222 r 96 - corrosion of metals in concrete

CORROSION OF METALS 222R-1
This committee report has been prepared to reﬂect the state
of the art of the corrosion of metals, and especially steel, in
concrete. Separate chapters are devoted to the mechanisms
of the corrosion of metals in concrete, protective measures
for new concrete construction, procedures for identifying
corrosive environments and active corrosion in concrete,
and remedial measures. A selected list of references is
included with each chapter.
Keywords: admixtures; aggregates; blended cements; bridge decks; calci-
um chlorides; carbonation; cathodic protection; cement pastes; chemical
analysis; chlorides; coatings; concrete durability; corrosion; corrosion re-
sistance; cracking (fracturing); deicers; deterioration; durability; marine at-
mospheres; parking structures; plastics, polymers and resins; portland
cements; prestressing steels; protective coatings; reinforced concrete; re-
inforcing steels; repairs; resurfacing; spalling; waterproof coatings.
CONTENTS
Chapter 1—Introduction, p. 222R-1
1.1—Background
1.2—Scope
1.3—References
Chapter 2—Mechanism of corrosion of steel in
concrete, p. 222R-3
2.1—Introduction
2.2—Principles of corrosion
2.3—Effects of the concrete environment on corrosion
2.4—References
Chapter 3—Protection against corrosion in new
construction, p. 222R-11
3.1—Introduction
3.2—Design and construction practices
3.3—Methods of excluding external sources of chloride
ion from concrete
3.4—Methods of protecting reinforcing steel from chlo-
ride ion
ACI 222R-96
Corrosion of Metals in Concrete
Reported by ACI Committee 222
ACI 222R-96 replaces ACI 222R-89 and became effective May 23, 1996.
Copyright (c) 1997, American Concrete Institute. All rights reserved including
rights of reproduction and use in any form or by any means, including the making of
copies by any photo process, or by any electronic or mechanical device, printed or
written or oral, or recording for sound or visual reproduction or for use in any knowl-
edge of retrieval system or device, unless permission in writing is obtained from the
copyright proprietors.
ACI Committee Reports, Guides, Standard Practices, and
Commentaries are intended for guidance in designing, plan-
ning, executing, or inspecting construction and in preparing
specifications. Reference to these documents shall not be
made in the Project Documents. If items found in these doc-
uments are desired to be part of the Project Documents, they
should be phrased in mandatory language and incorporated
in the Project Documents.
222R-1
ACI COMMITTEE 222
Corrosion of Metals in Concrete
Brian B. Hope
Chairman
Keith A. Pashina
Secretary
John P. Broomﬁeld Bret James Robert E. Price
Kenneth C. Clear Thomas D. Joseph D. V. Reddy
James R. Clifton David G. Manning William T. Scannell
Israel Cornet Walter J. McCoy David C. Stark
Marwan Daye Theodore L. Neff Wayne J. Swiat
Bernard Erlin Charles K. Nmai Thomas G. Weil
John K. Grant William F. Perenchio Richard E. Weyers
Kenneth C. Hover Randall W. Poston David A. Whiting
Associate Members
Stephen L. Amey Odd E. Gjorv Mohamad A. Nagi
Steven F. Dailey Clayford T. Grimm Morris Schupack
Stephen D. Disch Alan K. C. Ip Ephraim Senbetta
Hamad Farzam Andrew Kaminker Robert E. Shewmaker
Per Fidjestol Mohammad S. Khan Bruce A. Suprenant
Rodney R. Gerard Philip J. Leclaire William F. Van Siseren
Michael P. Gillen Joseph A. Lehmann Michael C. Wallrap

222R-2 ACI MANUAL OF CONCRETE PRACTICE
3.5—Corrosion control methods
3.6—References
Chapter 4—Procedures for identifying corrosion
environments and active corrosion in concrete,
p. 222R-21
4.1—Introduction
4.2—Methods of evaluation
4.3—References
Chapter 5—Remedial measures, p. 222R-23
5.1—Introduction
5.2—General
5.3—Applicability
5.4—The remedies and their limitations
5.5—Summary
5.6—References
Chapter 6—References to documents of standard-
producing organizations, p. 222R-28
Appendix A—Standard Test Method for Water-
Soluble Chloride Available for Corrosion of
Embedded Steel in Mortar and Concrete Using the
Soxhlet Extractor, p. 222R-28
CHAPTER 1—INTRODUCTION
1.1—Background
Concrete normally provides reinforcing steel with excel-
lent corrosion protection. The high alkaline environment in
concrete results in the formation of a tightly adhering film
which passivates the steel and protects it from corrosion. In
addition, concrete can be proportioned to have a low perme-
ability which minimizes the penetration of corrosion-inducing
substances. Low permeability also increases the electrical
resistivity of concrete which impedes the flow of electro-
chemical corrosion currents. Because of these inherent pro-
tective attributes, corrosion of steel does not occur in the
majority of concrete elements or structures. Corrosion of
steel, however, can occur if the concrete is not of adequate
quality, the structure was not properly designed for the ser-
vice environment, or the environment was not as anticipated
or changes during the service life of the concrete.
The corrosion of metals, especially steel, in concrete has
received increasing attention in recent years because of its
widespread occurrence in certain types of structures and the
high cost of repairs. The corrosion of steel reinforcement
was first observed in marine structures and chemical manu-
facturing plants.1.1,1.2,1.3
More recently, numerous reports of
its occurrence in bridge decks, parking structures, and other
structures exposed to chlorides has made the problem partic-
ularly prominent. The consequent extensive research on fac-
tors contributing to steel corrosion has increased our
understanding of corrosion, especially concerning the role of
chloride ions. It is anticipated that the application of the find-
ings of this research will result in fewer instances of corrosion
in new concrete structures and improved methods of repair-
ing corrosion-induced damage in existing structures.
For these improvements to occur, the information must be
disseminated to individuals responsible for the design,
construction, and maintenance of concrete structures. The
main purpose of this report is to present the state of the art.
While several metals may corrode under certain conditions
when embedded in concrete, the corrosion of steel reinforce-
ment is of the greatest concern, and, therefore, is the primary
subject of the report.
Chloride ions are considered to be the major cause of pre-
mature corrosion of steel reinforcement. Corrosion can oc-
cur in some circumstances in the absences of chloride ions,
however. For example, carbonation of concrete results in re-
duction of its alkalinity, thereby permitting corrosion of em-
bedded steel. Carbonation, however, is usually a slow
process in concrete which has a low water-cement ratio and
carbonation-induced corrosion is not as common as corro-
sion induced by chloride ions. Chloride ions are common in
nature and small amounts are usually unintentionally con-
tained in the mix ingredients of concrete.
Chloride ions also may be intentionally added, most often as
a constituent of accelerating admixtures. Dissolved chloride
ions also may penetrate unprotected hardened concrete in struc-
tures exposed to marine environments or to deicing salts.
The rate of corrosion of steel reinforcement embedded in
concrete is strongly influenced by environmental factors. Both
oxygen and moisture must be present if electrochemical cor-
rosion is to occur. Reinforced concrete with significant gradi-
ents in chloride ion content is vulnerable to macrocell
corrosion, especially if subjected to cycles of wetting and dry-
ing. Other factors that affect the rate and level of corrosion are
heterogeneities in the concrete and the steel, pH of the con-
crete pore water, carbonation of the portland cement paste,
cracks in the concrete, stray currents, and galvanic effects due
to contact between dissimilar metals. Design features also
play an important role in the corrosion of embedded steel. Mix
proportions, depth of cover over the steel, crack control mea-
sures, and implementation of measures designed specifically
for corrosion protection are some of the factors that control the
onset and rate of corrosion.
Deterioration of concrete due to corrosion results because
the products of corrosion (rust) occupy a greater volume than
the steel and exert substantial stresses on the surrounding
concrete. The outward manifestations of the rusting include
staining, cracking, and spalling of the concrete. Concurrent-
ly, the cross section of the steel is reduced. With time, structural
distress may occur either by loss of bond between the steel
and concrete due to cracking and spalling or as a result of the
reduced steel cross-sectional area. This latter effect can be of
special concern in structures containing high strength pre-
stressing steel in which a small amount of metal loss could
possibly induce tendon failure.
The research on corrosion to date has not produced a steel
or other type of reinforcement that will not corrode when used
in concrete and that is both economical and technically feasible.
However, research has pointed to the need for quality con-
crete, careful design, good construction practices, and reason-
able limits on the amount of chloride in the concrete mix
ingredients. Other measures that are being investigated in-
clude the use of corrosion inhibitors, protective coatings on
the steel, and cathodic protection. There has been some

success in each of these areas though problems resulting from
corrosion of embedded metals are far from eliminated.
1.2—Scope
This report discusses the factors that cause and control
corrosion of steel in concrete, measures for protecting em-
bedded metals in new construction, techniques for detecting
corrosion in structures in service, and remedial procedures.
Consideration of these factors, and application of the dis-
cussed measures, techniques, and procedures should assist in
reducing the occurrence of corrosion and result, in most in-
stances, in the satisfactory performance of reinforced and
prestressed concrete elements.
1.3—References
1.1. Tremper, Bailey; Beaton, John L; and Stratfull, R. F. “Causes and
Repair of Deterioration to a California Bridge Due to Corrosion of Rein-
forcing Steel in a Marine Environment II: Fundamental Factors Causing
Corrosion,” Bulletin No. 182, Highway Research Board, Washington, D.C.,
1958, pp. 18-41.
1.2. Evans. U. R., The Corrosion and Oxidation of Metals: Scientiﬁc Prin-
ciples and Practical Applications, Edward Arnold, London, 1960, 303 pp.
1.3. Biczók, Imre, Concrete Corrosion and Concrete Protection, 3rd
Edition, Académiai Kiadó, Budapest, 1964.
CHAPTER 2—MECHANISM OF CORROSION OF
STEEL IN CONCRETE
2.1—Introduction
This chapter is divided into two sections. In the first, em-
phasis is placed on the processes responsible for corrosion of
steel reinforcement in concrete. The corrosion mechanism is
generally accepted to be electrochemical in nature. Some of
the major causes of corrosion of steel in concrete are exam-
ined and related phenomena are discussed.
In the second part, a discussion is given on the concrete
variables that influence corrosion, including concrete mix
proportions, quality, and cover, and the effects of corrosion
inhibiting admixtures and carbonation.
2.2—Principles of corrosion
2.2.1 Corrosion—An electrochemical process—Al-
though iron can corrode by chemical attack, the most com-
mon form of corrosion in an aqueous medium is
electrochemical.2.1
The corrosion process is similar to the
action which takes place in a flashlight battery. An anode,
where electrochemical oxidation takes place, a cathode,
where electrochemical reduction occurs, an electrical con-
ductor, and an aqueous medium must be present. Any metal
surface on which corrosion is taking place is a composite of
anodes and cathodes electrically connected through the body
of the metal itself. Reactions at the anodes and cathodes are
broadly referred to as “half-cell reactions.”
At the anode, which is the negative pole, iron is oxidized
to ferrous ions.
Fe Fe++
+ 2e-
E˚ = - 0.440 Standard Redox Potential (2-1)
The Standard Redox Potential is the potential generated
when the metal is connected to a hydrogen electrode and is
one method of expressing electromotive forces. The Fe++ in
Eq. (2-1) is subsequently changed to oxides of iron by a
number of complex reactions. The volume of the reaction
products is several times the volume of the iron.
At the cathode, reduction takes place. In an acid medium,
the reaction taking place at the cathode is the reduction of
hydrogen ions to hydrogen. However, as will be shown later,
concrete is highly basic and usually has an adequate supply
of oxygen, so the cathodic reaction is
1/2 H2O + 1
/4 O2 + e-
OH -
E˚ = 0.401 Standard Redox Potential (2-2)
The corroding iron piece has an open circuit potential,
also called a rest potential, related to the Standard Redox
Potentials of the reactions in Eq. (2-1) and (2-2), to the com-
position of the aqueous medium, the temperature, and to the
“polarizations” of these half-cells. The rate of corrosion is
related to the “polarizations” as will be discussed later.
2.2.2 Availability of oxygen in concrete—Although the
availability of oxygen is one of the main controlling factors
for corrosion of steel, there appears to be little quantitative
information on its effect in concrete. Some information is
shown, however, in Fig. 2.1, where the rate of oxygen diffusion
through water-saturated concrete of varying quality and
thickness is illustrated.2.2
The rate of oxygen diffusion
through concrete is also significantly affected by the extent
to which the concrete is saturated with water. A number of
investigations indicate that if the steel passivity is destroyed,
conditions will be conducive to the corrosion of steel rein-
forcement in those parts of a concrete structure that are ex-
posed to periods of intermittent wetting and drying.
Investigations also indicate that, although chlorides are
present, the rate of steel corrosion will be very slow if the
concrete is continuously water-saturated.2.3
In wet concrete,
dissolved oxygen will primarily be diffusing in solution,
while in a partly dry concrete the diffusion of gaseous oxy-
gen is much faster. For oxygen to be consumed in a cathodic
reaction, however, the oxygen has to be in a dissolved state.
Since it is the concentration of dissolved oxygen that is im-
portant, all factors affecting the solubility of oxygen should
also affect its availability. The effect of salt on the corrosion
rate has been demonstrated by Griffin and Henry2.4
(see
Fig. 2.2). Corrosion increased as the sodium chloride con-
centration increased until a maximum was reached. Beyond
this, the rate of corrosion decreased despite the increased
chloride ion concentration. This change in relationship be-
tween corrosion and sodium chloride concentration is attrib-
uted to the reduced solubility and diffusivity of oxygen, and,
therefore, the availability of oxygen to sustain the corrosion
process. This represents corrosion in a salt solution; howev-
er, the availability of oxygen in wet concrete may be different.
Problems due to corrosion of embedded steel have seldom
been observed in concrete structures that are continuously
submerged, even as in seawater.
2.2.3 The importance of chloride ions—As will be discussed
later, concrete can form an efficient corrosion-preventive

environment for embedded steel. However, it is well
documented2.5-2.7
that the intrusion of chloride ions in rein-
forced concrete can cause steel corrosion if oxygen and
moisture are also available to sustain the reaction. No other
common contaminant is documented as extensively in the
literature as a cause of corrosion of metals in concrete. Chlo-
ride ions may be introduced into concrete in a variety of
ways. Some are intentional inclusion as an accelerating ad-
mixture; accidental inclusion as contaminants on aggregates;
or penetration by deicing salts, industrial brines, marine
spray, fog, or mist.
2.2.3.1 Incorporation in concrete during mixing. One of
the best known accelerators of the hydration of portland ce-
ment is calcium chloride. Generally, up to 2 percent solid
calcium chloride dihydrate based on the weight of cement is
added. Chlorides may be contained in other admixtures such
as some water-reducing admixtures where small amounts of
chloride are sometimes added to offset the set-retarding ef-
fect of the water reducer.
In some cases, where potable water is not available, seawa-
ter or water with a high chloride content is used as the mixing
water. In some areas of the world, aggregates exposed to sea-
water (or that were soaked in seawater at one time) can contain
a considerable quantity of chloride salts. Aggregates that are
porous can contain larger amounts of chloride.
2.2.3.2 Diffusion into mature concrete. Chlorides can
permeate through sound concrete, i.e., cracks are not neces-
sary for chlorides to enter the concrete.2.8
Approximate
determinations of the diffusion coefficients for chloride in
concrete have been published,2.9
but largely the literature ne-
glects the interaction between concrete and chloride.
2.2.3.3 Electrochemical role of free chloride ions. There
are three modern theories to explain the effects of chloride
ions on steel corrosion:
(a) The Oxide Film Theory—Some investigators believe
that an oxide film on a metal surface is responsible for pas-
sivity and thus protection against corrosion. This theory pos-
tulates that chloride ions penetrate the oxide film on steel
through pores or defects in the film easier than do other ions
(e.g., SO4
-
). Alternatively, the chloride ions may colloidally
disperse the oxide film, thereby making it easier to penetrate.
(b) The Adsorption Theory—Chloride ions are adsorbed on
the metal surface in competition with dissolved O2 or hydrox-
yl ions. The chloride ion promotes the hydration of the metal
ions and thus facilitates the dissolution of the metal ions.
(c) The Transitory Complex Theory—According to this
theory, chloride ions compete with hydroxyl ions for the
Fig. 2.2—Effect of concentration of sodium chloride
on corrosion rate2.4
Fig. 2.1—Effect of water-cement ratio and thickness on diffusion of oxygen through mor-
tar and concrete2.2

ferrous ions produced by corrosion. A soluble complex of
iron chloride forms.2.9
This complex can diffuse away from
the anode destroying the protective layer of Fe(OH)2, per-
mitting corrosion to continue. Some distance from the elec-
trode, the complex breaks down, iron hydroxide precipitates
and the chloride ion is free to transport more ferrous ions
from the anode. Evidence for this process can be observed
when concrete with active corrosion is broken open. A light
green semisolid reaction product is often found near the steel
which, on exposure to air, turns black and subsequently rust
red in color.
Since corrosion is not stifled, more iron ions continue to
migrate into the concrete away from the corrosion site and to
react with oxygen to form higher oxides that result in a four-
fold volume increase. It is the expansion of iron oxides as
they are transformed to higher oxidation states that produce
internal stress, which eventually cracks the concrete. Formation
of iron chloride complexes may also lead to disruptive forces.
2.2.4 Corrosion rate and pH—As illustrated in Fig. 2.3, the
corrosion rate of iron is reduced as the pH increases. Since
concrete has a pH higher than 12.5, it is usually an excellent
medium for protecting steel from corrosion. Only under con-
ditions where salts are present or the concrete cover has car-
bonated does the steel become vulnerable to corrosion.
2.2.5 Pourbaix diagrams—Pourbaix2.10
devised a com-
pact summary of thermodynamic data in the form of electri-
cal potential versus pH diagrams, which includes the
electrochemical and corrosion behavior of iron in an aqueous
medium. Fig. 2.4 indicates that iron is in a passive state at a
pH in the range of 8 to 13. This accounts for the protective
properties of concrete and the absence of steel corrosion in
concrete when no chloride ions, or only small amounts, are
present. A careful inspection of Fig. 2.4 indicates, however,
that corrosion may begin if the pH of the system is raised to
above about 13. In this case, a soluble ferrite, HFeO2
-
, forms.
Any mechanism where iron dissolves at an appreciable rate
in concrete can be considered serious corrosion. Thus, the
addition of materials that further increase the pH of concrete
may be detrimental; however, the occurrence of this phe-
nomenon in concrete has not been confirmed.
2.2.6 High-strength steels and other metals—There is lit-
tle information in the literature suggesting that the newer
high-strength steels are either more or less subject to corrosion
than the previous “standard” and lower strength steels. Al-
though information is lacking, it is expected that whatever
differences might exist would be secondary to the major fac-
tors influencing corrodibility of the steel.
Studies have been made on the corrosion of prestressed
steel in concrete2.11
that demonstrate the hazards of adding
chlorides to concrete containing prestressed steel.
Aluminum corrodes in concrete and the rate of corrosion
is higher if the aluminum is in contact with steel and chlo-
rides or if alkalies are present.2.12
Lead and tin (e.g., tin sol-
der) can corrode similarly.
At pH about 12.5, zinc reacts rapidly to form soluble
zincates and hydrogen gas is liberated
Zn + OH-
+ H2O -> HZnO2
-
+ H2 ↑ (2-3)
If galvanized steel is to be used in concrete, appropriate
measures should be taken to prevent hydrogen evolution in
the fresh concrete. A small amount of a chromate salt is gen-
erally used.2.13
In submerged concrete structures having freely exposed
steel components in metallic connection with the reinforcing
steel, galvanic cells may develop with the freely exposed
steel forming the anode and the embedded steel the cathode.
This may cause an increased corrosion rate on the freely ex-
posed steel.
Fig. 2.3—Effect of pH on corrosion of iron in aerated
soft water at room temperature2.1
Fig. 2.4—Pourbaix diagram for iron2.10

Copper, chromium, nickel, and silver generally do not
corrode in concrete. However, they are somewhat suscep-
tible to corrosion when the concrete is located in a marine
environment.2.14
2.2.7 Polarization of half-cells—The equilibrium of a
half-cell is characterized by its reversible potential which
depends on the Standard Redox Potential, on the concen-
trations (activities) of the species participating in the half-cell
reaction (e.g., OH-
, Fe++
, etc.), and on the temperature.
When a current flows through the half-cell, there is a shift
of its measured potential away from the reversible potential.
This shift is called polarization. For a given current, the po-
larization is large for half-cell reactions that are irreversible
or nearly so. For instance, the oxygen half-cell reaction
[Eq. (2-2)] is almost irreversible and can have a relatively
high degree of polarization.
Some corrosion processes, although thermodynamically
more favored than others as indicated by the reversible po-
tentials of the half-cells, are actually slower in practice be-
cause of polarization effects.
There are three general kinds of polarization that apply
to the anode as well as to the cathode. These are concen-
tration polarization, ohmic polarization, and activation
polarization. These three kinds of polarization can be
present simultaneously.
(a) Concentration polarization occurs when the concen-
tration of the electrolyte changes in the vicinity of the elec-
trode. An example of this would be depletion of oxygen
concentration at the cathode.
(b) Ohmic polarization occurs because of the ohmic resis-
tance of the electrolyte (e.g., moist concrete) and of any
films on the electrode surface. This produces an ohmic po-
tential drop in accordance with Ohm’s law (IR drop).
(c) Activation polarization occurs due to kinetic hindrance
of the rate controlling step of the electrode reaction.
Tafel2.1
has shown experimentally that for large currents
in the absence of concentration and ohmic polarization, the
measured polarization η, which is the activation polarization, is
directly proportional to the logarithm of the current density i
η = a + blog i
where a and b are the so-called Tafel intercept and Tafel
slope parameters, respectively. These parameters can be ob-
tained by plotting η versus i on semilogarithmic paper. The
Tafel intercept parameter a is related to the exchange current
density io, which is the equilibrium current flowing back and
forth through the electrode electrolyte interface at equilibri-
um and a measure of the reversibility of the reaction.
The Tafel slope parameter b, on the other hand, gives an
insight according to modern electrode reaction theory2.15
into the mechanism of the electrode reaction.
2.2.8 Passivity and transpassivity—Specific to the anodic
half-cell in the corrosion process are the concepts of passiv-
ity and transpassivity. Passivity of a metal is generally char-
acterized by a thin and tightly adherent oxide film on the
metal surface, which tends to protect the metal against fur-
ther corrosion. The exact composition of the thin and nor-
mally invisible film has been difficult to determine. It seems
clear, however, that it is made up of chemical combinations
of oxygen and is simply called an oxide film.
When a potential is applied to an iron electrode, the rate of
current flow depends on the state of passivity. Iron in con-
crete is generally passive, such that little current flows when
a potential is progressively increased, eventually the current
will flow. This is because, at this point, oxygen is evolved
and the electrode reaction involves the electrolysis of water
(see Curve I, Fig. 2.5).
2H2O -> 4H+
+ O2 + 4e-
(2-4)
This phenomenon is called “transpassivity.” The use of a
corrosion testing procedure involving an impressed potential
has been reported.2.17
Grimes et al.2.18
found that when a
high anodic voltage was applied to steel in concrete, the pH
around the steel was changed to values ranging between al-
most 0 and 4. He also found that a liquid pool formed quickly
around the reinforcing steel. Neither steel nor concrete is du-
rable in a low pH (acidic) environment.
2.2.9 Types of corrosion-controlling mechanisms—As
previously mentioned, it is necessary to have both a cathodic
and an anodic reaction for a corrosion process to occur.
However, different corrosion situations can be visualized by
plotting, on the same graph, the log of the absolute current
(I) versus potential (E) curves which would be obtained by
polarizing each half-cell with an auxiliary counter electrode
in absence of the other half-cell. For this discussion, we as-
sume (a) no IR drop between anodic and cathodic areas and
(b) simple algebraic addition of the anodic and cathodic cur-
rents. Under these conditions, the rest potential or open cir-
cuit potential of the corroding sample will be that at which
the two i versus E curves intercept. At this point no net, ex-
ternal current flows, and the absolute value of current at the
intercept is equal to the corrosion current.
If the cathodic process is the slower process (the one with
the larger polarization), the corrosion rate is considered to be
cathodically controlled (Fig. 2.6). Conversely, if the anodic
process is slower, the corrosion rate is said to be anodically
controlled (Fig. 2.7).
In concrete, one or two types of corrosion-rate-controlling
mechanisms normally dominate. One is cathodic diffusion,
where the rate of oxygen diffusion through the concrete
Fig. 2.5—Iron electrode in concrete2.16

determines the rate of corrosion. In Fig. 2.8, cathodic diffusion
control is shown for two different rates of oxygen diffusion.
The other type of controlling mechanism involves the de-
velopment of a high resistance path. When steel corrodes in
concrete, anodic and cathodic areas may be as much as several
feet apart; therefore, the resistance of the concrete may be of
great importance. Fig. 2.9 illustrates two cases of resistance
control where the potential available for corrosion is the dif-
ference in the potential between anode and cathode minus
the relevant IR loss.
2.2.10 Stray current corrosion—Stray electric currents are
those that follow paths other than the intended circuit. They
can greatly accelerate the corrosion of reinforcing steel. The
most common sources of these are electric railways, electro-
plating plants, and cathodic protection systems. Kondo et
al.2.9
reported corrosion of reinforcing steel embedded in
concrete used in an electric railroad.
2.2.11 Cathodic protection—The principle of cathodic
protection is to change the potential of a metal to reduce the
current flow and thereby the rate of corrosion. This is accom-
plished by the application of a protective current at a higher
voltage than that of the anodic surface. The current then
flows to the original anodic surface resulting in cathodic re-
actions occurring there. The difficulties in using this method,
however, are to determine the correct potential to apply to
the system and to make sure that it is applied uniformly.
2.2.12 Effects of other salt ions—It has been reported that
sulfate2.20
and carbonate2.21
salts can also cause reinforcing
steel to corrode. However, this has not been well document-
ed. Although the concrete may have cracked in certain cases
and the exposed steel may have appeared rusted, these salts
may not have been the primary cause. Their low solubility in
a high calcium ion environment would reduce their availabil-
ity. However, certain soluble salts, such as perchlorates, ac-
etates, and salts of halogens other than chlorine may be
corrosive to steel embedded in concrete. Hydrogen sulfide
has also been cited as a cause of corrosion.2.22
2.2.13 Stress corrosion cracking—Stress corrosion2.23
is
defined as the process in which the damage caused by stress
and corrosion acting together greatly exceeds that produced
when they act separately.
Fig. 2.8—Cathodic diffusion control
Fig. 2.9—Resistance controlFig. 2.7—Anodic control
Fig. 2.6—Cathodic control

In stressed steel, a small imperfection caused by corrosion
can lead to a serious loss in tensile strength as the corrosion
continues at the initial anode area.
Another form of corrosion that is related to stress corrosion
cracking is intergranular corrosion. In this case, a gas, usual-
ly hydrogen, is absorbed in the iron, causing a loss of ductil-
ity and cracking. Hydrogen cracking in connection with
cathodic protection will be discussed in Chapter 5. Other ma-
terials that may cause intergranular corrosion are hydrogen
sulfide and high concentrations of ammonia and nitrate salts.
The mechanism of how this type of corrosion proceeds is not
fully understood; however, it is believed that it involves the
reduction in the cohesive strength of the iron. Documented
examples of stress corrosion cracking of steel in concrete
have not been found in the literature.
2.3—Effects of the concrete environment on
corrosion
2.3.1 Portland cement—When portland cement hydrates,
the silicates react with water to produce calcium silicate hy-
drate and calcium hydroxide. The following simplified
equationsgivethemainreactionsofportlandcementwithwater
2(3CaO ⋅ SiO2) + 6H2O → 3CaO ⋅ 2SiO2 ⋅ 3H20 +
3Ca(OH)2 (2-5)
2(2CaO ⋅ SiO2) + 4H2O → 3CaO ⋅ 2SiO2 ⋅ 3H2O +
Ca(OH)2 (2-6)
As previously mentioned, the high alkalinity of the chem-
ical environment normally present in concrete protects the
embedded steel because of the formation of a protective ox-
ide film on the steel. The integrity and protective quality of
this film depends on the alkalinity (pH) of the environment.
Differences in the types of cement are a result of variation
in composition or fineness or both, and as such, not all types
of cement have the same ability to provide protection of
embedded steel. According to Pressler et al.2.24 a well-hy-
drated portland cement may contain from 15 to 30 percent
calcium hydroxide by weight of the original cement. This is
usually sufficient to maintain a solution at a pH about 13 in
the concrete independent of moisture content.
Rosenqvist2.25
has described an example of exceedingly
rapid steel corrosion in a tropical concrete wharf where a
pozzolan was added to the portland cement. Corrosion of the
reinforcement was observed shortly after construction was
completed. Measurements of pH in the extract from the con-
crete adjacent to the steel gave values from 5.7 to 8.5. When
the pozzolan was omitted, an increase in pH was obtained
and no damage was observed.
Other research2.26
showed that a pozzolan did not adverse-
ly affect the performance of steel in concrete that was partial-
ly immersed in a saturated salt solution. In this latter study,
the pozzolan studied was a calcined volcanic tuff that was
added to the concrete at dosages of 16 and 32 percent by
weight of the cement. It may be that the protective qualities
of the pozzolan are related to its source or generic type.
The use of blended cements might, under certain circum-
stances, be detrimental because of a reduction in alkalinity.
However, blended cements can give a substantial reduction in
permeability and also an increase in electrical resistivity espe-
cially where a reduction in the water-cement ratio is made pos-
sible. Also, such blended cements may give concrete as much
as two to five times higher resistance to chloride penetration
than concrete made with portland cements.2.27
The effects
would be beneficial as far as corrosion is concerned and in
some circumstances the benefits associated with blended ce-
ments more than offset the adverse effects.
Even for cements with the highest reserve basicity, the al-
kalinity may be reduced in a number of ways. Reduction of
alkalinity by leaching of soluble alkaline salts with water is
an obvious process. Partial neutralization by reaction with
carbon dioxide (carbonation), as present either in air or dis-
solved in water, is another common process.
The silicates are the major components in portland cement
imparting strength to the matrix. No reactions have been de-
tected between chloride ions and silicates. Calcium chloride
accelerates the hydration of the silicates when at least 1 per-
cent by weight is added.2.28
Calcium chloride seems to act as
an accelerator in the hydration of tricalcium silicate as well
as to promote the corrosion of steel.
Also present in portland cement are C3A and an alumino-
ferrite phase reported as C4AF. The C3A reacts rapidly in the
cement system to cause flash set unless it is retarded. Calci-
um sulfate is used as the retarder. Calcium sulfate forms a
coating of ettringite (C3A ⋅ 3CaSO4 ⋅ 32H2O) around the alu-
minate grains thereby retarding their reactivity (Fig. 2.10).
Calcium chloride also forms insoluble reaction products with
the aluminates in cement (see Fig. 2.11). The most commonly
noted complex is C3A ⋅ CaCl2 ⋅ xH2O, Friedel’s salt (Table 2.1).
The rate of formation of this material is slower than that of
ettringite (compare Fig. 2.10 with Fig. 2.11). The chloride-
aluminate complex forms after ettringite and prevents further
reactions of sulfate with the remaining aluminates.2.29
Fig. 2.10—Rate of reaction of gypsum with tricalcium alu-
minate in portland cement2.29

The chemical combining of C3A with chlorides is fre-
quently referred to as a beneficial effect in that it will reduce
the rate of chloride penetration into concrete. According to
Mehta,2.30
a chemical binding of penetrating chlorides can-
not be expected unless the C3A content is higher than about
8 percent. However, recent experimental observations have
shown that as much as 8.6 percent C3A does not effectively
reduce chloride penetration when concrete is immersed in
seawater2.31
and other studies have found more corrosion-
induced distress associated with high C3A contents.2.32
2.3.2 Aggregate—The aggregate generally has little effect
on the corrosion of steel in concrete. There are exceptions.
The most serious problems arise when the aggregates con-
tain chloride salts. This can happen when sand is dredged
from the sea or taken from seaside or arid locations. Porous
aggregates can absorb considerable quantities of salt.
Care should be exercised when using admixtures contain-
ing chloride in combination with lightweight aggregates.
Helms and Bowman2.33
found that reinforcing steel in light-
weight concrete was particularly susceptible to corrosion
when exposed to moisture and changes in temperature.
Lightweight aggregate containing sulfides can be damaging
to high-strength steel under stress.
2.3.3 Water—The effect of moisture content or degree of
water saturation on the rate of oxygen diffusion into concrete
has already been discussed. A high moisture content will
also substantially reduce the rate of diffusion of carbon diox-
ide and hence the rate of carbonation of the concrete.
An important effect of the moisture content of concrete is
its effect on the electrical resistivity of the concrete. Progres-
sive drying of initially water-saturated concrete results in
the electrical resistivity increasing from about 7 × 103
ohm cm to about 6000 × 103
ohm cm (Fig. 2.12).2.34
Field ob-
servations indicate that when the resistivity exceeds a lev-
el of 50 to 70 × 103
ohm cm, steel corrosion would be
negligible.2.35
Other authors quote values of resistivity of
10 x 103
ohm cm2.36
and 12 × 103
ohm cm2.37
above
which corrosion induced damage is unlikely even in the
presence of chloride ions, oxygen, and moisture.
2.3.4 Corrosion inhibiting admixtures—Numerous chemi-
cal admixtures, both organic and inorganic, have been sug-
gested as specific inhibitors of steel corrosion.2.13
Some of the
admixtures, however, may retard time of setting of the cement
or be detrimental at later ages. Many would be subject to
leaching and hence less effective in concrete that has lost sol-
uble material by leaching. Among those compounds reported
as inorganic inhibitors are potassium dichromate, stannous
chloride, zinc and lead chromates, calcium hypophosphite,
Fig. 2.11—Rate of reaction of calcium chloride with
portland cement2.29
Table 2.1—Comparison of producers analysis of
eight different portland cements with total amount of
calcium chloride that reacted with each cement2.29
1/3 SO3 content*
C3A + C4AF - 1/3SO3
Amount of calcium
chloride reacted
0.20 1.43 1.19
0.05 1.42 1.37
0.30 1.10 1.18
0.07 1.62 1.45
0.72 0.69 0.63
0.25 1.03 1.08
0.26 1.05 1.07
0.23 1.15 1.05
Average 1.18 1.13
*Data expressed in molar equivalents rather than usual percent for comparison pur-
poses. Wide differences in SO3 content obtained intentionally.
Fig. 2.12—Effect of water saturation on the resistivity
of concrete2.34

sodium nitrite, and calcium nitrite. Organic inhibitors suggest-
ed have included sodium benzoate, ethyl aniline, and mercap-
tobenzothiazole. With some inhibitors, inhibition occurs only
at addition rates sufficiently high to counteract the effects of
chlorides. Sodium nitrite has been used with apparent effec-
tiveness in Europe.2.38
Calcium nitrite2.39
is being studied in
the United States since several side effects of the sodium salt
would be avoided. Some of the side effects are low strength,
erratic times of setting, efflorescence, and enhanced suscepti-
bility to the alkali-aggregate reaction.2.40
2.3.5 Concrete quality—Concrete will offer more pro-
tection against corrosion of embedded steel if it is of a high
quality. A low water-cement ratio will slow the diffusion of
chlorides, carbon dioxide, and oxygen and also the increase in
strength of the concrete may extend the time before corrosion-
induced stresses cause cracking of the concrete. The pore vol-
ume and permeability can be reduced by lowering the water-
cement ratio. The type of cement or use of superplasticizing
and mineral admixtures may also be an important factor in
controlling the permeability and the ingress of chlorides.2.31
2.3.6 Thickness of concrete cover over steel—The amount
of concrete cover over the steel should be as large as possible,
consistent with good structural design, the severity of the ser-
vice environment, and cost. The effect of cover thickness is
more than a simple linear relationship. Considering the normal
diffusion of an electrolyte into a porous solid without chemi-
cal reaction, a relationship such as shown in Fig. 2.13 would
be anticipated. However, in the case of cement paste, the dif-
fusion of chloride ions into the paste is accompanied by both
physical adsorption and chemical binding. These effects re-
duce the concentration of chloride ion at any particular site
and hence the tendency for inward diffusion is further re-
duced. This is also illustrated in Fig. 2.13.
2.3.7 Carbonation—Carbonation occurs when the con-
crete reacts with carbon dioxide from the air or water and re-
duces the pH to about 8.5. At this low pH the steel is no
longer passive and corrosion may occur. For high-quality
concrete, in situations where the rate of carbonation is ex-
tremely slow, carbonation is normally not a problem unless
cracking of the concrete has occurred or the concrete cover
is defective or very thin. Carbonation is not a problem in
very dry concrete or in water-saturated concrete. Maximum
carbonation rates are observed at about 50 percent water sat-
uration. A more complete discussion of carbonation and the
corrosion of steel in carbonated concrete is given in Refer-
ences 2.17 and 2.21.
2.4—References
2.1. Uhlig, Herbert H., Corrosion and Corrosion Control, 2nd Edition,
John Wiley & Sons, New York, 1971, 419 pp.
2.2. Gjørv, O. E.; Vennesland, Ø; and El-Busaidy, A. H. S., “Diffusion of
Dissolved Oxygen through Concrete,” Paper No. 17, NACE Corrosion 76,
National Association of Corrosion Engineers, Houston, Mar. 1976, 13 pp.
2.3. Shalon, R., and Raphael, M., “Inﬂuence of Sea Water on Corrosion
of Reinforcement,” ACI JOURNAL, Proceedings V. 55, No. 8, Feb. 1959, pp.
1251-1268.
2.4. Grifﬁn, Donald F., and Henry, Robert L., “Effect of Salt in Concrete
on Compressive Strength, Water Vapor Transmission, and Corrosion of
Reinforcing Steel,” Proceedings, ASTM, V. 63, 1963, pp. 1046-1078.
2.5. Stratfull, R. F.; Jurkovich, W. J.; and Spellman, D. L., “Corrosion
Testing of Bridge Decks,” Transportation Research Record No. 539, Trans-
portation Research Board, 1975, pp. 50-59.
2.6. Browne, Roger D., “Mechanisms of Corrosion of Steel in Concrete
in Relation to Design, Inspection, and Repair of Offshore and Coastal
Structures,” Performance of Concrete in Marine Environment, SP-65,
American Concrete Institute, Detroit, 1980, pp. 169-204.
2.7. Aziz, M. A., and Mansur, M. A., “Deterioration of Marine Concrete
Structures with Special Emphasis on Corrosion of Steel and Its Remedies,”
Corrosion of Reinforcement in Concrete Construction, Ellis Horwood Lim-
ited, Chichester, 1983, pp. 91-99.
2.8. Gjørv, Odd E., “Durability of Concrete Structures in the Ocean
Environment,” Proceedings, FIP Symposium on Concrete Sea Structures
(Tbilisi, Sept. 1972), Federation Internationale de la Precontrainte, Lon-
don, 1973, pp. 141-145.
2.9. Foley, R. T., “Complex Ions and Corrosion,” Journal, Electrochemi-
cal Society, V. 122, No. 11, 1975, pp. 1493-1549.
2.10. Pourbaix, M., Atlas of Electrochemical Equilibrium in Aqueous
Solutions, Pergamon Press Limited, London, 1976.
2.11. Monfore, G. E., and Verbeck, G. J., “Corrosion of Prestressed Wire in
Concrete,” ACI JOURNAL, Proceedings V. 57, No. 5, Nov. 1960, pp. 491-516.
2.12. Monfore, G. E., and Ost, Borje, “Corrosion of Aluminum Conduit
in Concrete,” Journal, PCA Research and Development Laboratories, V. 7,
No. 1, Jan. 1967, pp. 10-22.
2.13. Boyd, W. K., and Tripler, A. B., “Corrosion of Reinforcing Steel
Bars in Concrete,” Materials Protection, V. 7, No. 10, 1968, pp. 40-47.
2.14. Baker, E. A.; Money, K. L.; and Sanborn, C. B., “Marine Corrosion
Behavior of Bare and Metallic-Coated Steel Reinforcing Rods in Con-
crete,” Chloride Corrosion of Steel in Concrete, STP-629, ASTM, Phila-
delphia, 1977, pp. 30-50.
2.15. Stern, M., and Geary, A. L., “Electrochemical Polarization No. 1
Theoretical Analysis of the Shape of Polarization Curves,” Journal, Elec-
trochemical Society, V. 104, No. 1, 1957, pp. 56-63.
2.16. Rosenberg, A. M., and Gaidis, J. M., “The Mechanism of Nitrite
Inhibition of Chloride Attack on Reinforcing Steel in Alkaline Aqueous
Environments,” Materials Performance, V. 18, No. 11, 1979, pp. 45-48.
2.17. “Corrosion of Reinforcement and Prestressing Tendons: A State of
the Art Report,” Materials and Structures/Research and Testing (RILEM,
Paris), V. 9, No. 51, May-June 1976, pp. 187-206.
2.18. Grimes, W. D.; Hartt, W. H.; and Turner, D. H., “Cracking of Con-
crete in Sea Water Due to Embedded Metal Corrosion,” Corrosion, V. 35,
No. 7, 1979, pp. 309-316.
2.19. Kondo, Yasuo; Takeda, Akihiko; and Hideshima, Setsuji, “Effect
of Admixtures on Electrolytic Corrosion of Steel Bars in Reinforced Con-
crete,” ACI JOURNAL, Proceedings V. 56, No. 4, Oct. 1959, pp. 299-312.
Fig. 2.13—Gradient of total chloride concentration depth
depends on whether chemical reaction occurs with cement2.41

2.20. Eickemeyer, J., “Stress Corrosion Cracking of a High-Strength
Steel in Saturated Ca(OH)2 Solutions Caused by Cl and SO4 Additions,”
Corrosion Science, V. 18, No. 4, 1978, pp. 397-400.
2.21. Hamada, M., “Neutralization (Carbonation) of Concrete and
Corrosion of Reinforcing Steel,” Proceedings, 5th International Sym-
posium on the Chemistry of Cement (Tokyo, 1968), Cement Associa-
tion of Japan, Tokyo, 1969, V. 3, pp. 343-360.
2.22. Treadaway, K. W. J., “Corrosion of Prestressed Steel Wire in
Concrete,” British Corrosion Journal (London), V. 6, 1971, pp. 66-72.
2.23. Sluijter, W. L., and Kreijger, P. C., “Potentio Dynamic Polar-
ization Curves and Steel Corrosion,” Heron (Delft), V. 22, No. 1, 1977,
pp. 13-27.
2.24. Weise, C. H., “Determination of the Free Calcium Hydroxide
Contents of Hydrated Portland Cements and Calcium Silicates,” Analyt-
ical Chemistry, V. 33, No. 7, June 1961, pp. 877-882. Also, Research
Department Bulletin No. 127, Portland Cement Association.
2.25. Rosenqvist, I. T., “Korrosjon av Armeringsstal i Betong,”
Teknisk Ukeblad (Oslo), 1961, pp. 793-795.
2.26. Spellman, D. L., and Stratfull, R. F., “Concrete Variables and
Corrosion Testing,” Highway Research Record No. 423, Highway
Research Board, 1973, pp. 27-45.
2.27. Page, C. L.; Short, N. R.; and El Tarras, A., “Diffusion of Chlo-
ride Ions in Hardened Cement Pastes,” Cement and Concrete Research,
V. 11, No. 3, May 1981, pp. 395-406.
2.28. Ramachandran, V. S., Calcium Chloride in Concrete, Applied
Science Publishers, London, 1976, 216 pp.
2.29. Rosenberg, Arnold M., “Study of the Mechanism through
Which Calcium Chloride Accelerates the Set of Portland Cement,” ACI
JOURNAL, Proceedings V. 61, No. 10, Oct. 1964, pp. 1261-1270.
2.30. Mehta, P. K., “Effect of Cement Composition on Corrosion of
Reinforcing Steel in Concrete,” Chloride Corrosion of Steel in Con-
crete, STP-629, ASTM, Philadelphia, 1977, pp. 12-19.
2.31. Gjørv, O. E., and Vennesland, Ø., “Diffusion of Chloride Ions
from Seawater into Concrete,” Cement and Concrete Research, V. 9,
No. 2, Mar. 1979, pp. 229-238.
2.32. Stratfull, R. F., Discussion of “Long-Time Study of Cement
Performance. Chapter 12—Concrete Exposed to Sea Water and Fresh
Water,” by I. L. Tyler, ACI JOURNAL, Proceedings V. 56, Part 2, Sept.
1960, pp. 1455-1458.
2.33. Helms, S. B., and Bowman, A. L., “Corrosion of Steel in Light-
weight Concrete Specimens,” ACI JOURNAL, Proceedings V. 65, No.
12, Dec. 1968, pp. 1011-1016.
2.34. Gjφrv, O. E.; Vennesland, Ø.; and El-Busaidy, A. H. S., “Elec-
trical Resistivity of Concrete in the Oceans,” OTC Paper No. 2803, 9th
Annual Offshore Technology Conference, Houston, May 1977, pp.
581-588.
2.35. Tremper, Bailey; Beaton, John L.; and Stratfull, R. F., “Causes
and Repair of Deterioration to a California Bridge Due to Corrosion of
Reinforcing Steel in a Marine Environment II: Fundamental Factors
Causing Corrosion,” Bulletin No. 182, Highway Research Board, Wash-
ington, D.C., 1958, pp. 18-41.
2.36. Browne, R. D., “Design Prediction of the Life for Reinforced
Concrete in Marine and Other Chloride Environments,” Durability of
Building Materials (Amsterdam), V. 1, 1982, pp. 113-125.
2.37. Cavalier, P. G., and Vassie, P. R., “Investigation and Repair of
Reinforcement Corrosion in a Bridge Deck,” Proceedings, Institution of
Civil Engineers (London), Part 1, V. 70, 1981, pp. 461-480.
2.38. Woods, Hubert, Durability of Concrete Construction, ACI
Monograph No. 4, American Concrete Institute/Iowa State University
Press, Detroit, 1968, p. 102.
2.39. Rosenberg, A. M.; Gaidis, J. M.; Kossivas, T. G.; and Previte,
R. W., “A Corrosion Inhibitor Formulated with Calcium Nitrite for Use
in Reinforced Concrete,” Chloride Corrosion of Steel in Concrete, STP-
629, ASTM, Philadelphia, 1977, pp. 89-99.
2.40. Craig, R. J., and Wood, L. E., “Effectiveness of Corrosion
Inhibitors and Their Inﬂuence on the Physical Properties of Portland
Cement Mortars,” Highway Research Record No. 328, Highway
Research Board, 1970, pp. 77-88.
2.41. Verbeck, George J., “Mechanisms of Corrosion of Steel in Con-
crete,” Corrosion of Metals in Concrete, SP-49, American Concrete
Institute, Detroit, 1975, pp. 21-38.
CHAPTER 3—PROTECTION AGAINST
CORROSION IN NEW CONSTRUCTION
3.1—Introduction
Measures which can be taken in reinforced concrete con-
struction to protect the steel against corrosion can be divided
into three categories:
(a) Design and construction practices that maximize the
protection afforded by the portland cement concrete.
(b) Treatments that penetrate or are applied on the surface
of the reinforced concrete member to exclude chloride ion
from the concrete.
(c) Techniques that prevent corrosion of the reinforce-
ment directly.
In the last case, two approaches are possible: to use
corrosion-resistant reinforcing steel or to nullify the effects
of chloride ions on unprotected reinforcement.
3.2—Design and construction practices
Through careful design and good construction practices,
the protection provided by portland cement concrete to em-
bedded reinforcing steel can be optimized. It is not the so-
phistication of the structural design that determines the
durability of a concrete member in a corrosive environment
but the detailing practices.3.1
The provision of adequate
drainage and a method of removing drainage water from the
structure are particularly important.
In reinforced concrete members exposed to chlorides and
subjected to intermittent wetting, the degree of protection
against corrosion is determined primarily by the depth of
cover to the reinforcing steel and the permeability of the con-
crete.3.2-3.6
Estimates of the increase in corrosion protection
provided by an increase in cover have ranged between slight-
ly more than a linear relationship3.3,3.7
to as much as the
square of the cover.3.8
The time to spalling is a function of
the ratio of cover to bar diameter,3.8
the reinforcement spac-
ing, and the concrete strength. Although conventional port-
land cement concrete is not impermeable, concrete with a
very low permeability can be made through the use of good
quality materials, a minimum water-cement ratio consistent
with placing requirements, good consolidation and finishing
practices, and proper curing.
In concrete that is continuously submerged, the rate of cor-
rosion is controlled by the rate of oxygen diffusion that is not
significantly affected by the concrete quality or the thickness
of cover.3.9 However, as noted in Chapter 2, corrosion of
embedded steel is a rare occurrence in continuously sub-
merged concrete structures.
Placing limits on the allowable amounts of chloride ion in
concrete is an issue still under active debate. On the one side
are the purists who would like to see essentially no chlorides
in concrete. On the other are the practitioners, including
those who must produce concrete under cold weather con-
ditions, precast concrete manufacturers who wish to mini-
mize curing times, producers of chloride-bearing aggregates,
and some admixture companies, who would prefer the least
restrictive limit possible. Since chlorides are present natural-
ly in most concrete-making materials, specifying a zero
chloride content for any of the mix ingredients is unrealistic.

222R-12 ACI COMMITTEE REPORT
However, it is also known that wherever chloride is present
in concrete, the risk of corrosion increases as the chloride
content increases. When the chloride content exceeds a cer-
tain value (termed the “chloride corrosion threshold”), unac-
ceptable corrosion may occur provided that other necessary
conditions, chiefly the presence of oxygen and moisture, ex-
ist to support the corrosion reactions, It is a difficult task to
establish a chloride content below which the risk of corrosion is
negligible, which is appropriate for all mix ingredients and
under all exposure conditions, and which can be measured
by a standard test.
Three different analytical values have been used to desig-
nate the chloride content of fresh concrete, hardened con-
crete, or any of the concrete mixture ingredients: (a) total,
(b) acid-soluble, and (c) water-soluble.
The total chloride content of concrete is measured by the
total amount of chlorine. Special analytical methods are
necessary to determine it, and acid-soluble chloride is often
mistakenly called total chloride. The acid-soluble method
is the test method in common use and measures that chlo-
ride that is soluble in nitric acid. Water-soluble chloride is
chloride extractable in water under defined conditions. The
result obtained is a function of the analytical test procedure,
particularly with respect to particle size, extraction time,
and temperature, and to the age and environmental expo-
sure of the concrete.
It is also important to distinguish clearly between chloride
content, sodium chloride content, calcium chloride content,
or any other chloride salt content. In this report, all referenc-
es to chloride content pertain to the amount of chloride ion
(Cl-
) present. Chloride contents are expressed in terms of the
mass of cement unless stated otherwise.
Lewis3.10
reported that, on the basis of polarization tests of
steel in saturated calcium hydroxide solution and water ex-
tracts of hydrated cement samples, corrosion occurred when
the chloride content was 0.33 percent acid-soluble chloride,
or 0.16 percent water-soluble chloride based on a 2 hr ex-
traction in water. However, it has been shown that the pore-
water in many “typical” portland cement concretes made
using relatively high-alkali cements is a strong solution of
sodium and potassium hydroxides with a pH approaching
14. Since the passivity of embedded steel is determined by
the ratio of the hydroxyl concentration to the chloride con-
centration,3.11
the amounts of chloride that can be tolerated
in concrete are higher than those that will cause pitting cor-
rosion in a saturated solution of calcium hydroxide.3.12
Work at the Federal Highway Administration
laboratories3.5
showed that for hardened concrete subject to
externally applied chlorides, the corrosion threshold was
0.20 percent acid-soluble chlorides. The average content of
water-soluble chloride in concrete was found to be 75 to 80
percent of the content of acid-soluble chloride in the same
concrete. This corrosion threshold value was subsequently
confirmed by field studies of bridge decks including those in
California3.13
and New York.3.14
These investigations show
that, under some conditions, a chloride content of as little as
0.15 percent water-soluble chloride (or 0.20 percent acid-
soluble chloride) is sufficient to initiate corrosion of embedded
steel in concrete exposed to chlorides in service. However, in
determining a limit on the chloride content of the mix ingre-
dients, several other factors need to be considered.
As noted in the figures already given, the water-soluble
chloride content is not a constant proportion of the acid-
soluble chloride content. It varies with the amount of chlo-
ride in the concrete,3.10
the mix ingredients, and the test meth-
od. All the materials used in concrete contain some chlorides,
and, in the case of cement, the chloride content in the hardened
concrete varies with cement composition. Although aggre-
gates do not usually contain significant amounts of chlo-
ride,3.15
there are exceptions. There are reports of aggregates
with an acid-soluble chloride content of more than 0.1 percent
of which less than one-third is water-soluble when the aggre-
gate is pulverized.3.16
The chloride is not soluble when the ag-
gregate is placed in water over an extended period and there is
no difference in the corrosion performance of structures in
southern Ontario made from this aggregate when compared to
other chloride-free aggregates in the region. However, this is
not always the case. Some aggregates, particularly those from
arid areas or dredged from the sea, may contribute sufficient
chloride to the concrete to initiate corrosion. A limit of 0.06
percent acid-soluble chloride ion in the combined fine and
coarse aggregate (by mass of the aggregate) has been suggest-
ed with a further proviso that the concrete should not contain
more than 0.4 percent chloride (by mass of the cement) de-
rived from the aggregate.3.17
There is thought to be a difference in the chloride corrosion
threshold value depending on whether the chloride is present
in the mix ingredients or penetrates the hardened concrete
from external sources. When chloride is added to the mix,
some will chemically combine with the hydrating cement
paste. The amount of chloride forming chloroaluminates is a
function of the C3A content of the cement.3.18
Chloride add-
ed to the mix will also tend to be distributed relatively uni-
formly and, therefore, has less tendency to cause the creation
of concentration cells.
Conversely, when chloride permeates from the surface of
hardened concrete, uniform chloride contents will not exist
around the steel because of differences in the concentration
of chlorides on the concrete surface resulting from poor
drainage, for example, local differences in permeability, and
variations in the depth of cover to the steel. All these factors
promote differences in the environment (oxygen, moisture,
and chloride content) along a given piece of reinforcement.
Furthermore, most structures contain reinforcement at dif-
ferent depths, and, because of the procedures used to fix the
steel, the steel is electrically connected. Thus, when chloride
penetrates the concrete, some of the steel is in contact with
chloride-contaminated concrete while other steel is in
chloride-free concrete. This creates a macroscopic corrosion
cell that can possess a large driving voltage and a large cath-
ode to small anode ratio which accelerates the rate of corrosion.
In seawater, it has been suggested that the permeability of
the concrete to chloride penetration is reduced by the precip-
itation of magnesium hydroxide.3.19
In laboratory studies3.20
in which sodium chloride was added
to the mix ingredients, it was found that a substantial increase in

corrosion rate occurred between 0.4 and 0.8 percent chloride,
although the moisture conditions of the test specimens were
not clearly defined. Other researchers have suggested3.21
that the critical level of chloride in the mix ingredients to
initiate corrosion is 0.3 percent and that this has a similar
effect to 0.4 percent chloride penetrating the hardened con-
crete from external sources. In studies in which calcium
chloride was added to portland cement concrete, it was
found3.22
that the chloride ion concentration in the pore so-
lution remained high during the first day of hydration. It de-
clined gradually, but it appeared that substantial
concentrations of chloride ion remained in solution indefinitely.
Chloride limits in national codes vary widely. ACI 318 al-
lows a maximum water-soluble chloride ion content of 0.06
percent in prestressed concrete, 0.15 percent for reinforced
concrete exposed to chloride in service, 1.00 percent for re-
inforced concrete that will be dry or protected from moisture
in service, and 0.30 percent for all other reinforced concrete
construction. The British Code, CP 110, allows an acid-sol-
uble chloride ion content of 0.35 percent for 95 percent of the
test results with no result greater than 0.50 percent. These
values are largely based on an examination of several struc-
tures in which it was found there was a low risk of corrosion
up to 0.4 percent chloride added to the mixture.3.23
However,
corrosion has occurred at values less than 0.4 percent,3.24
particularly where the chloride content was not uniform. The
Norwegian Code, NS 3474, allows an acid-soluble chloride
content of 0.6 percent for reinforced concrete made with nor-
mal portland cement but only 0.002 percent chloride ion for
prestressed concrete. Both these codes are under revision.
Other codes have different limits, though the rationale for
these limits is not well established.
Corrosion of prestressing steel is generally of greater
concern than corrosion of conventional reinforcement be-
cause of the possibility that corrosion may cause a local re-
duction in cross section and failure of the steel. The high
stresses in the steel also render it more vulnerable to stress-
corrosion cracking and, where the loading is cyclic, to cor-
rosion fatigue. However, most examples of failure of pre-
stressing steel that have been reported3.24-3.26
have resulted
from macrocell corrosion reducing the load carrying area
of the steel. Nevertheless, because of the greater vulnera-
bility and the consequences of corrosion of prestressing
steel, chloride limits in the mix ingredients are lower than
for conventional concrete. Based on the present state of
knowledge, the following chloride limits in concrete used
in new construction, expressed as a percentage by weight of
portland cement, are recommended to minimize the risk of
chloride-induced corrosion.
Normally concrete materials are tested for chloride con-
tent using either the acid-soluble test described in ASTM C
1152, “Acid-Soluble Chloride in Mortar and Concrete,” or
water-soluble test described in ASTM C 1218, “Water-
Soluble Chloride in Mortar and Concrete.” If the materials
meet the requirements given in either of the relevant col-
umns in the previous table they are acceptable. If they meet
neither of the relevant limits given in the table then they may
be tested using the Soxhlet test method. Some aggregates,
such as those discussed previously,3.16
contain a consider-
able amount of chloride that is sufficiently bound that it does
not initiate or contribute towards corrosion. The Soxhlet test
appears to measure only those chlorides that contribute to the
corrosion process, thus permitting the use of some aggre-
gates that would not be allowed if only the ASTM C 1152 or
ASTM C 1218 tests were used. If the materials fail the
Soxhlet test, then they are not suitable.
For prestressed and reinforced concrete that will be exposed
to chlorides in service, it is advisable to maintain the lowest pos-
sible chloride levels in the mix to maximize the service life of
the concrete before the critical chloride content is reached and a
high risk of corrosion develops. Consequently, chlorides should
not be intentionally added to the mix ingredients even if the
chloride content in the materials is less than the stated limits. In
many exposure conditions, such as highway and parking struc-
tures, marine environments, and industrial plants where chlo-
rides are present, additional protection against corrosion of
embedded steel is necessary.
Since moisture and oxygen are always necessary for elec-
trochemical corrosion, there are some exposure conditions
where the chloride levels may exceed the recommended val-
ues and corrosion will not occur. Concrete which is continu-
ously submerged in seawater rarely exhibits corrosion-
induced distress because there is insufficient oxygen present.
Similarly, where concrete is continuously dry, such as the in-
terior of a building, there is little risk of corrosion from chlo-
ride ions present in the hardened concrete. However, interior
locations that are wetted occasionally, such as kitchens or
laundry rooms or buildings constructed with lightweight ag-
gregate that is subsequently sealed (e.g., with tiles) before
the concrete dries out, are susceptible to corrosion damage.
Whereas the designer has little control over the change in use
of a building or the service environment, the chloride content
of the mix ingredients can be controlled. Estimates or judg-
ments of outdoor “dry” environments can be misleading.
Stratfull3.27
has reported the case of approximately 20 bridge
decks containing 2 percent calcium chloride built by the Cal-
ifornia Department of Transportation. The bridges were lo-
cated in an arid area where the annual rainfall was about 5
in., most of which fell at one time. Within 5 years of con-
struction, many were showing signs of corrosion-induced
spalling and most were removed from service within 10
years. For these reasons, the committee believes a conserva-
tive approach is necessary.
The maximum chloride limits suggested in this report dif-
fer from those suggested by ACI Committee 2013.2
and
those contained in the ACI Building Code. When making a
comparison between the figures, it should be noted that the
Category Chloride limit for new construction
Acid-soluble Water-soluble
Test method ASTM C 1152 ASTM C 1218 Soxhlet*
Prestressed concrete 0.08 0.06 0.06
Reinforced concrete
in wet conditions
0.10 0.08 0.08
Reinforced concrete
in dry conditions
0.20 0.15 0.15
*
The Soxhlet Method of Test is described in the appendix to this report.

figures in this report refer to acid-soluble chlorides while the
other documents make reference to water-soluble chlorides.
As noted previously, Committee 222 has taken a more con-
servative approach because of the serious consequences of cor-
rosion, the conflicting data on corrosion threshold values,
and the difficulty of defining the service environment
throughout the life of a structure.
Various nonferrous metals and alloys will corrode in damp
or wet concrete. Surface attack of aluminum occurs in the
presence of alkali-hydroxide solutions. Anodizing provides
no protection.
Much more serious corrosion can occur if the concrete
contains chloride ions, particularly if there is electrical (metal-
to-metal) contact between the aluminum and steel reinforce-
ment, because a galvanic cell is created. Serious cracking or
splitting of concrete over aluminum conduits has been re-
ported.3.28,3.29
Certain organic protective coatings have been
recommended3.30
when aluminum must be used and it is im-
practicable to avoid contamination by chlorides. Other met-
als such as zinc, nickel, and cadmium, which have been
evaluated for use as coatings for reinforcing steel, are dis-
cussed elsewhere in this chapter. Additional information is
contained in Reference 3.31.
Where concrete will be exposed to chloride, the concrete
should be made with the lowest water-cement ratio consistent
with achieving maximum consolidation and density. The ef-
fects of water-cement ratio and degree of consolidation on the
rate of ingress of chloride ions are shown in Fig. 3.1 and 3.2.
Concrete with a water-cement ratio of 0.40 was found to re-
sist penetration by deicing salts significantly better than con-
cretes with water-cement ratios of 0.50 and 0.60. A low
water-cement ratio is not, however, sufficient to insure low
permeability. As shown in Fig. 3.2, a concrete with a water-
cement ratio of 0.32 but with poor consolidation is less resis-
tant to chloride ion penetration than a concrete with a water-
cement ratio of 0.60. The combined effect of water-cement
ratio and depth of cover is shown in Fig. 3.3, which illus-
trates the number of daily applications of salt before the
chloride content reached the critical value (0.20 percent
acid-soluble) at the various depths. Thus, 40 mm (1.5 in.) of
0.40 water-cement ratio concrete was sufficient to protect
embedded reinforcing steel against corrosion for 800 appli-
cations of salt. Equivalent protection was afforded by 70 mm
(2.75 in.) of concrete with a water-cement ratio of 0.50 or 90
mm (3.5 in.) of 0.60 water-cement ratio concrete. On the ba-
sis of this work, ACI 201.2R recommends a minimum of 50
mm (2 in.) cover for bridge decks if the water-cement ratio
is 0.40 and 65 mm (2.5 in.) if the water-cement ratio is 0.45.
Even greater cover, or the provision of additional corrosion
protection treatments, may be required in some environ-
ments. These recommendations can also be applied to other
reinforced concrete components exposed to chloride ions
and intermittent wetting and drying.
Even where the recommended cover is specified, con-
struction practices must insure that the specified cover is
achieved. Conversely, placing tolerances, the method of
construction, and the level of inspection should be consid-
ered in determining the specified cover.
The role of cracks in the corrosion of reinforcing steel is
controversial. Two schools of thought exist.3.32,3.33
One
viewpoint is that cracks reduce the service life of structures
by permitting more rapid penetration of carbonation and a
means of access of chloride ions, moisture, and oxygen to the
reinforcing steel. The cracks thus accelerate the onset of the
corrosion processes and, at the same time, provide space for
the deposition of the corrosion products. The other viewpoint
is that while cracks may accelerate the onset of corrosion, such
Fig. 3.1—Effect of water-cement ratio on salt penetration3.5
Fig. 3.2—Effect of inadequate consolidation on salt
penetration3.5
Fig. 3.3—Effect of water-cement ratio and depth of cover
on relative time to corrosion3.5

corrosion is localized. Since the chloride ions eventually
penetrate even uncracked concrete and initiate more wide-
spread corrosion of the steel, the result is that after a few
years’ service there is little difference between the amount of
corrosion in cracked and uncracked concrete.
The differing viewpoints can be partly explained by the
fact that the effect of cracks is a function of their origin,
width, intensity, and orientation. Where the crack is perpen-
dicular to the reinforcement, the corroded length of inter-
cepted bars is likely to be no more than three bar
diameters.3.33
Cracks that follow the line of a reinforcement
bar (as might be the case with a plastic shrinkage crack, for
example) are much more damaging because the corroded
length of bar is much greater and the resistance of the con-
crete to spalling is reduced. Studies have shown that cracks
less than about 0.3 mm (0.012 in.) wide have little influence
on the corrosion of reinforcing steel.3.8
Other investigations
have shown that there is no relationship between crack width
and corrosion.3.34-3.36
Furthermore, there is no direct rela-
tionship between surface crack width and the internal crack
width. Consequently, it has been suggested that the control
of surface crack widths in design codes is not the most ratio-
nal approach.3.37
For the purposes of design, it is useful to differentiate be-
tween controlled and uncontrolled cracks. Controlled cracks
are those which can be reasonably predicted from a knowl-
edge of section geometry and loading. For cracking perpen-
dicular to the main reinforcement, the necessary conditions
for crack control are that there be sufficient steel so that it re-
mains in the elastic state under all loading conditions, and
that at the time of cracking, the steel is bonded (i.e., cracking
must occur after the concrete has set).
Examples of uncontrolled cracking are cracks resulting
from plastic shrinkage, settlement, or an overload condition.
Uncontrolled cracks are frequently wide and usually cause
concern, particularly if they are active. However, they can-
not be dealt with by conventional design procedures, and
measures have to be taken to avoid their occurrence or, if
they are unavoidable, to induce them at places where they
are unimportant or can be conveniently dealt with, by sealing
for example.
3.3—Methods of excluding external sources of
chloride ion from concrete
3.3.1 Waterproof membranes—Waterproof membranes
have been used extensively to minimize the ingress of chlo-
ride ions into concrete. Since external sources of chloride
ions are waterborne, a barrier to water will also act as a bar-
rier to any dissolved chloride ions.
The requirements for the ideal waterproofing system are
straightforward;3.38
it should:
(a) Be easy to install,
(b) Have good bond to the substrate,
(c) Be compatible with all the components of the system
including the substrate, prime coat, adhesives, and overlay
(where used),
(d) Maintain impermeability to chlorides and moisture un-
der service conditions, especially temperature extremes,
crack bridging, aging, and superimposed loads.
The number of types of products manufactured to satisfy
these requirements makes generalization difficult. Any sys-
tem of classification is arbitrary, though one of the most use-
ful is the distinction between the preformed sheet systems
and the liquid-applied materials.3.38
The preformed sheets
are manufactured under factory conditions but are often dif-
ficult to install, usually require adhesives, and are highly vul-
nerable to the quality of the workmanship at critical
locations in the installation. Although it is more difficult to
control the quality of the liquid-applied systems, they are
easier to apply and tend to be less expensive.
Given the different types and quality of available water-
proofing products, the differing degrees of workmanship,
and the wide variety of applications, it is not surprising that
laboratory3.39-3.41
and field3.14,3.42,3.43
evaluations of
membrane performance have also been variable and some-
times contradictory. Sheet systems generally perform better
than liquid-applied systems in laboratory screening tests be-
cause quality of workmanship is not a factor. Although there
has been little uniformity in both methods of test or accep-
tance criteria, permeability (usually determined by electrical
resistance measurements) has generally been adopted as the
most important criterion. However, some membranes do of-
fer substantial resistance to chloride and moisture intrusion,
even when pinholes or bubbles occur in the membrane.3.41
Field performance has been found to depend not only on the
type of waterproofing material used, but also on the quality of
workmanship, weather conditions at the time of installation, de-
sign details, and the service environment. Experience has
ranged from satisfactory3.43
to failures that have resulted in the
membrane having to be removed.3.44,3.45
Blistering, which affects both preformed sheets and liquid-
applied materials, is the single greatest problem encountered
in applying waterproofing membranes.3.46
It is caused by the
expansion of entrapped gases, solvents, and moisture in the
concrete after application of the membrane. The frequency
of blisters occurring is controlled by the porosity and mois-
ture content of the concrete3.47
and the atmospheric conditions.
Water or water vapor is not a necessary requirement for blis-
ter formation, but is often a contributing factor. Blisters may
also result from an increase in concrete temperature or a de-
crease in atmospheric pressure during or shortly after appli-
cation of membranes. The rapid expansion of vapors during
the application of hot-applied products sometimes causes
punctures (which are termed “blowholes”) in the membrane.
Membranes can be placed without blisters if the atmo-
spheric conditions are suitable during the curing period.
Once cured, the adhesion of the membrane to the concrete is
usually sufficient to resist blister formation. To insure good
adhesion, the concrete surface must be carefully prepared
and be dry and free from curing membranes, laitance, and
contaminants such as oil drippings. Sealing the concrete pri-
or to applying the membrane is possible, but rarely practi-
cal.3.48
Where the membrane is to be covered (for example,
with insulation or a protective layer), the risk of blister for-

mation can be reduced by minimizing the delay between
placement of the membrane and the overlay.
Venting layers have been used in Europe, but rarely in
North America, to prevent blister formation by allowing the
vapor pressures to disperse beneath the membrane. The dis-
advantages of using venting layers are that they require con-
trolled debonding of the membrane, leakage through the
membrane is not confined to the immediate area of the punc-
ture, and they increase cost.
3.3.2 Polymer impregnation—Polymer impregnation con-
sists of filling some of the voids in hardened concrete with a
monomer and polymerizing in situ. Laboratory studies have
demonstrated that polymer impregnated concrete (PIC) is
strong, durable, and almost impermeable. The properties of
PIC are largely determined by the polymer loading in the
concrete. Maximum polymer loadings are achieved by dry-
ing the concrete to remove nearly all the evaporable water,
removing air by vacuum techniques, saturation with a monomer
under pressure, and polymerizing the monomer in the voids
of the concrete while simultaneously preventing evaporation
of the monomer.
In the initial laboratory studies on PIC, polymerization
was accomplished by gamma radiation.3.49
However, this is
not practical for use in the field. Consequently, chemical ini-
tiators, which decompose under the action of heat or a chem-
ical promoter, have been used exclusively in field
applications. Multifunctional monomers are often used to in-
crease the rate of polymerization.
The physical properties of PIC are determined by the ex-
tent to which the ideal processing conditions are compro-
mised. Since prolonged heating and vacuum saturation are
difficult to achieve, and increase processing costs substan-
tially, most field applications have been aimed toward pro-
ducing only a surface polymer impregnation, usually to a
depth of about 25 mm (1 in.). Such partially impregnated
slabs have been found to have good resistance to chloride
penetration in laboratory studies, but field applications have
not always been satisfactory.3.5,3.50
There have been a few full-scale applications of PIC to
protect reinforcing steel against corrosion and it must still be
considered largely an experimental process. Some of the dis-
advantages of PIC are that the monomers are expensive and
that the processing is lengthy and costly. The principal defi-
ciency identified to date has been the tendency of the con-
crete to crack during heat treatment.
3.3.3 Polymer concrete overlays—Polymer concrete over-
lays consisting of aggregate in a polymer binder have been
placed experimentally.
Most monomers have a low tolerance to moisture and low
temperatures; hence the substrate must be dry and in excess
of 4 C (40 F). Improper mixing of the two (or more) compo-
nents of the polymer has been a common source of problems
in the field. Aggregates must be dry so as not to inhibit the
polymerization reaction.
Workers should wear protective clothing when working
with epoxies and some other polymers because of the potential
for skin sensitization and dermatitis.3.51
Manufacturers’ rec-
ommendations for safe storage and handling of the chemi-
cals must be followed.
A bond coat of neat polymer is usually applied ahead of
the polymer concrete. Blistering, which is a common phe-
nomenon in membranes, has also caused problems in the ap-
plication of polymer concrete overlays. A number of
applications were reported in the 1960s.3.52,3.53
Many lasted
only a few years. More recently, experimental polymer over-
lays based on a polyester-styrene monomer have been
placed, using heavy-duty finishing equipment to compact
and finish the concrete.3.54
3.3.4 Portland cement concrete overlays—Portland ce-
ment concrete overlays for new reinforced concrete are ap-
plied as part of a two-stage construction process. The overlay
may be placed before the first-stage concrete has set or sev-
eral days later, in which case a bonding layer is used between
the two lifts of concrete. The advantage of the first alterna-
tive is that the overall time of construction is shortened and
costs minimized. In the second alternative, cover to the
reinforcing steel can be assured, and small construction tol-
erances achieved because dead load deflections from the
overlay are very small. No matter which sequence of con-
struction is employed, materials can be incorporated in the
overlay to provide superior properties, such as resistance to
salt penetration and wear and skid resistance, than possible us-
ing single stage construction.
Where the second stage concrete is placed after the first
stage has hardened, sand or water blasting is required to
remove laitance and to produce a clean, sound surface.
Resin curing compounds should not be used on the first-
stage construction because they are difficult to remove.
Etching with acid was once a common means of surface
preparation.3.55,3.56
but is now rarely used because of the
possibility of contaminating the concrete with chlorides
and the difficulty of disposing of the runoff.
Several different types of concrete have been used as con-
crete overlays including conventional concrete,3.57
concrete
containing steel fibers,3.57 and internally sealed con-
crete.3.57,3.60
However, two types of concrete, low-slump
and latex-modified concrete, each designed to offer maxi-
mum resistance to penetration by chloride ions, have been
used most frequently.
3.3.5 Low-slump concrete overlays—The performance of
low-slump concrete is dependent solely on the use of con-
ventional materials and good quality workmanship. The
water-cement ratio is reduced to the minimum practical
(usually about 0.32) through the use of a high cement content
(over 470 kg/m3
or 800 lb/yd3
) and a water content sufficient
to produce a slump less than 25 mm (1 in.). The concrete is
air-entrained and a water-reducing admixture or mild retard-
er is normally used. The use of such a high cement factor and
low workability mixture dictates the method of mixing, plac-
ing, and curing the concrete.
Following preparation of the first-stage concrete, a bond-
ing agent of either mortar or cement paste is brushed into the
base concrete just before the application of the overlay. The
base concrete is not normally prewetted. The overlay con-
crete is mixed on site, using either a stationary paddle mixer

or a mobile continuous mixer, because truck mixers are not
suited to producing either the quantity or consistency of con-
crete required. The concrete must be compacted and screed-
ed to the required surface profile using equipment specially
designed to handle stiff mixtures. Such machines are much
heavier and less flexible than conventional finishing ma-
chines and have considerable vibratory capacity. The perme-
ability of the concrete to chloride ions is controlled by its
degree of consolidation, which is often checked with a nucle-
ar density meter as concrete placement proceeds. Wet burlap
is placed on the concrete as soon as practicable without dam-
aging the overlay (usually within 20 min of placing), and the
wet curing is continued for at least 72 hr. Curing compounds
are not used, since not only is externally available water re-
quired for more complete hydration of the cement, but the
thin overlay is susceptible to shrinkage cracking and the wet
burlap provides a cooling effect by evaporation of the water.
Low-slump concrete was originally proposed as a repair
material for concrete pavements3.55
and was developed for
patches and overlays on bridges3.61-3.63
in the early 1960s.
More recently, concrete overlays have been used as a pro-
tection against reinforcing steel corrosion in new bridges. In
general, the performance of low-slump overlays has been
good.3.64-3.66
Local bond failures have been reported, but
these have been ascribed to inadequate surface preparation3.64
or premature drying of the grout.3.66
The overlays are suscep-
tible to cracking, especially on continuous structures, though
this is a characteristic of all rigid overlays.
3.3.6 Latex-modified concrete overlays—Latex-modified
concrete is conventional portland cement concrete with the
addition of a polymeric latex emulsion. The water of sus-
pension in the emulsion hydrates the cement and the poly-
mer provides supplementary binding properties to produce a
concrete having a low water-cement ratio, good durability,
good bonding characteristics, and a high degree of resistance
to penetration by chloride ions, all of which are desirable
properties in a concrete overlay.
The latex is a colloidal dispersion of synthetic rubber par-
ticles in water. The particles are stabilized to prevent coagu-
lation, and antifoaming agents are added to prevent
excessive air entrapment during mixing. Styrene-butadiene
latexes have been used most widely. The rate of addition of
the latex is approximately 15 percent latex solids by weight
of the cement.
The construction procedures for latex-modified concrete
are similar to those for low-slump concrete with minor mod-
ifications. The principal differences are:
1. The base concrete must be prewetted for at least 1 hr pri-
or to placing the overlay, because the water aids penetration
of the base and delays film formation of the latex.
2. A separate bonding agent is not always used, because
sometimes a portion of the concrete itself is brushed over the
surface of the base.
3. The mixing equipment must have a means of storing
and dispensing the latex.
4. The latex-modified concrete has a high slump so that
conventional finishing equipment can be used.
5. Air entrainment of the concrete is believed not required
for resistance to freezing and thawing.
6. A combination of moist curing to hydrate the port-
land cement and air drying to develop the film forming
qualities of the latex are required. Typical curing times
are 24 hr wet curing, followed by 72 hr of dry curing. The
film-formation property of the latex is temperature sensi-
tive and film strengths develop slowly at temperatures be-
low 13 C (55 F). Curing periods at lower temperatures
may need to be extended and application at temperatures
less than 7 C (45 F) is not recommended.
Hot weather causes rapid drying of the latex-modified con-
crete, which makes finishing difficult and promotes shrinkage
cracking. Some contractors have placed overlays at night to
avoid these problems. The entrapment of excessive amounts
of air during mixing has also been a problem in the field. Most
specifications limit the total air content to 6.5 percent. Higher
air contents reduce the flexural, compressive, and bond
strengths of the overlay. Furthermore, the permeability to
chloride ions increases significantly at air contents greater
than about 9 percent. Where a texture is applied to the concrete
as, for example, grooves to impart good skid resistance, the
time of application of the texture is crucial. If applied too soon,
the edges of the grooves collapse because the concrete flows.
If the texturing operation is delayed until after the latex film
forms, the surface of the overlay tears and, since the film does
not reform, cracking often results.
High material costs and the superior performance of latex-
modified concrete in chloride penetration tests have led to
latex-modified concrete overlays being thinner than most
low-slump concrete overlays. Typical thicknesses are 40 and
50 mm (1.5 and 2 in.).
Although latex-modified overlays were first used in
1957,3.67
the majority of installations were placed after
1975. Performance has been generally satisfactory, though
extensive cracking and some debonding have been report-
ed,3.68
especially in overlays 20 mm (0.75 in.) thick that were
not applied at the time of the original deck construction. The
most serious deficiency reported has been the widespread oc-
currence of shrinkage cracking in the overlays. Many of these
cracks have been found not to extend through the overlay and it
is uncertain whether this will impair long-term performance.
3.4—Methods of protecting reinforcing steel from
chloride ion
The susceptibility to corrosion of mild steel reinforcement
in common use is not thought to be significantly affected by
its composition, grade, or the level of stress.3.69
Consequent-
ly, to prevent corrosion of the reinforcing steel in a corrosive
environment, either the reinforcement must be made of a
noncorrosive material, or conventional reinforcing steel
must be coated to isolate the steel from contact with oxygen,
moisture, and chlorides.
3.4.1 Noncorrosive steels—Natural weathering steels
commonly used for structural steelwork do not perform well
in concrete containing moisture and chloride3.2
and are not
suitable for reinforcement. Stainless steel reinforcement has
been used in special applications, especially as hardware for

attaching panels in precast concrete construction, but is
much too expensive to replace mild-steel reinforcement in
most applications. Stainless-steel-clad bars have been evalu-
ated in the FHWA time-to-corrosion studies, and found to re-
duce the frequency of corrosion-induced cracking compared
to black steel in the test slabs, but did not prevent it. Howev-
er, it was not determined whether the cracking was the result
of corrosion of the stainless steel or corrosion of the basis
steel at flaws in the cladding.
3.4.2 Coatings—Metallic coatings for steel reinforcement
fall into two categories, sacrificial and noble or nonsacrifi-
cial. In general, metals with a more negative corrosion po-
tential (less noble) than steel, such as zinc and cadmium,
give sacrificial protection to the steel. If the coating is dam-
aged, a galvanic couple is formed in which the coating is the
anode. Noble coatings such as copper and nickel protect the
steel only as long as the coating is unbroken, since any ex-
posed steel is anodic to the coating. Even where steel is not
exposed, macrocell corrosion of the coating may occur in
concrete through a mechanism similar to the corrosion of un-
coated steel.
Nickel,3.70,3.71
cadmium,3.72
and zinc3.70,3.73,3.74
have all
been shown to be capable of delaying, and in some cases pre-
venting, the corrosion of reinforcing steel in concrete, but
only zinc-coated (galvanized) bars are commonly available.
Results of the performance of galvanized bars have been
conflicting, in some cases extending the time-to-cracking of
laboratory specimens,3.75
in others reducing it3.76
and some-
times giving mixed results.3.77
It is known that the zinc will
corrode in concrete3.71,3.78
and that pitting can occur under
conditions of nonuniform exposure in the presence of high
chloride concentrations.3.79
Field studies of galvanized bars in service for many years in
either a marine environment or exposed to deicing salts have
failed to show any deficiencies in the concrete.3.74
However, in
these studies, chloride ion concentrations at the level of the re-
inforcing steel were low, such that the effectiveness could not
be established conclusively. Marine studies3.80
and accelerated
field studies3.81
have shown that galvanizing will delay the on-
set of delaminations and spalls but will not prevent them. In
general, it appears that only a slight increase in life will be ob-
tained in severe chloride environments.3.82
When galvanized
bars are used, all bars and hardware in the structure should be
coated with zinc to prevent galvanic coupling between coated
and uncoated steel.3.82
Numerous nonmetallic coatings for steel reinforcement
have been evaluated,3.83-3.86
but only fusion-bonded epoxy
powder coatings are produced commercially and widely
used. The epoxy coating isolates the steel from contact with
oxygen, moisture, and chloride.
The process of coating the reinforcing steel with the epoxy
consists of electrostatically applying finely divided epoxy
powder to thoroughly cleaned and heated bars. Many plants
operate a continuous production line and many have been
constructed specifically for coating reinforcing steel. Integ-
rity of the coating is monitored by a holiday detector in-
stalled directly on the production line. The use of epoxy-
coated reinforcing steel has increased substantially since it
was first used in 1973.
The chief difficulty in using epoxy-coated bars has been
preventing damage to the coating in transportation and han-
dling. Cracking of the coating has also been observed during
fabrication where there has been inadequate cleaning of the
bar prior to coating or the thickness of the coating has been
outside specified tolerances. Padded bundling bands, fre-
quent supports, and nonmetallic slings are required to pre-
vent damage during transportation. Coated tie wires and bar
supports are needed to prevent damage during placing. Ac-
celerated time-to-corrosion studies have shown that nicks
and cuts in the coating do not cause rapid corrosion of the ex-
posed steel and subsequent distress in the concrete.3.87
It
should be noted, however, that the damaged bars were not
electrically connected to uncoated steel in the early acceler-
ated tests. Subsequent tests3.88
showed that even in the case
of electrical coupling to large amounts of uncoated steel, the
performance of damaged and nonspecification bars was
good, but not as good as when all the steel was coated.
Consequently, for long life in severe chloride environments,
consideration should be given to coating all the reinforcing
steel. If only some of the steel is coated, precautions should
be taken to assure that the coated bars are not electrically
coupled to large quantities of uncoated steel. A damaged
coating can be repaired using a two-component liquid epoxy,
but it is more satisfactory to adopt practices that prevent
damage to the coating and limit touch-up only to bars where
the damage exceeds approximately 2 percent of the area of
the bar.
Studies have demonstrated that epoxy-coated, deformed
reinforcing bars embedded in concrete can have bond
strengths and creep behavior equivalent to those of uncoated
bars.3.89,3.90
Another study3.91
reported that epoxy-coated
reinforcing has less slip resistance than normal mill scale re-
inforcing, although, for the particular specimens tested, the
epoxy-coated bars attained stress levels compatible with
ACI development requirements.
3.5—Corrosion control methods
3.5.1 Chemical inhibitors—A corrosion inhibitor is an ad-
mixture to the concrete used to prevent the corrosion of em-
bedded metal. The mechanism of inhibition is complex and
there is no general theory applicable to all situations.
The effectiveness of numerous chemicals as corrosion in-
hibitors for steel in concrete3.69,3.92-3.94
has been studied.
The compound groups investigated have been primarily
chromates, phosphates, hypophosphites, alkalies, nitrites,
and fluorides. Some of these chemicals have been suggested
as being effective; others have produced conflicting results
in laboratory screening tests.
Many inhibitors that appear to be chemically effective pro-
duce adverse effects on the physical properties of the concrete,
such as a significant reduction in compressive strength. More
recently, calcium nitrite has been reported to be an effective
corrosion inhibitor3.95
and studies are continuing.3.88

222 r 96 - corrosion of metals in concrete

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