Built in 1810, Tickford Bridge near Newport Pagnell in Buckinghamshire is believed to be the world's oldest operational cast iron highway bridge and is a scheduled ancient monument. Six cast iron arch ribs spanning 18.2m carry the two lane B526 over the River Ouzel. Six years ago however, a routine assessment by Buckinghamshire County Council found that at low temperatures the tensile stresses in the lower chords were dangerously high. It was therefore recommended that a 3t weight limit should be placed on the structure.
Maunsell was given the task of coming up with a way of strengthening the ancient structure so that it could carry full highway loading - one that would meet the strict requirements of English Heritage at reasonable cost and without disrupting traffic across the bridge. The solution adopted was, in the opinion of the judges, innovative, elegant and well-engineered. It used the fast developing technology of carbon fibre reinforcement to solve a particularly tricky problem in a sympathetic manner that preserves a historic structure for future generations.
Early cast iron is a notoriously variable material. Large sections are particularly unreliable, and current codes of practice tend towards the conservative. Tensile strength is assumed to belittle more than 30% of compressive strength. This brittleness makes carbon fibre, with its high tensile strength, a very compatible partner, and indeed there have been earlier examples of the use of carbon fibre to strengthen cast iron structures.
'But none of these were arched structures,' points out Maunsell project manager Will Duckett. 'Tickford Bridge is, as far as we know, the first cast iron arched structure to be repaired this way.'
Maunsell had two main concerns about the use of carbon fibre in this context. The first was the ability of the adhesive to absorb energy from any cracks which might develop in the cast iron. In other words, would cracks beginning in the cast iron propagate through the adhesive and destroy the essential bond between cast iron and carbon fibre.
Second, given that carbon fibre is an excellent conductor of electricity, Maunsell felt there was a significant risk of galvanic action taking place. This possibility was excluded by the specification of an insulating 'veil', a layer of polyester fabric between the iron and the carbon fibre. A particularly innovative fracture mechanics approach to the design process and a series of tests were needed to provide reassurance on the fracture toughness of the adhesive layer.
'Basically, the design was carried out at the ultimate limit state assuming the cast iron carried no tensile forces,' Duckett explains. 'In some critical areas the only way to ensure adequate energy absorption rated in the adhesive was to accelerate the curing by raising adhesive temperature to 60degreesC with hot air blowers.'
The Japanese Replark repair system was chosen for the project, mainly because its partially pre-impregnated sheets of carbon fibre cloth were flexible and easy to fix to the curved arch surfaces. Work began on site in April last year, and, after many layers of paint had been grit-blasted off, up to 14 layers of carbon fibre fabric were applied to the three largest spandrel rings and the lower main chord.
This reinforcement was still no more than 10mm thick, and after the bridge had been repainted even the sharpest eyes would be hard pushed to detect a change in appearance. Main contractor Topbond was on site for only two months and Maunsell claims the repairs were carried out at a fraction of the cost of more conventional methods.