CARBON FIBRE REINFORCED POLYMERS

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.

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.

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.

THE MAIN USES OF CFRP IN STRUCTURES

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.

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.

3. CFRP Laminates

Low Thermal Expansion CFRP Laminates Are Used For Structural Strengthening.

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.

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.

CFRP in launch vehicles

Fig. CFRP in launch vehicles

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.