Hard to Beat
14 Sep 2004
After years of unpopularity, precast concrete is now being used to clad many new buildings. What is responsible for the shift in attitudes towards the material?
As a cladding material which is solid, prefabricated, suitable for fast-track construction yet with a subtle, stone like texture, precast concrete is hard to beat.
To me, precast concrete is a building material with ‘gravitas’; its solidity and strength recalls traditional concepts of enclosure, yet is has all the advantages of a modern, factory-produced product. Many architects recognise its qualities; Ian Simpson Architects’ new 47-storey Beetham Tower in Manchester will have a four-storey podium of polished precast concrete. Formerly the Free Trade Hall, the new Radisson Edwardian Hotel by architect Stephenson Bell has a new 15-storey block clad with precast concrete panels faced with Jura limestone. Two new apartment towers, one in Old Hall Street, Liverpool, the other next to the Lowry Centre in Manchester, are both clad with precast concrete panels.
Yet concrete cladding has suffered from decades of unpopularity; some architects are still biased by the outdated notion – originating in the poor performance of 1960s precast-clad tower blocks – that precast does not weather well. This is no longer true; unlike early 1960s cladding, today’s precast components are made of concrete so strong as to be virtually impervious (in excess of 40 N/mm2) and are routinely detailed to eliminate streaking.
Precast cladding is a heavyweight structural material and ideal for tall buildings – it can be shaped to form mullions and spandrels or storey height panels and windows or other components can be pre-fitted at the precast factory. It is also uniquely versatile. Its composition, based on stone aggregate mixes, can be altered to produce a great variety of colours, textures and finishes, including a marble-like polished finish. As a cladding panel precast is often cast from a mix which will produce the appearance and texture of natural stone – a specification generally known as reconstructed stone. Such mixes make the material acceptable in environmentally sensitive areas where new projects are required to blend in with traditional stone buildings. Precast cladding panels can also be faced with brick, natural stone slabs or terracotta tiles.
Precast concrete is a prefabricated product; it is produced and finished in the factory under controlled conditions. Most precasters have batches of samples which give a range of colours, textures and finishes; they will also cast a full-size sample panel so that it can be approved by client and architect before installation, removing any causes of delay on site.
But what gives precast the edge over other cladding materials is its buildability. Here are some of its advantages:
ï The quality achieved by prefabricated manufacture in controlled environment, unaffected by weather and labour shortages. This permits rigorous selection and inspection before installation, removing causes of delay on site.
ï The prefabrication and phased delivery to site accelerates the construction programme and achieves a weathertight building enclosure at the earliest opportunity.
ï On-site ‘wet trades’ are reduced; if internal precast elements are prefinished, wet trades can be virtually eliminated.
ï Site installation by single team of skilled workers. No scaffolding is required.
ï Glazing, fixings, thermal insulation and vapour control layer can be incorporated in the factory before delivery to site.
ï The fabric energy storage potential of precast; thermal mass, especially when used as exposed precast floor slab, helps to control building temperatures.
ï Precast cladding panels produce a thinner external wall than conventional double-skin walls, increasing the lettable floor area.
Case Study – Hotel, apartment tower and office block, Old Hall Street, Liverpool
As you approach Liverpool’s city centre from the north you now see a striking new landmark, a 28-storey tower flanked by a luxury hotel and an 8-storey office block. The tower acts as a gateway to the city and the development, designed by the Liverpool office of Aedas Architects, is one of the first major new developments in the city’s regeneration.
The buildings occupy a triangular site with the tower at the extreme edge of the triangular site to act as a focus. The Radisson Four-star Plus hotel has 200 beds, a conference suite, restaurant, bars and health club, together with all the usual back-of-house facilities. Public and guest rooms are arranged around a central atrium with nine tiers of open galleries set around it to give access. The tower is linked to the health club so that owners as well as hotel residents can use it.
The tower contains133 apartments and is constructed of slip-form cast in-situ concrete. The elevations comprise a vertical slot of inset curtain wall glazing and balconies to living rooms, flanked on both sides with a series of precast concrete cladding panels. ‘We had a very short contract period’ explains Helen Donleavy, the project architect. ‘The fact that the precast concrete panels were manufactured off-site, brought in and craned into position without the need for scaffolding made a big difference in speeding up the construction programme.’ The storey-height panels were manufactured by Techrete at its Brigg, Lincolnshire factory and to save additional time a large number were factory-fitted with windows. They are a pale grey in colour with a lightly acid-etched finish. The top floors of the tower are set back and contrast with the lower precast panels by having a more robust sand-blasted finish.
The hotel has a cast in-situ concrete frame from basement to second floor; upper levels were designed with structural pre-cast concrete internal cross-wall panels, giving the acoustic and fire ratings required and allowing fast construction. The cross-wall panels and precast concrete cladding were produced by Histon Concrete. In addition pre-designed, pre-finished bathroom pods were delivered shrink-wrapped to site and craned into position at the same time as the cross-walls.
The adjacent eight-storey office block has a steel-frame structure and is clad with precast concrete panels by Techrete in the same colour and texture as the apartment tower, alternating with glazed curtain walling. The precast panels are storey-height and between 6 – 7metres wide to achieve a watertight building shell at an early stage of construction, together with fast completion time.
The architect and Carillion Building, the contractor, involved the precast concrete manufacturers at an early stage, which allowed the job to run smoothly.
Case study – Beetham Tower,
Construction has just started on the 47-storey Beetham Tower, the tallest structure in Manchester and the highest living space in the UK, designed by Ian Simpson Architects. The tower will combine 17,185 sq m of residential space, 6,506 sq m of quality office space and a 285-bed five-star Hilton hotel.
The four-storey podium at the base of the tower, containing the public rooms of the hotel, will be clad with polished precast panels manufactured by Trent Concrete. Before being polished the panels will be cast with vertical grooves set in a random pattern. They will be cast with a mix of English stone -black basalt aggregate from Clee Hill near Ludlow and green Crigeon granite from North Wales, with grey black-pigmented cement. ‘When polished the granite has wonderful variations in colour which catch the light’ explains Gary Colleran, project architect. Precast was chosen for its monolithic quality and so as to reflect the urban context of the podium.
Case study – Apartment building,
The new “Waterside at the Lowry’ development is an 11-storey tower housing 165 apartments and penthouses on a site overlooking Salford Quays and just across the water from the Lowry Centre. It is clad with panels of precast concrete and rests on a two-storey podium of retail shopping and car parking which is clad with natural sandstone ashlar masonry. Techrete, the precast manufacturer, worked with architect DLA of Wakefield to design a finish with a colour and texture which would be sympathetic to the large range of colours inherent in the natural stone on the podium below. The final mix contains limestone and quartz with a pigmented white cement matrix. The moulds were cast at Techrete’s Brigg factory on steel vibrating tables; this is an alternative system to the traditional method of casting – in timber moulds; it allows changes in panel sizes to be made very easily, making casting faster and reducing cost. Each precast panel was fitted with two windows, pre-glazed at the factory. They were lifted in place by crane and the projecting balconies were installed at each floor level immediately after the cladding panel, with the precaster sealing the interface between panel and balcony.
Written by Susan Dawson
Cast in Concrete, by Susan Dawson is a new technical handbook on how to use precast concrete. It can be obtained from the BPCF, 60 Charles St, Leicester LE1 1FB (price £25, enclose cheque to be made payable to BPCF) or by credit card from www.concretebookshop.com
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