Corrosion of Concrete’s Steel Reinforcement

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


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- 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


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.


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 .


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


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.



Fig.8: 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


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.


Figure 3 – the effect of corrosion on concrete


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


Fig.12: The effects of corrosion on concrete

2.2.5 Protections against Corrosion

Protection

Protective

coatings

Corrosion

Inhibitors

Cathodic

Protection


Protective

coatings

Fig.13: The difference between protected steel and unprotected steel concerning resistivity against chloride attack


Cathodic

Protection

Sacrificial

anode

Impressed

current

Sacrificial

anode


Fig.14: The mechanism of Sacrificial anode

Impressed

current


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- 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



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.


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


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


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.


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