FIBRE REINFORCED POLYMER: An FIBRE REINFORCED POLYMER composite is defined as a polymer that is reinforced with fibre. The primary function of fibre reinforcement is to carry the load along the length of the fibre and to provide strength and stiffness in one direction.
FRP represents a class of materials that fall 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 appear 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.
COMPOSITE COMPONENTS of FIBRE REINFORCED POLYMER
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 figure1 in comparison with an ordinary steel bar and a steel tendon.
Properties of different fibres and typical reinforcing steel. ACI Committee 440(1996) and Dejke (2001)
Mechanical properties of common strengthening material
|Material||Modulus of elasticity[GPa]||Compressive Strength[MPa]||Tensile Strength[MPa]||Density [kg/m³]|
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 20 degree 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 2. Epoxies have good strength, bond, creep properties and chemical resistance.
Table 2 Properties of matrix materials
TYPES OF FIBRE REINFORCED POLYMERS
The different types of fibre reinforced polymer are glass fibre, carbon, aramid, ultra-high molecular weight polyethene, 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.
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.
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.
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 withstand 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.
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 moduli of the fibres are 70-200 GPa with an ultimate elongation of 1.5-5% depending on the quality.
Aramid has 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.
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 aluminium 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.
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