CFRP for Strengthening Structures

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 Engineering of the University of Kerala

Submitted by

MENA.G.PILLAI S7C3 Roll No.17322 Guided by

Mrs. DEEPARAJ.S Lecturer Department of civil Engineering College of Engineering Thiruvananthapuram

DEPARTMENT OF CIVIL ENGINEERING COLLEGE OF ENGINEERING

THIRUVANANTHAPURAM 2007

DEPARTMENT OF CIVIL ENGINEERING COLLEGE OF ENGINEERING THIRUVANANTHAPURAM 2007

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

U G Professor

Mrs DEEPARAJ S

Dr. V SYAM PRAKASH

Lecturer Department of Civil Engineering College of Engineering Thiruvananthapuram

Professor Department of Civil Engineering. College of Engineering Thiruvananthapuram

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

3.

2.1

CHALLENGES

2.2

DUCTILITY

2.3

THE NEED FOR STRENGTHENING

2.4

STRATEGIES FOR STRENGTHENING

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 4.

ADVANTAGES OF FIBRE REINFORCED POLYMER

CARBON FIBRE REINFORCED POLYMER 4.1

GENERAL

5.

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

NEAR

SURFACE

MOUNTED

LAMINATES

FOR

STRENGTHENING OF CONCRETE BEAMS

6.

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

CONCLUSION REFERENCES

SHEAR

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

Compressiv

Tensile

elasticity[G

e

Strength[M

Pa]

Strength[M

Pa]

Density [kg/m³]

Pa] Concret

20-40

5-60

1-3

2400

200-210

240-690

240-690

7800

200-800

NA

2500-6000

1750-1950

e Steel Carbon fibre

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

Tensile strength

Tensile modulus

Failure strain

[kg/m³]

[MPa]

[GPa]

[%]

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 70200 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 1315°C (2400°F). 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, nonhomogenous 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:

Simple bonding of factory made profiles with adhesive

Thickness

or cement mortar in presawed slots in the concrete

Width

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. 5.5.1 Failure 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 5.5.2 Failure 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.

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