonation; chloride

attack; electrical resistivity; corrosion measurements; corrosion rate


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Acknowledgments

I would like to express my deepest gratitude to my supervisor, Professor Beatriz Martin-Perez, for all her

support, expert guidance, understanding and encouragement throughout this Direct Studies course, my

study, and research. In addition, I would like to thank my colleague, Mr. Ahmad Shahroodi, for his

continual support of my research and his friendship.


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Table of Contents

Abstract ......................................................................................................................................................... 2

Acknowledgments ......................................................................................................................................... 3

List of Tables ................................................................................................................................................. 6

List of Tables ................................................................................................................................................. 7

List of Equations ............................................................................................................................................ 7

1.0 Understanding the Transport Process in Concrete ................................................................................. 8

1.1 Diffusion .............................................................................................................................................. 8

1.1.1 Stationary Diffusion ..................................................................................................................... 8

1.1.2 Nonstationary Diffusion ............................................................................................................... 8

1.2 Capillary Action ................................................................................................................................... 9

1.3 Permeation.......................................................................................................................................... 9

1.4 Migration ........................................................................................................................................... 10

1.5 Correlation between Mechanisms of Transportation ...................................................................... 10

2.0 Causes of Reinforcement Corrosion ..................................................................................................... 11

2.1 The Corrosion Process – Electrochemistry ....................................................................................... 11

2.1.1 Phases ........................................................................................................................................ 14

3.0 Corrosion Mechanisms ......................................................................................................................... 14

3.1 Carbonation ...................................................................................................................................... 15

3.1.1 Corrosion Initiation .................................................................................................................... 15

3.1.2 Corrosion Propagation ............................................................................................................... 16

3.1.3 Carbonation Alleviation ............................................................................................................. 16

3.1.4 Carbonation Detection ............................................................................................................... 17

3.2 Chloride Attack .................................................................................................................................. 17

3.2.1 Chloride Initiation ...................................................................................................................... 17

3.2.2 Chloride Penetration .................................................................................................................. 17

3.2.3 Chloride Binding ......................................................................................................................... 18

3.2.4 Threshold Value ......................................................................................................................... 19

3.2.5 Corrosion Propagation ............................................................................................................... 20

3.3 Corrosion Damage ............................................................................................................................ 21

4.0 Condition Evaluation ............................................................................................................................. 21

4.1 Electrochemical Inspection Techniques ............................................................................................ 23

4.1.1 Reference Electrode Potential ................................................................................................... 23


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4.1.2 Electrical Resistivity .................................................................................................................... 25

4.1.2.1 Surface Electrical Resistivity ................................................................................................ 25

4.1.2.2 Bulk Electrical Resistivity ..................................................................................................... 26

4.1.2.3 Factors that Affect Electrical Resistivity .............................................................................. 28

4.1.2.3.1 Internal Factors ............................................................................................................ 28

4.1.2.3.2 External Factors ........................................................................................................... 29

5.0 Corrosion Measurements ..................................................................................................................... 30

5.1 Linear Polarization Resistance Technique......................................................................................... 31

5.2 Alternating Current Impedance Technique ...................................................................................... 33

5.3 Corrosion Rate Range ....................................................................................................................... 35

5.4 Electrical resistivity and Corrosion Rate ........................................................................................... 35

5.4.1 Factors that Affect Corrosion Rate............................................................................................. 38

6.0 Conclusion ............................................................................................................................................. 38

References .................................................................................................................................................. 40


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List of Tables

Figure 1: Sampling of Parameter Correlations ........................................................................................... 10

Figure 2: Electrochemical Mechanism of Corrosion ................................................................................... 12

Figure 3: Pourbaix Diagram ......................................................................................................................... 13

Figure 4: Service Life Periods ...................................................................................................................... 14

Figure 5: Relationship between Corrosion Rate and Resistivity of Concrete ............................................. 16

Figure 6: Classical Chloride Diffusion Curve ................................................................................................ 18

Figure 7: Corrosion Rate vs. [Cl-]/[OH-] ..................................................................................................... 20

Figure 8: Schematic Representation of Pitting .......................................................................................... 20

Figure 9: Corrosion of Steel Reinforcement in Concrete ............................................................................ 21

Figure 10: Methods for Condition Surveying .............................................................................................. 22

Figure 11: Information Obtained from Various Electrochemical Techniques ............................................ 23

Figure 12: Schematic of Half-Cell Potential Setup ..................................................................................... 24

Figure 13: Potential Measurements ........................................................................................................... 24

Figure 14: Schematic of Wenner Technology ............................................................................................. 26

Figure 15: Bulk Electrical Resistivity ............................................................................................................ 27

Figure 16: Concrete Resistivity Reference Values....................................................................................... 28

Figure 17: Electrical Resistivity Ranges for Concrete Mixes with and without Admixtures ...................... 29

Figure 18: Polarization Curve ...................................................................................................................... 31

Figure 19: Linear Portion of Polarization Curve .......................................................................................... 32

Figure 20: Schematic Setup for LPR ............................................................................................................ 32

Figure 21: Nyquist Diagram......................................................................................................................... 34

Figure 22: Corrosion Rate Ranges as they relate to Corrosion Level .......................................................... 35

Figure 23: Literature Comparison between Resistivity and Corrosion Rate ............................................... 36

Figure 24: Resistivity Range as it related to Risk of Corrosion .................................................................... 38


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List of Tables

Table 1: Comparison of Equations Relating Electrical Resistivity and Corrosion Rate ............................... 37

List of Equations

Equation 1: Fick’s First Law of Diffusion ....................................................................................................... 8

Equation 2: Fick’s Second Law of Diffusion ................................................................................................... 8

Equation 3: Sorptivity.................................................................................................................................... 9

Equation 4: Darcy’s Law for Permeation ...................................................................................................... 9

Equation 5: Migration ................................................................................................................................. 10

Equation 6: Anodic Reaction ....................................................................................................................... 12

Equation 7: Cathodic Reaction……………………. ............................................................................................. 12

Equation 8: Ferrous Hydroxide. .................................................................................................................. 12

Equation 9: Ferric Hydroxide………………….. ................................................................................................ .12

Equation 10: Hydrated Ferric Oxide (Rust) ................................................................................................. 13

Equation 11: Carbonic Acid ......................................................................................................................... 15

Equation 12: Carbonation……………………………………… .................................................................................. 15


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1.0 Understanding the Transport Process in Concrete

Concrete is a porous material. Unfortunately, this concrete property allows unwanted gases, liquids, and

substances to penetrate into the concrete. To understand how these unwanted substances are

transported through the concrete, the following mechanisms are explained: diffusion, capillary action,

permeation, and migration.

1.1 Diffusion

Diffusion is a mechanism defined by a concentration gradient, where a higher concentration of a

substance on the concrete surface moves to a lower concentration through the concrete. Two forms of

diffusion are found: stationary diffusion and nonstationary diffusion.

1.1.1 Stationary Diffusion

Stationary diffusion is unidirectional and constant, as defined by Fick’s first law of diffusion:

= − Equation 1: Fick’s First Law of Diffusion

where: F is the flux in kg/m2s, C is the concentration of the diffusing substance in kg/m3 present at x

distance from the surface, and D is the diffusion coefficient in m2/s. The diffusion coefficient is dependent

on the substance that is being diffused into the concrete, the concrete properties, and the environmental

conditions. Experimental tests are used to determine what the diffusion coefficient value is with regard

to different diffusion substances and concrete properties.

1.1.2 Nonstationary Diffusion

Nonstationary diffusion, as commonly seen in concrete structures, is defined by Fick’s second law of

diffusion and incorporates the assumptions listed below:

• the concentration of the diffusing substance on the concrete surface is constant with time

• the coefficient of diffusion does not vary with time

• the coefficient of diffusion does not vary with thickness

• there are no chlorides initially

Under the above assumptions, the solution to Fick’s second law of diffusion is as follow:

, = [1 − ] Equation 2: Fick’s Second Law of Diffusion


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where C(x,t) is the chloride concrentration at depth x and time t. Dapp is the apparent diffusion

coefficienttaken from experimental data, and Cs is the surface chloride content which is dependant on the

composition of the concrete, the position of the structure, and the orientation of its surface and micro-

environment, the chloride contration in the environment, and finally the general conditions of exposure

(Bertolini L. , 2008). The equation above is most commonly used to determine the chloride penetration

profile in concrete structures.

However, the solution to Fick’s second law of diffusion does not exactly represent actual diffusion of

reinforced concrete structures. Substances brought into the concrete through diffusion will bind and react

with components from the concrete mix thus deviating away from the above diffusion equation. (Bertolini

L. , 2013).

1.2 Capillary Action

Water at the concrete surface is affected by capillary suction. This action is dependent on a number of

factors including: surface tension, viscosity, angle of contact between the pore wall and the liquid, radius

of pore, and finally the density of the liquid (Bertolini L. , 2013). From experimental data, the following

empirically derived equation represents the capillary suction:

= √ Equation 3: Sorptivity

where: i is the mass of liquid absorbed per unit of surface, S is the sorptivity as a parameter characterized

by the rate of capillary suction, and t is the time. The numerical value of sorptivity is determined from

experimental testing and is influenced by concrete properties and mix design. S can be expressed as

g/m2s0.5, for a change in mass, or m/s0.5, for a determined absorbed volume (Bertolini L. , 2013).

1.3 Permeation

Permeation is based on a pressure gradient. In the case of concrete structures, the coefficient of

permeability is measure with water (Bertolini L. , 2013). Following Darcy’s law for liquids, assumed

incompressible and entirely viscous, flow is measured by the equation below:

= !"#

$ Equation 4: Darcy’s Law for Permeation

Where: dq/dt is the flow in m3/s, H is the height of the column of water pressure differential across the

sample, and k represents the coefficient of permeability in m/s.


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1.4 Migration

Migration is defined as the transport of ions in solution under an electric field. Ion mobility is related to

the diffusion coefficient through the following equation:

= %&'/|*| Equation 5: Migration

Ions are only able to move through the water-filled, inter-connected concrete pores which are tortuous

in nature. With that said, electrical current flow by ion migration in concrete is a phenomenon that can

be measured in terms of electrical resistivity. Electrical resistivity (ρ) will be discussed in more detail later

in this report.

1.5 Correlation between Mechanisms of Transportation

It has been seen that the mentioned parameters in the transport process, capillary absorption (S),

permeation (K), diffusion (D), and electrical resistivity (ρ), are related to the corrosion of reinforcement

steels (Bertolini L. , 2013). Relating the transport parameters to the various deterioration mechanisms of

concrete is not a straightforward task since such correlations are affected by other factors. A sampling of

correlations can be seen in the figure below:

Figure 1: Sampling of Parameter Correlations (Bertolini L. , 2013)


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2.0 Causes of Reinforcement Corrosion

Corrosion is the inevitable process that occurs when refined metals, the reinforcement within reinforced

concrete structures, returns to their more stable combined forms as oxides, sulphides, and chlorides.

(Scientific, Technical Training: Corrosion Measurement Techniques, 2016) This natural process is the

reason behind reinforced concrete structures deterioration and is a central challenge for engineers today.

Concrete is an alkaline environment. It has microscopic pores containing high concentrations of soluble

calcium, sodium, and potassium oxides. In the presence of water, during the hydration of cement,

hydroxides are formed which are alkaline. This alkaline condition causes the formation of a passive and

protective layer surrounding the steel reinforcement in concrete structures. Along with other minor

chemical protectors, this layer protects the steel reinforcement from oxidation (corrosion). In an

uninterrupted environment, this passive layer will maintain and repair itself (Broomfield, 2007). However,

most reinforced concrete structures are subject to the environmental conditions that break down this

passive layer.

2.1 The Corrosion Process – Electrochemistry

It is important to understand the corrosion of steel is an electrochemical process, i.e. chemical reactions

produce electricity. The electrochemical mechanism of corrosion can be broken down into four

electrochemical reactions: (1) oxidation of iron (anodic process) which releases electrons and corresponds

to the formation of irons whose hydrolysis produces acidity, (2) reduction of oxygen (cathodic process)

which consumes those released electrons and produces alkalinity, (3) transportation of electrons within

the metals from the anodic areas (available to transport) to the cathodic areas (able to consume), which

produces an electrical current flow from the cathodic area to the anodic area, and finally (4) the flow of

currents from the anodic areas to the cathodic areas, to complete the electrical circuit (Bertolini L. , 2013).


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Figure 2: Electrochemical Mechanism of Corrosion (Pedeferri & Bertolini, 2000 as cited in Bertolini, 2013)

The most common chemical reactions that occur during this electrochemical process are described below:

As steel in concrete corrodes, it dissolved in the pore water and gives up electrons as seen in the anodic

reaction below:

⟶ 2+ + 2− Equation 6: Anodic Reaction

The released electrons must be consumed elsewhere to preserve electrical neutrality of the system. This

is transformed into the cathodic reaction:

2/ + 00 + 23 ⟶ 230 − Equation 7: Cathodic Reaction

Hydroxides are produced with the consumption of water and oxygen. If this was the end of the chemical

process, then the resulting hydroxyl ions would strengthen the passive layer with an increase in local

alkalinity (Broomfield, 2007). However, the ferrous compound from the anodic reaction combines with

the hydroxides from the cathodic reaction to start a chain reaction that creates rust (iron oxide):

4 + 230−⟶ 30 Equation 8: Ferrous Hydroxide

430 + 200 + 3 ⟶ 430 6 Equation 9: Ferric Hydroxide


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230 6 ⟶ 3600 + 200 Equation 10: Hydrated Ferric Oxide (Rust)

Created rust takes up about twice the volume of that is replaces at fully density (Broomfield, 2007). With

the additional swelling that comes from hydration, this volume increase exerts expansive stressed on the

surrounding concrete which results in cracking and spalling.

It is important to note that the above chemical reactions to reach rust are just one example. There are

many different anodic and cathodic reactions combination that depend on the pH of the cement paste,

pore solution, and the availability of oxygen. In order to determine which of these reactions for rust will

occur, the Pourbaix diagram is utilized. The Pourbaix diagram for iron in an aqueous solution, see in Figure

3, outlines the thermodynamic areas of stability as a function of electrochemical potential since the rate

at which anodic and cathodic process depends on the electrochemical potential. The electrochemical

potential is defined as the “measure of the ease of electron charge transfer between a metal and its

environment; it is a property of the steel/concrete interface” (ACI Committee 222, 2001) .

Figure 3: Pourbaix Diagram (ACI Committee 222, 2001)

Where, the anodic reaction, 2H0 + O + 4e/ ⟶ 4OH /, occurs when the potential is below the upper

dashed line and the anodic reaction, 4H4 + 2e/ ⟶0, occurs with the potential is below the lower

dashed line (ACI Committee 222, 2001).


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Corrosion of reinforcement within a structure is commonly described by polarization curves that relate

the potential and the anodic or cathodic current density. These polarization curves are difficult to

determine and are dependent on type of corrosion-induced damage and the type of metal used in the

reinforcement. However, the principle behind polarization curves, as they relate to potential, will be seen

later in this report with electrochemical condition evaluation techniques.

2.1.1 Phases

In the context of time or service life of a reinforced structure, there are two main phases of note. The

initiation phase is when the reinforcement as a whole is passive but environmental attacks can lead to

some loss of passivity. At the end of this phase, corrosion begins (steel becomes depassivated). When the

passive layer breaks down, the propagation phase begins. This phase continues until the effects of

corrosion can no longer be tolerated by the reinforced structure (Bertolini L. , 2013). An illustration of the

periods in the service life of a reinforced structure can be seen in Figure 4.

Figure 4: Service Life Periods (Tuutti, 1982 as cited in Bertolini, 2013)

3.0 Corrosion Mechanisms

The main corrosion mechanisms in reinforced concrete members are carbonation and chloride

penetration. These mechanisms do not attack the concrete itself but instead pass through the concrete

pores to attack the steel (Broomfield, 2007).


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3.1 Carbonation

Carbonation is defined as the “neutralization of the alkalinity of concrete due to carbon dioxide in the

atmosphere” (Bertolini L. , 2008). At a basic chemical level, carbonation is the result of the interaction of

carbon dioxide gas from the atmosphere with the alkaline hydroxide found in concrete. The chemical

reactions are as follows:

0 +00 ⟶ 006 Equation 11: Carbonic Acid

036 + :30 ⟶ :36 + 203 Equation 12: Carbonation

The carbon dioxide gas dissolves in water to form carbonic acid. This acid does not attack the cement

paste but instead neutralizes the alkalinity in the pore water, forming calcium carbonate. With a surplus

of calcium hydroxide in the pores, the pH maintains its alkaline level of 12-13. However, when the carbon

dioxide reacts with the calcium hydroxides, which are in excess, the calcium hydroxide react which leads

to the precipitation of calcium carbonate and thus the pH level drops (Broomfield, 2007). With a decrease

in pH level, steel begins to corrode.

3.1.1 Corrosion Initiation

Ingress of carbonation follows Fick’s law of diffusion where the rate of carbonation is proportional to the

distance from the surface. Since carbonation alters the concrete pore structure as it proceeds, cracks and

changes in moisture and concrete composition cause the diffusion equation to fall short. Through

integration of Fick’s law, the following equation for the calculation of the carbonation depth as a function

of carbonation rate, concrete quality, and environment is seen below:

d = K√t Equation 13: Carbonation Depth

In the above equation, d represents the depth of carbonation in mm, t is time in years, and K is the

measure of the rate of penetration of carbonation, or the carbonation coefficient in mm/year(1/2). The

carbonation coefficient, K, is a function of the diffusion coefficient D0, the CO2 concentration at the

concrete surface, and the amount of alkaline components that have to be consumed by the CO2. (Bertolini

L. , 2013). It is important to note that the carbonation depth equation mentioned here is in its simplest

form. Modified equations are present in the literature that take into account a multitude of different

parameters, including but not limited to environmental influences, surface finish, and water to cement

ratio (Parrott, 1987).


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3.1.2 Corrosion Propagation

Once the carbonation front has reached the steel reinforcement, depassivation of protective film occurs

and corrosion begins. The corrosion rate due to carbonation is controlled by the availability of oxygen and

water at the reinforcement surface (Andrade C. A., 1994). Generally, there is an inverse relationship

between electrical resistivity and corrosion rate of steel due to carbonation that can be used to monitor

the corrosion propagation, as seen in Figure 5. Electrical resistivity will be further discussed in latter

sections of this report.

Figure 5: Relationship between Corrosion Rate and Resistivity of Concrete (Alonso, Andrade, & Gonzalez, 1988 as cited in

Bertolini, 2013)

3.1.3 Carbonation Alleviation

The best way to reduce the effects of carbonation prior to poring is to provide good concrete cover since

the carbonation rate is a function of its thickness, as seen in the law of diffusion. In addition, high cement

content is preferred as this helps prevent neutralizing of the alkalinity in the concrete, good compaction

helps make the concrete have less permeable, and finally properly cured concrete has lower connectivity

of pores, so less penetrability. All of these factors help lessen the effects of carbonation in reinforced

concrete structures.

During the design of reinforced concrete structures, structural location plays an important role in the

deterioration of the structure. Wet dry cycling (i.e. long dry seasons proceeded by long wet seasons) will

accelerate the carbonation process since this type of cycling more easily provides carbon dioxide and

water to penetrate the concrete. (Broomfield, 2007)


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3.1.4 Carbonation Detection

Since the carbonation front is defined by a decrease in alkalinity from pH 11-13 to less than pH 8, it can

be detected by a pH indicator. The most common of which is phenolphthalein in a solution of water and

alcohol (Broomfield, 2007). The indicator changes from clear to pink when high pH is found. Therefore,

the carbonation front is detected when the color remains unchanged.

3.2 Chloride Attack

The most widely seen problem affecting reinforced structures today is the chloride attack. Chloride affects

reinforced structures by means of chlorides cast into the concrete, either through contaminated

aggregates, use of seawater, or accelerators, or by means of chlorides diffused into the concrete from

environmental factors, either through de-icing salts, chemicals, or contact with seawater. The presence

of chloride ions in the pore solution of the concrete may lead to localized corrosion of the reinforcement

steel knows as pitting. In an alkaline concrete environment, this occurs when the concentration of the

chloride ions has reached the threshold value, the ion concentration needed to break down the passive

layer (Bertolini L. , 2008). It is important to note that corrosion due to chloride attack is known as a self-

feeding process which makes it more severe over time.

3.2.1 Chloride Initiation

The initiation period for chloride attack depends on the rate of chloride penetration, the chloride

threshold value, and the thickness of the concrete cover (Bertolini L. , 2008).

3.2.2 Chloride Penetration

Unlike carbonation which has a front, chloride attack has a concentration profile. In Figure 6, the classical

diffusion curve is shown, as a function of chloride depth by mass percent of cement. It shows that chloride

concentration is the highest at the concrete surface and decreases with depth (Broomfield, 2007). It is

important to note that most reinforced structures follow a more complicated and erratic diffusion curve.


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Figure 6: Classical Chloride Diffusion Curve (Broomfield, Field Survey of Cathodic Protection on North American Bridges, 1992)

The rate of chloride ingress is affected by many factors including the diffusion, capillary suction,

permeation, migration mechanisms, and chemical reactions that occur within the concrete that hinders

the chloride movement (Bertolini L. , 2008). Understanding the penetration of chloride ions is a matter of

selecting the relevant mechanisms of chloride penetration, finding an appropriate value of the parameter

describing the rate of penetration (D, S, k, etc.), and finally calculating the evolution of the chloride

concentration in time as it propagates through the concrete. Unfortunately, determining the true chloride

penetration is more complicated than that because all transport mechanisms are dependent on the

complex concrete microstructure. In addition, chloride transport usually involves a combination of

transport mechanisms (Bertolini L. , 2008).

However, in general, the chloride profile can be reasonable descripted by Fick’s second saw of diffusion

(Equation 2), with the previously mentioned assumptions, after fitting by the ‘erf-function’ and with

suitable values for Cs and D (Bertolini L. , 2008). Important note, this reasonable description only applies

solely with diffusion. Suitable values for Cs and D are usually determined from experimental data, where

Dapp is the value of D from interpolated experimental data. Dapp, the diffusion coefficient, is often used in

the assessment of the risk of corrosion since studies have shown that the apparent diffusion coefficient

and electrical resistivity are inversely related (Lataste J.-F. , 2010). The rate of diffusion of chlorides from

the environment is dependent on the following factors: the water to cementitious materials ratio, type of

cement, the temperature, the maturity of the concrete, and the specific cation associated with the

chloride (ACI Committee 222, 2001).

3.2.3 Chloride Binding

It is important to note that not all chlorides present in the concrete contribute to corrosion. The term

“total chlorides” refers to all chlorides in the concrete. From the total chlorides, a portion of these


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chlorides become bound to constituents of the cement paste to form “bound chlorides”. For example,

chlorides react with calcium aluminates to form calcium chloroaluminates (ACI Committee 222, 2001).

Another portion of the chlorides becomes physical trapped by unconnected pores or by absorption (Bentz,

Evans, & Thomas, 1996 as cited in ACI Committee 222, 2001). The final portion of the chlorides available

in the pore solution that contribute to the corrosion process and are termed “free chlorides”. The

following factors attribute to the amount of free chlorides present: pH, water to cementitious material

ratio, tricalcium aluminate and tetracalcium aluminoferritte contents, and finally whether the chlorides

were added to the mix or entered the concrete from the environment (ACI Committee 222, 2001).

There is an ongoing discussion on whether chlorides that are dissolved in the pore solution are only

involved in corrosion while chlorides bound to constituents of the cement paste are not. It is important

to note that in practice, total chloride content is easily measured so for this reason, chloride threshold

value is expressed as critical total chloride content, where critical total chloride content is expressed as a

percentage of chlorides with respect to the mass of cement (Bertolini L. , 2013).

3.2.4 Threshold Value

Chlorides can be present in the concrete while the passive layer is intact. With that said, at what point

does the percentage of chlorides within the concrete initiate pitting corrosion? Several correlations have

been linked to the critical total chloride content: the concentration of hydroxyl ions, the electrochemical

potential of the steel, and the presence of voids at the interface of the reinforcement and concrete

(Bertolini L. , 2013). With regard to the hydroxyl ion concentration in the pore solution, there are several

critical ratio values given in the literature. For instance one such reference states that the chloride ions

start to break down the passive layer when the chloride concentration exceeds 0.6 the hydroxyl

concentration (Broomfield, 2007). The below graph shows the general relationship between the corrosion

rate and the molar ratio of chloride and hydroxides. It is important to note that the hydroxyl ion

concentration depends on the type of cement and admixture. Therefore, it has been summarized that

threshold values based on chloride/hydroxyl ions should be defined by statistical data and not by a

blantant value (Bertolini L. , 2013).


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Figure 7: Corrosion Rate vs. [Cl-]/[OH-] (Page, Lambert, & Vassie, 1991 as cited in Bertolini L. , 2013)

3.2.5 Corrosion Propagation

Once the chlorides have reached the reinforcement surface, the critical total chloride threshold has been

reacted and the protective passive layer has been broken down. The two most common results of chloride

attack are microcell and macrocell formation. Microcell formation, commonly referred to as localized

chloride attack or pitting, occurs when the high levels of chlorides are concentrated on a reasonable small

area of reinforcement (Bertolini L. , 2008). This area, no longer protected by the passive layer, becomes

the active zone, or anode. The unaffected area surrounding, on the other hand acts like the passive zone,

or cathode. The interaction between this active and passive zone is where the cathodic reaction of oxygen

reduction occurs.

Figure 8: Schematic Representation of Pitting (Bertolini L. , 2013)


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Electrical current flow from the anodic areas to the cathodic areas increases the chloride content,

followed by lowering the alkalinity. On the other end, the passive layer is strengthened since the chlorides

are moving towards the anodic area. This stabilization accelerates the corrosion process and is the reason

why pitting is so dangerous (Bertolini L. , 2013). At the end of the day, pitting leads to drastic loss of section

in the reinforcement.

When there is a separation between the anodic and cathodic reaction, seen when there are small

corroded areas followed with significantly larger areas of non-corroded reinforcement, macrocell

formation occurs. For ions to transport between such large distances, high levels of moisture within the

pore structure must be present, as seen in chloride attack (Broomfield, 2007). An overall increase in the

corrosion rate on the active steel is a strong indication that macrocell formation has occurred (Bertolini L.

, 2008).

3.3 Corrosion Damage

It is important to mention that most damages due to corrosion, either from carbonation or chloride attack,

result in an increase of iron oxide content. Since these oxides are immobile, very porous, and large in

volume, it creates substantial expansive stresses on the surrounding concrete leading to cracking and

spalling (Broomfield, 2007). Once these cracks reach the surface, it allows easier transport of water,

oxygen, and chlorides to the reinforcment within, subsquently feeding the corrosion process. It is also

important to remember that reinforcement section loss due to pitting is very dangeruos because such a

phenonum is difficult to detect and it can lead to serious decrease in the tensile strenght of the reinforced

concrete structure.

Figure 9: Corrosion of Steel Reinforcement in Concrete (Carbonation of Concrete (Corrosion), 2010)

4.0 Condition Evaluation

Evaluation of any reinforced structure usually involves two steps:


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1. Preliminary survey to define the nature of the problem and provide for more effective planning

2. Detailed survey which confirms the nature of the problem and determines the extent of the

problem.

It is important to note that this report closely examines one possible deterioration mechanism, corrosion.

However, there are far more mechanism found, including but not limited to: alkali-silica reactivity,

sulphate attack, Thaumasite attack, freeze thaw, delayed ettringite formation, thermal movement and

settlement (Broomfield, 2007).

There are various techniques used in practice today that provides valuable on-site or in-lab information

on the condition of a reinforced structure. The following table provides a good summary of the most

commonly used methods, what the method detects, and the approximate speed at which information can

be gathered.

Figure 10: Methods for Condition Surveying (Broomfield, 2007)

For the purpose of this Direct Studies course, this report will focus of condition evaluations that deal

primarily with electrochemical inspection techniques and corrosion measurements.


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4.1 Electrochemical Inspection Techniques

As discussed previously, electrochemical inspection techniques incorporate the principles behind the

electrochemistry of corrosion and electrochemical potentials. The various techniques provide phase

specific information, as seen in the figure below:

Figure 11: Information Obtained from Various Electrochemical Techniques (Bertolini L. , 2013)

4.1.1 Reference Electrode Potential

Also known as half-cell potential mapping, reference electrode potential is the most widely accepted form

of non-destructive corrosion monitoring and condition assessment that can be used before symptoms of

corrosion are seen on the concrete surface. This technique follows the ASTM C876 standard. The concept

of half-cell potential stems from the understanding that passive and corroding reinforcement have a

difference in corrosion potential. In order to determine the location of corroding reinforcement, a

reference electrode is placed on the concrete surface and an electric field coupled with the corrosion

current between the passive and active areas of the reinforcement are measured, resulting in

equipotential lines (Bertolini L. , 2013). The half-cell device follows the concept of a simple electrical

Daniell cell. A schematic of the half-cell on-site test setup can be seen below:


24

Figure 12: Schematic of Half-Cell Potential Setup (Bertolini L. , 2013)

The values acquired from the half-cell device depend on the reference electrode used. The most

commonly used reference electrode is copper/copper sulfate (CSE). Regardless of the reference electrode

used, the potential difference to the standard hydrogen electrode (SHE) is required so that the recorded

data takes into account the offset due to different reference electrodes (Broomfield, 2007). In general,

locations of corroding reinforcement are found when the half-cell device reads potential negative

measurement that are more negative. Broad summaries of the potential measurements values along with

the probability of corrosion are seen Figure 13, taken from the industry standard, ASTM C876, for

reinforced concrete structures exposed to the atmosphere. It is important to note that the potential

ranges provided in ASTM C876 were derived empirically from a set of specific structures. In addition, these

potential ranges do not take into account all factors that influence half-cell potential readings such as

moisture content, chloride content, temperature during test, carbonation of concrete, and cover

thickness. For these reasons, experimental studies looked into the former mentioned factors and has

shown that the ASTM ranges are indeed inaccurate (Bertolini L. , 2013).

Figure 13: Potential Measurements (ASTM C876 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in

Concrete, 2009 as cited in Bertolini L. , 2013)


25

ASTM C876 requires data presentation to be in the form of equipotential contour mapping or cumulative

frequency distribution (ASTM C876 Standard Test Method for Corrosion Potentials of Uncoated

Reinforcing Steel in Concrete, 2009). The latter incorporates statistics in the analysis of the corrosion data.

Research into improving the half-cell test has shown that these statistical analyses of potential mapping

results, which produce potential boundaries between active and passive steel, are more accurate

(Bertolini L. , 2013).

In addition to incorporating statistical analysis, a good application practice for this half-cell potential

technique is to develop a history for the reinforced structure by preforming the half-cell potential test at

regular intervals. This collection of data as it relates to time provides for a more detailed understanding

of the corrosion activity (ACI Committee 222, 2001).

In addition to the above mentioned factors that influence the half-cell potential measurements, problems

of interpretation occur when carbonation, stray currents, electrochemical treatment, cracks, or saturated

structures are found.

Half-cell potential measurements cannot be directly related to rate of corrosion but instead provide an

indication of the state of corrosion (ACI Committee 222, 2001). In summary, it is commom for sections of

the structure that show very negative potential measurements to be exposed, so that the condition of the

structure is defined with great certainity.

4.1.2 Electrical Resistivity

Resistivity is defined as “a measure of the ability of an electrical current to flow within a material, and is

thus an indicator of a material’s transfer properties” (Lataste J.-F. , 2010). As it relates to reinforcement

corrosion within concrete, electrical resistivity (ER) has a bearing on the corrosion rate since during the

corrosion mechanism, ionic current passes from the anodic to cathode areas (Broomfield, 2007).

At its most basic, ER is an indication of the amount of moisture in the concrete pore structure, and the

tortuosity and size of that pore structure. The pore structure is related to the concrete mix properties like

cement content, water/cement ratio, curing, and additives used. As it relates to chlorides, chlorides

encourage concrete to hold water so at the end of the day, ER can also be an indication of chloride content

(Broomfield, 2007).

4.1.2.1 Surface Electrical Resistivity

Electrical resistivity is measured in a number of different ways. Commonly, surface ER is measured using

the disc method, two, or a four probe (also known as a Wenner probe) system as seen is ASTM G57. For


26

a Wenner probe, the probe system passes a current though the concrete using the outer probes while

simultaneously measuring the voltage difference with the inner probes (Broomfield, 2007). A general

schematic of a Wenner technology setup can be seen below:

Figure 14: Schematic of Wenner Technology (Lataste J.-F. , 2010)

The following equation is used to represent this phenoneom for a semi-infinite, homogenous material:

ρ = 2πa AB Equation 14: Surface Electrical Resistivity

where: ρ represents the resistivity, a is the electrode spacing, V is the voltage measure between the inner

probes, and I is the current applied across the outer probes. The term CD is known as the resistance (R) or

the impedance (Z) value. Modifications to the above equation are needed in situations where the outside

probe are not spaced at the same interval as in the inner probes (ASTM G57 Standard Test Method for

Field Measurements of Soil Resistivity Using the Wenner Four-Electrode Method, 2012). It is important to

note that the probe spacing should be larger than the maximum aggregate size so to avoid measuring the

ER of a single aggregate piece (Broomfield, 2007).

As surface ER is a commonly used on-site, it is important to note that reinforcement actually conducts

current better than concrete. For this reason, measurements taken near reinforcement will yield results

that are not ideal and erroneous (Polder, 2000).

4.1.2.2 Bulk Electrical Resistivity

Bulk electrical resistivity measurement technique establishes a regular potential gradient by applying an

electrical field intensity. The measurement of that gradient allows for the assessment of resistivity (Lataste

J.-F. , 2010). The setup is commonly seen as a system of steel plates that enclose a concrete specium with

wetted sponges as seen in the figure below:


27

Figure 15: Bulk Electrical Resistivity

The resistivity is calculated using the equation below where ρ is the resistivity in Ωm, R is the resistance

(stated previously as voltage/current) between plates in Ω, and C is a geometric constant equal to cross-

sectional area of the concrete specimen divided by the length of the specimen (Bertolini L. , 2013).

ρ = RC Equation 15: Bulk Electrical Resistivity

When this electrical resistivity measurement technique is used, several operational factors can play a role

in the results. Concrete specimens should be fully saturated to avoid errors due to water content. Good

contact between the probes and the concrete specimen should be established. This can be achieved by

assuring that the same pressure is applied during testing, keeping in mind that increased pressure

provides for less variability in the results. It is recommended that the contact solutions used, whether a

water or gel solution, should contain 1M sodium chloride (Newlands, Jones, Kandasami, & Harrison, 2008).

In addition, studies have concluded that longer samples have provided for more representative samples

(Lataste J.-F. , 2010). Care should be taken to ensure that each test is conducted in the same manner.

Bulk ER data on its own provides little information. It is common to take concrete core samples from an

existing structure, preform bulk ER testing, and compare the data to existing data for a similar concrete

type (Polder, 2000). The figure below provides an example of this.


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Figure 16: Concrete Resistivity Reference Values (Polder, 2000)

4.1.2.3 Factors that Affect Electrical Resistivity

Both internal and external factors are discussed.

4.1.2.3.1 Internal Factors

The most basic aspect of concrete that affects how electrical resistivity flows through the concrete is the

concrete microstructure. Therefore, factors affecting the microstructure are influencing factors of

electrical properties of concrete.

Porosity is the main intrinsic factor that affects ER. In general, the higher the porosity, the lower resistivity

is. As a subset of porosity, correlations can also be made for pore distribution, pore volume, pore radii,

and the pore network, where pore network is defined by interconnectivity and tortuosity (Lataste J.-F. ,

2010). It is important to make note of the fact that the term total porosity is used to include the

summation of the open and closed pores. However, resistivity is only affected by open porosity, or in

other words, only the electrically interconnected pores.

Several concrete properties are linked to electrical resistivity: water/cement ratio (w/c), cement type,

aggregate, and admixtures. By definition, electrical current is carried by ions dissolved in the pore liquid.

For this reason, the presence of more water and larger pores correlates to a higher w/c ratio (Polder,

2000). As a result, an inverse relationship between ER and w/c exists: the higher the water/cement ratio,

the lower the resistivity (Lataste J.-F. , 2010).

Cement type is one of the more complex factors because there are many different cement types with

various chemical compositions available. As a basic level, cement is the source of chemical elements and

ions that support the electrical flow, elements and ions such as Ca, Na, K, OH, and SO4. It is for this reason

that studies relating electrical resistivity to corrosion usually provide data in categories separated by the


29

cement type, where such electrical resistivity ratios range from one to one hundred for multiple cement

types tested under the same conditions and methods (Neville, 1996).

For the most part, aggregates have been labeled to be non-conducting particles surrounded by ionically

conducting cement paste (McCarter, Ford, & Whittington, 1981 as cited in Lataste, 2010). Aggregates can

be subdivided by the following properties: aggregate mass/cement mass, minerology, and aggregate

distribution. Studies have shown that these properties have various effects on ER, but the generally

accepted correlation between electrical resistivity and aggregates is that the more aggregate in a mix, the

more the concrete is resistive.

Finally, supplementary cementing materials play a critical role in electrical resistivity measurements.

Experimental studies have shown that, in general, electrical resistivity increases with the presence of

admixtures. This correlation has been shown experimental where two typical admixtures, fly ash (FA) and

silica fume (SF) are compared to concrete without admixtures. This is seen below:

Figure 17: Electrical Resistivity Ranges for Concrete Mixes with and without Admixtures (Lataste, Breysse, Sirieix, & Naar, 2006)

4.1.2.3.2 External Factors

Changes in temperature have been found to have a drastic effect on ER. Overall, temperature and

electrical resistivity are inversely related. That is to say that as one increases, the other decreases (Polder,

2000). At the microstructure level, this is due to changes in the mobility of the ions and the ion-solid

interactions in the cement paste (Bertolini L. , 2013). Arrhenius equation is the basic equation used to

describe the relationship between temperature and ER:


30

σTI = σTJ KLb N 2OP − 2

OQRS Equation 16: Arrhenius

where: σ is conductivity, 1/Ω m, To is the reference temperature in K, Ti is the actual temperature in K, and

b is an empirical factor found to be between 1500 to 4500 K.

5.0 Corrosion Measurements

Quantifying the rate of deterioration in a reinforced concrete structure is the best way to determine the

life of the structure, if rehabilitation (restoring to proper condition), or repair (replacing or fixing parts) is

needed. There are many techniques for measuring the corrosion rate of reinforcement such as Tafel

extrapolation, alternating current (AC) impedance, electrochemical noise technique, and linear

polarization resistance (LPR). The two most common in-field techniques are linear polarization resistance

and AC impedance; these will be discussed in detail.

By measuring the electric current generated by the anodic reaction, this current can be converted to a

rate of loss of metal from the surface of the steel using Faraday’s law of metal loss:

m = UIVWX Equation 17: Faraday’s law of metal loss

where: m is the mass of steel consumed, i is the current in amperes, t is time in seconds, z is the ionic

charge or valency, M is the atomic mass of the metal, and F is Faraday’s constant (Broomfield, 2007).

Faraday’s constant is approximately 96,500 coulombs/equivalent mass (ACI Committee 222, 2001). Taking

the mass of metal dissolved, M, and dividing by the density, the mass can then be converted to thickness

of the dissolved layer. For iron, 1μA/cm2 = 11.6 μm/yr (Broomfield, 2007). Since the current density cannot

be determined directly, an external potential must be imposed on the system to displace it from

equilibrium and the resultant net current measured. The difference between the original corrosion

potential Ecorr and the applied potential E is termed polarization (η). Using the following equation, it has

been determined that for values of η between ±100 to 22 mV, η is proportional to the logarithm of the

current density (ACI Committee 222, 2001):

η = a + blogi Equation 18: Tafel

where: a is a constant, and b is the Tafel slope. The corrosion current density value icorr is obtained by

extrapolating the linear part of the curves to Ecorr as shown in the Figure 18 for an actively corroding system

without limits from diffusion:


31

Figure 18: Polarization Curve (ACI Committee 222, 2001)

It is important to note that two different definitions of corrosion rate for steel within reinforced concrete

structures are used: average corrosion rate and instantaneous corrosion rate. The average corrosion rate

is measured by determining the average reinforcement mass loss or loss of cross-section over a long

period of time. Average corrosion rate is very difficult to determine as the time of depassivation is not

known. For the most part, average corrosion rate is used in calculating the service life. The instantaneous

corrosion rate (icorr) is measured using the electrochemical method of polarization resistance (Rp), which

will be discussed in the section below (Bertolini L. , 2008).

5.1 Linear Polarization Resistance Technique

The LPR, steady-state, technique is based on the observation that the polarization curve close to the

corrosion potential is linear (Bertolini L. , 2013). In order words, the change in potential (ΔE) divided by

the change in current (ΔI), is defined as the polarization resistance (Rp,) which has a slope with a linear

relation. This relationship is seen in the figure below:


32

Figure 19: Linear Portion of Polarization Curve (Bertolini L. , 2013)

The LPR technique setup incorporates a working electrode (the reinforcement), a counter electrode

(nonreactive metal), and a reference electrode. An LPR device applies a voltage to the working electrode

via the counter electrode and the corresponding voltage response is measured. The reference electrode

measures the initial corrosion potential and any shift in potential of the working electrode. The voltage

data is then used to obtain the polarization resistance which is fitted to the Stern-Gray equation. An

illustration of this setup is seen below:

Figure 20: Schematic Setup for LPR

The polarization resistance (Rp) is inversely related to the corrosion current density (icorr) as seen in the

Stern-Gray equation below:

I_J`` = abc Equation 19: Stern-Gray


33

where: K is the proportionality constant which is a function of the anodic and cathodic Tafel slopes (ie.

the relationship between current and voltage levels outside the linear region) and is represented in the

following equation:

K = βeβ_/2.3βe + β_ Equation 20: Proportionality Constant

where: hi and hj are the anodic and cathodic Tafel constants (ACI Committee 222, 2001). Two values for

K are commonly used, for actively corroding steel, K=26mV and for passive steel, K=52mV (Bertolini L. ,

2013). Units for the above parameters are as follows: Icorr is in units of μA/cm2 (the instantaneous corrosion

current density), K in units of mV, and Rp in units of Ω *cm2.

It is important to note several items:

• to maintain a linear relation, the change in potential must be kept to less than 20mV

• concrete resistance between the reference electrode and the working electrode, known as iR

drop, need to be accounted for

The corrosion rate or corrosion velocity, Vcorr, represents the volumetric metal loss by unit of area and

time. Using Faraday’s law, the density of steel, and the corrosion current density, the following equation

for corrosion rate is seen below:

V_J`` = 0.0116i_J`` Equation 21: Corrosion Rate

where: Vcorr is in units of mm/year (Andrade & Alonso, Test Methods for On-site Corrosion Rate

Measurements of Steel Reinforcement in Concrete by Means of Polarization Resistance Method, 2004).

The result from on-site LPR testing, usually in conjunction with half-cell potential readings, can provide

the reasonable precise locations of high corrosion activity and thus allows engineers to predict future

deterioration and the overall service life of the structure (Bertolini L. , 2013). Since environmental factors,

such as temperature and relative humidity, can alter the readings for corrosion current density, it is

recommended that multiple measurements are taken over regular time intervals.

5.2 Alternating Current Impedance Technique

Alternating current (AC) impedance technique, also known as AC impedance spectroscopy, involves

measuring the electrical properties of concrete at varying frequencies. The aim of this transient technique

is to determine the resistive and capacitive response, both descriptors of the electrical concrete behavior


34

(Lataste J.-F. , 2010). These results are represented by the Nyquist diagram which incorporates the

imaginary part of impedance as a function of the real part of impedance, seen in the figure below:

Figure 21: Nyquist Diagram (Lataste J.-F. , 2010)

For the Nyquist diagram, the diameter of the semi-circle is the polarization resistance, Rp. On the real

resistance axis, the high-frequency intercept is the solution (concrete) resistance, Rs, and the low-

frequency intercept is the total impedance of the system, Rs+Rp. By subtracting the high-frequency

intercept from the low-frequency intercept, the polarization resistance is determined as thus the

corrosion rate can be calculated (ACI Committee 222, 2001).

The electrical behaviors are defined by the various semi-circular loops and sizes. The values obtained at

low frequency reflect pure resistance behavior whereas the high frequency values reflect capacitive

responses. Each loop describes a specific microstructure characteristic (Lataste J.-F. , 2010). In practice,

the curve deviates from the idealized curve seen above.

On-site, a complete frequency scan is time-consuming and expensive. For this reason and for the reason

that only the low-frequency and high-frequency values are of interest to the determination of the

polarization resistance, is assumed that only the low and high frequency are needed to be measurement

on site. However, this is an incorrect assumption because in order to determine the fundamental

characteristics of a particular system, low-frequency that defines total impedance and high-frequency that

defines solution resistance, a complete frequency scan is required (ACI Committee 222, 2001). Therefore,


35

this technique is indeed time-consuming and expensive and not entirely adaptive to investigating a real,

complete reinforced concrete structure (Lataste J.-F. , 2010). There are however, application of the AC

impedance technique for monitoring of the reinforced structure as this requires less measurement to be

taken. In addition, this technique has been shown as a good resource for in-lab testing (Lataste J.-F. ,

2010).

5.3 Corrosion Rate Range

No matter the electrochemical technique used to measure the corrosion rate, a broad criterion has been

established between the corrosion current density, the corrosion rate, and the significance of the

corrosion level within the reinforced concrete structure. These values deviate slightly in the literature but

the in general, the range is as follows:

Figure 22: Corrosion Rate Ranges as they relate to Corrosion Level (Andrade & Alonso, Test Methods for On-Site Corrosion Rate

Measurement of Steel Reinforcement in Concrete by Means of the Polarization Resistance Method, 2004)

It is important to note that the above data was compiled from experimental studies that focused on

ordinary Portland cement.

5.4 Electrical resistivity and Corrosion Rate

Generally speaking, low electrical resistivity is related to high risk of corrosion (Polder, 2000). This general

relationship is universally accepted throughout the literature. However, this general relationship came

about from multiple studies where the parameters outlining the experimental work differ. The difference

parameters found in these studies include, but not limited to, cement type, corrosion cause, specimen

geometry, reinforcement type, cover depth, measurement technique, and exposure conditions. In

addition, the analysis of this relationship between resistivity and corrosion rate vary by properties, scales,

scatter, and correlations (Hornbostel, Larsen, & Geiker, 2013). A comparison of the regression lines for


36

the relationship between electrical resistivity in concrete and the corrosion rate collected from multiple

studies can be seen in the graph below:

Figure 23: Literature Comparison between Resistivity and Corrosion Rate (Hornbostel, Larsen, & Geiker, 2013)

It is important to note that the most important parameter affecting the relationship between electrical

resistivity and corrosion rate, as outlined by a comprehensive literature review, is cement type

(Hornbostel, Larsen, & Geiker, 2013).

In the literature, there are multiple studies putting forth equations relating electrical resistivity to

corrosion rate. As stated before, there is a generally accepted inverse relationship between the two but

not a universally accepted equation. With that said, a sampling of empirically determined equations

establishing a relationship between the electrical resistivity and the intensity of the current of corrosion

can be seen below in Table 1:


37

Table 1: Comparison of Equations Relating Electrical Resistivity and Corrosion Rate

(Andrade & Alonso,

2004) (Ghods, Isgor, & Pour-Ghaz, 2007) (Ahmad, 2014)

mjnoo = 310pq

jnoo = −1.3310/6 + 3.0q − 3.83x10/plnCn

+ 0.333 lnu2 q

mjnoo = 15.39q x.y2z

Units: μA/cm2; Ωcm

Units: A/m2; Ωm

Where: Co2 = 0.00075 (average)

Units: μA/cm2;

KΩcm

Using the above equation to calculate the corrosion rate with an electrical resistance, ρ, of 100 Ωm and

1000 Ωm, it can be seen that these equations do provide corrosion rate values that fit within the

regression lines from the literature comparison plot seen above.

As can be inferred from the vast number of studies, the relationship between electrical resistivity and

corrosion rate is still being studied. Each of the above equations are dependent on many factors

surrounding how the experimental test were done. From this fact, caution needs to be taken when

applying these equations to existing reinforced concrete structures for the purpose of condition

evaluation.

In addition to the universally accepted inverse relationship between electrical resistivity and corrosion

rate, several resistivity ranges corresponding to risk of corrosion have seen published. The following table

presents these relationship values. It is important to note that these ranges differ slightly in the literature

but generally speaking, these ranges are universally accepted.


38

Figure 24: Resistivity Range as it related to Risk of Corrosion (Lataste J.-F. , 2010)

5.4.1 Factors that Affect Corrosion Rate

At a basic chemical level, corrosion is fed by oxygen and water. Thus both of these two environmental

factors must be present to feed the corrosion process. In addition, the following primary factors affect the

rate of corrosion of steel reinforcement: pH of the concrete pore water, electrical resistivity, relative

humidity, and temperature (ACI Committee 222, 2001).

It is also important to be aware of corrosion during the design and construction of the structure. Care

should be taken to ensure that water runoff removes the water from the reinforced structure and avoids

splashing of water. In addition, a few ways to reduce the possibility of reinforcement corrosion would be

to ensure that chlorides found in admixtures are not part of the mix design, adequate concrete cover is

provided, crack-control measures are established, and corrosion protection systems are incorporated.

6.0 Conclusion

It is important to note that this report, as it related to the Direct Studies course, does not include all topics

related to the corrosion of steel in concrete. This report does not mean to diminish the importance of

those excluded topics.

This report focused on the basics behind the corrosion of steel in concrete. The corrosion process involves

many transport processes that allow for unwanted substances to penetrate the concrete surface, move

through the concrete, and to deteriorate the steel reinforcement. Two common corrosion mechanisms,

carbonation and chloride attack, have unique initiation and propagation phases that affect the reinforced

concrete structure differently. Over time, both mechanisms will deteriorate the structure.

There are many condition evaluation techniques used to assess how carbonation and chloride attack are

affecting the structure. Two common electrochemical condition evaluation techniques, reference


39

electrode potential and electrical resistivity are widely used. Both techniques provide relatively accurate

data that can be used to make a generalized assessment of the condition of the structure. However, both

techniques are affected by many environmental factors that make specific assessments difficult. Research

should be conducted to refine both.

Out of the many corrosion measurement techniques used to determine the corrosion rate, the most

common two are linear polarization resistance and alternating current impedance. Using these

techniques, broad criterion has been established between the corrosion current density, the corrosion

rate, and the significance of the corrosion level within the reinforced concrete structure.

After numerous studies, a general inverse relationship has been established between electrical resistivity

and corrosion rate, and from that several empirical equations have been put forth. There are countless

factors that affect electrical resistivity and corrosion rate. All factors were not simultaneously taken into

account during these studies so future research into refining the relationship needs to be done.

There is no one way to eliminate the problems arising from the corrosion of embedded reinforcing steel.

There are, however, recommendations to provide for quality concrete, careful engineering design, and

good construction practices. Furthermore, there are recommendations for corrosion inhibitors,

reinforcement protection, and cathodic protection that will ease the problem of corrosion. Taking all of

this into account, the study into reinforcement corrosion is still a field that requires more attention.


40

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