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Corrosion of Steel Reinforcementin Concrete
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Overview
Introduction
Mechanisms of Steel Corrosion
Control of Corrosion
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Introduction
One of the principal causes of concretedeterioration in KSA.
The damage is especially large in the structures
exposed to marine environment , contaminatedground 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 alsodeficient. The total repair cost was estimated at$ 90 billion dollars.
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CRACKING OF CONCRETE
Heat of hydration
Alkali-aggregate reactivity
Carbonation
Sulfate attack
Acid and chemicals Reinforcement corrosion
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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
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MECHANISMS OFREINFORCEMENT CORROSION
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FACTORS AFFECTINGREINFORCEMENT CORROSION
Depassivation of steel
Potential variation
Availability of the reaction products,namely oxygen and moisture
Electrical resistivity of concrete
Moisture
Chloride and sulfate contamination
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FACTORS INFLUENCINGREINFORCEMENT
CORROSION Carbonation
Chlorides
Moisture
Oxygen diffusion
Concrete mix variables
Construction variables
Temperature
Humidity
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Chloride-induced
Reinforcement Corrosion
Due to the external chlorides in
substructures Due to chloride contamination from the
mix constituents in the superstructures
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Chloride Limits
ACI 318 (0.1 0.15%; water soluble)
ACI 224 (0.2%; acid soluble)
BS 8110 (0.4%; total)
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Damage to Concrete
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Mechanisms of Steel Corrosion
Corrosion of steel in concrete is anelectrochemical process.
The electrochemical potentials to form thecorrosion cells may be generated in two
ways:1. Two dissimilar metals are embedded in concrete,
such as steel rebars and aluminum conduit pipes, orwhen significant variations exist in surfacecharacteristics of the steel.
2. In the vicinity of reinforcing steel concentration cellsmay be formed due to differences in theconcentration of dissolved ions, such as alkalies andchlorides.
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Mechanisms of Steel Corrosion
As a result, one of the two metals (orsome parts of the metal when only onetype of metal is present) becomes anodic
and the other cathodic.
The fundamental chemical changesoccurring at the anodic and cathodic areas
are as follows:
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Electrochemical Process of SteelCorrosion
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Anodic and Cathodic Reactions
Anode: Fe 2e- + Fe2+
(metallic iron)
FeO (H2O)xrust
Cathode: () O2+ H2O + 2e- 2(OH)-
air water
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Oxidation State vs. Increase ofVolume
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Corrosion Process
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Corrosion Cells
Anodic reaction (involving ionization ofmetallic iron) will not progress far unlessthe electron flow to the cathode is
maintained by the consumption ofelectrons.
For the cathode process, therefore thepresence of both air and water at thesurface of the cathode is absolutelynecessary.
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Steel Passivity
Ordinary iron and steel products arenormally covered by a thin iron oxidefilmthat becomes impermeable and
strongly adherent to the steel surface in analkaline environment, thus making thesteel passive to corrosion.
This means that metallic iron is notavailable for the anodic reaction until thepassivity of steel has been destroyed.
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Destroying Passive LayerIn absence of chloride ions in the solution
Protective film on steel is stable as long asthe pH of the solution stays above 11.5.
When concrete has high permeability and
when alkalies and most of the calciumhydroxide have either been carbonated orleached away), the pH of concrete in thevicinity of steel may have been reduced toless than 11.5.
This would destroy the passivity of steel.
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Destroying Passive LayerIn presence of chloride ions
Depending on the Cl-/OH- ratio, theprotective film is destroyed even at pHvalues considerably above 11.5.
When Cl-/OH-molar ratio is higher than0.6, steel is no longer protected, probablybecause the iron-oxide film becomes
either permeable or unstable under theseconditions.
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Destroying Passive LayerIn presence of chloride ions
The threshold chloride content to initiatecorrosion is reported to be in the range0.6 to 0.9 kg Cl
-per cubic meterof
concrete. When large amounts of chloride are
present, concrete tends to hold moremoisture, which also increases the risk ofsteel corrosion by lowering the electricalresistivity of concrete.
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After the Destroy of Passivity
Rate of corrosion will be controlled by:
The electrical resistivity. [significantcorrosion is not observed as long as theelectrical resistivity of concrete is above50 to 3 70 10 .cm].
The availability of oxygen.
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Sources of Chloride in Concrete
admixtures,
salt-contaminated aggregate,
Penetration of seawater, groundwater, or
deicing salt solutions.
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Corrosion of the Steel Reinfo rced
Concrete Struc tures
MARINE STRUCTURES BURIED UTILITIES
FOUNDATIONS BRIDGES & CULVERTS
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Corrosion o f the Reinfo rcing Steel in a
Spand rel Beams(17 years of service)
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CARBONATION
Ca(OH)2+ CO2CaCO3+ H2O
Reduction in pH (up to 8.5)
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Carbonation in uncontaminated cementmortar
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Carbonation in OPC mortar specimens
contaminated with chloride plus sulfate
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Carbonation in fly ash cement mortarcontaminated with chloride plus sulfate
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Control of Corrosion
Permeability of concrete is the key tocontrol the various processes involved inthe phenomena.
Concrete mixture parameters to ensure lowpermeability, e.g., low water-cement ratio,adequate cement content, control ofaggregate size and grading, and use ofmineral admixtures.
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Control of Corrosion
Maximum permissible chloride content of concretemixtures 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, cementitiousmaterials, and admixtures) should not exceed 0.06 % by weight of cement for prestressed concrete,
0.15 % by weight of cement for reinforced concrete exposed tochloride in service,,
and 0.30 % by mass of cement for other reinforced concretes,respectively.
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Control of Corrosion
ACI Building Code 318 specifies minimumconcrete cover of 50 mm for walls and slabs,and 63 mm for other members is
recommended. Current practice for coastalstructures in the North Sea requires aminimum 50 mm of cover on conventionalreinforcement, and 70 mm on prestressing
steel. RCJY and other agencies requires 75 mm
minimum concrete cover.
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Control of Corrosion
ACI 224R specifies 0.15 mm as the maximumpermissible crack width at the tensile face ofreinforced concrete structures subject to wetting-dryingor seawater spray.
The CEB Model Code recommends limiting the crack
widths to 0.1mm at the steel surface for concretemembers exposed to frequent flexural loads, and 0.2mm to others.
By increasing the permeability of concrete and exposingit to numerous physical-chemical processes of
deterioration, the presence of a network ofinterconnected cracks and microcracks would have adeleterious effect.
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Control of Corrosion
Waterproof membranes: are used when theyare protected from physical damage by asphalticconcrete wearing surfaces; therefore, their
surface life is limited to the life of the asphalticconcrete, which is about 15 years.
Overlay of watertight concrete:37.5 to 63 mmthick, provides a more durable protection to the
penetration of aggressive fluids into reinforcedor prestressed concrete members.
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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-coatedsteel), long-time performance of epoxy-coatedrebars is still under investigation in manycountries.
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Epoxy-coated Steel
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Control of Corrosion
Cathodic protection techniques involvesuppression of current flow in thecorrosion cell, either by:
Supplying externally a current flow in theopposite direction
or by using sacrificial anodes.
Due to its complex and high cost thesystem is finding limited applications.