Concrete advantage

When Phase 1 (of five) of st george wharf was nearly complete, developer St George sought the programming benefits, and the kudos, of turning its high-profile development on the Thames into a Movement for Innovation (M4I) demonstration project.

To do this, St George worked with the Building Research Establishment (BRE) to put into practice lessons learned from research at Cardington, Bedfordshire, where, in 1998, a seven-storey, flat-slab concrete frame was built in situ and subjected to various tests.

Cardington was closed last year, but key lessons were fed into phase 2 of the St George Wharf development, leading to a 39% cut in man-days and a drop in average floor cycle time from 10.5 days to 8.8 days.

Rational reinforcement
Acting on the lessons from Cardington, St George cut the amount of variation in its steel reinforcement bar (rebar) design. It's called rationalisation. Instead of 20 different rebar sizes, you design for five or six.

Traditionally, to cut steel costs, designers stipulate the thinnest bar where the thinnest bar will do. This can result in up to 60 rebar variations in a single slab. Slab by slab it may save money but over an entire project it complicates things. Guides published after Cardington (see www.rcc-info.org.uk) admit that rationalising rebar design may mean using more steel, but simplification will lead to overall savings.

St George found this to be the case in phase 2. James Gaffney, senior construction manager at St George Wharf, says everything became easier, from handling the delivered bundles to fixing them in place. Cardington suggested the benefits could extend to design and manufacture as well.

Applying the lessons of Cardington led to a 39% cut in man-days and a drop in average floor cycle time from 10.5 days to 8.8 days

Strike action
Cardington also proved it was possible to save time, without compromising the slab, by striking formwork earlier than usual. Experiments pushed the striking time forward to less than three days after pouring. Some concrete specialists knew this already, but contractors are understandably cautious about testing this theory on paying clients. Cardington emboldened St George to try it out. In phase 1 the formwork was struck after five days. In phase 2 they struck after three days and the project team intend to shave that down even further in the future.

The team justifies early striking with strength tests, of course, and they are trying new methods here as well. Instead of the cube test, in which an air-dried lump of concrete is sent to a lab and crushed, St George inserts a bolt in the slab when it's poured and then pulls it out when the concrete hardens, measuring the resistance.

They don't rely on the pull-out test completely yet, running it instead alongside the cube test to see if they get the same results. But if it works, says Gaffney, they could forgo sending cubes to a lab and proceed with confidence on a test done there and then on site.

Laboratory
As well as implementing the Cardington lessons, the project is a bit like a laboratory itself. Robert Vollum, who lectures in civil engineering at London's Imperial College, developed a model for predicting concrete slab deflection at Cardington, a model that incorporates the effects of construction loading. He uses St George Wharf, with its innovation-minded management and oft-repeated phases, to test that model. Vollum's work could be a step forward because current analytical models ignore how construction loading affects deflection.

Richard Moss, senior consultant at the BRE, puts forward innovations and co-ordinates their implementation at St George Wharf in conjunction with the project team.

Always stipulating the thinnest bar where the thinnest bar will do can result in up to 60 rebar variations in a single slab

He says it's been a process of sitting at meetings with St George, structural engineer White Young Green, specialist concrete contractor Stephensons and others, and kicking around things to try.

"Among other things we hope to try out is a kind of specialist jointing material called CRC jointcast," he says. "This allows much shorter lapping of rebar, which could make precast construction more like monolithic, in situ poured concrete construction, greatly speeding up construction work."

Hard work
Gaffney says that St George can handle M4I demonstration status, with all the extra work that that entails, because the developer is both client and construction manager and so does not need to persuade disparate parties of the benefits of innovation. The St George Wharf project also lends itself to a certain amount of experimentation because it has many repetitive phases.

At the end of the day, innovation is hard work and St George has the resources to try new things and test them. The developer has committed 120 man-days throughout the life of the case study (phase 3 of the development) to monitoring the new methods, collecting data and sitting in meetings.

Gaffney appears genuinely enthusiastic about the innovations and insists that construction managers in general can benefit from the lessons learned, although it will be up to a year before the BRE starts to disseminate the research through seminars and papers.