Highways England (the UK’s largest bridge owning authority) and Network Rail are responsible for maintaining around 12,000 concrete bridges, some built as far back as the 1920s. Since their construction, not only have building standards changed, but our population, private car ownership and lorry loads have hugely increased. These bridges have endured years of weathering and erosion, putting into doubt their ability to withstand these heavy loads.
Deciding the fate of our troubled bridges
Condemning vast sections of our national infrastructure would be costly, disruptive and in some circumstances, completely impractical. Highways England estimates that to close one car lane costs the UK government somewhere between ￡10,000 and ￡50,000 per day.
Despite the significant stresses these bridges are subjected to, in many cases there is no real need to scrap these older structures. They may well already meet modern building standards or can be reinforced to meet them. We just need to know how to assess their load capacity and improve this where it’s needed.
By prolonging the life of these structures, we can save the economy millions of pounds each year and use fewer resources. This means we can protect the environment without compromising our safety.
Assessing existing load capacity
At Bath, we've spent over 20 years testing how concrete structures behave under different loads and developing novel approaches to prevent them from failing.
Many bridges built before the 1970s suffer from a loss of concrete and reinforcement corrosion issues, particularly highway bridges that use prestressed reinforcement. Working with Balfour Beatty and Mott MacDonald (on behalf of Highways England) we set out to investigate how concrete deterioration affects this reinforcement in critical anchorage regions.
We found that even after severe concrete deterioration, a significant amount of strength capacity remained, which had previously been neglected in assessment standards. Once this capacity is taken into account within a structural design, in many situations, the capacity of these bridges actually meets modern building standards. In other words, these bridges were much stronger than once thought and did not need costly and disruptive strengthening works.
Strengthening with advanced composite materials
For those bridges that were found to have an inadequate load capacity, we developed different strengthening methods using an advanced composite material: carbon fibre reinforced polymer (CFRP).
CFRP is a lightweight, strong and durable composite material created by combining carbon fibres into a polymer resin. It is the same material used in Formula 1 racing cars and the Airbus A380.
We found CFRP can be used to increase the strength of columns, beams, slabs and bridge decks. It can be glued to the surface of the concrete or within slots or holes cut into the concrete to act as additional reinforcement. This allows structures to carry increased loading.