Composite structural beams could revolutionise the way long-span structures such as stadiums or bridges are built. They are less than a quarter of the weight of traditional reinforced beams and have the same ultimate load capacity. Their light weight is also an advantage in situations where transporting and installing them is particularly difficult.

Taylor Woodrow Engineering is developing a composite beam called ComCon, made from resin-bound carbon and glass fibres combined with concrete. A beam under a heavy load tends to deflect and bow downwards, so each section of the ComCon has been carefully designed to cope with the type of stress to which it will be subjected. The base of the beam is under tension, or stretched, and the top part is under compression, or squeezed. The lower part of ComCon is made using a hollow carbon-fibre composite section, which is very strong in tension. Only the top section of the beam is filled with concrete, which performs very well under compression.

A U-shaped metal former is used to mould the composite. This is rather like a giant square profile gutter: the top third of the former steps out so it is wider at the top than the bottom. The composite material is called “prepreg” because the fibres are already impregnated with the resin. It will harden when heated.

The base of the mould is lined with carbon-fibre prepreg, the fibres aligned longitudinally to resist the tensile forces. A vacuum bag is used to press the prepreg against the sides of the former and the whole thing is heated at 65 °C to harden the resin. Once the resin is cured, the beam is turned out of the mould.

The test beam was 15 m long, weighed just 200 kg and could be carried by four people. A commercially developed composite beam could be delivered to site at this stage, ready for finishing on site. Shuttering ply is placed in the beam so it rests on the stepped section, and an adhesive is applied to the sides of the beam to anchor the concrete in place. Concrete is poured in until it is flush with the top of the beam.

When load-tested, the beam’s deflection was proportional to the load imposed on it, which means designing with composite is no more difficult than with concrete or steel. The test beam deflected rather more than a concrete beam, but use of a stiffer composite material will solve this. Creep testing has been carried out to see if the beam permanently deforms under load over time, and the results indicate that the composite performs better than the concrete.

The next step is to use the beam for a live project.

The plan is to use ComCon for footbridges across the canal adjacent to Taylor Woodrow’s test facility in Southall, Middlesex.