corrosion of concrete’s steel reinforcement
TRANSCRIPT
Beirut Arab University
Faculty of Engineering
Civil & Environmental Engineering Department
Corrosion of Concrete’s Steel Reinforcement
Adham Aboul Hosn
Hassan Kichli
Jana Abou Shakra
Marwan El Masri
Mahmoud Hamdan
Rouba Joumblat
Under supervision of
Prof. Dr. Adel Ahmed Elkordi
Dr. Meheddine Mashaka
Spring 2014/2015
Aim
To study the effect of the concrete cover (different
bar diameters), the cement content (300, 400, and 500
kg/m3), and steel bar coating on steel corrosion in
concrete.
Twelve separate mixes are designed and several
specimen will be undergo the accelerated corrosion test
and compressive strength test to study the structural
behavior of specimens according to the change in
material properties.
Chapter 1
Introduction
1.1 Introduction
chloride ions
from
marine environments
deicing salt
chloride
contaminated aggregates
It occurs due to attacks of aggressive agents
Corrosion of steel reinforcement is a major problem influencing the long-term
performance of reinforced concrete structures.
1.1 Introduction
Reinforcing steel in concrete is normally protected from corrosion by the
passive film formed at the steel/concrete border. In the presence of chloride
and other causing agents, the steel protective passive layer is locally
destroyed and unprotected steel areas dissolve.
Fig.1:The protective layer present around the steel bar in an alkaline medium
1.2 Problem Statement
The problem of corrosion
the formation of corrosion products
substantial volume increase
expansive stresses are induced
possible cracking spalling of concrete cover loss of bond
between
steel/concrete
1.3 Objectives
Main objectives
studying the effect of:
1. concrete cover
2. bar coating
3. cement content
on corrosion of steel in
concrete within a
chlorinated solution.
investigating the
mechanical properties of
the test specimens:
1. Weight loss
2. Compressive strength
3. Corrosion (by ACT)
Chapter 2
Corrosion in Concrete Reinforcement
2.1 Introduction
• Reinforced concrete is a composite material of
steel embedded in a hardened concrete.
• It is a durable material that can be deformed into
different shapes.
• In order to insure composite action between the
steel and concrete, proper or full bond should be
provided between these materials.
2.2 Effect of Corrosion on Concrete Generally
Corrosion
degradation of metals by chemical reaction
with their environment
deterioration of physical properties of the material
loss of cross-sectional area
Weakening of material
2.2.1 Factors Affecting Corrosion
Factors
Steel
Concrete
Resistivity
Components
of concrete
Moisture
Alkali
Aggregate
Reactions
Permeability
2.2.1 Factors Affecting Corrosion
1- Steel
It is well known fact that some metals will corrode
faster that others.
It is a less known fact that variations in size and
shape of metal can indirectly affect is corrosion
resistance.
Thick structural sections are more susceptible to
corrosive attack that thin sections because
variations in physical characteristics are greater.
2.2.1 Factors Affecting Corrosion
2- Permeability
Permeability of concrete is mainly determined by the porosity
of concrete and its pore size distribution which are dependent
on the ratio of w/c.
Low w/c ,better compaction ,and use of mineral admixtures
could lower the permeability of the cover concrete, therefore
they are the option to improve the corrosion resistance of
reinforced concrete.
Fig.2: A Permeable Concrete structure where Corrosion Inhibiters are applied
2.2.1 Factors Affecting Corrosion
3- Pore solution of concrete
The pore solution in concrete is an electrolyte which is
physically absorbed in the pores of the concrete.
It reacts with the steel reinforcement and under certain
conditions can lead to the corrosion damage at the
steel surface.
Fig. 3: Schematic illustration of the ingress of chloride ions from an exposure
solution (e.g. seawater) into a reinforced concrete structure.
2.2.1 Factors Affecting Corrosion
4- Alkali Aggregate Reactions
OPC contains alkalies like sodium oxide and
potassium Oxide to some extent.
These alkalies chemically reacts with reactive
siliceous minerals in some aggregate and cause
expansion, cracking and disintegration of
concrete give rise to the corrosion of
reinforcement.
Fig.4: Reaction with reactive siliceous minerals in some aggregate causing expansion and
cracking .
2.2.1 Factors Affecting Corrosion
5- Moisture
The moisture of concrete has a complicated influence on the
corrosion of steel in the concrete. The water absorption into
concrete from outside environment can rapidly increase the rate
of corrosion of reinforcing steel to the level that will cause
cracking and spalling.
Presence of moisture is a precondition for corrosion to take
place because concrete can act as electrolyte in electrochemical
cell only if it contains some moisture in pores.
Corrosion can neither occurs in dry concrete or in submerged
concrete.
Fig.5: Accumulation of moisture at the sealing of a warehouse
2.2.1 Factors Affecting Corrosion
6- Components of Concrete
The additives containing chloride have a detrimental
effect on corrosion of steel in concrete and can
accelerate the localized corrosion.
Solubility sulphates react with tricalcium aluminate
(C3A) content of cement in the presence of moisture
and from products which occupy much bigger volume
than the original constituent. This expansive reaction
results in weakening of concrete, formation of cracks as
well as corrosion of reinforcement as long as concrete
remains damp.
2.2.2 Causes of Corrosion in Concrete Reinforcement
Causes
Carbonation Chlorination
Reduction of alkalinity
Destruction of passive layer
Chloride penetrates
protective layer
CORROSION
The alkaline environment of concrete (pH = 12) provides steel
with corrosion protection.
At the high pH, a thin oxide layer forms on the steel and
prevents metal atoms from dissolving.
This passive film does not actually stop corrosion; it reduces
the corrosion rate to an insignificant level.
Without the passive film, the steel would corrode at rates at
least 1,000 times higher
2.2.2.1 Corrosion by Carbonation
Fig. 6: Carbon Dioxide from the atmosphere diffuse inside the concrete, react with calcium
hydroxide to form calcium carbonate Carbonation lowers the alkalinity of concrete and
reduce its effectiveness as protective medium.
The intrusion of chloride ions into reinforced concrete can
cause steel corrosion if oxygen and moisture are also available.
Chlorides dissolved in water can permeate through concrete.
The mechanism by which chlorides promote corrosion is that
chloride ions penetrate the protective oxide film easier than
other ions, leaving the steel vulnerable to corrosion.
2.2.2.2 Corrosion by Chlorination
The risk of corrosion increases as the chloride content of
concrete increases
The primary rate-controlling factors are the availability of
oxygen, the electrical resistivity and relative humidity of the
concrete, and the pH and temperature.
2.2.2.2 Corrosion by Chlorination
Fig. 7: The chloride ions (Cl-) attack the iron oxide film leading to corrosion.
Severity of sulphate attack depends upon permeability of concrete, amount of C3A content
in cement and duration for which concrete remains damp.
2.2.3 The Mechanism of Corrosion
Corrosion is an electrochemical process involving the flow of
charges (electrons and ions).
The Anodic Reaction:
At active sites on the bar, called anodes, iron
atoms lose electrons and move into the surrounding
concrete as ferrous ions. The electrons remain in the bar
and flow to sites called cathodes, where they combine
with water and oxygen in the concrete.
To maintain neutrality, the Fe2+ migrate through the concrete
pore water to the cathodic sites where they combine to form
FeOH, or rust.
This hydroxide tends to react further with oxygen to form
higher oxides.
The increases in volume as the reaction products react further
with dissolved oxygen leads to internal stress within the
concrete that may be sufficient to cause cracking and spalling
of the concrete cover.
2.2.3 The Mechanism of Corrosion
2.2.3 The Mechanism of Corrosion
Fig.8: The mechanism of corrosion
2.2.3 The Mechanism of Corrosion
Fig.9: The different factors causing corrosion and its effect on concrete
2.2.4 The Effect of Corrosion on Concrete Properties
Effects
Pitting
Strength
loss
Reduction of
bond strength
Spalling, failure,
And cracking
2.2.4 The Effect of Corrosion on Concrete Properties
Pitting Strength
loss
Reduction of
bond strength Spalling, failure,
And cracking
1- Pitting
Pitting corrosion is a localized form of corrosion by which
cavities or "holes" are produced in the material. Corrosion pits
can be harmful by acting as stress risers.
Fatigue and stress corrosion cracking may initiate at the base
of corrosion pits. One pit in a large system can be enough to
produce the catastrophic failure of that system.
2- Strength loss
Corrosion is known for reducing the cross-sectional area of the
steel bars embedded in concrete. This reduction affects the role
of the rebars, causing strength loss.
3- Loss of bond between concrete and steel
The effect of corrosion on the behavior of concrete and its durability is very essential.
Corrosion has a significant influence on the bonding performance of steel reinforcing in concrete with over 50% reductions in bond strength observed associated with 16% reduction in average cross-section due to corrosion.
This loss of bond can cause cracking and spalling as well.
4- Spalling cracking and failure
As moisture starts entering into concrete through pores, rust
begins to form around the steel bar.
As it continuously accumulates, stress is induced causing
cracks and spalling in the concrete which can lead to failure in
some cases.
2.2.4 The Effect of Corrosion on Concrete Properties
Figure 3 – the effect of corrosion on concrete
2.2.4 The Effect of Corrosion on Concrete Properties
Fig..10: Photographs of steel bar embedded in 30 MPa concrete strength
after accelerated corrosion for three different periods
2.2.4 The Effect of Corrosion on Steel Properties
Fig.11: Photographs of steel bar embedded in 30 MPa concrete strength after
accelerated corrosion for three different periods
2.2.4 The Effect of Corrosion on Concrete Properties
Fig.12: The effects of corrosion on concrete
2.2.5 Protections against Corrosion
Protection
Protective
coatings
Corrosion
Inhibitors
Cathodic
Protection
2.2.5 Protections against Corrosion
Protective
coatings
Fig.13: The difference between protected steel and unprotected steel concerning resistivity against chloride attack
2.2.5 Protections against Corrosion
Cathodic
Protection
Sacrificial
anode
Impressed
current
Sacrificial
anode
2.2.5 Protections against Corrosion
Fig.14: The mechanism of Sacrificial anode
Impressed
current
2.2.5 Protections against Corrosion
Fig.15: Mechanism of Impressed Current
2.2.6 Can Corrosion be totally inhibited?
Corrosion can be
inhibited .. IF
Concrete is
always wet
Concrete is
always dry
Cathodic
protection
Is implemented
Steel bars
are coated
Concrete section
is coated
2.2.6 Can Corrosion be totally inhibited?
BUT… These cases can never be applied in real life,
so there will always be aggressive environments
And there will be agents penetrating into the
concrete causing corrosion
2.2.7 Repair of Corroded Concrete Sections
Repair
Cement
Based repairs
Surface
Coatings
Sealing of
Cracks
Large volume
Repair
Fig.18: Column Jacketing is done to improve the load carrying capacity of the column.
2.2.7 Repair of Corroded Concrete Sections
Fig.19: surface coating; epoxy injection; shotcrete
2.2.8 Effect of Different Parameters on Steel Corrosion
1- Concrete Cover
Concrete cover is the distance from the surface of
the concrete to the surface of the reinforcing bars
embedded in the concrete.
Ensuring sufficient concrete cover is critical for the
durability of some concrete structures subject to
poor environment during their service life.
As we increase the cover over the reinforcement, the
corrosion initiation is delayed.
Fig.20: If Concrete cover to reinforcement is inadequate, reinforcement is liable to get
corroded soon due to various factors such as Carbonation, ingress of sea water,
moisture penetration etc.
0
100
200
300
400
500
600
0 0.5 1 1.5 2 2.5
Corr
osi
on
In
itia
tion
/Days
Cover Over Reinforcement/Inches
Effect Of Cover Over Reinforcement on Corrosion
Effect Of Cover Over Reinforcement on
Corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
2- Cement Content
Too low a cement content may cause inadequate structural
capability, and may not provide a durable protective
environment for the steel reinforcement, permitting rapid
carbonation and subsequent loss of the protective alkaline
environment for the steel.
Too high a cement content may cause excessive shrinkage,
particularly if inadequately cured, thermal cracking from the
heat of hydration if large pourings are involved, or the risk of
alkali silica reaction if a susceptible aggregate has been used
and the cement is not a low alkali type.
90
100
110
120
130
140
150
300 320 340 360 380 400 420 440 460 480
Corr
osu
ion
In
itia
tion
(days)
Cement Content( KG/CU.M)
Effect of Cement Content on Corrosion
Effect of Cement Content on Corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
3- Water Cement Ratio
The water–cement ratio is the ratio of the weight of
water to the weight of cement used in a concrete mix
and has an important influence on the quality of
concrete produced.
As the w/c is increased, the concrete will have more
pores and humidity, thus increasing the risk of the
occurrence of corrosion.
2.2.8 Effect of Different Parameters on Steel Corrosion
50
100
150
200
250
300
350
0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7
Corr
osi
on
In
itia
tion
(Days)
Water-Cement Ratio
Effect of Water-Cement Ratio on Corrosion
Effect of Water-Cement Ratio on
Corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
4- C3A Content of cement
High Tricalcium aluminate content of cement has a significant beneficial effect on reinforcement corrosion resistance performance of concrete structures.
On an average, a Type I cement performs better than a C3A Type V cement in terms of corrosion initiation time for embedded reinforcement.
This appears to be due to the complexing ability of C3A with free chlorides in cement.
90
110
130
150
170
190
210
230
250
270
2 4 6 8 10 12 14 16
Corr
osi
on
In
itia
tion
(Days)
C3A Content of Cement
Effect of Cement Type on Corrosion
Linear (Effect of Cement Type on
Corrosion)
2.2.8 Effect of Different Parameters on Steel
Effect of C3A content of cement on corrosion
2.2.8 Effect of Different Parameters on Steel Corrosion
5- Exposure Temperature
In general, Corrosion rates increase with increasing
temperature.
Temperature affects the corrosion rate of metals in
electrolytes primary through its effect on the oxygen
solubility and oxygen diffusion coefficient.
As temperature increases the diffusion coefficient of
oxygen also increases which tends to increase the
corrosion rate.
2.2.8 Effect of Different Parameters on Steel Corrosion
0
0.5
1
1.5
2
2.5
20 30 40 50 60 70 80
Corr
osi
on
In
itia
tio
n(D
ays)
Exposure Temperature(ᴄ̊ )
Effect of Temperature of Reinforcement Corrosion
Effect of Temperature on Reinforcement
Corrosion
Chapter 3
Experimental Investigation
Parameters
Cement content
300 kg/m3
400 kg/m3
500 kg/m3
Silica Fume
5%
Steel Diameters
(mm)
14
22
W/C
0.4
Zinc Coating
With
Without
Chloride Solution
5%
3.2 Concrete Mix Design
Fig.26: Mix Design Parameters
Mix Vmix
(m3)
cement
(Kg/m3)
Silica
(Kg/m3)
water
(Kg/m3)
CA
(Kg/m3)
FA
(Kg/m3)
Zinc
Protection
Steelbar
diameter
(mm)
w/c Unitweight
(Kg/m3)
1 1 285 15 120 1040 866.76 yes 14 0.4 2326.76
2 1 285 15 120 1040 866.76 yes 22 0.4 2326.76
3 1 285 15 120 1040 866.76 no 14 0.4 2326.76
4 1 285 15 120 1040 866.76 no 22 0.4 2326.76
5 1 380 20 160 1040 691.98 yes 14 0.4 2291.98
6 1 380 20 160 1040 691.98 yes 22 0.4 2291.98
7 1 380 20 160 1040 691.98 no 14 0.4 2291.98
8 1 380 20 160 1040 691.98 no 22 0.4 2291.98
9 1 475 25 200 1040 517.18 yes 14 0.4 2431.18
10 1 475 25 200 1040 517.18 yes 22 0.4 2431.18
11 1 475 25 200 1040 517.18 no 14 0.4 2431.18
12 1 475 25 200 1040 517.18 no 22 0.4 2431.18
3.2 Concrete Mix Design
Table.1: Results of the 12 mix designs and their unit weights
3.4 Testing Procedure
Tests
Tests on Fresh Concrete
Raw Materials Tests
Tests on Hardened Concrete
Accelerated Corrosion Test
3.4.1 Raw Materials Tests
Raw Materials
Tests
Moisture Content
of
Concrete Aggregate
Bulk Unit Weight
and
Voids in Aggregate
Sieve Analysis
Of Fine and
Coarse Aggregate
Specific Gravity
and Absorption of
Coarse Aggregate
Specific Gravity
and Absorption of
Fine Aggregate
3.4.2 Tests on Hardened Concrete
Compressive Strength of
Cylindrical Concrete Specimens
Tests on Hardened Concrete
Fig.27: Concrete cylindrical specimen being tested for compressive strength.
3.4.3 Accelerated Corrosion Test
Impressed Voltage Test
Fig.27: Accelerated Corrosion experimental set-up
3.4.4 Tests on Fresh Concrete
Slump of Hydraulic-Cement Concrete
Tests on Fresh Concrete
Fig.28: Measuring the slump of freshly mixed concrete
Our vision for FYP 2
Apply research knowledge earned in FYP 1
Design 12 mix design according to our chosen parameters
Perform the assigned tests on the specimen
Record, analyze, and discuss the results
Conclude the project and set recommendations for further
research
Our appreciation is dedicated to the Dean of the
Faculty of Engineering Prof. Adel Elkordi and to Dr.
Mehedine Mashaka, who continually and persuasively
conveyed patience, motivation and excitement in regard to
finishing this project.
Acknowledgement