With its six repetitive phases of high rise 17-storey apartment blocks, the £200M St George Wharf project in south London has the makings of an ideal testbed for new technology. Add a highly unusual developer and the result is a unique opportunity for real progress.
A mixed use 100,000m 2development comprising 750 apartments plus, cafes, restaurants, offices and an international hotel, St George Wharf is scheduled to take six years to complete. Its brownfield riverside site offered several geotechnical challenges (see box) but it is the innovations taking place above ground that have led to its adoption as a M4i Housing Forum demonstration project.
St George construction director Tony Pressley explains why his company is different. 'You could call us developer/contractors, ' he says. 'We buy the land, obtain planning permission, commission designs, act as construction managers, market and sell.
'To offer the purchasers maximum choice we operate as main contractor on the final fit-out.
And we're led by Tom Carey, who is an engineer, not an accountant. If we decide that a new technology offers significant advantages we adopt it.'
Flat slab insitu concrete construction was chosen before the Cardington influence came into play, Pressley adds. Speed was the driver. An insitu concrete frame offered short lead times compared to pre-cast or steel alternatives, plus superior acoustic performance to a steel frame - a significant advantage on a largely residential development. Pre-cast concrete was preferred for the 'very repetitive' balconies, where the planners demanded a high standard of exposed concrete finish.
'The way we operate means we are constantly pushing at the limits of the programme, ' Pressley says. 'We made it clear to our structural engineers that speed of construction and buildability were the key considerations - which meant flat slab construction, table forms, large jump shutters and the minimum of scaffolding.'
For the contractors the frame packages include everything above the top of the piles - including the detail design of the reinforcement. 'Within constraints, ' adds Pressley, 'structural dimensions have to stay the same.
'And the contractors carry full design liability, which means they have to check the designs produced by our engineers. But we have had excellent results from this form of procurement.
There are no arguments about late delivery of bending schedules - and the contractors are able to really speed up the rebar fixing.'
On the first 'pre-Cardington' phase, very large 7m x 3.5m table forms were used. But any theoretical advantages were more than outweighed by practical and logistical drawbacks, says St George project manager Marcus Blake.
'We needed eight men to fly the forms and two to land them, and the men had to get in very early and work very late. It also tied up the craneage for long periods.'
One decision that did work well and will be repeated on all subsequent phases was the flooring system in the residential areas. Instead of power floating the slabs, to enable carpets or tiles to be laid directly on to the concrete, chipboard mounted on an adjustable plastic saddlefloor system was selected. An easyfloat finish was all that was needed - 'much more cost effective overall' says Pressley.
Direct involvement with the BRE team working on the Cardington project began about 12 months ago, when the planning for Phase II was well advanced.
The real influence of the BRE research will be seen on Phase III, which is due to start in early May. But there were several key changes on Phase II that were influenced by the Cardington results.
'The most important was a switch to smaller, lighter aluminium table forms, ' Blake reports. 'These were only 3.5m by 3.5m and needed no more than a four-man team to fly and land them. Overtime working was eliminated and there was less demand on the cranes.'
Another radical change was to increase the percentage of reinforcement in the concrete frame in order to maximise structural stiffness and minimise deflections under live load. 'It's more economical to spend money on extra rebar than complex deflection-tolerant connections between cladding and frame, ' says Pressley. Concrete specification remained unchanged, a conventional C40 mix being preferred, with placing by pump or crane and skip as appropriate.
Overall, the savings in manhours and cycle times between Phases I and II were dramatic (see diagram). A massive 39% reduction in man days was achieved, with floor cycle times down from an average 10.5 to 8.8 days with a best of 5.5 days. 'By the last phase we hope our man day savings to be better than 50%', Pressley adds.
Blake is particularly excited by the potential of the Cardington-proven CRC Jointcast on Phase III. (See box).
'The gullwing detail at roof level looks great when it's cast, but doing it insitu involves an enormous amount of shuttering and scaffolding. We're looking at precasting 'banana' segments offsite and bonding them together with CRC Jointcast in position.'
Phase III is also likely to feature significant improvements in rebar fixing and shutter striking times. The pull-out tests for early concrete strengths investigated during the Cardington trials will be tried out for real at St George Wharf.
All this real life data, coupled with that from Cardington, will be made available to the whole construction industry.
Groundworks Few brownfield sites in the heart of London are free from serious problems and the St George project is no exception.
The 3ha Thameside plot immediately to the west of Vauxhall bridge was once home to a gasworks.
More recently, a large coach park and car pound covered most of the site, and on its western end loomed the corroding concrete hulk of the former Nine Elms cold store - 'the largest building without windows in London, ' according to St George construction director Tony Pressley.
But it was the eastern end, where construction was planned to start, that posed the biggest challenge to the developers. Running from north to south some 8m below ground level were the twin tunnels of the Victoria Line.
'These involved us in enormous groundworks for the first two phases, ' recalls Pressley.
'We had to slot bored piles 65m long and up to 2.1m diameter actually between the running tunnels, and as close as 2m each side. The Vauxhall station tunnels were very close as well. In fact we were operating at the limits of current piling technology.'
Piling contractor Amec carefully probed each pile before driving began. Strain gauges were installed in the tunnels to monitor the effects both of direct piling forces and of the removal of overburden during undercroft excavation.
In the end all 80 large diameter piles were installed without problems.
Despite the proximity of the tidal Thames, water was not a problem on the site, thanks to the truly massive riverwall.
But, when smaller diameter piling operations behind the riverwall hit a layer of coarse gravel, the increased vibration from driving pile casings did cause some problems for sensitive equipment in the neighbouring Governmentowned office complex to the east.
Less exotic but more challenging problems were presented by the demolition of the cold store. Pressley describes it as 'exceedingly difficult and dangerous to demolish, thanks to its system of construction.' The only safe option was to employ very long reach excavators fitted with nibblers and slowly munch the building down over an eight month period.
All 9,000m 3of concrete from the cold store was crushed and recycled on site. Minimising waste and maximising recycling is a contract priority.
Even the plasterboard waste is taken away for British Gypsum to recycle.
External finish On such a high profile project as St George Wharf, where some flats sell for well into seven figures, the quality of the external finishes were of paramount importance to planners and developer alike.
Curtain walling is used on part of the scheme but most external cladding is pre-cast architectural concrete, in a range of colours from classic white through a 'Portland Stone' effect to a reddish-pink sandstone finish at lower levels.
All the many balconies are architectural quality precast concrete. Precast producer is Weston-Super-Mare-based The Marble Mosaic Company.
'We had no real problems matching the specified colours, ' says Marble Mosaic managing director Stephen Maddalena. 'The real challenge came from the fixings.
'Most of the development is residential, which means there are none of the raised access floors and suspended ceilings you get in commercial property. So we had to develop a bespoke fixing solution which would work in the very restricted depth available.'
To speed erection, the precaster lisased with specialist subcontractor Contano to preframe and glaze the window apertures into concrete panels in the Weston works. Insulation is fitted after erection as part of the dry lining package. The Marble Mosaic Company has supplied more than 2,000 cladding panels and 150 balcony sets, worth around £4.5M.
Key lessons from Cardington
Improving the construction process by eliminating all nonproductive activities can produce time savings of over 30% and man hour savings of over 45% compared with traditional practice.
Early striking of formwork (at less than three days) and improved backpropping procedures increase construction efficiency.
The early age concrete strength can be assessed using LOK inserts cast into the concrete.
Rationalisation of reinforcement can provide significant savings in overall costs. For flexural (or main) reinforcement, rationalised arrangements of loose bar using elastic design should be used on all but larger buildings, where there are benefits in rationalising using two-way mats.
Where punching shear reinforcement is required, proprietary shear systems, such as stud rails and shear ladders, should be used. These are between three and 10 times quicker to fix than traditional links.
Structural steel shearheads are an effective solution where large service holes are required adjacent to the column support.
Ready-mixed concrete suppliers have extensive databases that will allow them to rapidly produce a mix design to meet the requirements of most in-situ frames. In some cases, however, some mix design development will be necessary. Examples include high strength and selfcompacting concretes.
Thin flat slab construction is particularly economic for spans ranging from 5 to 9m, especially where a square or near square grid is used. For spans in excess of 9m, posttensioning should be considered.
In-situ concrete flat slab construction without drops has very considerable advantages in terms of enabling rapid floor construction and giving maximum flexibility to the end user. Drops should be avoided even if this leads to some form of column-head reinforcement.