Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation
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286 High-performance organic coatings
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Part III
Applications for high-performance organic coatings
© 2008, Woodhead Publishing Limited
289
14
Organic coatings for concrete and
rebars in reinforced concrete structures
A S KHANNA,
Indian Institute of Technology Bombay, India
14.1 Introduction
The word ‘concrete’ is derived from a Latin verb which literally means to
grow together [1]. Correctly engineered and constructed, concrete is one
of the strongest construction materials available, which will last longer.
Basically a composite material, concrete is produced by mixing together
by itself has high compressive strength, but has low strength when loaded
in tension. Reinforced concrete uses steel to provide the much-needed
tensile properties in structural concrete. Rebars are thus steel rods, embed-
ded in concrete in such a manner that the composite material resists the
in which reinforcement bars or rebars have been incorporated to strengthen
the concrete material that would otherwise be brittle. Initially employed
for projects of modest scale, reinforced concrete is now one of the major
structural materials available and its usage by mankind is second only to
that of water.
Two conditions are essential for durability of reinforced concrete:
1. The concrete should be of high quality and should not undergo envi-
ronmental degradation.
2. The reinforcing steel must not be allowed to corrode.
However, due to the varied environmental exposure conditions, concrete
may undergo degradation and reinforcing steel may also corrode. If left
unchecked, together they lead to loss of serviceability and integrity of the
structure.
14.2 Deterioration of reinforced concrete
It is important to understand the mechanism of the deterioration of con-
crete and the corrosion of reinforcing steel within the concrete. Direct
Portland type cement, aggregates (coarser and finer) and water. Concrete
load on the concrete. Reinforced concrete can thus be defined as concrete
© 2008, Woodhead Publishing Limited
290 High-performance organic coatings
deterioration of concrete may be physical, structural or chemical. Physical
impact occurring in tidal zones, occurrence of extreme temperatures, mois-
ture, etc. Chemicals attack by destroying the cement paste, thus weakening
the concrete. Sulphate attacks, acid attack, the alkali–silica reaction within
the concrete, and chemical attack by natural and industrial liquids are some
examples. In many cases the resistance of concrete to these forms of attack
is determined primarily by its penetrability. Structural degradation may be
due to poor mix design, type of cement and aggregates used, impact and
overload effects, etc.
Concrete in general provides protection to steel reinforcement for the
following two reasons:
1. Concrete provides a highly alkaline environment to steel reinforcement
which passivates the steel surface and hence prevents it from
corrosion.
2. Concrete prevents the ingress of corrosion species like oxygen, chloride
ions, carbon dioxide and water.
However, the deterioration of concrete as mentioned earlier may lead to
loss of such protection as a result of loss of effective concrete cover. In
addition, changes to the concrete environment itself may render it corro-
sive. Subsequent corrosion of steel produces corrosion products which have
2–4 times the volume of the original steel [2, 3]. This in turn generates
stresses, causing cracking and spalling of the concrete.
14.3 Corrosion of steel reinforcement in concrete
The hydraulic binder in concrete, usually Portland cement, reacts with
water to form a porous matrix of hydration products between the aggregate
particles and around the reinforcement. Thus the reinforcing steel is usually
in contact with moisture containing dissolved oxygen and the necessary
cement hydration is that the aqueous phase rapidly acquires a high pH.
Furthermore, the material contains a substantial portion of reserve basicity
in the form of sparingly soluble Ca(OH)
2
and the system is therefore buff-
ered to resist downward pH changes at a value of 12.6 [2, 3].
When steel is in contact with an alkaline solution of pH values in this
a barrier to further metal dissolution. The protective oxide layers that
develop are either Fe
2
O
3
or Fe
3
O
4
, both of which are stable in concrete.
The most stable layer in concrete is Fe
2
O
3
written as γ-FeOOH, formed by
the interaction of O
2
with Fe(OH)
2
[2]. When the local environment at the
rebar/concrete interface cannot maintain the passive state of reinforcing
deterioration may be due to the effects of freeze/thaw, fire damage, wave
reactants are present to permit corrosion. However, a significant feature of
range, it is normally passive. A thin oxide film covers the surface, presenting
© 2008, Woodhead Publishing Limited
Organic coatings for concrete 291
steel, active corrosion in either uniform or localized form (pitting) will
occur. Corrosion of the reinforcement, forming rust, leads to a loss of bond
between the steel and the concrete and subsequent delamination and
spalling [2].
14.4 The corrosion process
Corrosion of steel embedded in concrete is an electrochemical process [2,
3]. The surface of the corroding steel functions as a mixed electrode that is
a composite of anodes and cathodes electrically connected through the
body of the steel itself, upon which coupled anodic and cathodic reactions
take place. Concrete pore water functions as an aqueous medium, i.e., a
complex electrolyte. Therefore, a reinforcement corrosion cell is formed,
as shown in Fig. 14.1.
Reactions at the anodes and cathodes are broadly referred to as ‘half-cell
reactions’. The ‘anodic reaction’ is the oxidation process, which results in
dissolution or loss of metal, while the ‘cathodic reaction’ is the reduction
process which results in reduction of dissolved oxygen forming hydroxyl
ions.
For steel embedded in concrete, the following are the possible anodic
reactions depending on the pH of interstitial electrolyte, the presence of
aggressive anions, and the existence of an appropriate electrochemical
potential at the steel surface:
3Fe + 4H
2
O → Fe
3
O
4
+ 8H
+
+ 8e
−
2Fe + 3H
2
O → Fe
2
O
3
+ 6H
+
+ 6e
−
Fe + 2H
2
O → HFeO
2
−
+ 3H
+
+ 2e
−
Fe → Fe
2+
+ 2e
−
Corrosion
product
Reinforcing
steel
Cl
–
CO
2
Fe
Fe
++
2e
–
2e
–
1
2
O
2
+H
2
O+2e
–
2OH
–
Anode
Cathode
Concrete
Fe
++
+ 2OH
–
Fe(OH)
2
2OH
H
2
O
14.1 Schematic illustration of the corrosion of reinforcement steel in
concrete as an electrochemical process.
© 2008, Woodhead Publishing Limited
292 High-performance organic coatings
The possible cathodic reactions depend on the availability of O
2
and on the
pH in the vicinity of the steel surface. The most likely reactions are as
follows:
2H
2
O + O
2
+ 4e
−
→ 4OH
−
or
2H
+
+ 2e
−
→ H
2
−
14.5 Corrosion initiation and propagation mechanism
Corrosion initiation and propagation occur mainly due to the diffusion of
Cl
−
, CO
2
, SO
2
, O
2
and moisture through the reinforced concrete. The two
major factors destroying the passivation of steels in concrete are:
1. Reduction in pH level by ingress of atmospheric carbon dioxide
2. Penetration of chloride from the environment [2, 3].
14.5.1 Carbonation/ingress of carbon dioxide
The pore solution has a very high concentration of hydroxide (high pH).
[Ca(OH)
2
] has a limited solubility which is markedly dependent on the
OH
−
concentration in the solution. Na and K, on the other hand, are more
soluble and are almost completely dissolved in the concrete pore solution.
As a result, very little [Ca(OH)
2
] is dissolved in the concrete pore solution
[2–4].
Carbonation of concrete results when atmospheric carbon dioxide (CO
2
)
dissolves in the cement pore solution to form carbonic acid (H
2
CO
3
):
CO
2
+ H
2
O → H
2
CO
3
This reacts with some of the products of cement hydration as well as neu-
tralizing the calcium hydroxide [Ca(OH)
2
] present:
H
2
CO
3
+ Ca(OH)
2
→ CaCO
3
·2H
2
O
As the reserve levels of Ca(OH)
2
are depleted, a zone of low pH (the car-
bonated zone) extends from the surface of the concrete. This fall in pH
pH, corrosion is initiated by destruction of the protective oxide layer (Fe
2
O
3
or Fe
3
O
4
).
of reinforcement cover, moisture conditions, humidity, temperature, and
the type and quantity of cement used. The carbonation process follows an
equation X = k·t
½
, where X = penetration depth, t = exposure time and k is
from 14.0 to about 8.0 renders the passive film unstable, since at this low
The secondary factors which influence the carbonation process are depth
© 2008, Woodhead Publishing Limited
Organic coatings for concrete 293
a constant which is dependent on the diffusivity of CO
2
through the con-
crete, the concentration difference and the quantity of bound CO
2
[4]
.
14.5.2 Chloride ion effect/ingress of chloride ions
Chloride may be present in concrete from different sources. The following
are some of the sources:
• Aggregates containing chloride salines
• Mixing water
• Sea-coastal environment
• Road de-icing salts, etc.
The chloride ions attack the passive layer and catalyze the corrosion process.
Chloride ions break down the passive layer and cause pitting of the steel
reinforcement [2–4]. There are two processes which proceed simultane-
−
and a breakdown process by chloride ions. The chloride ions achieve a
critical concentration, dependent on the pH and pH concentration before
it breaks down the passive Fe
2
O
3
or Fe
3
O
4
layer. In general, the chloride
threshold is taken to be 0.15% of the soluble chloride by weight of cement
[2–4].
The following are the major effects of chloride on the corrosion of rein-
forcing steel:
1. Chloride is absorbed in the protective oxide layer.
2. The oxidized iron reacts to form soluble intermediate iron complex at
the anode:
Fe → Fe
2+
+ 2e
−
Fe
2+
+ 4Cl
−
→ (FeCl
4
)
−2
3. The complex reacts with moisture to form Fe(OH)
2
:
(FeCl
4
)
−2
+ 2H
2
O → Fe(OH)
2
+ 2H
+
+ 4Cl
−
The pH is lowered and the concentration of chloride is increased. Once
the chlorides are released on the metal surface, the process becomes
self-generating and no further chloride ions are required. The repeated
cycle of effects 2 and 3 continues until the protective layer of Fe
2
O
3
or
Fe
3
O
4
is completely destroyed. One of the major forms of corrosion
brought by the chloride ion is pitting of reinforcing steel. Pitting is an
autocatalytic process which continues until a hole is created in the rein-
forcing concrete.
ously: a repair process of the disrupted protective oxide film by OH ions,
© 2008, Woodhead Publishing Limited
294 High-performance organic coatings
Besides carbonation and chloride ingress attack, the presence of oxygen,
the different types of cement used, potential differences between two sites
on the rebar surface, permeability of concrete, and the construction prac-
concrete [2–4].
14.6 Methods of corrosion control and protection
Despite the impressive progress made in controlling the corrosion of steel
in concrete in the last 30 years, reinforcement corrosion in concrete struc-
tures is still a challenging problem. Important methods to mitigate rein-
forcement corrosion can be broadly categorized as:
reinforcement
• Use of admixtures
• Use of corrosion inhibitors
• Cathodic protection method
• Protective coatings: concrete coatings, rebar coatings.
Mild steel bars conforming to IS:432 and cold-worked steel high-strength
deformed bars conforming to IS:1786 (grade Fe 415 and grade Fe 500,
where 415 and 500 indicate yield stresses of 415 N/mm
2
and 500 N/mm
2
respectively) are the commonly used reinforcements in concrete. However,
the corrosion aspect of these steels drives the search for alternatives that
will guarantee performance in every sense.
To keep pace with the rapidly changing needs and technological require-
ments of steels for the construction industries, continuous efforts have been
made towards development of reinforcement bars (rebars) with higher
strength levels and superior product attributes. Cost-effective steel, having
its composition controlled to impart intrinsically improved corrosion resis-
tance properties, reducing the rate of corrosion from the time of construc-
tion, and thus lengthening the life of the structure, is one such practical
solution addressing the corrosion issue.
Thermomechanically treated steel known as TMT steel can be described
as new-generation high-strength steel having superior properties such as
weldability, strength, ductility and bendability and meeting the highest
quality standards at international level. A tempered layer of martensite
that is formed at the outer surface of the rebar protects it from mechanical
damage, thus enhancing its corrosion properties indirectly.
tices, all play a significant, vital role in initiation of corrosion in reinforced
• Modification of steel composition or use of a better alloy as
14.7 Modification of steel composition
© 2008, Woodhead Publishing Limited
Organic coatings for concrete 295
Another development is the production of corrosion resistant steel,
called CRS. Carbon content in this steel is restricted to 0.18%, manganese
is absent, silicon is 0.45% and the percentage of corrosion resistant ele-
ments such as chromium is as high as 1.5%. Cu, Ni, Mo, Nb, B, Zr, W, Ti
and P are the other alloying elements imparting the corrosion-resistant
property to these steels [4].
Innovations and discoveries on this front are on a rise with a variety of
customary old black mild steel rebars.
14.8 Admixtures in concrete
Concrete admixtures are used to improve the behavior of concrete under
a variety of conditions and are of two main types: chemical and mineral [5].
Chemical admixtures reduce the cost of construction, modify properties of
hardened concrete, ensure quality of concrete during mixing/transporting/
placing/curing, and overcome certain emergencies during concrete opera-
tions [5]. They fall into the categories of air entrainers, water reducers, set
retarders, set accelerators, superplasticizers, and specialty admixtures which
include corrosion inhibitors, shrinkage control, alkali–silica reactivity inhib-
itors, and coloring.
Mineral admixtures make mixtures more economical, reduce permeabil-
admixtures affect the nature of the hardened concrete through hydraulic
or pozzolanic activity [5]. Pozzolans are cementitious materials and include
ash and silica fume. They can be used with Portland cement, or blended
cement, either individually or in combination.
14.9 Corrosion inhibitors in concrete
The most practical and economical approach to minimize or eliminate the
costly maintenance problem may well be to use better materials in new
construction. This can be accomplished by adding corrosion inhibitors to a
quality concrete mix.
Corrosion inhibitors can be divided into three types: anodic, cathodic and
mixed, depending on whether they interfere with the corrosion preferen-
tially at the anodic or cathodic sites or whether both are involved [3, 6]. In
the class of anodic inhibitors, calcium nitrite, sodium nitrite, sodium benzo-
ate and sodium chromate are commonly used. Cathodic inhibitors mainly
consist of amines, phosphates, zincates, aniline and its chloroalkyl nitro-
substituted form, and aminoethanol groups. Some work to create a protec-
tive barrier that stabilizes the layer of rust surrounding the steel; others
modified steels available, which are far better in performance than the
ity, increase strength, and influence other concrete properties. Mineral
natural pozzolans (such as the volcanic ash used in Roman concrete), fly
© 2008, Woodhead Publishing Limited
296 High-performance organic coatings
provide a thin protective coating that prevents chlorides from reacting with
the steel. Yet others provide inhibitors that react with the iron to form a
steel, or reduces the corrosion of steel by acting as an oxygen barrier. Table
14.1 shows the development of concrete reinforcement corrosion inhibitor
technology and application methods.
on a large scale for reinforced concrete [6]. Calcium nitrite inhibits corro-
inhibitive action of calcium nitrite depends on its reaction with Fe
2+
ions
according to the following reaction:
2Fe
2+
+ 2OH
−
+ 2NO
2
−
→ 2NO↑ + Fe
2
O
3
+ H
2
O
Calcium nitrite competes with the chloride ions for ferrous ions produced
in concrete and incorporates them into a passive layer on the iron surface,
thus preventing further corrosion. Long-term corrosion studies showed that
cantly reduced.
Corrosion inhibitors can also be categorized on the basis of their addition
at the time of construction as integral/cast-in type and those which are
added to rehabilate or repair a corroding structure, i.e. the migrating type.
Integral inhibitors are liquids or solids that are batched and mixed with the
Table 14.1 Development of concrete reinforcement corrosion inhibitor
technology and application methods [6]
Year Type of
inhibitor
Chemical composition Application
1979 Anodic Calcium nitrite Admixture
1986 Mixed Alkanolamines Admixture
1990 Mixed Water-based amine
carboxylate
Surface applied
1990 Mixed Water-based, organic
(amines and esters)
Admixture
1993 Anodic Calcium nitrite Surface applied
1994 Anodic Organic and
inorganic
Admixture
1996 Mixed Water based, organic Surface applied
1997 Mixed Amino alcohol
admixture
Surface applied
1997 Anodic Sodium Surface applied
1998 Mixed Ethanol amine Pellets into predrilled holes
protective film or coating that either prevents chloride from reaching the
Calcium nitrite is the first corrosion inhibitor admixture commercialized
sion by reacting with ferrous ions to form a protective ferric oxide film. The
monofluorophosphate
in spite of the decrease in concrete resistivity, corrosion rates were signifi-
© 2008, Woodhead Publishing Limited