CARBON FIBRE REINFORCED POLYMERS FOR

STRENGTHENING OF STRUCTURAL ELEMENTS

SEMINAR REPORT

Submitted in partial fulfillment of the requirement for the award of B.Tech. Degree in

Civil Engineeringof the University of Kerala

Submitted by

MENA.G.PILLAI S7C3

Roll No.17322

Guided by

Mrs. DEEPARAJ.SLecturer

Department of civil EngineeringCollege of EngineeringThiruvananthapuram

DEPARTMENT OF CIVIL ENGINEERING

COLLEGE OF ENGINEERING

THIRUVANANTHAPURAM

2007

DEPARTMENT OF CIVIL ENGINEERING

COLLEGE OF ENGINEERING

THIRUVANANTHAPURAM2007

Certificate

This is to certify that this Report entitled “CARBON FIBRE

REINFORCED POLYMERS FOR STRENGTHENING OF STRUCTURAL

ELEMENTS” is a bonafide record of the seminar presented by MENA G

PILLAI towards the partial fulfillment of the requirements for the award

of B. Tech. Degree in Civil Engineering of the University of Kerala during

the year 2007-2008

Guided by

Mrs DEEPARAJ S Lecturer Department of Civil Engineering College of Engineering Thiruvananthapuram

Dr. V SYAM PRAKASH Professor Department of Civil Engineering. College of Engineering Thiruvananthapuram

U G Professor

Acknowledgement

I hereby express my deep and sincere gratitude to my guide Mrs.Deeparaj S

Lecturer,Department of Civil Engineering, College of

Engineering,Thiruvananthapuram,for the expert guidance creative suggestions and advice

in presenting this seminar.

I express my sincere thanks to Dr.R Sathikumar, Head of the Department of Civil

Engineering,College of Engineering, Thiruvananthapuram for all necessary help extended

by him in the fulfillment of this work.

I also express my sincere and heartfelt gratitude to Dr. V Syamprakash, U.G

Professor, Dr. S Latheswary,Dr.Ruby Abraham, Seminar Co-ordinators and

Mr.K G Jaiprakash JainAssistant Professor,Mrs Jeenu G Lecturer Mr Mithra D C

Lecturer, Department of Civil Engineering, College of Engineering,Thiruvananthapuram

for their sincere help and valuale suggestions.

I am also grateful to all my friends and classmates for their help and support in

carrying out this work successfully.

Last but not the least I would wish to record my gratefulness to The Almighty God

who had given me the strength to prepare this and who made this work flawless.

ABSTRACT

Concrete is a building material with a high compressive strength and poor

tensile strength. A structure with out any form of reinforcement will crack and fail

when subjected to a relatively small load. The failure occurs in most cases suddenly

and in a brittle manner. Structural weakness, overloading, design and construction

faults, vibration settlements, change of structural system etc can cause failure of

concrete structures. Hence arises the need for strengthening of concrete structures in

flexure as well as in shear. Epoxy plate bonding with Carbon Fibre Reinforced

Polymers (CFRP) has been shown to be a competitive method for strengthening of

existing structures so as to enhance the load carrying capacity.

By bonding CFRP to a structure in sawn grooves, some advantages

compared to traditional plate bonding may be achieved The reinforcement will get

some protection, installation may be easier and quality may be improved. The rods

can also be bonded by grout. Normally in strengthening application CFRPs are used

as additional tensile reinforcement. This paper presents a discussion on Carbon

Fiber Reinforced Polymers and its application in strengthening of concrete

structures.

CONTENTS

1. INTRODUCTION

2. STRENGTHENING

2.1 CHALLENGES

2.2 DUCTILITY

2.3 THE NEED FOR STRENGTHENING

2.4 STRATEGIES FOR STRENGTHENING

3. FIBRE REINFORCED POLYMERS

3.1 GENERAL

3.2 COMPOSITE COMPONENTS

3.2.1 Fibres

3.2.2 Matrices

3.3 TYPES OF FIBRE REINFORCED POLYMERS

3.3.1 Glass Fibre Reinforced Polymer

3.3.2 Carbon Fibre Reinforced Polymer

3.3.3 Aramid Fibre Reinforced Polymer

3.4 ADVANTAGES OF FIBRE REINFORCED POLYMER

4. CARBON FIBRE REINFORCED POLYMER

4.1 GENERAL

4.2 MANUFACTURING OF CARBON FIBRE REINFORCED POLYMERS

4.3 PROPERTIES OF CARBON FIBRE REINFORCED POLYMERS

4.4 THE MAIN USES OF CFRP IN STRUCTURES

4.4.1 CFRP Strips

4.4.2 CFRP Wraps

4.4.3 CFRP Laminates

4.5 BARRIERS

4.6 STATUS

4.7 NEW PRODUCTS DEVELOPED FROM CFRP

5. NEAR SURFACE MOUNTED LAMINATES FOR SHEAR

STRENGTHENING OF CONCRETE BEAMS

5.1 GENERAL

5.2 ASSUMPTIONS IN NSMR STRENGTHENING

5.3 STRENGTHENING TECHNIQUES

5.4 ADVANTAGES AND DISADVANTAGES OF NSMR

5.5 FAILURE MODES

5.5.1 Failure modes of externally bonded FRP reinforcements

5.5.2 Failure modes of Near Surface mounted FRP reinforcement

6. CONCLUSION

REFERENCES

LIST OF FIGURES

Fig. 2.1 Performance history of a structure

Fig. 2.2 Deterioration and strategies for strengthening

Fig. 3.1 Properties of different fibres and typical reinforcing steel

Fig. 3.2 Glass fibre reinforced polymer sheet

Fig. 3.3 Carbon fibre reinforced polymer

Fig. 4.1 Carbon fibre reinforced polymers used in launch vehicles

Fig. 5.1 Near surface mounted FRP,rectangular shapes and rods.

Fig. 5.2 Strengthening techniques

Fig. 5.3 Spacing of NSM reinforcement

Fig. 5.4 Shear strengthening

Fig. 5.5 Procedure for strengthening with NSMR

Fig. 5.6 Strengthening of a concrete joint with NSMR

Fig. 5.7 Interface bond failure modes for EBR FRP strips

Fig. 5.8 Failure at epoxy concrete interface

Fig. 5.9 Cover splitting failure of NSM round bars

LIST OF TABLES

Table 3.1 Mechanical properties of common strengthening material

Table 3.2 Properties of matrix materials

Table 5.1 Characteristics and aspects of externally bonded FRP reinforcement

1. INTRODUCTION

Since the first structures were formed, whether by nature or early human beings,

they have been plagued by detonation and destruction. Deterioration and destruction are

laws of nature that affects even the most modern structures. Modern structures, like

skyscrapers and bridges are costly to build and the construction period may sometimes be

disturbing to people and society. Therefore it is of interest to have with long life and low

maintenance costs. Maintenance is not only about costs but also a necessity to keep a

structure at a defined performance level. The definition of performance includes load

carrying capacity, durability, function and aesthetic appearance. A structure that fulfils all

demands of load carrying capacities might at the same time not satisfy durability demands.

Absence of, or incorrect maintenance will in most cases increase the speed of the

degradation process and therefore lower the performance of the structure. If the

performance level has become too low, then repair is needed to restore the structure to its

original performance. Structures with long lifespan, which most of the civil and building

should have, will meet changed demands placed on them from the owners, users or

surrounding society. A structure with satisfactory load bearing capacity, aesthetic

appearance and durability might not fulfill the function demands. A bridge can be for

instance too narrow. To meet changed demands; a structure may be upgraded, which

furthermore can be a way to increase life, durability and reliability of the structure.

The society around us is changing and so are the demands on existing structures.

Transportation has become heavier and more frequent during the last decades and will

probably continue that way in the future. The vehicle speed has increased which also leads

to higher loads by dynamic effects. The knowledge in structural behavior has also increased

and sometimes led to awareness of unreliable structures. Structures are sometimes damaged

by accidents. Ships cars or for example trucks can collide with bridges and the structure

may be damaged. Some times structures are insufficient to carry loads either due to

incorrect design or mistakes during construction. Further more reasons for repairing or

upgrading a structure may be widening of bridges or problems initiated by temporary over

load. If the performance level of a structure becomes inadequate, it might be possible to

keep it in service with the restrictions of use. Other wise the structure has to be upgraded or

replaced. One way to upgrade, the performance level is by strengthening.

Maintenance repair and strengthening of old concrete structures are becoming

increasingly common. If one considers the capital that has been invested in existing

infrastructures, there is not always economically viable to replace an existing structure with

a new one. The challenge must be taken to develop relatively simple measures to keep

increase a structure performance level throughout its life. In comparison to building a new

structure, strengthening an existing one is more complicated since the structural conditions

are already set. In addition, it is not always easy to reach the areas that need to be

strengthened; often there is also limited space. Traditional methods such as different kinds

of overlays, shortcrete or post tensioned cables placed on the outside of the structure

normally needs much space. Existing structures have an intended lifespan and are supposed

to fulfill a certain function during that time. Strengthening can make it possible to extend

the life span of the structure to an optimum. This may be reached by an administrative

upgrading where refined calculation models are used in connection with higher exactness

for material parameters to show that the existing structure has a higher load carrying

capacity than what was earlier assumed. This can also in some cases be used to show that

the structure can fulfill new demands.

If strengthening is needed there exists a variety of methods, for example adding on

new structural material, or changing the structural system. These methods have been proven

to work well in many situations. However they may, in some cases have draw backs that

make the method too expensive to use or not as effective as wanted in terms of time and

structural behavior. Due to different advantages and draw backs of strengthening methods,

designers must closely evaluate all alternatives including the possibility that upgrading may

not be the best choice and replacement is the alternative. During the last decade, it has

become more and more customary to strengthen concrete structures by bonding advanced

composite materials to their surfaces. The method involves a material with high tensile

strength and relatively high stiffness being bonded to the surface of a structural element to

serve as additional reinforcement. The most common material used is carbon fiber fabric or

laminate. In the future it will probably be even more common with strengthening as new

methods are developed and as the knowledge on environmental aspects and lifecycle cost

increase.

2. STRENGTHENING

2.1 CHALLENGES

For an uninitiated person an upgrade must be considered to be a small and simple

alteration of an existing structure. Concerns must be taken to existing materials, often in

deteriorated condition, loads during strengthening and to existing geometry. In some

cases it can also be difficult to reach the areas that need to be strengthened.

When strengthening is going to be undertaken all failure modes must be evaluated.

Strengthening a structure for flexure may lead to shear failure instead of giving the

desired increased load bearing capacity. It should also be noted that not only the failure

mode of the strengthened member is important. If a critical member in a structure is

strengthened, another member can become the critical one. Because of changed stiffness

in an undetermined structural system the whole structure must be investigated. The

strengthening should also designed with consideration to minimize the maintenance and

repair needs. When a strengthening is designed the consequences from loss of

strengthening effectiveness by fire, vandalism, collision etc. must in addition be

considered.

Furthermore, the existing documentation of the structure is often very poor and

sometimes even wrong. It might be necessary to redesign the structure with the probable

former codes that were active when the structure was built. This can give enough

knowledge about the structural mode of action, otherwise field investigations must be

undertaken to provide an understanding of the structure. The design of a strengthening

however must fulfill requirements in the codes of today.

It is not only the financial and structural aspects that should form the basis for

decisions of strengthening method, but environmental and aesthetic aspects must also be

considered.

2.2 DUCTILITY

Most fibre composites are linear-elastic material without any defined yield point.

Structures on the other hand should be designed to fail in a ductile way or atleast with

adequate warning signals preceding a potential collapse. Ductility can be defined as

capability of a structure to deform while still carrying the load even when the maximum

load bearing capacity is exceeded. It is important to distinguish between material ductility

and structural ductility. Steel bars with short anchorage can be an example of brittle failures

even though steel is considered to be a ductile material. Material properties and structural

ductility are not directly dependent, and linear-elastic materials may increase the ductility

of a structure.

A concrete beam reinforced in bending with steel bars is often considered to have a

very ductile behavior. However, consider the same beam subjected to a fatigue load that

causes high strains in the steel both in compression and tension. The loading will make the

structure to fail in a brittle way, but even worse the normal behavior of the structure.

Consider the same beam, with fatigue load, and strengthened with a linear elastic material,

due to strengthening the stresses in the steel bars will decrease and it will not fail in fatigue,

instead the ductile behavior is regranted.

Work has been carried out on many different types of structures to restore or

increase the flexural capacity, which gives that the structure will be loaded close to its

maximum shear capacity. One of the chief concerns is that shear failures are often very

brittle with no, or only small warnings preceeding the failure because of the higher elastic

energy built up compared to what it had before strengthening. On the other hand, a

structure with a brittle failure in shear may be strengthened so that the failure mode will

change to a more ductile and friendly mode.

2.3 THE NEED OF STRENGTHENING

Concrete structures need to be strengthened for any of the following reasons:

.Load increases due to higher live loads, increased wheel loads, installations of

heavy machinery, or vibrations.

.Damage to structural parts due to aging of construction materials or fire damage,

corrosion of steel reinforcement, and/or impact of vehicles.

.Improvements in suitability for use due to limitation of deflections, reduction of

stress in steel reinforcement and/or reduction of crack widths.

.Modification of structural system due to elimination of walls/columns and/or

openings cut through slabs.

.Errors in planning or construction due to insufficient design dimensions and/or

insufficient reinforcing steel.

2.4 STRATEGIES FOR STRENGTHENING

When a structure is going to be strengthened there are several aspects to consider. The

figure shows a schematic example of a structure that had inadequate load bearing capacity

due to a design fault already present before it was taken into service. It was then

strengthened slightly above the desired performance level. After some time the structure

was damaged due to an accident, collision, fire or overload that damaged the system to a

level where performance requirements were not fulfilled. The damages were then repaired

to a new satisfactory performance level. Later, the demands on the structure were changed,

higher load bearing capacity was required, and the structure needed to be strengthened to a

higher performance level to meet these demands By a third strengthening it was possible to

meet the new demands and keep the structure in service.

Fig 2.1 Performance History of a Structure

Without considering the deterioration the need for strengthening may not be that

complicated. Insufficient performance due to a design fault, accident or increased demands

can quite clearly be identified. When deterioration is significantly prevalent it becomes

more complicated. For a new structure that is inadequate due to a design or constructional

fault, the size of the problem is more or less well known and the desired life of the structure

can also be quite clearly expressed. The selection of suitable strengthening methods can

nevertheless be complicated. For older structures in need of strengthening, the situation

becomes even more complex. One important issue is the remaining life of the structure. It is

not always valid to strengthen a structural part to give it 50 years remaining life if the

foundations, for instance, will only function for another 10years.For example a road

network may be changed within 5 years due to a larger infrastructure project. If a bridge on

the existing road needs to be repaired to provide satisfactory reliability in the meantime, it

would be very cost ineffective to replace the old bridge with a new. In this case the bridge,

if possible, should be repaired and the repaired bridge does not need to have a life span

longer than 5 years.

Fig 2.2 Deterioration and Strategies for Strengthening

With deterioration in mind the strategy for strengthening becomes more

complicated. This is schematically illustrated in the figure 2.The performance level of the

structure is slowly decreasing, but it still fulfills its performance requirements. New

demands are placed on the structure, but the time being it still fulfills the performance

requirements. The decrease of performance will in the nearby future result in the structure

being inadequate, marked by X.The rate of degradation can be different in different cases.

However, the structure must be upgraded, in this case by strengthening.

3. FIBRE REINFORCED POLYMER

3.1 GENERAL

An FRP composite is defined as a polymer that is reinforced

with a fibre. The primary function of fibre reinforcement is to carry

load along the length of the fiber and to provide strength and

stiffness in one direction. FRP represents a class of materials that

falls into a category referred to as composite materials. Composite

materials consist of two or more materials that retain their

respective chemical and physical characteristics when combined

together. FRP composites are different from traditional construction

materials like steel or aluminium. FRP composites are anisotropic

(properties apparent in the direction of applied load) whereas steel

or aluminium is isotropic (uniform properties in all directions,

independent of applied load). Therefore FRP composites properties

are directional, meaning that the best mechanical properties are in

the direction of the fibre placement.

3.2 COMPOSITE COMPONENTS

3.2.1 Fibres

The composite’s properties are mainly influenced by the

choice of fibres. In civil engineering three types of fibres dominate.

These are carbon, glass, and aramid fibres and the composite is

often named by the reinforcing fibre, e.g.CFRP for Carbon Fibre

Reinforced Polymer. They have different properties. For

strengthening purposes carbon fibres are the most suitable. All

fibres have generally higher stress capacity than the ordinary steel

and are linear elastic until failure.The most important properties

that differ between the fibre types are stiffness and tensile strain.

The three fibre types are schematically presented in figure3-1 in

comparison with an ordinary steel bar and a steel tendon.

Fig. 3.1 Properties of different fibres and typical reinforcing steel. ACI

Committee 440(1996) and Dejke (2001)

Table 3.1 Mechanical properties of common strengthening

material

Material

Modulus of

elasticity[G

Pa]

Compressiv

e

Strength[M

Pa]

Tensile

Strength[M

Pa]

Density

[kg/m³]

Concret

e

20-40 5-60 1-3 2400

Steel 200-210 240-690 240-690 7800

Carbon

fibre

200-800 NA 2500-6000 1750-1950

3.2.2 Matrices

The matrix should transfer forces between the fibres and

protect the fibres from the environment. In civil engineering,

thermosetting resins (thermosets) are almost exclusively used. Of

the thermo sets vinyl ester and epoxy are the most common

matrices. Epoxy is mostly favoured above vinyl ester but is also

more costly. Epoxy has a pot life around 30 minutes at 20degree

Celsius but can be changed with different formulations. The curing

goes faster with increased temperature. Material properties for

polyester and epoxy are shown in table 3-2. Epoxies have good

strength, bond, creep properties and chemical resistance.

Table 3.2 Properties of matrix materials

Material Density

[kg/m³]

Tensile strength

[MPa]

Tensile

modulus

[GPa]

Failure strain

[%]

Polyester 1000-1450 20-100 2.1-4.1 1.0-6.5

Epoxy 1100-1300 55-130 2.5-4.1 1.5-9.0

3.3. TYPES OF FIBRE REINFORCED POLYMERS

The different types of fibre reinforced polymer are : glass

fibre, carbon, aramid, ultra high molecular weight polyethylene,

polypropylene, polyester and nylon. The change in properties of

these fibres is due to the raw materials and the temperature at

which the fibre is formed.

3.3.1 Glass fibre reinforced polymer

Glass fibres are basically made by mixing silica sand,

limestone, folic acid and other minor ingredients. The mix is heated

until it melts at about 1260°C. The molten glass is then allowed to

flow through fine holes in a platinum plate. The glass strands are

cooled, gathered and wound. The fibres are drawn to increase the

directional strength. The fibres are then woven into various forms

for use in composites.

Fig 3.2 Glass fiber reinforced polymer sheet

Based on an aluminium lime borosilicate composition glass

produced fibres are considered the predominant reinforcement for

polymer matrix composites due to their high electrical insulating

properties, low susceptibility to moisture and high mechanical

properties. Glass is generally a good impact resistant fibre but

weighs more than carbon or aramid. Glass fibres have excellent

characteristics equal to or better than steel in certain forms.

3.3.2 Carbon Fibre Reinforced Polymer

Carbon fibres have a high modulus of elasticity, 200-800 GPa.

The ultimate elongation is 0.3-2.5 % where the lower elongation

corresponds to the higher stiffness and vice versa.Carbon fibres do

not absorb water and are resistant to many chemical solutions . They

with stand fatigue excellently, do not stress corrode and do not

show any creep or relaxation, having less relaxation compared to

low relaxation high tensile prestressing steel strands.

Carbon fibre is electrically conductive and, therefore might

give galvanic corrosion in direct contact with steel.

Fig 3.3 Carbon Fibre Reinforced Polymer

3.3.3 Aramid Fibre Reinforced Polymer

Aramid is the short form for aromatic polyamide. A well known

trademark of aramid fibres is Kevlar but there exists other brands

too,e.g Twaron, Technora and SVM.The modulli of the fibres are 70-

200 GPa with ultimate elongation of 1.5-5% depending on the

quality.Aramid has a high fracture energy and is therefore used for

helmets and bullet-proof garments.Aramid fibres are sensitive to

elevated temperatures, moisture and ultraviolet radiation and

therefore not widely used in civil engineering applications.Further

aramid fibres do have problems with relaxation and stress corrosion.

3.4 ADVANTAGES OF FIBRE REINFORCED POLYMERS

The advantages of FRP are

FRP can provide a maximum material stiffness to density ratio

of 3.5 to 5 times that of aluminum or steel.

It has high fatigue endurance limits

It can absorb impact energies

The material properties can be strengthened where required

The corrosion potential is reduced

Joints and fasteners are eliminated or simplified.

4. CARBON FIBRE REINFORCED POLYMERS

4.1 GENERAL

The main impetus for development of carbon fibres has come from the aerospace

industry with its need for a material with combination of high strength, high stiffness and

low weight. Recently, civil engineers and construction industry have begun to realize that

this material (CFRP) have potential to provide remedies for many problems associated with

the deterioration and strengthening of infrastructure. Effective use of carbon fibre

reinforced polymer could significantly increase the life of structures, minimizing the

maintenance requirements.

Carbon fibre reinforced polymer is a type of fibre composite material in which

carbon fibres constitutes the fibre phase. Carbon fibre are a group of fibrous materials

comprising essentially elemental carbon. This is prepared by pyrolysis of organic fibres.

PAN-based (PAN-poly acrylo nitrile) carbon fibres contains 93-95 percentage carbons, and

it is produced at 1315C (2400F). Carbon fibres have been used as reinforcement for

albative plastics and for reinforcements for lightweight, high strength and high stiffness

structures. Carbon fibres are also produced by growing single crystals carbon electric arc

under high-pressure inert gas or by growth from a vapour state by thermal decomposition

of a hydrocarbon gas.

CFRP materials possess good rigidity, high strength, low density, corrosion

resistance, vibration resistance, high ultimate strain, high fatigue resistance, and low

thermal conductivity. They are bad conductors of electricity and are non-magnetic.

Carbon fibre reinforced polymer (CFRP) is currently used world wide to retrofit and

repair structurally deficient infrastructures such as bridges and buildings. Using CFRP

reinforcing bars in new concrete can eliminate potential corrosion problems and

substantially increase a member’s structural strength. When reinforced concrete (RC)

members are strengthened with externally bonded CFRP, the bond between the CFRP and

RC substrate significantly affects the members load carrying capacity.

Strengthening measures are required in structures when they are required to

accommodate increased loads. Also when there are changes in the use of structures,

individual supports and walls may need to be removed. This leads to a redistribution of

forces and the need for local reinforcement. In addition, structural strengthening may

become necessary owing to wear and deterioration arising from normal usage or

environmental factors.

The usage of composite materials like CFRP is still not widely recognized. The lack

of knowledge of technology using CFRP and the simplicity of it will make some people

hesitant to use it.

4.2 MANUFACTURING OF CARBON FIBER REINFORCED POLYMER

There are different methods of manufacturing polymer composites. They are listed as

below:

Continuous reinforcement process

Filament winding

Pultrusion

Hand lay-up processes

Moulding processes

Matched-die moulding

Autoclave moulding

Vacuum bagging

Resin injection processes

Resin transfer moulding

Reaction injection moulding

Integrated manufacturing systems

Carbon fibre reinforced polymer strip is mainly manufactured by the process called

pultrusion. . The pultrusion principle is comparable with a continuous press. Normally

24,000 parallel filaments are pulled through the impregnated bath, formed into strips under

heat, and hardened. These strips are uni-directional; the fibres are oriented only in the

longitudinal direction. Correspondingly, the strip strength in this direction is proportional to

the fibre strength and, thus, very high.

The composite materials are very difficult to machine due to anisotropic, non-

homogenous and reinforcing fibres tend to be abrasive. During machining defects tend to

be abrasive. During machining defects are introduced in work piece and tools wear rapidly.

Traditional machining techniques like drilling and screwing can be used with modified tool

design and operating conditions. Also some sophisticated processes like laser and ultrasonic

machining and electric discharge techniques are also used. For unidirectional CFRP, the

tools are of PCD (poly crystalline diamond) and carbide. For multidirectional CFRP, the

tools used are made of carbide.

4.3 PROPERTIES OF CARBON FIBRE REINFORCED POLYMER

Carbon fibre reinforced polymer (CFRP) is alkali resistant.

Carbon fibre reinforced polymers (CFRP) are resistant to corrosion; hence they are

used for corrosion control and rehabilitation of reinforced concrete structures.

Carbon fibre reinforced polymer composite (CFRPC) has low thermal conductivity.

CFRPC has high strength to weight ratio and hence it eliminates requirements of

heavy construction equipment and supporting structures.

CFRPC is available in rolls of very long length. Therefore, they need very few

joints, avoiding laps and splices, and its transportation is also very easy.

CFRPC has a short curing time. Therefore, the application takes a shorter time. This

reduces the project duration and down time of the structure to a great extent.

Application of CFRPC does not require bulky and dusty materials in a large

quantity; therefore, the site remains tidier.

CFRPC possess high ultimate strain; therefore, they offer ductility to the structure

and they are suitable for earthquake resistant applications.

CFRPC has high fatigue resistance. So they do not degrade, which easily alleviates

the requirement of frequent maintenance.

CFRPC is bad conductor of electricity and is non-magnetic.

Due to the lightweight of prefabricated components in CFRPC, they can be easily

transported. This thus encourage prefabricated construction, reduce site errection,

labour cost and capital investment requirements.

4.4 THE MAIN USES OF CFRP IN STRUCTURES

4.4.1 CFRP Strips

CFRP strips or laminates are used for strengthening of structures. The performance

of CFRP strips depends on the strength of the adhesive used to bond the strips to the

concrete surface and the degree of stress at the interface of the concrete and strips, which

governs the onset of delamination. Critical modes of failure, such as, debonding of strips

from the concrete (due to failure at the concrete adhesive interface) and shear-tension

failure (delamination of concrete cover), can limit improvements in structures strengthened

with CFRP. Also, these structures may require a higher factor of safety in their design.

Minimizing the chances of potential failure can optimize the benefits of CFRP strips,

allowing a strong, ductile, and durable structural system to be achieved. One possible

solution to minimizing failure problems is an efficient mechanical-interlocking-anchorage

system for bonding CFRP strips to the concrete surface. Experiments have been done and it

is found that deep grooves are cut (6mm) in the top surface of beam, perpendicular to the

beam length and 150mm intervals, and filling the grooves with epoxy adhesive. The

grooves are intended to provide a better interlocking mechanism between the concrete

surface and CFRP strips. To create a stronger surface at the ends of the beam for proper

bonding of strips, CFRP fabric sheets are attached at both ends before strip application.

CFRP is used to strengthen steel road bridges more easily and cheaply. The CFRP

strips are only 20% of the weight of the strips of similar products made from high-strength

steel but are at least four times as strong. Their high-strength-to- weight ratio makes the

CFRP strips easily to handle and reduces installation costs.

4.4.2 CFRP Wraps

CFRP wrapping is used for rehabilitation of masonry columns. CFRP wraps are

used for corrosion control and rehabilitation of reinforced concrete columns. They are also

used for construction of earthquake resistant structures.

4.4.3 CFRP Laminates

Low Thermal Expansion CFRP Laminates Are Used For Structural Strengthening.

4.5 BARRIERS

Although, the technology of using CFRP for strengthening of structures has been

used successfully in Japan and Europe the usage of composite materials like CFRP is still

not widely recognized in the industry. The lack of knowledge of the technology and the

simplicity of it will make some people hesitant to use it. The technology of CFRP is still to

develop.

4.6 STATUS

Recently, CFRP is used to strengthen structural parts of RC bridges. CFRP is used

to strengthen steel road bridges more quickly, cheaply and easily. Strips of CFRP

measuring just 8 mm in thickness have been used to strengthen a road bridge in Rochdale,

UK. Worldwide research development work, the use of CFRP strips to rehabilitate

structures is already routine for many firms in Western Europe and Japan. In the US, Sika

has introduced Sika CarboDur, which is a CFRP laminate used to strengthen concrete,

steel, or wooden structures. CFRP materials will not replace traditional construction

materials, but will be used increasingly to supplement them as needed.

A research team led by Dr.Abdul-Hamid Zureick, professor of civil and

environmental engineering at Georgia Institute of Technology, Atlanta, GA, has performed

an integrated field/laboratory approach to rehabilitate the Lee Road Bridge over Interstate

20 in Douglas County, GA, using CFRP. This project is funded by Georgia Department of

Transportation (GDOT) in cooperation with the Federal Highway Administration (FHWA).

The project took workers less than a day to complete what could have taken several weeks

to do traditionally and, so far, laboratory tests have determined that CFRP can make

bridges 30 to 40 percent stronger than the original design.

Results obtained from current experimental research at Carleton University have

demonstrated the feasibility and the advantages of using carbon fiber reinforced plastic

(CFRP) sheets as external reinforcements in the repair and strengthening of concrete shear

wall structures. The addition of CFRP sheets greatly increases the ultimate flexural moment

capacity of the retrofitted shear wall. However, in order for the FRP sheet to carry the high

axial loads resulting from the bending moment imposed on the shear wall, the CFRP sheets

must be adequately anchored at the base of the wall.

In India CFRP composites are mainly used in launch vehicles (for example in

GSLV and PSLV) for making them much lighter.

Fig. 4.1 CFRP in launch vehicles

4.7 NEW PRODUCTS DEVELOPED FROM CFRP

“ALBIS” have developed an exceptional range of high performance compounds

under the name “ALCOM”. Carbon fibre reinforcement promotes strength and rigidity

coupled with dimensional stability. They are characterized by high abrasion resistance and

available as hybrid modifications with technical fillers and fibers.

“SGL Carbon group” has come with three products:

PANOX - The oxidized fiber, for textile applications.

SIGRAFIL T – A partially carbonized fibre ideally suited for industrial gaskets and

packing.

SIGRAFIL C – heavy tow carbon fibre- it is an essential component to make

materials electrically conductive or mechanically reinforce. E.g. thermoplastics,

thermosetting resins, cement systems and floorings.

5. NEAR SURFACE MOUNTED CFRP LAMINATES FOR

SHEAR STRENGTHENING OF CONCRETE BEAMS.

5.1 GENERAL

The use of Near Surface mounted Reinforcement for concrete structures are not a

new invention. A type of NSMR has been used since the 1940s, where steel reinforcement

is placed in slots in the concrete cover or in addition concrete cover that is cast onto the

structure. Here steel bars are placed in slots in the concrete structure and then the slots are

grouted. It has also been quite common to use steel bars, fastened to the outside of the

structure covered with shotcrete. However in these applications it is often difficult to get a

good bond to the original structure and in some cases, it is not always easy to cast the

concrete around the whole steel reinforcing bars. From 1960s the development of strong

adhesives such as epoxies, for the construction industry moved the method further ahead by

bonding the steel bars in sawed slots in the concrete cover. However, due to the corrosion

sensitivity of steel bars an additional concrete cover is still needed. For these applications,

epoxy coated steel bars are not always corrosion resistant for various reasons that will not

be discussed here. The use of steel NSMR cannot be said to have shown great success.

Nevertheless, by using CFRP NSMR some of these drawbacks that steel NSMR posses can

be overcome.

Fig. 5.1. Near surface mounted FRP, rectangular shapes and

rods

Firstly, CFRP NSMR does not corrode, so thick concrete covers are not needed.

Secondly, the CFRP laminate can be tailor made for near surface applications and moreover

the lightweight of the CFRP laminates makes them easy to mount. Finally, depending on

the form of the form of the laminate air voids behind the laminates can be avoided. Both

epoxies and systems using high quality cement mortar can be used. However, before

proceeding, a short description of how to undertake a strengthening work with NSMR will

be given. In practical execution the following steps must in general be performed during

strengthening:

Sawing slots in the concrete cover, with the depth depending upon product used and

the depth of concrete cover.

Careful cleaning of the slots after sawing using high-pressurized water,

approximately 100-150 bars is recommended. No saw mud is allowed in the slot.

If an epoxy system is used, the slot must be dry before bonding. If a cement system

is used it is generally recommended that the existing surfaces are wet at the time of

concrete mortar casting.

Adhesive is applied in the slot, or with a cement system, cement mortar is applied in

the slot.

Table 5.1 Characteristics and aspects of externally bonded FRP reinforcement.

Properties NSMR

Shape Rectangular strips or laminates

Dimensions:

Thickness

Width

Simple bonding of factory made profiles with adhesive

or cement mortar in presawed slots in the concrete

cover

Application aspects For flat surfaces

Depends on the distance to steel reinforcement

A slot needs to be sawn up in the concrete cover

The slot needs careful cleaning before bonding

Bonded with a thixotropic adhesive.

Possible to use cement mortar for bonding

Protected against impact and vandalism

Suitable for strengthening in bending

Minor protection against fire

5.2 ASSUMPTIONS IN NSMR STRENGTHENING

In the design for strengthening with NSMR the following assumptions are made:

Bernoulli’s hypothesis applies, i.e.; linear strain across the section varies

rectilinearly. This implies that the linear strain in the concrete, steel reinforcement

and laminate that occurring at the same level is of the same size. Composite action

applies between all the materials involved.

Concrete stresses are obtained from the materials characteristic curve. Concrete

compression strain is limited to an approved failure strain of €=3.5%.

For a cracked cross section, the concrete’s residual ensile strength is ignored.

The stress in tensile and compression steel reinforcement are taken from the

reinforcement’s characteristics curve corresponding to the total strain. The total

strain in laminate may not exceed the failure strain.

The laminate is assumed to be linearly elastic until breakage i.e.; Hooke’s law

applies.

In addition to this it is important to notice that if there exists a strain field on the

structure, due to for example the dead load, this must be considered in design.

5.3 STRENGTHENING TECHNIQUES

The NSMR strengthening is based on fixing,by epoxy adhesive,Carbon Fibre

Reinforced Polymers into precut slits opened in the concrete cover of lateral surfaces of

beams.The EBR and NSM strengthening techniques are represented infig6 .

Fig.5.2 Strengthening techniques (a) external bonded,(b)near surface mounted

Detailing of the near surface mounted reinforcement is an

important issue; we need to select the most suitable FRP cross-section

and adhesive. In design there should be considered the minimum

distance between adjacent reinforcement to avoid horizontal

propagation of the splitting cracks, and the minimum distance from the

edge of the member to avoid edge splitting effect.

Application of near surface mounted FRP reinforcement consists

of the following working steps:

In the first step a groove is cut using a saw with one or two diamond

blades or a grinder with dimensions in function of the reinforcement size

and type. Further preparation of the groove consists of cleaning the

surface from dust and lose parts using vacuum or compressed air, then

the groove is filled halfway with adhesive, afterwards the FRP rod/strip is

inserted and lightly pressed to let the adhesive flow around the FRP.

Finally, the groove is filled with more paste and the surface is

levelled .The minimum dimension of the grooves should be taken atleast

1.5 times the diameter of the FRP bar. When a rectangularbar (strip)

with large aspect ratio is used, the minimum dimensionsmust be 3 times

the bar width and 1.5 times the bar height. In other instances, the

minimum groove dimension could be the result of installation

requirements rather thanengineering. For example the groove width

may be limited bythe minimum blade size and the depth by the concrete

cover.We should always avoid cutting of the existing steel

reinforcement.Optimal dimensions of the groove may depend on

characteristicsof the adhesive, surface treatment of FRP, and concrete

tensile strength, surface aggregates

Fig. 5.3 Spacing of the NSM reinforcement

Spacing of FRP shear reinforcement should not exceed lnet /2or 600 mm

Fig. 5.4 Shear Strengthening

Fig.5.5 Procedure for strengthening with NSMR

Fig.5.6 Strengthening of a concrete joint with NSMR

5.4. ADVANTAGES AND DISADVANTAGES OF NSMR

The advantages of Near Surface Mounted Reinforcements are:-

a low weight of the fibre makes it easy to handle without lifting equipment at the

site

negligible changes of crosssection, self weight and free height of a structure.

quick to apply

There also exist disadvantages such as:

Without protection the reinforcement is fire and impact sensitive.

Design consultants, contractors and clients have limited experience.

Depending on the structure going to be strengthened, different aspects might arise. For

all strengthening methods it is of utmost importance to understand how the strengthening

will affect the final structure.

5.5 FAILURE MODES

Several failure modes are known in general for elements

strengthened with FRP. Their understanding is important, because they

have significant effect on the ultimate load.

5.5.1Failure Modes of Externally Bonded FRP Reinforcements

Bond is necessary to transfer forces from the concrete in to the FRP,

bond failure implies complete loss of composite action. Four different

bonding failures are discussed below:

debonding in the concrete cover near the surface along a

weakened layer,

debonding at the interface between concrete and adhesive,

debonding in the adhesive, and

debonding between adhesive and FRP

Peeling-off failure is associated with the propagation of the localized

debonding. Peeling-off failures can be distinguished according to the

initiation of debonding. Debonding can result in peeling-off at: flexural

cracks, shear cracks, unevenness of the concrete surface and in the

anchorage zones

Fig. 5.7 Interface bond failure modes for EBR FRP strips

5.5.2Failure Modes of Near Surface Mounted FRP Reinforcement

5.5.2.1 Interfacial Failure Modes

Interfacial failure modes can develop in two modes as a pure

interfacial failure or as a cohesive shear failure in the adhesive. Pure

interfacial failure can be identified by the absence of adhesive remained

at the FRP surface after failure. Cohesive shear failure can be identified

by the presence of adhesive on both FRPand concrete after failure.

(1) Failure at reinforcement adhesive interface

The pure interfacial mode can be critical for bars with smooth or

lightly sand-blasted surfaces, when the bond relies on adhesion instead

of mechanical interlock between bar and adhesive.

(2) Failure at the epoxy concrete interface

Interfacial failure was found critical only in case of precast grooves

due to their even surface. When this type of failure develops the bond

stress is lower than usual, but failure is more ductile due to the residual

friction at adhesive and concrete interface.

5.5.2.2 Cover Splitting

The mechanism of cover splitting in case of round bars is similar to the

splitting bond failure of steel deformed bars, but due to the softer

deformations of the FRP bars the splitting tendency is not as intense.

Splitting is caused by the radial component of the bond stress. Multiple

types of cover splitting were observed, incase of epoxy adhesive

concrete cracking and concrete cracking accompanied by longitudinal

splitting of the adhesive, in case of cementitious mortar adhesive

splitting of the adhesive was dominant influenced by the low tensile

strength of the filler material. However, in case of NSM strips the

perpendicular component of interactional stress acts towards the thick

lateral concrete (exception are reinforcements close to the edge) so

splitting failure is less likely to appear

Fig. 5.8 Failure at epoxy concrete interface

[

Fig. 5.9 . Cover splitting failure of NSM round bars a) concrete

cracking b)

concrete cracking accompanied by longitudinal splitting

of the adhesive c) splitting of the adhesive

5.5.2.3 Edge Splitting

Edge splitting failure can be critical in elements where the

reinforcement is close to the edge of the concrete member. It is induced

also by the development of interactional stress. Edge splitting failure can

be avoided by keeping a minimum distance from the edge; this should

be considered in design Thermal expansion differences between epoxy

and concrete can influence edge splitting.

5.5.2.4 FRP Tensile Rupture

Tensile rupture (it has been rarely observed by non prestressed

strengthening) should be avoided according to its explosive nature.

Structures strengthened with prestressed FRP more frequently fail by

fibre tensile rupture because by prestressing the FRP we use a portion of

its strain capacity .

6. CONCLUSIONS

Originally, developed for aerospace and defense applications (aircraft and missile

parts) CFRP materials now find wide spread use across a number of industries. Apart from

construction field, CFRP materials are used in sporting goods industry, such as golf club

shafts, fishing rods, fences, tennis rackets etc. What’s more, CFRP materials are now

almost indispensable in the field of medical equipment and general machine construction.

As composite materials find increasing use in infrastructure applications were the design

lives are typically long, the issue of durability becomes more critical. The most damaging

factor faced by CFRP reinforcement is the environment from which steel reinforcement in

concrete is shielded automatically. The various external factors such as quality of concrete

surface, temperature conditions, humidity effect, effect of dynamic response with the

movement of traffic, impact resistance of composites that affect the process of

strengthening of concrete structures using CFRP should be studied in detail before

commencement of work.

Carbon Fibre Reinforced Polymers are a real boon in the field of

strengthening of structures and concrete repair owing to its good

rigidity, high strength, low density, corrosion resistance vibration

resistance and low conductivity. The application of carbon FRP laminates

is very effective for flexural strengthening of reinforced concrete beams,

provided proper anchorage of the laminate is ensured. As the amount of

steel reinforcement increases, the additional strength provided by the

carbon FRP external reinforcement decreases. Mechanical clamping or

wrapping with FRP fabric combined with adhesion is effective in

anchoring the FRP laminate and increases the anchorage capacity above

that expected for adhesive bond only. If proper anchorage is provided,

such as by wrapping or clamping, the effective strain limit (or stress

level) currently proposed informally-for FRP reinforcement by ACI 440 is

close to being achievable for this particular type of carbon FRP. For

lightly (steel) reinforced beams, this design stress level in the FRP can

add substantially and economically to the beam strength.

REFERENCES

Debaiky, A. S., Green, M. F. and Hope, B. F., “Carbon Fiber-Reinforced Polymer Wraps

for Corrosion Control and Rehabilitation of Reinforced Concrete Columns”, journal of ACI

Materials, March-April 2002, pp.129-137.

2. Anders Carolin.,”Carbon Fibre Reinforced Polymers for strengthening of

Structural elements’.(Doctoral Thesis) 2003.

3. J.A.O Barros, S.J.E Dias.,” Near Surface Mounted CFRP Laminates for Shear

Strengthening of Concrete Beams”, journal of Cement and Concrete

Composites28 (2006) 276-292, January 2005.

4. Zsombor Kalman Szabo., Gyorgy.L .Balazs., ”Near Surface Mounted FRP

Reinforcement for Strengthening of Concrete Structures”,(Research

Article),Periodica Polytechnica April 2006,pp33-38

5. www.sciencedirect.com

6.. www.wikipedia.org