Kingston Bridge Widening & Strengthening
Back in the seventeenth century the river crossing at Kingston was the second most important river crossing in England the only other downstream bridge across the Thames being London bridge, even today it is one of the busiest bridges across the Thames, carrying 40,000 vehicles per day, with 2000 an hour at peak times. The bridge in west London is a 7 arch brick built bridge spanning the river (Fig 5). The original masonry bridge, which replaced the old timber one, was built in 1824 with a second bridge built adjacent to it in 1914 which in effect doubled its width. In 1993 an appraisal by Travers Morgan indicated that a 3tonne weight limit should be imposed this gave the local authorities a huge potential problem and the decision was made after a number of alternatives to add a further bridge to increase capacity by 30%. This was to be combined with a strengthening programme for the 1800 and 1900 bridges along with the associated drainage.
The structure is grade two listed so any work was carried out under the close scrutiny of English heritage who insisted that the new bridge was built using precast concrete arch shells which exactly matched the existing arches a considerable feat when considering that the bridge was not symmetrical about the centre arches span, but was slightly different for every arch. The newly constructed bridge was built with a structural lightweight aggregate saddle over the aches with the void between road level and the arches needing to be filled with as lightweight a foamed concrete as possible yet still returning a strength of 1N/mm2. The two existing bridges were to be strengthened in a similar way with the bridges excavated down to the original arches then a lightweight aggregate concrete saddle cast over them with the lightweight foamed concrete infill then placed over this saddle (after applying suitable waterproof and drainage membranes). The lightweight foamed concrete (fc2) was poured to within 700mm of the road level (Fig 6), with the remaining level made up by the use of a higher strength foamed concrete (fc1). This fc1 foamed material again had to be as light as possible but with a much higher strength of 7N/mm2 @ 28 days.
The requirement for both foamed concretes to be as light as possible within there strength parameters was to keep the loading on all bridges to an absolute minimum whilst increasing the structures integrity. These requirements had to be allied to the fact that the client required large volumes of both grades of foamed material to be produced at any time, so the option of a hand batching operation was discarded. This left the readymix scenario with our base materials bought to site readymixed. This fact limited our choice of mix designs, as there was a limit on the cement content that the local readymix suppliers were able to supply.
As a rule of thumb the lower the density the lower the strength with very low density materials requiring very high cement contents prior to the foam being added to compensate for this reduction in strength. In the case of Kingston the amount of cement in the base materials was limited to 1000kg of cement in a cubic metre and consequently our mix design had to reflect this. Drawing from experience gained at Canary wharf where a similar set of parameters were enforced a series of trials were undertaken which resulting in mixes being designed, tested and specified.
The mixes were accepted and subsequently appeared in the tender documents for the project. Propump Engineering were awarded the supply contract and worked closely with the main contractor MJ Gleeson PLC on all aspects of the foamed concrete.
The depth of pour for the fc2 material was limited to 1.5 metres to avoid compression of the material, with at any one time no more than a 1 metre differential in material level between two adjacent arches, to avoid any undue lateral loadings on the arch structures (Fig 7).The construction of the new bridge was finished early 2001, the strengthening of the 1914 section was completed in May 01, with the final fc1 material poured on the original 1828 bridge in June 2001 and the new bridge re-opened August 2001.