Date post: | 18-Jul-2015 |
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Presentation by: Niraj Thapa
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Service Life Prediction of RC Structure
With Respect to Corrosion of Steel
Introduction
Damage caused by corrosion
Corrosion mechanism
Carbonation induced corrosion
Chloride induced corrosion
Corrosion control
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What is Corrosion?
Concrete corrosion is the chemical, colloidal or
physicochemical deterioration and disintegration of
solid concrete components and structures, due to
attack by reactive liquids and gases.
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ASTM terminology defines corrosion as “the
chemical or electrochemical reaction between a
material, usually a metal, and its environment that
produces a deterioration of the material and its
properties.”
Corrosion of steel reinforcements represents the
major cause of degradation of reinforced concrete
structures.
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The corrosion of steel can only be delayed or slowed
down but can never be stopped.
A passive oxide film on the surface of the steel
protects the reinforcing steel in concrete from
corrosion. But with time cover concretes cannot
provide enough resistance due to the degradation of
concrete and the ingress of corrosive species from
environment.
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Corrosion leads to several coupled effects:
longitudinal cracking of concrete cover due to
expansive corrosion products.
steel cross-section reduction.
the degradation of steel–concrete bond.
Thus the service life and the load-bearing capacity of
reinforced concrete elements are considerably
reduced.
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Steel in sound concrete is protected by passivation .
Corrosion can take place when the passive film is
removed or is locally damaged. This may take place
due to carbonation of concrete or to chloride
penetration.
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Carbonation penetrates the concrete cover and
corrosion takes place.
Corrosion then initiates when the carbonation front
reaches the steel reinforcement. With time, corrosion
products cause cracking, spalling and delamination
of the concrete cover, which may compromise the
serviceability and the stability of the structure.
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For corrosion of steel,
Service Life = Initiation time + Propagation time
The initiation period is the time required for the
carbonation depth to equal the concrete cover
thickness. The propagation period begins when the
steel is depassivated, and finishes when a given limit
state is reached.
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The presence of chloride ions in the pore solution of
concrete may induce a form of localized corrosion onthe embedded steel called pitting corrosion.
Corrosion of steel in concrete is an electrochemical
process. When there is a difference in electrical
potential along the steel reinforcement in concrete, an
electrochemical cell is set up. In the steel, one part
becomes anode and other part becomes cathode
connected by electrolyte in the form of pore water inthe hardened cement paste.
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At cathode:
Fe Fe++ + 2e–
Fe++ + 2(OH)–Fe(OH)2 (Ferrous hydroxide)
4 Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (Ferric oxide)
At anode:
4e– + O2 + H2O 4(OH)–
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It has been reported that 40% of failure of structures
is on account of corrosion of embedded steel
reinforcement in concrete. Therefore corrosion
control of steel reinforcement is of paramount
importance.
To prevent corrosion:
Use lowest possible water/cement ratio with regard to
workability.
Superplasticizers should be used to cut down w/c
ratio.
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Adopt proper mix design, use of right quality and
quantity of cement for different exposure conditions.
Use of supplementary cementitious materials such as
fly ash, ground granulated blastfurnace slag, silica
fume etc. are required
In short, if we make good concrete with low
permeability and improved microstructure, it will be
durable by itself and also it can take care of the
reinforcement contained in it to a great extent.
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For other serious problems, following methods are
used:
Metallurgical methods
Corrosion inhibitors
Coatings to reinforcement
Cathodic protection
Coatings to concrete
Design and detailing
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Metallurgical methods:
Steel can be made more corrosion resistant by
altering its structure through metallurgical processes:
quenching of the hot bars by series of water jets.
keeping the hot steel bars for a short time in a water
bath.
Corrosion inhibitors:
by using certain corrosion inhibiting chemicals such
as nitrites, phosphates, benzoates etc.
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Coatings to reinforcement:
Simple cement slurry coating is a cheap method for
temporary protection against rusting of reinforcement
in storage.
The steps suggested by CECRI are:
Derusting
Phosphating
Cement coating
Sealing
Fusion Bonded Epoxy Coating
Galvanised reinforcement
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Cathodic Protection:
Most effective, well-known and extensively used
method.
High cost and long term monitoring required.
In this method current is applied to an electrode laid
on the concrete above steel reinforcement. This
electrode serves as anode and the steel reinforcement
which is connected to the negative terminal of a DC
source acts as a cathode. Here the external anode is
subjected to corrode and the cathodic reinforcement
is protected against corrosion and hence the name
“Cathodic protection”.
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Coatings to Concrete:
In addition to the coating of reinforcement byappropriate material, a surface coating to the concretemember is given to increase the durability further.
Design and Detailing:
Precautions should be taken in designing anddetailing with respect to:
spacing between bars for the concrete to flowbetween reinforcements
to facilitate vibration of concrete.
to give proper cover to the steel reinforcements.
to restrict the crack width.
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References:
Bertolini, L., Steel corrosion and service life of reinforced
concrete structures.
Liang, M.T., Wang, K.L., Liang, C.H., Service life prediction ofreinforced concrete structures.
Vidal, T., Castel, A., Francois, R., Corrosion process and structuralperformance of a 17 year old reinforced concrete beam stored inchloride environment.
Guangling, S., Ahmad, S., Corrosion of steel in concrete: causes,detection and prediction
Shetty M.S.(1982), Concrete Technology Theory and Practice, S.Chand & Company Ltd., pp 402-410
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Thank you!
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