Copy of Steel Corrosion

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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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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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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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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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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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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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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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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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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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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.

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