Brussels, Jun 2003
A fibre-reinforced composite river bridge able to carry 40-tonne vehicles has been built in the UK as part of the ASSET project to help improve Europe's ageing road system. It is made from strong but light structural elements.
Roads and railways built in the boom years of the 1970s are reaching a critical age when they need more maintenance or even replacement. The growing weight and volume of the traffic these systems have to bear increase wear and damage. Yet the share of Europe's GDP spent on such infrastructure has fallen by half in the last 50 years.
In 1998, the four-year European Commission funded ASSET project set out to produce an economical but reliable repair system, exploiting the strength and lightness of fibre-reinforced polymer sections. The goal was to design structural elements that could be used to reinforce or replace worn or damaged parts of the transport infrastructure more efficiently than by traditional methods.
"We have created strong and versatile products that can be used in bridges, tunnels, buildings and other structures," comments Dr Sam Luke of UK consultant Mouchel, the lead partner and project manager for ASSET.
A structured project
Fibre-reinforced polymer (FRP), a resin matrix strengthened with glass or carbon fibres, is the structural material used in the project. FRP can be shaped as it is made by the pultrusion process, in which the fibres are continuously fed through a molten resin mixture, which is then pulled out through a heated die.
The first stage of ASSET developed a prismatic profile for an FRP structural component of sufficient strength to carry heavy loads. It has a cross section with triangular rather than hexagonal cells. The design was then worked up into an economical manufacturing process suitable for accurate mass production. The profiles can be used for bridge decking and structural reinforcement.
To build a bridge, the project also had to design and test long beam sections and to design joints and connection systems to transfer the weight of traffic from the FRP beams to concrete or steel support structures. It developed construction tools and optimised the fibre architecture of the profiles.
A consortium of experts
"Partners in ASSET between them supplied all the skills and techniques needed for its success," says Dr Luke. Fiberline Composites of Denmark, one of Europe's leading pultruders, made all the FRP members, developing the dies and optimising the process. Research partners KTH of Sweden and IETCC of Spain undertook the small- and large-scale testing respectively of ASSET profiles and connections. They confirmed the numerical analysis and modelling work with tests to establish the properties of the FRP structural sections.
A UK local authority, Oxfordshire County Council (OCC), joined the project as client partner for the bridge, which now forms part of its road network, and gave technical approval to the bridge design and construction. Another partner was HIM Chemie of the Netherlands, which developed a road surface to apply to the composite bridge deck. As lead partner, consulting engineer Mouchel were responsible for the design, analysis and optimisation of the pultruded profile, the design of the road bridge and site supervision while it was built. "We have also drawn up design rules and guidelines for the structural use of pultruded profiles," adds Dr Luke.
A fully monitored FRP bridge
When all the elements had been thoroughly tested, all was ready for the final stage – to erect a fully monitored FRP bridge and use it in real traffic. Construction was the responsibility of ASSET partner Skansa. OCC chose to replace a 19th century cast iron bridge on the B4508 secondary road near Shrivenham in Oxfordshire.
The original 3.5-m wide bridge could only take a single lane of traffic. Its 10-m span composite replacement is 6.8-m wide with two lanes and two footpaths on new concrete abutments. Support members are four 11-m long box composite beams, each assembled by bonding together four smaller beams of square cross-section. They were then covered with a carbon fibre composite to give the bridge enough flexural rigidity to meet the UK BS5400 standard. The carriageway decking of ASSET profiles was then laid on the beams and bonded in place.
Profiles had been partly pre-assembled in Denmark by Fiberline Composites, with the work completed in a tent at the side of the bridge. The bonding required six days during a period of two weeks. Only a small crane was required to lift the elements into place, because the material is so light in relation to its strength. Optical fibre sensors and conventional strain gauges were inserted during manufacture so that the performance of the bridge could be monitored over time.
Before being opened to traffic the bridge was load tested with a 34-tonne Sherman tank and an equivalent 40-tonne lorry. The test showed that the design was meeting its targets, and that there was composite action between the main beams and the ASSET profiles, so that future bridges of this type may not need such large beams. "Since October 2002, West Mill Bridge has carried about 1,200 vehicles a day, some of them 44-tonne articulated lorries servicing local car showrooms and garages in the area," reports Dr Luke. "The maximum deflection measured so far is less than 4 mm, well within the design limits.
"This project has proved the value of composites in structural work," he adds. "Their higher material cost is more than offset by cheaper installation and longer life. We expect the ASSET profile to play a significant role in maintaining the European infrastructure."