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Structural Repair A bridge to success
Concrete Repair Association Chairman, Ken Dykes, highlights the importance of employing concrete repair professionals for bridge repairs
Concrete is an extremely successful construction material that is versatile, relatively low in cost and readily available. However, with many of the UK’s existing concrete structures dating from the 1960’s and 70’s, a number of them are now showing significant signs of deterioration and are in need of repair.
While various builders and contractors may claim to be able to conduct concrete repairs, unless they have the necessary specialist knowledge and experience, it is unlikely that repairs carried out will be safe, fit for purpose and durable.
Selecting a member of the Concrete Repair Association can help to ensure that repairs are completed properly, are fit for purpose and appropriate to the service life expectations for the structure.
HAMMERSMITH FLYOVER A recent high profile project to repair the Hammersmith Flyover demonstrates the complexity of a typical bridge repair project, highlighting the need to appoint an experi- enced and professional contractor to undertake the work.
The Hammersmith Flyover is a 623m long, 17m wide, 16-span structure, carrying a dual two-lane carriageway over the Hammersmith Broadway gyratory system and railway lines. It
is formed from 2.6m long precast segments, joined with small insitu stitches and compressed together with a semi- internal / semi-external grouted post-tensioning system.
The flyover was closed just before Christmas 2011 after inspections revealed a significant corrosion problem with the existing post-tensioning installed in the early 1960’s. Although it was re-opened with reduced load limits and lane closures in early January, the target was to re-open to full traffic prior to the 2012 London Olympic Games.
Having investigated several different strengthening strategies, Transport for London (TfL) decided to install new tendons on the six most damaged spans. The objective was to install new tendons over the piers in the central reservation, with five above the top flange at each pier and below it.
The new post-tensioning was anchored into new anchorage blisters that were poured insitu above and below the top slab. These were fixed to the slab by way of high tensile steel stress-bars and the top slab locally thickened by 200mm to help to distribute the concentrated forces from the blisters back to the structure’s webs.
CRA member Freyssinet was appointed to the project. The company started work in February 2012 with the removal of about 200mm of the existing concrete central reservation using hydrodemolition and wall sawing techniques. After concreting the top slab thickening, holes for the stress-bars were cored vertically through the top slab to miss the existing reinforcement.
A matching pair of top and bottom blisters was cast insitu before stress-bars were inserted. A combination of Freyssibar and Macalloy bars were used to anchor these blisters, which were then stressed from the top of the deck to clamp the blisters solidly in place.
An HDPE duct for the external post-tensioning system was then assembled. This 7mm wall pipe provides the first layer of corrosion protection. Ducts were installed at each pier. Four tendons were 33.5m long and covered eleven of the bridge’s precast segments, two were 21.3m crossing seven segments and four were 9.1m crossing three segments.
Once each duct had been assembled, 19 greased and plastic coated strands were inserted; the grease and the plastic coating providing two further corrosion barriers. Parex Cable Grout was injected into the main duct, providing a fourth corrosion barrier, and the Freyssinet 19C15 tendons stressed in a carefully balanced sequence, one strand at a time. The tensioning of the 50 tendons was finished two days ahead of schedule on 14th April.
GETTING IT RIGHT FIRST TIME By selecting a CRA member, the contractor was assured a professional service. All CRA members are BS EN ISO 90001 and BS EN ISO 14001 accredited; can demonstrate a proven track record and capability; and comply with the CRA’s stringent codes of practice.
For peace of mind, get it right; get a professional; and insist on the CRA.
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www.cra.org.uk 01420 471615
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