A composite material is a combination of two or more constituent materials which have improved characteristics when together than they do apart. Composites are often composed of a 'matrix' and reinforcement fibres.
The matrix is often a form of resin which keeps the reinforcement fibres in position and bonding them together so that loads can be effectively transferred. The properties of the composite can be influenced by cutting, aligning and placing the reinforcement fibres in different ways.One of the main advantage of using composites is that the reinforcement and matrix combination can be altered according to the required properties
There are many different types of composites which can be used for a wide range of construction and engineering purposes. Concrete is the most common composite material, consisting of aggregate held with cement as the matrix. Other common types of composites include:
- Fibre-reinforced polymer (FRP).
- Carbon-fibre-reinforced polymer (CFRP).
- Glass-fibre-reinforced plastic (GFRP).
- Aramid fibres, such as Kevlar, that are heat-resistant and strong synthetic fibres often used in aerospace and the military applications.
- Bio-derived polymers or biocomposites.
- PVC polyestyer.
- PTFE glass.
 Fabrication methods
There are several methods for fabricating composite materials, depending upon the intended use of the components and the required properties.
The process commonly involves wetting, mixing or saturating the reinforcement with the matrix. A heat or chemical reaction then causes the matrix to bind together into a rigid structure. This is usually done in an open or closed forming mould, but the exact process varies considerably.
 Wet lay-up
The reinforcement is laid into a mould of the final component and the matrix resin poured on before being left to cure. Other agents can be added as required, such as for the surface finish or to assist with removing after curing.
 Filament winding
Used for producing hollow tubes. The direction of winding the fibres contributes to the performance properties required.
Reinforcement fibres already impregnated with resin are placed in an open, heated mould. Another mould is placed on top using a combination of heat and pressure shaping before being left to cool.
A woven fabric reinforcement is placed into a mould into which resin is injected at pressure. This ‘wets out’ the fibres. It is then left to cure.
 Vacuum bagging
In this process, the woven fabric reinforcement is placed into a mould, which can be pre-coated with a release agent and/or gel coat. The resin is then rolled on top and a plastic film placed over it. A vacuum extracts the air which helps with consolidation.
Pultrusion is used for producing long, continuous components such as cable trays. Multiple strands of reinforcement fibres are pulled into a heater and coated in resin. These strands are then pulled through a moulding die and cut to the desired length.
 Use of composites
The construction industry has made increasing use of composites since the 1960s. Applications include:
Architectural features such as facades, cladding, domes, roofing and structures such as cupolas, can be made effectively using composites. They can be lighter, more efficient, more durable and require less maintenance than traditional materials. If combined with other core materials such as steel or plastics, they are capable of meeting high structural, fire, security and sound insulation requirements.
Composites can be used in the construction of entire bridge structures, bridge decks and bridge enclosures. They are useful for their high stiffness-to-weight and strength-to-weight ratios in comparison with conventional materials such as steel and reinforced concrete.
Composites are often used in modular structures, masts, towers, pipes, tanks, access covers and water control structures. They are also commonly used in rail applications such as trackbeds, platform systems, and gantries.
 Advantages of composites
There are a wide range of advantages offered by composite materials, including:
- Greater durability for use in extreme environments.
- Light weight composition.
- Faster construction times.
- Structures can often be repaired in situ.
- Low maintenance.
- Flexible in terms of colour, shape and texture.
- Can be made fire resistant.
- High ratios of strength and stiffness to weight.
 Related articles on Designing Buildings Wiki
- Carbon fibre.
- Concrete-steel composite structures.
- Construction materials.
- Curtain wall systems.
- Metal fabrication.
- Recyclable construction materials.
- Reinforced concrete.
- Sustainable materials.
- The development of structural membranes.
- Thermoplastic materials in buildings.
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