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Building with composites

technical

Advanced composites offer benefits to the building industry in terms of their strength and mouldability, and their durability in high- humidity or marine environments.

Initially developed for the aerospace industry, they were for many years considered unsuitable for building applications because of the high cost of their basic ingredient, carbon fibre, and the manufacturing process. However, now that the automotive industry has developed low-cost moulding techniques and low temperature curing, the technology can now be transferred to create large strong lightweight panels for building.

Materials mix

Advanced composites are a mixture of fibres (normally glass, aramid or carbon) in a resin (normally epoxy). Initially these were produced with unidirectional fibres as the reinforcement material in strip form, but developments now allow production of a pre-impregnated matrix of woven fibres in the form of a mat (a bit like ready-made Filo pastry). The mats are partially cured. When needed they are laid onto a mould, consolidated in position using a simple vacuum bag and cured at 50degreesC.

Previously, advanced composites had to be moulded using highly automated autoclave processes with metal moulds to avoid distortion at the high pressures and temperatures used (typically 3 bar and up to 180degreesC). Composites can now be made either in the factory or even on site, using a vacuum bag press and no more than a tent with electrical storage heaters for curing. This means a lower possibility of mould distortion, allowing the use of low-cost moulding materials, including timber or even machinable foam.

Through careful design and engineering combined with low-cost moulding techniques it is possible to produce economic, large, lightweight structures with flexibility of form and non-corrodable materials.

To achieve economic use of the materials, particularly the amount of carbon fibre, it is now possible to combine woven and unidirectional carbon fibres, Kevlar fibres, honeycomb cores, reinforcement of hard points and machined metal items. In this way the reinforcement can be used where it is most effective, not unlike the design of reinforced concrete. One of the most complex examples of such integration is the Formula 1 racing car chassis.

Creating advanced composites does not have the problems of wet lay-up of glass reinforced fibre when making grp - crazing or blistering of the gel coat due to variable quality of the mix and variations in curing conditions. With advanced composites the formulations are predictable and the void content is lower. Although manufacture is still dependent on hand lay- up of the pre-impregnated mats, this can be done using relatively unskilled labour, with the final consolidation of the layers of mats achieved through the use of vacuum bag moulding.

Fire performance

Composites usually contain a high proportion of fibres and/or fillers, neither of which support combustion. However, epoxy resin clearly does not normally offer good fire performance although it can be mixed with fire-retardant additives to make the matrix self-extinguishing, with low smoke emission. Comprehensive fire tests have been carried out for its use on railway carriages (internal and external) and for aerospace applications. These test results need to be interpreted to the requirements of BS 476.

Finishes

The appearance of the self-finished material can be varied as required. The multi-directional fibre automatically gives a patterned effect, either closed or open weave, within the resin-rich surface. Using carbon fibre the moulding is normally black but other colours can be achieved by pigmenting the resin. After moulding, non-woven carbon tissue will give a matt finish which can either be left as a self-finish or painted.

Architectural applications

The new techniques of production, plus developments in pre-impregnated matrices, enable advanced composites to be used for large, irregular components of varying section or for detailed features which in other materials may be prohibitively expensive to form. This, coupled with their high-strength:low- weight ratio, offers the potential for shell roofs to atria, shopping malls or leisure facilities. Possible applications range from large monocoque panels to small and complex bracketry.

The practicalities of using advanced composites to construct long-span structures are being explored in a new research project, funded through the detr's Partners in Technology programme. This two-year investigation, which began in April 1998, is being undertaken by architect and technology consultant Brookes Stacey Randall, design and technology consultancy Taywood Engineering and manufacturer Advanced Composites Group.

Current long-span roof systems using beams, stays or arches overclad with solid panels are often not as efficient as shell structures, which use one material both for cladding and structural support. Our research aims to demonstrate the benefits of using high-strength lightweight composites in this type of project. Roof panels, cladding panels and staircases are already favourites for the composite treatment and other applications are now being identified.

Composites production

The production process starts using carbon, glass or aramid fibres wound onto a number of bobbins. Fibre is drawn into a 'pre-preg' machine which is a series of heated and chilled rollers that apply pressure to the material while drawing it through. The fibres are mixed with the resin, curing agents and fillers then accurately spread as a thin layer on to paper (which is removed before use). A pre-impregnated mat is created in either unidirectional or woven form. This mat is then stored in such a way as to retard the curing process until the material is ready for use.

By prepregging the materials, all of the health, safety and quality problems associated with traditional wet lay-up (where resin is mixed on the job) are avoided. Also, the product has an extremely accurate fibre-to-fibre ratio which can be optimised to the design requirement right back at the pre-pregging stage. It is very easy to adjust the process to produce a range of pre-pregs, from resin-rich to almost resin-starved, depending on the design of the product.

While the traditional autoclave processes associated with high temperature moulding prototypes and small production runs can be prohibitively expensive, with the low temperature moulding process and simple tooling, prototyping of components is much easier and cheaper.

Quality control and testing

The quality requirements for manufacture of composite materials are in many ways governed by aerospace industry standards. That industry has strict requirements for consistency of product from batch to batch and year to year. Testing may involve moulding of test pieces and their mechanical testing.

The architect's role

As with other materials it is important for architects to understand the production process. With advanced composites the range of mix formulations, additives and fibre reinforcements is limitless. Further, there are none of the size limitations found with some other materials where size of production lines or curing boxes may limit their overall dimensions. Large sections are possible using a system of moulds joined together. Such large lightweight components can be lifted into place without the use of particularly heavy plant. In the future cadcam may be able to link the design and engineering with the production and control of the mould-making tools.

Use of advanced composites is dependent on high levels of design and engineering skill to create a specific formulation for a particular application. At present there is a gap both in the technical understanding of advanced composites applications within the building industry, and a need for greater dialogue between building designers and composites subcontractors.

As with other new materials or applications, design input is often required by the specialist subcontractor at the early stages of a project. Its cost and the contractual implications of that input must be recognised.

Alan Brookes is a partner in Brookes Stacey Randall. The author acknowledges contributions by Adrian Potts of Advanced Composites Group and Martin Wilson of Taywood Engineering

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