1. corrosion of reinforcement - faculty of civil...

MAB 1033Structural Assessment and Repair

1. CORROSION OF REINFORCEMENT

Professor Dr. Mohammad bin IsmailC09-313

Learning Outcome

At the end of the course students should be able

to understandto understand

• Mechanism of corrosion of reinforcement

• Factors that influence corrosion propagation

• Differences general and pitting corrosion

• Method of rectification

Effect Cause

Leakage

Deflection

Wear

SettlementDefect

Damage

Design

Materials

Construction

Overloading

Chemical spill

MAB 1033 Structural Assessment & Repair 3

Spalling

Disintegration

Cracking

Scaling

Delamination

Damage

Deterioration

Earthquake

Fire

Erosion

Corrosion of

metals

AAR

Sulphate Attack

• Corrosion of reinforcement is indeed one of the major cause of deterioration to concrete structures in many parts of the world

• The main cause is largely related to :

– The use of de-icing salts

INTRODUCTION


The use of de-icing salts

– Chloride :

a) Exposure chloride containing environments (marine environments)

b) Previous use of chloride based accelerator

c) Chloride contaminated materials

– Due to reduction in alkalinity of concrete as a result of carbonation of concrete from exposure to CO2 in the atmosphere

MECHANISMS OF CORROSION OF

STEEL IN CONCRETE (1)

• Definition of corrosion : Degradation of metals by an electrochemical reaction with the environment

• The electrochemical corrosion cell has 4 components :

– Anode : Site where corrosion occurs and electrons flow from


– Anode : Site where corrosion occurs and electrons flow from

– Cathode : Site where no corrosion occurs and electron flow to

– Electrolyte : the aqueous environment, in contact with both the anode and cathode to provide a path for ionic conduction

– The electrical connection between the anode and the cathode to allow electrons to flow between them



Electron Flow


Salt Water

Flow

Copper (Cathode)

Zinc (Anode)



Reinforcement Cracking

Cracking, Spalling and Delamination

MAB 1033Structural Assessment & Repair

7

Reinforcement Cracking

Reinforcement Spalling

Reinforcement Delamination




Fe

Fe3O4

Fe(OH)2

Fe(OH)3

Fe(OH)3. H2O

0 1 2 3 4 5 6 Volume (cm3)

Volume Change



In order for corrosion to occur the 4 basic elements

(anode, cathode, electrolyte & electrical connection)

are required plus the supply of O2 & H2O

If any of these required elements is absent,


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If any of these required elements is absent,

corrosion will not occur

Corrosion cell :� Anodic reaction : Fe (solid) � Fe2+ (ions) + 2e

� Cathodic reaction : O2 + 2H2O + 4e � 4OH-

Corrosion Process

• Concrete high alkalinity material (pH 12-13)

• Passive film protect Steel γ-Fe2O3

• When passive film disrupted, corrosion may take place

• Corrosion is defined as the deterioration of metal by


• Corrosion is defined as the deterioration of metal by reaction with species in the environment to form chemical compound

• Corrosion is a electrochemical process requiring an anode, a cathode an electrolyte

The Three-Stage

Model of Corrosion Damage

Initiation

Period

Propagation

Period

Accelerated

Period

No evidence of

Exte

nt

of

Dam

ag

e


0 15 30

evidence of Damage

Corrosion with minor damage

Corrosion initiated by chlorides or carbonation

Widespread cracking and spalling of cover

Age of Structure (Years)

Exte

nt

of

Dam

ag

e

Corrosion

Inhibitors

High quality

concrete

High pH (Alkalinity)

concrete protects

steel surface from

corrosion

Corrosion Promoters:

- Oxygen.

- Water

- Stray electrical

currents.

- Uneven chemical

environment around

reinforcement.

- Environments that

lower

the pH (alkalinity).

- Chlorides.


Carbonation

• Carbonation is a reaction between acidic gases in the atmosphere and the products of cement hydration

CO2 + H2O H2CO3

H2CO3 + Ca(OH)2 CaCO3 + 2H2O

• Carbon dioxide diffuse in concrete react with calcium


• Carbon dioxide diffuse in concrete react with calcium hydroxide and reduce pH value (pH < 10)

• Protective layer of the steel destroyed

Corrosion Rate

0.4

0.5

0.6

0.7

0.8

Acidic Alkaline


Rate mm/yr

0

0

2 4 6 8 10 12 14

0.1

0.2

0.3

pH of Concrete Relationship

between pH & Corrosion rate

CARBONATION – Induced corrosion

CO2 from the atmosphere penetrates the concrete

1. CO2 react with Ca(OH)2 to form CaCO3

Presence of O2 & H2O

MAB 1033 Structural Assessment & Repair 15Reinforcement concrete

3

Alkalinity < 12.5 – 8.5

Steel reinforcement

Passive oxide layer

Reinforcement - corroded

lost its ability to protect the steel

Carbonation process

CO2

H2O

Delamination


Years

Corrosion takes place faster when the pH is lowered.

Corrosion of Reinforcement

(Carbonation)


Chloride penetration

• Chloride in concrete may arise from external and internal source

• External – ingress from sea-water, salt laden mist, deicing salt

• Internal – added as admixture (accelerator)


• Internal – added as admixture (accelerator)

• Chloride attack the passive layer on steel

• As rust layer builds, tensile forces generated by expansion of the oxide cause concrete to crack and delaminate

CHLORIDE – Induced corrosion

Chloride penetrate the concrete from de-icing salts / seawater

Existing chloride – admixtures / contaminated aggregates etc.

Presence of O2 & moisture

MAB 1033 Structural Assessment & Repair 19Reinforcement concrete

Steel reinforcement

Passive oxide layer

Reinforcement - corroded

lost its ability to protect the steel

Chloride penetration

When chlorides penetrate to reinforcing steel corrosion begins.

Delamination/Sp

all


Cast-in Chloride

• Introduced deliberately as an accelerator

• Natural ingredient found in some aggregates

• Concrete made from beach sand or mix using

sea-water


sea-water

• Chlorides occur in either water soluble or acid

soluble

Corrosion of Reinforcement

(Chloride)


Limit of chloride ion in concrete

Service condition % of Cl to weight

of cement

Prestressed concrete 0.06


Conventionally reinforced concrete in

a moist environment and exposed to

chloride

0.10

Conventionally reinforced concrete

not exposed to chloride

0.15

Above-ground building construction

where concrete will stay dry

No limit

Cracks and Chloride

• Cracks and construction joints permit corrosive

chemicals to access reinforcement

• ACI 224R-90 present the following table of tolerable

crack width


crack width

Exposure Condition Tolerable crack Width

Dry air, protective membrane 0.41mm

Humidity, moist air, soil 0.3

De-icing chemicals 0.18

Seawater, seawater spray 0.15

Water retaining structures 0.1

Corrosion induced

cracking and spalling

• Cracking and spalling is a function of

– Concrete tensile strength

– Quality of concrete cover

– Bond/condition of interface between rebar and


– Bond/condition of interface between rebar and

surrounding concrete

– Diameter of reinforcing bar

– Percentage of corrosion by weight of reinforcement

C/D Ratio Cover

(mm)

Bar size Corrosion % to

cause cracking

7 89 #4 4%

3 38 #4 1%

Reduction in Structural Capacity

• The structural capacity of a concrete member

is affected by bar corrosion and cracking of

surrounding concrete

• Steel with more than 1.5% corrosion, the ult


• Steel with more than 1.5% corrosion, the ult

load capacity began to fall, and at 4.5%

corrosion, the ult load reduced by 12%.

• Corrosion can take place in concrete when two different metals are cast into a concrete structure

Dissimilar metal Corrosion

(galvanic)

_Electron Flow

Note: shaded area denotes level of moisture penetration and active electrolyte. If chlorides are present, the process is accelerated.


concrete structure

1. Zinc

2. Aluminium

3. Steel

4. Iron

5. Nickel

+_

Electron Flow

Ion OH Flow

Cathode Anode

Post-Tension Strand Corrosion

• Unbonded post-tension strands are protected

by protective grease and sheathing

• Aggressive agents can penetrate when

inadequate cover damage by heavy loads


inadequate cover damage by heavy loads

• Common problem - poor corrosion protection

of the end anchorages due to porous or

cracked anchorage plug grout

Unprotected Strand

without Protective

SheathingLeakage Paths

into Strand

System


Protective Sheathing

TypesIndividual Wires Grease

(typical)

Push-Thru

Heat-

Sealed Extruded

System

7 Wire

StrandAnchorage

Plug Grout

Wedge

sEnd Anchor

Casting

Breakout

Bars

Structural Steel Member

Corrosion

• Steel beam cast into concrete to form a

composite member

• To provide fire protection

• Top flange of beam is susceptible to corrosion


• Top flange of beam is susceptible to corrosion

when a crack or construction joint intersect

the flange

. .. ....

. ...

...... .

Aggressive Environment

Crack or Construction

Joint over Embedded

Structural Steel


CONSEQUENCES OF CORROSION

Reduction in the steel cross-sectional area

Cracking, spalling & delamination of the concrete

cover (due to expansive nature of the iron oxides)


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cover (due to expansive nature of the iron oxides)

A decrease of the steel/concrete bond

Possible reduction in load carrying capacity of

structural member

ASSESSMENT OF CORROSION DAMAGED

CONCRETE STRUCTURES

Objectives – to find the causes/as well as the extent of the corrosion problem

Normally done in 2 stages :

� Visual inspection + limited testing

� Detailed testing

Testing :


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� Covermeter survey

� Carbonation depth measurement

� Chloride ion content measurement

� Half-cell potential measurement

� Resistivity measurement

� Degree of corrosion

� Other tests (rate of corrosion, analyses for cement content)

REPAIR OF CORROSION DAMAGED

CONCRETE

Patch repair


39


CONCRETE (Cont.)

Guniting / Shotcreting


Shotcreting on R.C. wallPreparing slab for guniting


CONCRETE (Cont.)

Pressure grouting

Confuse.


AfterIn progressBefore

Confuse..

Hardworking


CONCRETE

Another repair options� Preplaced aggregates & Pressure grouting

� Preventive measures

� Surface protection

Electrochemical methods


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� Electrochemical methods

� Strengthening

� Plate bonding (steel, CFRP)

� Jacketing

� External prestressing

� Give up – Demolish and rebuild to new & improved specification

CORROSION PREVENTION FOR

CONCRETE STRUCTURES (1)

Use of sufficient cover

Use of impermeable good quality concrete

� Lower water binder ratio

� Use of mineral admixtures


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� Use of optimum cement content

� Optimum compaction

� Early and comprehensive curing

� Apply surface treatments

� Use of durability related tests for compliance (gas &

water permeability, chloride permeability, chloride

diffusion)



Isolation of reinforcement from the chemical

effect of corrosion by means of physical barrier or

chemical inhibition

� Use of epoxy coated reinforcement


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� Use of epoxy coated reinforcement

� Use of galvanised reinforcement

� Use of stainless steel reinforcement

� Use of bar primer

� Use of zinc rich paint



Reversing the effect of corrosion by cathodic

protection (CP)

It works based on the principles of eliminating the

anodic sites (corrosion sites) by progressing the


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anodic sites (corrosion sites) by progressing the

steel to a cathodic state

� Sacrificial anodes CP

� Impressed current (CP)



Preserving or restoring passivity (reserving the

effect carbonation and chloride attack by

electrochemical processes)

� Realkalization : Technique to introduce alkaline

solution into concrete to arrest and prevent further


46

solution into concrete to arrest and prevent further

deterioration due to carbonation. Produce hydroxyl ions

& restoring pH levels

� Chloride extraction (Desalination) : Technique to

remove ingressed or cast in chlorides in order to arrest

deterioration due to carbonation

CONCLUSION

The risk of reinforcement corrosion occuring in new

construction could be reduced by understanding the cause

and mechanism of corrosion and taking appropriate

preventives measures in the planning and construction

stages

The most appropriate measure to reduce the risk of


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The most appropriate measure to reduce the risk of

corrosion is to produce durable concrete in the first place

by choosing proper materials and mix proportions as well

as appropriate construction practices

Successful repair to deteriorated concrete also require an

understanding of the causes and mechanism of the

deterioration, so that the most appropriate repair materials

and techniques could be applied

Department of Structures and Materials,

Faculty of Civil Engineering

UTM

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1. corrosion of reinforcement - faculty of civil...

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