Overnight sensation

Time is money but, in the hotel business, SO is space. That's why the City Inn hotel chain used tunnelform concrete to build its new hotel in London's Westminster. Traditional methods of in situ concrete involve pouring the walls and floor slabs separately: tunnelform constructs both in the same pour (see pictures). "Pouring separately risks the elements being out of position," says Carillion design manager Graeme Tucker. "Tunnelform's repetitive dimensional accuracy is perfect for this project."

A City Inn is pared down to perfection. The chain aims to cram as many rooms as possible into each costly urban footprint, with no suspended or drylined ceilings or raised access floors to eat into the number of storeys in a development. Construction has to be accurate to the millimetre: "If a bedroom's 30-40mm too narrow, then the furniture won't fit," says Tucker. In City Inn's world, even plasterboard is too thick - and with only a 2-3mm skim of plaster on the walls and ceilings between the concrete and the paint, and the thinnest of latex layers between concrete and carpet, nothing other than a perfect finish will do.

Room surface
Finish quality is the reason Tucker gave the thumbs-up to tunnelform. Steel shuttering gives a higher quality finish than plywood, and, unlike steel-shuttered traditional concrete, each tunnelform mould is a single, continuous piece of steel, with no joints. "You can literally rub your hand on the wall and feel no imperfection," he says.

As well as giving quality and accuracy, tunnelform slashed the programme. Carillion originally planned to build the 400 bedrooms in 30 weeks, overlapping the construction of the 12 storeys, each of which would take four weeks to finish. But with two sets of tunnelform moulds from Ischebeck Titan, the concrete subcontractor promised to build one floor every 10 days, finishing the job within 24 weeks. In the event, by reusing the moulds every 24 hours, the concreter finished in 18 weeks.

The regime isn't an easy one. Each set of moulds needs its own tower crane, and if it's winded off or breaks down, that's it until the next day. If the moulds, bracing, kickers and formers aren't ready for the day's pour by 3pm then there isn't time for the concrete to gain strength overnight, and you can't move and reuse the moulds till it has. At City Inn, the plan to powerfloat the concrete had to be scrapped because of noise restrictions after 6pm. Instead, it had to be ground next day when stiff in order to make the floor smooth enough for the latex.

Tunnelform setup costs are high. You've got to buy the moulds, and they hog the hook time on tower cranes. But for multiple repetitive structures - and around 75% of City Inn's bedrooms are identical, except for handedness - the savings in labour and site time can be considerable.