Hammersmith Flyover is an elevated roadway in westLondonwhich carries the A4 dual carriageway over the central Hammersmith gyratory system. Completed in 1961, it is one of the first examples of an elevated road constructed using reinforced concrete.
Routine and special inspections
revealed a significant corrosion problem with the existing post-tensioning
installed in the early 1960’s. This necessitated an emergency closure
of the bridge just before Christmas 2011 to allow further investigations.
The bridge was re-opened a few weeks later in early January but with reduced
load limits and lane closures.
In December 2011 Freyssinet were appointed by Amey to assist with the design phase. Various options were considered to strengthen the bridge, with a target to complete the work in order to open the bridge to full traffic prior to the 2012 London Olympic Games. The solution adopted was to install additional post-tensioning (PT) tendons above and below the top slab within a new widened central reserve.
Freyssinet were awarded the contract to supply and install the additional post-tensioning. They commenced work on site in late January 2012, with the removal of around 200m of the existing concrete central reservation using hydrodemolition and wall sawing techniques. Works progressed well on site using a double shift pattern helped by unseasonably fine weather in February and March.
Once the reservation had been removed, a survey of the existing reinforcement in the deck was completed. An additional top slab was cast in the central reserve. Freyssinet carried out coring for the installation and stressing of vertical PT ‘Freyssibars’, and cast new PT concrete blisters above and below the top slab.
Freyssinet’s skilled post tensioning operatives then installed and stressed horizontal tendons, un-bonded in pre-grouted HDPE (High Density Polyethylene) ducts. After stressing operations were complete, the Freyssibar ends were finished and capped and treated with corrosion protection.
Using specially developed formwork and post tensioning support systems the teams were able to remain ahead of the target programme and finish on time in a race to reopen the flyover before the London Olympics. Freyssinet works were completed in early May 2012. The works completed to date brought Hammersmith Flyover back to full load capacity, and it was re-opened on May 28th 2012.