Lecture No.

30
Subject: Concrete Durability Problems
Reinforcement Corrosion
Objectives of Lecture:

To explain the reinforcement corrosion

mechanism, effects, factors affecting, and
control measures

Mechanism of Reinforcement Corrosion

Corrosion cell
Corrosion of steel in concrete is an
electrochemical process, which takes place as a
result of the formation of a corrosion cell.
The electrochemical potentials to form the
corrosion cell may be generated in two ways:
Composition cells may be formed when two
dissimilar metals are embedded in concrete,
such as steel rebars and aluminum conduit
pipes, or when significant variations exist in
surface characteristics of the steel
Concentration cells may be formed due to
differences in concentration of dissolved ions
 in the vicinity of steel, such as alkalies,
chlorides, and oxygen
The corrosion cell causing reinforcement
corrosion, as shown in the following Fig.,
consists of the following components:
Passive steel Corrosion
as cathode current
O2 O2

OH Fe++ OH

Reinforcing e e
steel Concrete

H2 O Anodic dissolution of iron

(Concrete pore water)

(i) Anode (one of the two metals or some

parts of the metal when only one
metal is present
(ii) Cathode (one of the two metals or
some parts of the metal when only one
metal is present
(iii) Electrolyte (concrete pore water)

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Anodic and Cathodic Reactions

Anodic reactions Cathodic reactions

3Fe + 4H2O Fe3O4 + 8H+ + 8e-
2H2O + O2 + 4e- 4OH-
2Fe + 3H2O Fe2O3 + 6H+ + 6e- or,
2H + 2e-
+
H2

Fe + 2H2O HFeO2- +3H+ + 2e-

Fe Fe++ + 2e-

The anodic reactions result into the transformation of

metallic iron (Fe) to rust. The rust formation on the
surface of reinforcement is accompanied by an increase
in volume, may be as large as 6 times the volume of Fe,
as shown in the following Fig.:

3
This volume increase causes concrete expansion and
cracking
Pourbaix diagrams illustrating the state of
reinforcement corrosion depending on the electrode
potential and electrolytic pH of the corrosion cell

1.6 Boundaries of H2O stability with

A pH2 < 1atm. And B pO2 < 1atm
E , V ( SHE scale )

0.2

Fe2O3
Fe2+
0.0 A

(Corrosion) (Passivation)

-0.8 (Immunity) Fe OOH

Fe (Corrosion)

0 2 4 6 8 10 12 14
pH

For Fe H2 O system at 25C

0.8
E , V ( S H E s c a le )

Pitting
General
corrosion
0.0 A Passivity

-0.8 Immunity

0 2 4 6 8 10 12 14

pH 4
 Effect of chloride ions introduced into the Fe
H2O system at 25C
Corrosion Measurement Parameters

1. Half-cell Potential (Ecorr)

Half-cell potential data is used to evaluate the
probability of reinforcement corrosion and to
delineate anodic (i.e. corroding) and cathodic
(non-corroding or passive) areas on the
structure
Half-cell potential measurement especially for
rebar corrosion detection has been duly
covered in ASTM C876-1991. The circuitry is
shown in the following Fig.:

5
Interpretation of half-cell potential test results
ASTM: C876

Half-cell potential (mV) relative to Percentage chance

Cu/CuSO4 reference electrode of active corrosion
< - 350 90%
-200 to 350 50%
> -200 10%

2. Concrete resistivity ()

Resistivity of concrete, used as indication of

likelihood of significant corrosion. Resistivity
survey data gives an indication of whether the
concrete condition is favorable for the easy
movements of ions leading to more corrosion
Resistivity of concrete is commonly measured
by four-electrode method, as shown in the
following Fig.:

6
Interpretation of concrete resistivity test
results
Resistivity Likelihood of significant corrosion
(Ohm-cm) (non-saturated concrete when steel activated)
< 5000 Very high
5000 - 10000 High
10000- 20000 Low/ moderate
> 20000 Low

3. Corrosion current density (Icorr)

Corrosion rate test results are useful in
order to know the extent of damage
quantitatively
The corrosion rate of rebar in terms of
corrosion current density, Icorr, may be
determined by conducting the linear
polarization test
The circuitry for linear polarization
measurements is shown in the following
Fig.:

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8
Using the measured value of Icorr, the state of
reinforcement corrosion can be ascertained
as:
If Icorr = 10-9 to 10-7 Amp/cm2 the state of
reinforcement corrosion is passive
If Icorr = 10-6 to 10-5 Amp/cm2 the state of
reinforcement corrosion is active

Electrochemically measured value of Icorr can

be converted to the instantaneous corrosion
rate, Jr, and penetration rate, Pr, through
Faradays law, as follows:
W
Jr I corr
F
W
Pr I corr
F st

Where

W = equivalent weight of steel = 55.85/2 = 27.925 gm

F = Faradays constant = 96487 Coulombs (Amp-sec)
st = density of steel (7.85 gm/cm3)
Icorr = corrosion current density (Amp/cm2)
Jr = instantaneous corrosion rate (gm/cm2/sec)
Pr = penetration rate (cm/sec)

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Initiation and Progress of Reinforcement
Corrosion

Degree of corrosion

Final state (tcr)

Propagation Life time

Initiation
D e p a ssiv a tio n

period (tp)
Steady-state
corrosion period (tcor)

The stages of rebar corrosion

tcr = tp + tcor

Effect of Reinforcement Corrosion on

Structural Behavior:
Rust produced as a result of corrosion has
a volume 2-6 times more than that of steel
Volume expansion develops tensile stresses
in concrete, which ultimately results in
cracking and spalling of the cover
concrete, as shown in the following Fig.:

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11
 Corrosion reduces the cross-sections of
the reinforcing bars
Due to the loss of cover concrete and loss
of cross-section of the reinforcing bars
there is significant reduction in the load
bearing capacity of the structure
Steel becomes more accessible to the
aggressive agents leading towards further
corrosion at an accelerated rate
The pitting (i.e., localized) corrosion of the
rebar is more dangerous than uniform
corrosion because it reduces the cross-
section area of rebar to a point leading to
a catastrophic failure of the structure.

Causes of Reinforcement Corrosion

Reinforcement corrosion is mainly caused by:

Carbonation, reducing the pH of concrete
Chloride ions

Effect of carbonation

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 In the absence of chloride ions in concrete
there is formation of a thin iron-oxide
protective film on steel surface
The protective film on steel is reported to
be impermeable and stable (i.e. strongly
adherent to the steel surface) as long as the
pH of concrete stays above 11.5
The effect of carbonation on
reinforcement corrosion lies in the fact
that the carbonation lowers the pH of
concrete below 9.5 at which the destruction
of protective (passive) film on steel starts
Protective film disappears at a pH of
about 8 and if the value falls below 7
catastrophic corrosion can occur

Effect of chloride ions

Chloride may enter into concrete from

three major sources: from CaCl2 added as
an accelerating admixture; from de-icing
salts used on pavements and bridge decks;
and from sea water or salt spray
exposures

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 In the presence of chloride ions, the
protective film on steel gets destroyed
even at pH values considerably above 11.5

The amount of chloride required to

initiate corrosion depends on the pH of
concrete in contact with the steel, as
shown in the following Fig.:

For the typical concrete mixtures

normally used in practice, the threshold
chloride content to initiate corrosion is
reported to be in the range 0.6 to 1.2 kg of
Cl- per cubic meter of concrete.

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 The threshold limit to initiate chloride
corrosion is also expressed in terms of the
Cl-/OH- molar ratio
When Cl-/OH- molar ratio is higher than
0.6, the corrosion is initiated
Once the protective film is destroyed and
the reinforcement corrosion is initiated, it
is the electrical resistivity of concrete and
availability of oxygen that control the rate
of corrosion
Control of Reinforcement Corrosion
Since water, oxygen, carbonation, and
chloride ions play important roles in the
corrosion of embedded steel and cracking
of concrete, it is obvious that permeability
of concrete is the key to control corrosion
Permeability of concrete could be minimized
through proper selection of the concrete mix
parameters, as: low w/c ratio; adequate cement
content; control of aggregate size and grading; and
use of mineral admixtures
The other corrosion controlling measures
are as follows:
1. Limiting chloride content of concrete
Maximum water-soluble Cl- ion concentration in
hardened concrete (% by wt. of cement), at an age

15
of 28 days, from all ingredients (including
aggregates, cement, and admixtures) should not
exceed: 0.06 for prestressed concrete; 0.15 for
reinforced concrete exposed to chloride in service;
and 0.30% for other reinforced concrete

2. Providing adequate concrete cover

For corrosive environment, the ACI Building Code
318-83 recommends a minimum concrete cover of
50 mm for walls and slabs, and 63 mm for other
members

3. Limiting the crack width on concrete

surface
ACI 224R-80 specifies 0.15 mm as the maximum
permissible crack width at the tensile face of
reinforced concrete structures subject to wetting-
drying or seawater spray

4. Providing waterproof membranes on

concrete surface

5. Providing 37.5 to 63 mm thick overlays

of watertight concrete on concrete
surface

6. Applying protective coatings on steel

surface
Zinc-coating is the example of anodic coating

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 Epoxy-coating is the example of barrier
coating

7. Installing cathodic protection system

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