Used in anything from snowboards to aeroplanes, fibre reinforced polymer (FRP) is an incredibly versatile material.
It is an inert material which can be used in a range of environments where conventional materials fear to tread.
Lightweight and mouldable into a huge range of shapes and sizes, it allows designers a freedom of form not previously available.
One by one, the barriers to using FRP in the construction industry are being peeled away and clients, designers and contractors are now starting to reap the benefits that this material brings.
Mondial House on the banks of the River Thames in London was built in 1974. The building had a concrete frame, but was clad entirely with FRP. Deemed to be slightly incongruous on the river frontage, it was demolished in 2006 to be replaced with a slightly more eye pleasing structure – but several of the cladding panels were rescued and tested to see how they had performed.
The results were positive. Aside from a little dirt, they were in the same condition as they were when it was built 40 years before.
FRP technology has advanced significantly since then and its place in the construction industry has continued to grow, albeit more slowly than in other areas such as the aerospace and automotive industries. FRP trade association Composites UK’s operations manager Sue Halliwell thinks one of the reasons for the slow growth has been the lack of prescriptive codes to guide engineers through the design process.
Some of the barriers which we’ve looked at over the years are the codes and standards
“Some of the barriers which we’ve looked at over the years are the codes and standards,” says Halliwell. “Obviously the construction industry is very standards driven, but there are now guidelines out there which can be referred to.”
She hopes the introduction of the new composites Eurocode for bridge design due out in the near future will make the material less of a specialist’s domain and more easily accessible.
Another of the barriers has been the lack of data about the performance of the material with time.
We all sit in an aircraft where the wings are glued on, and have done for years, but we still regard it as new technology in the civil engineering world
This too is changing, with accelerated weathering testing, and monitoring equipment being installed on FRP bridges to give real time information on performance.
Indeed, the material’s benefits are being realised by clients like Network Rail, which now has several FRP or FRP hybrid structures on its asset books, including at Dawlish (see box).
While the initial cost of the material may be higher than its more conventional counterparts, the whole life cost is usually lower. Because FRP components are about 75% lighter than concrete or steel, it’s use can reduce the cost of the foundations for a structure.
In addition, the fact that FRP parts are fabricated offsite means there is less waste and less construction time on site. Less cranage is required to lift the material and it does not require as much maintenance as traditional materials.
Old problems such as yellowing of the FRP due to sun damage can be prevented by using a more UV-resistant resin and better UV additives and, given careful design and the right choice of resins, additives and fillers, FRP can be used to make structures with good fire resistance. FRPs containing phenolic resins can be used to increase fire resistance.
Generally FRPs are poor thermal conductors, so they do not help the heat of a fire to spread in the way that can occur with metals.
End of life considerations
End of life considerations for FRP are also now being addressed. Carbon fibre can already be recovered and recycled from carbon fibre reinforced plastic (CFRP) with around half a dozen companies around the world specialising in this work.
Glass fibre reinforced plastic (GFRP) is less commonly recycled due to the cheapness of glass. However, it can be used as fuel or raw material in cement kilns and this is currently adopted where GFRP cannot be landfilled.
FRP can typically weigh about 75% less than similar conventional materials. It’s not as stiff as steel, though, so the designs tend to be deflection rather than strength driven.
In the UK, FRP parts are most commonly produced either by pultrusion or infusion processes and use a variety of different resins and fibre types (see box).
Certainly there are those who have used it, such as Tony Gee & Partners group director Ian Smith.
“We all sit in an aircraft where the wings are glued on, and have done for years, but we still regard it as new technology in the civil engineering world,” he says.
Case study: Dawlish Station Bridge
Dawlish station on the Great Western railway between Exeter and Torquay is possibly one of the closest railway stations to the sea.
This punishing salt water environment, has taken its toll on the station, leaving the structural steel rusted and unsightly.
In 2010, an inspection of the Grade II listed, 17.5m long footbridge linking the platforms revealed that some previously highlighted areas had deteriorated significantly and it was necessary to repair or replace the bridge.
Originally designed by Brunel in 1830, the bridge had been replaced using the same design in 1937. Due to its listed status, any repairs or replacement had to follow the same form. The initial design proposed a like for like repair, but such was the extent of the restoration, the structure would effectively have been new.
Consultant Tony Gee & Partners then proposed an FRP alternative design. With contractor Bam Nuttall and FRP fabricator Pipex, the consultant designed an FRP bridge which was aesthetically identical to the original steel structure but which was significantly lighter and more durable.
“It’s a Network Rail asset, so they did an option selection,” says Pipex engineering services manager Casper Kruger. “The main factors were that they couldn’t get a massive crane onto site because it is a small village, so we took a 15t structure and replaced it with a 5t FRP one. There is also a big drive in the rail industry to make longer life bridges with less maintenance.”
The bridge’s structure is principally the same as the previous design, with two main, 1.66m deep I-girders spanning 17.5m across the tracks, a deck which spans between the bottom flanges of the girders, and U-frames wrapping around the bridge at 1.7m centres tying into the roof and providing lateral stability. At the ends of the bridge, the deck stops 2.7m short of the full length as the stair rises between the beams. In these areas, an inverted U-frame in the roof provides the stability.
But instead of being steel, all the components were made of FRP.
“Each girder is formed from foam cored shear webs, moulded by film infusion using an epoxy resin and biaxial glass fibre reinforcement, capped top and bottom with pultruded angles and plates to form the flanges,” explains Tony Gee & Partners group director and head of special projects group Ian Smith.
“The deck is formed from lightweight ‘Composolite’ pultruded panels with a skin thickness of only 3mm. Suspended staircases at each end of the deck are moulded using a combination of solid laminate and sandwich panel with a core of balsa wood.”
The only non-FRP areas are the additional stainless steel bolts through the panels. “All the primary connections are bonded, but the bolts give an additional degree of confidence to the client due to the relatively novel innovation of the technology,” says Smith.
To match the original bridge, the steel bolt heads were capped to emulate rivets, to match the spacing of the rivets on the original bridge, mock caps were bonded onto the FRP to match the appearance.
As the bridge is so light, the team had to design it to withstand aerodynamic buffeting from trains passing underneath it, in addition to the normal pedestrian and dynamic loading criteria. It also includes special measures to stop the heat from the trains stopping at the station underneath having a detrimental effect on the FRP.
“There are heat deflector plates which are wider than the bottom flange, when the train is underneath and waiting for the passengers to board – the polyester flanges won’t heat up,” says Kruger.
The whole bridge, including the stair design was independently checked by Parsons Brinckerhoff before fabrication. However, the design was put to the test when, rather dramatically in February 2014, several severe storms battered the coastal railway destroying large parts of it and the station. The new FRP footbridge withstood the pummelling and remained unscathed despite being battered by rocks and sand tossed from the adjacent beach.
Client: Network Rail
Designers: Tony Gee
Contractor: Bam Nuttall
FRP fabricators: Pipex
Case study: Church Lane Road Bridge
Church Lane Road Bridge is located in Frampton Cotterell near Bristol. The 8m single span bridge over a small river was constructed in 1968 and was formed from a reinforced concrete slab.
Over four decades the bridge deteriorated significantly, with low cover and reinforcement corrosion apparent. Subject to a load restriction, it was clear that a new bridge was required.
Client South Gloucestershire Council set a key objective – develop a solution that would minimise closure time to limit impact on local residents.
FRP offered offsite fabrication to reduce the construction period on site and significant lifecycle durability benefits. The new, lighter, bridge could be supported on the existing abutments, further reducing the construction time and enabling it to be built within the six week school summer holiday period.
South Gloucestershire Council used its own street care team for construction. No wet joints were allowed onsite so complete off site fabrication was required. Pultruded sections fitted the bill.
Using pultrusion also created a secondary challenge for the supply chain as different manufacturers and suppliers had very different design solutions. As a result, comparable quotations were requested on a “design and fabricate” basis. This allowed the FRP manufacturers to be as innovative as possible in the deployment of their products.
The winning tender came from CTS Bridges using pultrusions sourced from Fiberline Composites in Denmark.
“We used a glass fibre, polyester resin composite with an Asset bridge deck pultrusion made by Fiberline,” says Atkins technical authority for composites in construction James Henderson. “It came off the shelf but then it was a bespoke application.
“The deck is made up of triangular sections which are about 225mm deep and bonded together.
“Then we put some box beams underneath. They act like stiffening ribs. There are then 2mm thick FRP plates bonded to the bottom of the beams along their length.”
The pultruded profiles were shipped to CTS Bridges in Huddersfield, partially manufactured by CTS and then transported down to the National Composites Centre (NCC) for final fabrication. The hybrid FRP Deck was then transported to site on 24 August 2014 and lifted into place in one operation.
“The bridge was installed in a day,” says Henderson. “Resurfacing of the road took place the next day and then traffic was running over it the next. It’s one of the biggest FRP road bridges in the UK at the moment.”
With the bridge open to traffic and a 7km diversion removed, a masonry parapet was then constructed to meet conservation requirements. Watch the timelapse video here.
As a legacy, Atkins and the University of Bristol arranged for sensors to be installed on the bridge. With time, these will provide valuable data about the behaviour of the FRP deck to inform future designs.
Client: South Gloucestershire Council
Contractor: Street Care
FRP manufacturer/bridge fabricator: Fibreline Composites/CTS Bridges
The main methods for manufacturing a section are pultrusion and infusion.
Pultrusion is a continuous moulding process which produces long constant cross sections which can be cut to any length, but which is usually limited by what is transportable to site.
Long lengths of fibre are drawn through a liquid polymer resin and then passed through a guide which holds them in the correct shape.
The taut fibres are then heated to set the resin and saw cut to the correct length. The sections generally have to be bonded or bolted together to form larger sections.
Infusion involves laying a mat of reinforcement in a mould, the mould is then sealed and a vacuum applied.
The vacuum sucks the resin into the laminate and helps to distribute it evenly. This process produces less waste and can produce a variety of complex shapes that are not possible to create with other methods.
Resins and Fibres used
In the UK, the most commonly used fibres tend to be glass – which is generally used with a polyester resin – and carbon, which is typically used with a vinylester or epoxy resin.
Case study: Chor Aqueduct
The newly constructed FRP Chor aqueduct carries the river Chor over a railway line. It replaced the original Victorian cast iron bridge which was supported on three masonry arches. These failed to provide the clearance needed for the forthcoming electrification of the line between Bolton and Preston.
An initial steel design was dismissed because of the bridge’s location and the difficulty of getting a crane big enough to lift the structure into place. An FRP solution was chosen for its light weight, low maintenance and ease of installation in a difficult access location. It also turned out to be the most cost effective solution.
“On this project the initial cost and the whole life cost of the FRP solution were the most favourable of all of the materials considered,” says Murphy development manager Martin Halpin. “The FRP was a very similar cost to steel but the savings were made in the installation costs.”
The 38m long, 2m wide and 1.3m high aqueduct is U-shaped in cross section. The new structure was split into three to make it easier to transport to site. As part of the work, the spans on either side of the central section were filled in and the central section of the FRP bridge spanned over the 10m clear span onto either side. Then the two remaining FRP bridge sections were lifted into place on either side. Construction was quick, taking only three hours to lift the bridge into place.
With the three sections in place, the joints were vacuum resin infused together on site to form one structure.
“The jointing process is a bit like a full penetration butt weld in steel, as the bond is as strong as the parent material itself,” says Halpin. “You’re using the same process as you made it to bond it.”
The bridge has a design life of 120 years.
Client Network Rail
Contractor J Murphy & Sons
FRP fabricators Delft Infra Composites
Case study: Streatham Bridge
There has been rapid growth in structural strengthening structures using FRP since it was introduced in the 1980s. Concrete and cast iron lend themselves to the technique of bonding layers of FRP to often accessible undersides of beams or slabs due to its ability to increase their tensile capacity. But the technique is not limited to cast iron or concrete, it can restore other materials such as masonry, timber and metallic structures.
One structure which benefited from this technique was Streatham Station Bridge which carries the A23 road over a twin track railway in south London. The bridge was originally constructed in 1850, but throughout its 160 year life it has been chopped and changed to cope with the ever increasing demands on its performance.
Each change was carried out with the latest technology of the time, resulting in a complex blend of structural solutions.
In previous modifications, some of the original cast iron beams in the central section had been replaced with steel beams encased in concrete to increase the load capacity, allowing bus lanes to be introduced. Deep kerbs and railings kept the buses on the strengthened part of the bridge only. More recently, the local authority wanted to increase the number of bus lanes and therefore needed to strengthen the original cast iron beams on either side of the previously strengthened central section. This time the potential for disruption ruled out the replacement of the sections. But as the bottom flanges of the beams were easily accessible, strengthening using FRP was chosen.
“We didn’t need to lift surfacing or do any work from above and that was a benefit as there were quite a few services buried within the construction of the bridge, such as gas mains, which can be troublesome,” says Tony Gee & Partners group director and head of special projects group Ian Smith. “The other particular advantage was that there was a minimal change to the headroom underneath the bridge and every bit that had been modified.”
The design was optimised to each of the 10 cast iron beams being strengthened. Strips of FRP “laminates” between 7.3m and 6.2m long, 200mm wide and with a maximum thickness of 25mm to 32mm –tapering to 1mm thick at the ends – were bonded to the underside of the beams. Each strengthened beam received either one or two laminates depending on the strengthening requirement.
The result was the rapid solution with minimal disruption to the road users above and rail passengers below.
“All the preparation work was done in short midweek night possessions with the installations of the carbon fibre laminates installed in a weekend possession,” says Smith. “Because of the lightness of the individual components, everything could be manually handled, so we could use conventional scaffolding.”
Client Network Rail on behalf of Transport for London
Designers Tony Gee
Contractor Concrete Repairs
FRP suppliers Sika UK
Case study: Startlink House
FRP is helping housing developers produce more affordable housing.
FRP specialists Exel Composites with Larkfleet Group, Warwick University, John Hutchinson, Odour Control Systems and Costain have designed and built a fully composite house in Lincolnshire, which requires only the addition of insulation.
The Startlink house is a pultruded glass reinforced composite component kit which can be assembled into a building without the need for metal fastenings. From the 2.5m long piles to wall panels to the stairs, the two-storey building was constructed out of FRP and erected on site in just two weeks. The 18t house’s component parts were transported to site on the back of an ordinary flatbed trailer. The joints are a series of locking pins which hold the panels in place.
Exel Composites technical director John Hartley says the construction time could be reduced next time around.
At the moment some new houses are built with some FRP components such as chimneys, roof edging systems, guttering and window frames. But the benefits to the housing industry could be so much more if FRP was used to make other components says Composites UK operations manager Sue Halliwell. She lists a whole range of advantages of having fully FRP houses. These include offsite fabrication and modular construction, easier transportation, faster construction time, superior durability, improved thermal capacities and removal of thermal bridging.
She also says the lower weight characteristics of FRP mean developers using it could use land which could not previously be developed.
Delivering Differently | Fibre reinforced polymers