Corrosion of Steel Reinforcement in Concrete

spyfleaUrban and Civil

Nov 25, 2013 (3 years and 9 months ago)

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Corrosion of Steel Reinforcement
in Concrete

Overview


Introduction


Mechanisms of Steel Corrosion


Control of Corrosion


Introduction


One of the principal causes of concrete
deterioration in KSA.


The damage is especially large in the structures
exposed to marine environment , contaminated
ground water, or deicing chemicals.


1991 report FHWA in U. S. reported that 134,00
(23% of the total) bridges required immediate
repair and 226,000 (39% of the total) were also
deficient. The total repair cost was estimated at
$ 90 billion dollars.


CRACKING OF CONCRETE


Heat of hydration


Alkali
-
aggregate reactivity


Carbonation


Sulfate attack


Acid and chemicals


Reinforcement corrosion

REINFORCEMENT CORROSION


Passivity


High pH leading to formation of passive layer


Chemical binding of chlorides


Dense and impermeable structure of concrete


Depassivation


Chloride ingress


Carbonation


MECHANISMS OF
REINFORCEMENT CORROSION

FACTORS AFFECTING
REINFORCEMENT CORROSION


Depassivation of steel


Potential variation


Availability of the reaction products,
namely oxygen and moisture


Electrical resistivity of concrete


Moisture


Chloride and sulfate contamination

FACTORS INFLUENCING
REINFORCEMENT
CORROSION


Carbonation


Chlorides


Moisture


Oxygen diffusion


Concrete mix variables


Construction variables


Temperature


Humidity

Chloride
-
induced
Reinforcement Corrosion


Due to the external chlorides in
substructures


Due to chloride contamination from the
mix constituents in the superstructures


Chloride Limits


ACI 318 (0.1


0.15%; water soluble)


ACI 224 (0.2%; acid soluble)


BS 8110 (0.4%; total)


Damage to Concrete

Mechanisms of Steel Corrosion


Corrosion of steel in concrete is an
electrochemical process
.


The electrochemical potentials to form the
corrosion cells
may be generated in two
ways:

1.
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.

2.
In the vicinity of reinforcing steel concentration cells
may be formed due to differences in the
concentration of dissolved ions, such as alkalies and
chlorides.


Mechanisms of Steel Corrosion


As a result, one of the two metals (or
some parts of the metal when only one
type of metal is present) becomes anodic
and the other cathodic.


The fundamental chemical changes
occurring at the anodic and cathodic areas
are as follows:


Electrochemical Process of Steel
Corrosion

Anodic and Cathodic Reactions

Anode: Fe 2e
-

+ Fe
2+

(metallic iron)


FeO (H
2
O)
x




rust


Cathode: (½) O
2

+ H
2
O + 2e
-

2(OH)
-


air water



Oxidation State vs. Increase of
Volume

Corrosion Process

Corrosion Cells


Anodic reaction (involving ionization of
metallic iron) will not progress far unless
the electron flow to the cathode is
maintained
by the consumption of
electrons
.


For the cathode process, therefore
the
presence of both air and water at the
surface of the cathode is absolutely
necessary
.


Steel Passivity


Ordinary iron and steel products are
normally covered by a
thin iron oxide
film

that becomes impermeable and
strongly adherent to the steel surface in an
alkaline environment, thus
making the
steel passive to corrosion
.


This means that metallic iron is not
available for the anodic reaction until the
passivity of steel has been destroyed.


Destroying Passive Layer

In absence of chloride ions in the solution


Protective film on steel is stable as long as
the pH of the solution stays above 11.5.


When concrete has high permeability and
when alkalies and most of the calcium
hydroxide have either been carbonated or
leached away), the pH of concrete in the
vicinity of steel may have been reduced to
less than 11.5.


This would destroy the passivity of steel.



Destroying Passive Layer

In presence of chloride ions


Depending on the Cl
-

/OH
-

ratio, the
protective film is destroyed even at pH
values considerably above 11.5.


When Cl
-

/OH
-

molar ratio is higher than
0.6, steel is no longer protected, probably
because the iron
-
oxide film becomes
either permeable or unstable under these
conditions.

Destroying Passive Layer

In presence of chloride ions


The threshold chloride content to initiate
corrosion is reported to be in
the range
0.6 to 0.9 kg Cl
-

per cubic meter

of
concrete.


When large amounts of chloride are
present, concrete tends to hold more
moisture, which also increases the risk of
steel corrosion by lowering the
electrical
resistivity of concrete
.


After the Destroy of Passivity

Rate of corrosion will be controlled by:



The electrical resistivity. [significant
corrosion is not observed as long as
the
electrical resistivity of concrete
is above
50 to 3 70 10
Ω
.cm].


The availability of oxygen.

Sources of Chloride in Concrete


admixtures,


salt
-
contaminated aggregate,


Penetration of seawater, groundwater, or
deicing salt solutions.


Corrosion of the Steel Reinforced
Concrete Structures

MARINE STRUCTURES

BURIED UTILITIES

FOUNDATIONS

BRIDGES & CULVERTS

Corrosion of the Reinforcing Steel in a
Spandrel Beams

(17 years of service)

CARBONATION


Ca(OH)
2

+ CO
2



CaCO
3

+ H
2
O


Reduction in pH (up to 8.5)

Carbonation in uncontaminated cement
mortar

Carbonation in OPC mortar specimens
contaminated with chloride plus sulfate


Carbonation in fly ash cement mortar
contaminated with chloride plus sulfate

Control of Corrosion


Permeability of concrete
is the key to
control the various processes involved in
the phenomena.


Concrete mixture parameters to ensure low
permeability, e.g., low water
-
cement ratio,
adequate cement content, control of
aggregate size and grading, and use of
mineral admixtures.



Control of Corrosion


Maximum permissible chloride content
of concrete
mixtures is also specified by ACI Building Code 318.


Maximum water
-
soluble Cl
-

ion concentration in
hardened concrete, at an age of 28 days, from all
ingredients (including aggregates, cementitious
materials, and admixtures) should not exceed


0.06 % by weight of cement for prestressed concrete,


0.15 % by weight of cement for reinforced concrete exposed to
chloride in service,,


and 0.30 % by mass of cement for other reinforced concretes,
respectively.



Control of Corrosion


ACI Building Code 318 specifies
minimum
concrete cover
of 50 mm for walls and slabs,
and 63 mm for other members is
recommended. Current practice for coastal
structures in the North Sea requires a
minimum 50 mm of cover on conventional
reinforcement, and 70 mm on prestressing
steel.


RCJY and other agencies requires 75 mm
minimum concrete cover.

Control of Corrosion


ACI 224R 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.


The CEB Model Code recommends limiting the crack
widths to 0.1mm at the steel surface for concrete
members exposed to frequent flexural loads, and 0.2
mm to others.


By increasing the permeability of concrete and exposing
it to numerous physical
-
chemical processes of
deterioration, the presence of a network of
interconnected cracks and microcracks would have a
deleterious effect.

Control of Corrosion


Waterproof membranes:
are used when they
are protected from physical damage by asphaltic
concrete wearing surfaces; therefore, their
surface life is limited to the life of the asphaltic
concrete, which is about 15 years.


Overlay of watertight concrete:

37.5 to 63 mm
thick, provides a more durable protection to the
penetration of aggressive fluids into reinforced
or prestressed concrete members.

Control of Corrosion


Protective coatings for reinforcing steel
are of two types:


anodic coatings (e.g., zinc
-
coated steel) very
limited use due to concern regarding the long
-
term durability.


and barrier coatings (e.g., epoxy
-
coated
steel), long
-
time performance of epoxy
-
coated
rebars is still under investigation in many
countries.


Epoxy
-
coated Steel

Control of Corrosion


Cathodic protection
techniques involve
suppression of current flow in the
corrosion cell, either by:



Supplying externally a current flow in the
opposite direction


or by using sacrificial anodes.


Due to its complex and high cost the
system is finding limited applications.