MAB 1033 Structural Assessment and Repair 1. CORROSION OF REINFORCEMENT Professor Dr. Mohammad bin Ismail C09-313
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
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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
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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
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– 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
MECHANISMS OF CORROSION OF
STEEL IN CONCRETE (2)
Electron Flow
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Salt Water
Flow
Copper (Cathode)
Zinc (Anode)
MECHANISMS OF CORROSION OF
STEEL IN CONCRETE (3)
Reinforcement Cracking
Cracking, Spalling and Delamination
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Reinforcement Cracking
Reinforcement Spalling
Reinforcement Delamination
MECHANISMS OF CORROSION OF
STEEL IN CONCRETE (4)
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Fe
Fe3O4
Fe(OH)2
Fe(OH)3
Fe(OH)3. H2O
0 1 2 3 4 5 6 Volume (cm3)
Volume Change
MECHANISMS OF CORROSION OF
STEEL IN CONCRETE (5)
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
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• 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
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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.
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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
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• 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
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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
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Years
Corrosion takes place faster when the pH is lowered.
Corrosion of Reinforcement
(Carbonation)
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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)
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• 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
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Cast-in Chloride
• Introduced deliberately as an accelerator
• Natural ingredient found in some aggregates
• Concrete made from beach sand or mix using
sea-water
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sea-water
• Chlorides occur in either water soluble or acid
soluble
Corrosion of Reinforcement
(Chloride)
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Limit of chloride ion in concrete
Service condition % of Cl to weight
of cement
Prestressed concrete 0.06
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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
24
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Cracks and Chloride
• Cracks and construction joints permit corrosive
chemicals to access reinforcement
• ACI 224R-90 present the following table of tolerable
crack width
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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
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– 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
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• 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.
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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
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inadequate cover damage by heavy loads
• Common problem - poor corrosion protection
of the end anchorages due to porous or
cracked anchorage plug grout
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Unprotected Strand
without Protective
SheathingLeakage Paths
into Strand
System
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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
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• 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
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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
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REPAIR OF CORROSION DAMAGED
CONCRETE (Cont.)
Guniting / Shotcreting
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Shotcreting on R.C. wallPreparing slab for guniting
REPAIR OF CORROSION DAMAGED
CONCRETE (Cont.)
Pressure grouting
Confuse.
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AfterIn progressBefore
Confuse..
Hardworking
REPAIR OF CORROSION DAMAGED
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)
CORROSION PREVENTION FOR
CONCRETE STRUCTURES (2)
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
CORROSION PREVENTION FOR
CONCRETE STRUCTURES (3)
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)
CORROSION PREVENTION FOR
CONCRETE STRUCTURES (4)
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
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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
49