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Cost-effective cladding

Concrete is producing a high quality result on a privately financed NHS hospital project. Report and pictures by Mike Winney.

Cost, construction speed, quality and durability defined the choice of concrete for the frame and most of the cladding envelope at the £148M Princess Margaret Hospital relocation project in Swindon.

Insitu reinforced concrete with columns and flat slabs won over steel and precast concrete for the frame. 'Reconstructed stone' large panel precast concrete was chosen for the cladding as giving a high quality, low cost alternative to insitu masonry or metal framed curtain walling.

With both these elements of the hospital structure now completed, the building team is delighted with the result. Carillion project manager Paul Dempster rates the frame built by Duffy Construction as the 'best' of the six concrete structures he has been involved with in his contracting career. And precast panel supplier Trent Concrete production director Peter King agrees. He is reported as saying it is the best insitu concrete frame he has ever worked on.

Quality of the dense facing concrete and its aesthetic appearance is superb in almost every respect. And it should be pretty well perfect once normal weathering has purged the lime bloom on the newest panels - a normal and temporary characteristic of precast concrete (see box).

Reasons cited by Dempster for not using a steel frame include 'the perception that it might be susceptible to bounce' and hence too lively for delicate equipment of a hospital. More important was the fact that in a fire critical building such as a hospital, a steel frame would require extensive fire protection - something which comes as part of the package with insitu reinforced concrete.

Floor finishes were also key.

'We were looking for no screeds, ' says Dempster. As a result neither precast nor composite floors were favoured.

Duffy Construction has supplied Carillion with a power floated surface that requires a minimum of further work prior to laying flooring finishes. Walking around the shell of the hospital it is obvious that only localised grinding has been required to remove occasional high spots.

And as a drawing board for the crews now setting out the partitions the clean and flat concrete surface is near perfect.

The 30,000m 3of the insitu frame was poured in 36 weeks 'ahead of programme and completed to price, ' says Dempster.

Large panel precast concrete cladding followed from the decisions to have no external independent scaffold, on cost and safety considerations, and to be reliant on a minimum of local resources in trades such as bricklaying. Typical panels are a full bay width and storey height, 7.2m by 4m and weighing up to 10.5t.

Preferred supplier Trent was brought in before the 5 October 1997 financial close of the PFI project to work on a two-stage tender. Preliminary concept and design work was paid for by the main contractor.

'Reconstructed stone' is the precast industry's description of the material used. In fact it is a very high quality concrete made with 400kg of white Portland Cement per cubic metre, and gap graded Gloucestershire limestone aggregates. Main aggregate is 10mm down to 5mm Horcott with fines of Horcott sharp sand.

Water/cement ratio was 0.42.

Mix design was driven by the need to lift the units from the moulds at about 16 hours after casting, when a compressive strength of 12N/mm 2was required. Although a nominal C40 mix, actual 28 day test results averaged 55N/mm 2.Aggregate was lightly exposed in a highly skilled, intensely manual operation of etching with 7% hydrochloric acid. Cills were ground down 3mm to make a subtle feature of attractive aggregate. Overall, the panels have a rich creamy colour.

The 35 weeks of production work at Trent's Nottingham works started in January 2000 for deliveries to site starting in the second week of September.

The decision was made to pre-fit the window frames, glazing and some internal insulation for all the upper floors. Glazing specialist Broderick Structures worked on a special line set up at the precast factory.

Consequently, all window frame to concrete weather joints could be sealed in comfortable working conditions near ground level. Access to the fascia of the building was then only needed via cherrypickers for placing the two-stage silicone seals in the nominal 20mm gaps between adjacent panels. Decorative false joints on the faces break up the large expanse and create patterns echoing restrained art deco - a pleasing feature emphasised by the elongated multipaned green-framed windows.

On site panel erection with mobile cranes began on 11 September and was completed in 16 weeks - without a single window being broken. Trailers carrying the units were delivered to site on a just-in-time basis and up to eight panels a day were placed by Trent's crew of four.

Fixings consist of stainless steel-shimmed cast-on concrete corbels bearing on the floors with all stainless steel cast-in sockets to the units and columns. Panels are tied in by stainless steel studs, nuts and angle brackets, allowing for easy on-site adjustment. In all, 563 panels were supplied to site in a limited number of basically identical patterns - but running to 301 types because of minor, designed, differences in socket positions.

Trent's quote of £1.7M for a net area of 7, 300m 2works out at about £233/m 2and compares with an entry level for curtain walling of about £400/m 2and top price of around £700/m 2. With the structure weathertight work is now building up on the complex M&E installation characteristic of a state of the art hospital.

Letting nature take its course Lime bloom is a characteristic, temporary and purely cosmetic, condition affecting all high cement content concrete, says Trent Concrete managing director Mike Downing.

'When cement hydrates, one of the byproducts is calcium hydroxide - what we call free lime.

In the early months of curing, the free lime reacts with carbon dioxide in the air to form calcium carbonate. This is deposited on the surface as a white powder, ' is how he summarises the condition.

Efforts put into cleaning away the deposit can be wasted unless the chemical process is complete and may even exacerbate variations in appearance. Speed of the reaction will vary according to the age of individual castings, weather conditions and the exposure of the concrete, says Downing.

Normally Trent advises its clients to let nature take its course and that a minimum of six to12 months after casting may be required until similarly manufactured concrete panels begin to assume a uniform appearance.

High temperatures on southern aspect exposures in spring and summer speed the process, while winter and shaded or northern exposure on a structure slow it.

The chalky surface dust is obviously of relatively little PFI project Princess Margaret Hospital Relocation, Swindon is a £148M private finance initiative project on six floors totalling 55,000m 2within a 13.6ha site near Junction 15 on the M4.

The PFI deal involves a design and build contract of 37 months followed by a 27year facilities management contract starting on 5 November 2002. Then 52 departments of Swindon's acute general hospital will begin to move in over a 60-day period and the project will be renamed Great Western Hospital.

Client: The Hospital Company, a special purpose vehicle whose partners are Carillion plc, United Medical Enterprises and Barclays Capital.

Engineer: TPS Consult Architect: Whicheloe Macfarlane HDR Main contractor:

Carillion Building Special Projects M&E specialist contractor:

Crown House Engineering Insitu concrete frame trade contractor:

Duffy Construction Precast cladding trade contractor:

Trent Concrete Recycling: a top priority Separation and on site recycling of waste materials gets high priority at PMH Swindon.

General waste, surplus and offcut materials from the various trades on site have to be deposited in designated skips which are carried by a forklift truck to the recycling centre.

Reusable material is dispatched for appropriate use elsewhere and the intention is to minimise the amount going to landfill.

To date, with the shell of the building complete, some 280t has gone to landfill and 323t has been recycled. The project target is for a maximum of 1,700t of landfill disposal.

Significant recycling so far includes disposal of the timber and plywood shutters used for the concrete frame construction. These were ground up in a chipping machine and used for chipboard and temporary footpaths.

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