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Ward round

The contractor's decision to go for an all-concrete frame for a new PFI hospital in Halifax is allowing early structural completion. David Hayward has the story.

Mickey Mouse's concrete ears should be complete this month - some eight weeks early. This success is due to a combination of converting his distinctive circular appendages into a square grid, plus a concreting contractor casting floor bays on a tight repetitive cycle.

In reality, the reference to the cartoon hero's most readily recognisable feature by the site team building a new Halifax general hospital, is just a novel way of describing two large circular pods housing six-bedded wards. That the architect which designed the mouse-shaped hospital footprint is headquartered in Dallas, Texas, gives increased licence to the sobriquet.

The pods, or ears, are attached, appropriately enough, to the top two corners of the rectangular main block of the £78M hospital now nearing structural completion.

'I have never seen insitu concrete floors built so fast,' claims Steve Gunning, associate director for consultant Oscar Faber. 'The programmed 28 day completion cycle for each 50m3 floor bay is being bettered by at least a week, allowing us early structural completion.'

The basis for his praise can be traced back to the decision by local authority client, Calderdale Healthcare, to double the size of its Victorian general hospital with the help of Government's private finance initiative.

This allowed winning PFI consortium Catalyst -a grouping of contractor Bovis Construction, the 30 year maintenance operator Bovis Facilities Management and the service company offering everything from bed linen to food, RCO - to sit down from day one with its consultant Oscar Faber. In front of them was a blank sheet of paper headed only by the word buildability.

Half the old infirmary was to be refurbished and the other half demolished to make way for a new 600 bed extension, plus new consulting and outpatients departments.

Two factors were uppermost during the joint design and build team's early debates. With barely two years from contract award in September 1998 to first patients, fast track construction - familiar to most PFI schemes - was a prerequisite to any programme. But the second buzzword was more job specific - secondary builders' work openings.

Hospitals, by their nature, are riddled with services and Halifax will boast some £20M worth. Primary requirements can be fed through main services risers and false ceilings, but secondary routes to beds or operating theatre tables are only fully identified after structural completion.

The flexibility to cut through completed floors and accommodate some 10,000 plus holes in exactly the required location for these secondary services, was a prime concern during design discussions and in responding to the question: Do we want a steel or concrete frame?

'In practice, from day one, steel never entered the equation,' recalls Gunning. 'Concrete offered us the flexibility we needed to cut service holes more or less where we wanted.'

Forced to compare the two materials now, nearly a year into construction, Gunning admits that overall, there would have been little cost or time difference between them. But he echoes the words of Bovis project manager, Mark Hanson, who says: 'Concrete just felt right from the start'.

The slightly faster initial erection time for a steel frame would have been counterbalanced by the need for 50% increased 1.5hr fire protection around it simply because it is a hospital.

The brief to concreting subcontractor Heyrod was to create a £6M insitu concrete frame up to five storeys high throughout the main block and its two circular ward pods.

The decision to go for circular columns - offering maximum internal space, no corners to damage and flexibility in partition angles - was straightforward. And Bovis' request that all 150 columns should have the same diameter to economise on shuttering was easily met.

But what the columns should support was less easy to agree on. The consultant wanted a ribbed slab - lighter and offering smaller foundations. The contractor preferred a conventional flat slab - quicker and easier to build, more concrete and rebar, but less shuttering and not so labour intensive.

Both designs had advantages but, in the end, that unfamiliar phrase -secondary builders' holes - was the deciding factor.

'With a ribbed slab you can make holes anywhere except in the ribs themselves and these are readily visible,' Gunning explains. 'But beams within a flat slab are less obvious, so cutting through it would have been more restrictive.'

The chosen ribbed slab is 325mm deep, with 200mm wide ribs positioned to a 7.2m overall rectangular grid. But to set out this rectangular grid shape on the circular plan area of the 48m diameter ward pods, would call for lots of 'special' side sections - unless of course you somehow turned the circular footprint into a square.

This is exactly what the design team did, but only, says Gunning; 'after a lot of agonising debate with the architect'.

The distinctive five-storey circular ward pods were a major feature of architect RTKL's conceptual design - and had already received the local planner's approval stamp.

The solution was a clever compromise. The circular drum profile remained the underlying shape but, at roughly quarter quadrants, it was punched outwards into balcony-like extensions to accommodate rows of the rectangular grid.

At roof level, a prominent 1.8m deep circular parapet was added so that from the ground the overall appearance of each pod still resembles a drum.

Having satisfied architect and planner, the contractor was delighted. A common floor grid throughout the building allowed repetitive, virtual conveyor belt style construction.

Heyrod chose relatively small concreting bays of around 50m3 by dividing each pod floor into roughly 20 sections. This reduced formwork volumes and allowed a total 28 day cycle for each of the hospital's 640 floor bays.

Virtually all 2,100t of rebar is pre-assembled in a special compound alongside the building into cages for columns or floor ribs. Upward of 3,000 prefabricated ribs are lifted into position in the slab complete with loose continuity steel. This is later slid out from the ribs to overlap the slab's top steel mesh which is added insitu.

Standardised falsework towers support conventional timber formwork which lasts the whole project. Large plastic void formers, laid between the ribs, enjoy at least 10 uses. Concrete is conventional C35, ready mixed and delivered by pump direct to the pour.

Only then does the main concreting challenge arrive - obtaining the required floor finish without resorting to a secondary surface skim. Most of the hospital's floor surface will be vinyl coated and a concrete surface tolerance of just 3mm is needed throughout.

Power floating is the obvious way to achieve such accuracy. But the technique tends to polish and seal the surface, delaying it from totally drying out.To stick down the vinyl, its adhesive demands negligible moisture within the concrete surface area.

The floor covering is on the project's overall critical path, so only six months was allowed before the vinyl must be laid. Trial finishing techniques at Halifax included conventional vibrating tamping bars and a large 6m wide hand operated skip float. But the best option to date is a small, relatively light, circular 1m diameter powered puddle float which is attempting to smooth the surface without sealing it.

Regular moisture content readings are taken in the concrete surface and the fall-back option would be to lay a very thin breathable fibre glass membrane onto which the vinyl can be stuck.

This month's completion of the concrete shell heralds the arrival in force of follow-on trades charged with having beds and everything else in place for the December 2000 opening ceremony.

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