Carbon fibre strengthening cities

Materials World magazine
,
1 Jun 2016

Simon Frost asks Jamie Dempster of Fibrwrap why carbon fibre reinforcement is becoming more common in renovation projects. 

The use of carbon fibre strengthening systems in the refurbishment and redevelopment sector is ‘growing exponentially’, according to Jamie Dempster, Managing Director of specialist structural renovation company Fibrwrap UK. The speed and minimal disruption of strengthening with carbon fibre, compared with that of steel and concrete, he says, is leading planners to increasingly favour carbon fibre for repairs and redevelopments, especially in London. 

Yorkshire-based Fibrwrap, which specialises in fibre-reinforced polymer (FRP) systems for structural repairs, recently collaborated with Adept Consulting Engineers, Leeds, to strengthen two very different structures – one, a concrete bridge opened in North Yorkshire in 1932, the other, a 1970s office block in London under redevelopment into luxury apartments. 

‘Scarborough’s Peasholm Glen Bridge was a project for North Yorkshire County Council – a bridge deck strengthening for a highways upgrade up to a 40-tonne capacity,’ Dempster told Materials World. The reinforcement required more than 900m2 of carbon fibre reinforcement on the underside of the bridge deck. 

‘The traditional methods would be to use either additional concrete or steel, which both take time to install and are heavy, adding undesirable weight to the structure,’ he explained. ‘By using carbon fibre, which is two-to-three times stronger than steel in its tensile properties but around five-to-six times lighter, we were able to bring it up to its required capacity much more quickly and without adding significant weight.’ 

Built for speed

The speed with which carbon fibre can be installed is particularly pertinent in London, where the cost of maintenance ‘downtime’ is typically higher. ‘Every construction site in London has to be done quickly and economically,’ said Dempster. Fibrwrap and Adept recently completed a project strengthening Hill House, an 11-storey, early 1970s office block near Archway Station in London, which is being turned into luxury apartments. 

‘For the Hill House project, we had a group of five guys on site. At one stage, they were installing 100m2 of carbon fibre a day, which a team of five couldn’t possibly achieve with steel,’ said Dempster.

Two solid concrete stair cores within the building were acting in shear. ‘Because the client plans to add two storeys to the building, we needed to strengthen the sheer cores not only to take the additional loading, but also to counteract the wind loading.’ 

To avoid the heavy, labour-intensive addition of steel and cement, Fibrwrap and Adept used carbon fibre from the second to eleventh floors. ‘It was quickly installed and smaller in dimension, so there was minimal disruption to the space, both inside and outside the stair core. At the lower levels, where forces were quite high, we used a combination of carbon fibre and steel plate – but by using carbon fibre for the higher levels, we were able to reduce the size and thickness of the steel plates and, therefore, the impact on the space.’ 

Wet layup

The carbon fibre was installed using a wet layup method, where the dry fibre and epoxy resin is taken to site and, after the concrete surface is prepared, the fibre is saturated with the resin and applied wet directly to the surface, to bond and cure in situ. 

On both the Peasholm Bridge and Hill House projects, the company’s Tyfo SCH-41 composite – a uni-directional carbon fabric – and Tyfo S Epoxy were used. Its ultimate tensile strength in the primary fibre direction is measured at 834MPa, with a tensile modulus of 82GPa and flexural strength of 104.8MPa, at a thickness of 1mm.

Dempster notes that the company only uses its own materials, developed at its R&D department in San Diego, USA, and also has academic links within the UK. ‘We have worked with several universities in the UK – mainly PhD students wishing to work with carbon fibre. We support them with materials and installation so they can do their applications and testing with our products. If it is a good research subject, we will fully support the student, and if their paper is published, it will mention our products.’ Useful for a company whose work is designed to be hidden.