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What is a fibre reinforced polymer composite?
Composite materials consist of two or more distinct physical phases, one of which, the fibrous, is dispersed in a continuous matrix phase. Composites offer the designer a combination of properties not available in traditional materials. It is possible to introduce the fibres in the polymer matrix at highly stressed regions in a certain position, direction and volume in order to obtain the maximum efficiency from the reinforcement, and then, within the same member to reduce the reinforcement to a minimal amount at regions of low stress value. Other advantages offered by the material are lightness, resistance to corrosion, resilience, translucency and greater efficiency in construction compared with the more conventional materials.
What are the materials used?
Resins
There are two main types of polymer used for resins: thermosets and thermoplastics. The thermosetting polymers used in the construction industry are the polyesters and the epoxides. There are many thermoplastic resins used in composite manufacture: polyolefins, polyamides, vinylic polymers, polyacetals, polysulphones, polycarbonates, polyphenylenes and polyimides.
Fibres
A wide range of amorphous and crystalline materials can be used as the fibre. In the construction industry the most common fibre used is glass fibre (there are 4 types of glass fibre: E-glass, AR-glass, A-glass and high strength glass). Carbon fibre, of which there are 3 types (Type I, II, III) can be used separately or in conjunction with the glass fibre as a hybrid to increase the stiffness of a structural member or the area within a structure, so that the stiffness exceeds the value possible using only glass fibre. Aramid fibres can be used instead of glass fibres to give increased stiffness to the composite.
Bundles of filaments are called strands and these are usually combined to form thicker parallel bundles called rovings. Assembled rovings are used in processes involving chopping of the fibres during the production of a composite e.g. SMC, spray-up, continuous sheet manufacture. Rovings are also manufactured by a direct technique in which all the filaments needed in the final roving (up to 4800) are all drawn simultaneously from one bushing. These rovings are called direct rovings and are used in weaving, pultrusion and filament winding. Strands may also be twisted to form several types of yarn; rovings or yarns may be used either individually or in the form of a woven fabric.
Additives
For structural applications it is mandatory to achieve some degree of flame retardance. Fire retardants are usually incorporated in the resin itself or as an applied gel-coat. Fillers and pigments are also used in resins for a variety of purposes, the former principally to improve mechanical properties and the latter for appearance and protective action.
Mechanism of Reinforcement
The reinforcement of a low modulus polymer with a high modulus, high strength fibre uses the plastic flow of the polymeric material under stress to transfer the load to the fibre; this results in a high strength, high modulus composite. The aim of the combination is to produce a two-phase material in which the primary phase (i.e. the fibres) is well dispersed and bonded by a weak secondary phase (i.e. the polymer matrix). The principal constituents influencing the strength and stiffness of composites are the reinforcing fibres, the matrix and the interface between the fibres and the matrix.
History of FRPs in construction
Fibre reinforced polymer (FRP) composites were first developed during the 1940âs, for military and aerospace applications. Considerable advances have been made since then in the use of this material and applications developed in the construction sector. FRPs have been successfully used in many construction applications including load bearing and infill panels, pressure pipes, tank liners, roofs, and complete structures where FRP units are connected together to form the complete system in which the shape provides the rigidity.
In the last decade, polymer composites have found application in the construction sector in areas such as bridge repair, bridge design, mooring cables, structural strengthening and stand-alone components. These composites are materials often referred to as advanced composites and have properties considerably superior to those of earlier composites. The term is ambiguous, however, because it does not identify any specific material combination. In the construction industry, the term is generally used for polymers reinforced with high strength high modulus continuous fibres of glass, carbon or aramid laid up in layers to form an engineered material.
Further information can be obtained from the following references:
Feldman D. "Polymeric Building Materials". Elsevier Applied Science, 1989, ISBN 1-85166-269-3.
Halliwell S. "Polymer Composites in Construction", BRE Report 405, CRC Publishers, 2000, ISBN 1-86081-429-8
Hollaway l. (Ed). "Polymers and Polymer Composites in Construction". Thomas Telford Ltd, London 1990, ISBN 0-7277-15216.
Hull D. "An Introduction to Composite Materials". Cambridge University Press, Cambridge, 1981. |