V5 7746jap Facade

Winter 2004/2005

Off-site prefabrication Chairman of the ACA, Stephen Maddalena considers the advantages of precast cladding

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lthough topical in consequence of the Latham-inspired initiatives and increasingly recognised as being a merited solution to the challenge of providing optimum value, off-site prefabrication is not a new concept. Architectural precast concrete panels have been produced off-site for many decades and are increasingly being specified to clad a wide range of new buildings including commercial and civic office schemes, residential, retail and leisure developments as well as hospitals and schools. Despite this trend, there are many building projects that fail to benefit because the advantages of precast cladding construction are not fully considered early enough. The direct and incidental advantages of precast cladding construction are many and varied. Importantly, they valuably contribute

to enhancing quality and safety, to improving efficiency and productivity and to providing programme and price certainty.

Rising high at Discovery Dock

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esigned by Chantrey Davis and Shepherd Design and built by Shepherd Construction, the Discovery Dock development in Canary Wharf, is predominantly clad in reconstructed stone panels supplied and erected by Techrete (UK) Ltd.

The largely residential development, comprises three blocks; 14 storey, 16 storey and the 25 storey tower. The panels are in the main double-punched window units, and were delivered to the site, following the installation of the windows at Techrete’s factory by Parry Bowen, the curtain wall subcontractor. Typically 6m x 3m and weighing up to 9 tonnes the panels were produced with cut-outs to accommodate the cantilever support beams for the steel balconies. Although there was very good panel repetition, it was necessary to manufacture several ‘challenging’ shapes, which in some cases resulted in the adoption of a two-stage casting process.

With such expansive areas of cladding it was important to minimise panelto-panel colour variation and a new mix was developed by Techrete to provide as much colour stability as can be achieved, using natural materials The panels were fixed to the reinforced concrete frame, where the load was transmitted via integral corbels on the rear of the panels adjacent to the columns. The restraint fixing was via channels typically cast into the underside of the post-tensioned floor slab, where accurate positioning of cast-in fixings has a substantial bearing on the economic and smooth running of the cladding system. Insulation and horizontal fire stopping was also included within the precast concrete cladding package.

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Design matters Innovation in production methods and significant investment in labour and plant resources enable architectural precast concrete panels to be provided in an increasing range of finishes and facings, shapes and sizes to suit the requirements of individual projects. Finishes and facings A comprehensive range of cladding finishes is available ranging from lightly textured reconstructed stone and polished concrete finishes to coarser washed exposed aggregate surfaces. Aggregates and cements can be selected to suit colour preferences. Precasting

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Off-site prefabrication continued from page 1 effectively allows the pre-assembly of natural stone, granite, brick and terracotta tile facings with a reinforced concrete backing, a single panel perhaps incorporating 100+ facing stones or 1000+ bricks. Natural stones and granites are used more economically than with traditional construction methods as they can be produced in thinner sections. Panels may include a mix of finishes and facings that would otherwise demand separate trades. Shapes and sizes Fresh concrete is an extremely mouldable material allowing complex shapes to be formed. Panels can economically incorporate cill, coping, soffit, window reveal and special sections, especially where repetitive. Large panels avoid the need for secondary structure for their vertical support and lateral restraint. Grid-width units can reduce mid-span loadings onto the structural slab edge. Panel sizes are usually limited by site craneage or transport constraints. Limitations may be overcome by using low-loader trailers that allow storey-height panels to be delivered vertically. Reduced wall thicknesses with precast cladding panels can increase net lettable floor area.

Quality matters Panels are produced by skilled craftsmen, motivated by longterm employment prospects, in a purpose-built factory environment using specialist equipment. Each stage of manufacture is subject to inspection in accordance with an independently certified quality system, and finish and dimensional accuracy are checked before delivery. Sustainability matters Precast panel construction produces negligible waste, just enough concrete being batched to fill available moulds. And at the end of the economic life of a building, the precast panels may be refurbished for re-use or crushed for recycling. Functional matters Precast concrete cladding produced in accordance with BS 8297:2000 has a life expectancy of 60+ years and is generally maintenance-free. Dense concrete is airand water-tight, giving superior weathering performance and corrosion resistance. Joints between panels are usually filled with sealant. The thermal mass of concrete serves to reduce peak heating and cooling loads. Insulation can readily be incorporated either as a lining to the internal face of a panel or as a core between two concrete layers, achieving Uvalue requirements of 0.35W/m2K and better. In addition, concrete is an inherent sound insulator, noncombustible and fire-resistant. Construction matters Precast cladding panels are erected on site by specialist teams trained in safe handling and fixing, typically without the need for an external scaffold. Panels are delivered on a just-in-time basis, allowing large

areas of the frame to be rapidly enclosed to give an early start to weather-sensitive following trades. Window apertures within the panels can be framed, glazed and sealed before delivery. In addition to issues of quality and practicality, each of the above points correspondingly contributes financial advantages. Offsite prefabrication and increased on-site productivity, especially using the largest units that are practical to deliver and handle at site, facilitate optimum value. Similarly, decreasing the dependence on site-based activities helps stabilise the total construction cost and shortens construction periods, at the same time reducing financing costs. However, it should be recognised that all these advantages can only be attained by early involvement of the specialist precaster.

TECHRETE INVESTS IN UK PLANT

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echrete, a leading manufacturer of architectural precast concrete in Ireland and the UK, has invested €5 million in an extension to its UK works at Brigg, making it one of the most modern factories in Europe. This reflects the company’s increasing volume of business in the UK. Plant director John Stothard commented, “This expansion will create more space for

conventional production, as well as housing a new robotic plant to streamline the production of architectural concrete.” A key feature will be a carousel system in which steel moulds will travel to the operative rather than visa versa. The aim is that all curing and finishing will be contained within the building, thus creating a better working environment and a controlled curing regime for the panels. According to Stothard, “The advantages

are that each product will be finished in a controlled environment regardless of the season and external areas will only be used for storage of completed units. Achieving this is quite a challenge as much of the equipment is bespoke and must be adapted for the special needs of architectural concrete.” The new building and offices are constructed of Techrete’s precast concrete sandwich panels, which provide a complete wall solution.

Combined natural and reconstructed Portland stone for Belgrave House facade The project also benefited from maximising the use of precast and specification of a large-panel scheme. This enabled rapid construction and the early enclosure of a weatherproof envelope, allowing follow-on trades to start sooner. Client: Belgrave House Development Ltd & JER Victoria Ltd Contractor: Sir Robert McAlpine Ltd Architect: Squire and Partners Engineer: Whitby Bird and Partners

Award for Trent

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rent Concrete recently completed its £2.3m cladding contract at Belgrave House, a high-profile office development at the rear of Victoria Station on Buckingham Palace Road in London. The contract was awarded following a two-stage tender process, with Trent’s previous successful cladding work with contractors Sir Robert McAlpine (on the new Bull Ring development in Birmingham) proving influential. Trent designed, manufactured and erected 350 units in total. In the main these were two-storey mullions, 7.5m high, with a facing

of deeply profiled natural Portland stone, and 8.5m long spandrels with projecting Portland stone sills. Trent was also responsible for 600m2 of handset in-situ Portland stone cladding. In addition, 1000m2 of matching reconstructed stone cladding for the service core and plant room were produced. This combined natural and reconstructed stone approach provides a high-quality, costeffective solution. Natural stone is used on the main elevations, whilst reconstructed stone is used elsewhere and provides an extremely good imitation of the natural stone in both colour and texture.

Rising high at Discovery Dock continued from page 1 Erection of the 880 or so panels was completed in approximately seven months, with a month or so ‘overlap’ as the double mastic pointing was applied by a youthful but very professional team of abseilers. Serious consideration has to be given to wind conditions at Canary Wharf, when formulating a programme for this type of system; crane down-time of 40% is not uncommon. A further challenge to be addressed was negotiating the cantilever formwork at roof levels, this was achieved by using a ‘C’ hook which circumnavigated the obstruction and resulted in a smoother operation than had been envisaged.

The Experian Data Centre in Nottingham is the winner of the Building category in this year’s Concrete Society Awards held at the London Marriott Hotel on 4 November. The judge’s commented “Viewed from the front, the building is striking: indeed its innovation is in the appearance. Low rise, precast, pigmented concrete units have been differentially etched and are of various lengths to provide a bomb-resistant screen with unspectacular but effective windows. The building is unobtrusive and entirely in harmony with the surroundings.” Architect for the project was Sheppard Robson, main contractor Bowmer & Kirkland Ltd and the precaster Trent Concrete Ltd.

An Innovative Solution

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rup Associates’ design of this new office block for British Land incorporates an innovative solution to the construction of its facade, albeit founded on the traditional principle of a load-bearing structure. The facade includes storey-height panels of Bavarian limestone and reconstructed stone finished precast concrete spandrel units, both acting as structural elements. The natural stone panels, which serve as the perimeter columns, were provided by Stone Cladding International. Each column was constructed as a set of five solid blocks of stone that were pre-tensioned with steel tiebars to prevent their splaying under load. The precast concrete spandrel units at each floor level provide the connection between the stone panels and the steel beams that tie the facade back to the main structure. The precast spandrels were manufactured and delivered by The Marble Mosaic Co Ltd, typically as 6m-long units. The concrete mix and surface finish specifications for their manufacture were developed to match

the appearance of the natural stone. Each spandrel unit required a number of stainless steel boxes to be cast-in with exceptional accuracy to suit the irregular locations of the

fixing positions on both the stone panel and steel beam. The stone panels and precast spandrels were erected by William Hare for Bovis Lend Lease.

Welcome to new member

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ith over twenty years experience of providing architectural concrete solutions on the Continent, Belgium based Decomo, entered the UK market in 1998.

Offering complete design, supply and installation packages, the company’s ethos is based on providing client satisfaction and it is always striving to give that “little bit extra” in order to enhance both its product and service.

With such prestigious projects as the European Parliament Building in Strasbourg and the Hilton Hotel in Paris to its credit, it was not long before the company’s expertise and flair became recognised by many leading UK architects and contractors.

This is evident, not only in the extensive range of finishes and colours available, but also from the substantial investments made in production facilities, equipment and people.

An impressive portfolio of completed projects, this side of the Channel, built up over the last six years, is something Decomo UK Ltd is now rightly proud of. Amongst these are the Guinness Headquarters in London, the Fenwick Store in Canterbury and the Royal Academy of Music in London. Currently, three major projects are being undertaken, at Wembley Park Station, Chesham House and the University College, London.

This year, alone, a new finishing building, covering an additional 3500m2 has been opened in Mouscron, Belgium, where all production is undertaken. A new office building and showroom is also planned for construction next year. In the UK, offices have recently been opened in London, where Paul Desmet has been joined by Andrew Tyrrell, formerly with Histon Concrete Products Ltd, to provide closer client support as the company realises its targets for continued growth in a market which it believes holds much potential.

Architectural Cladding Association 60 Charles Street, Leicester LE1 1FB. Tel: 0116 253 6161 Fax: 0116 251 4568 Website: www.britishprecast.org/aca Member Companies Decomo UK Ltd, 180-186 Kings Cross Road, London, WC1X 9DE. Tel: 0207 689 8058. Contact: Paul Desmet. Redland Precast Concrete Products Ltd, 15/F Kai Seng Commercial Centre, 4-6 Han Kow Road, Tsimshatsui, Kowloon, Hong Kong. Tel: 00852 2590 0328. Contact: Trevor Gregory. Techrete (UK) Ltd, Warren Court, Feldspar Close, Warren Park Way, Enderby, Leics, LE4 5SD. Tel: 0116 275 0785. Contact: David Kennell. The Marble Mosaic Company Ltd, Winterstoke Road, Weston-super-Mare, BS23 3YE. Tel: 01934 419941. Contact: Stephen Maddalena. Trent Concrete Ltd, Colwick, Nottingham, NG4 2BG. Tel: 0115 987 9747. Contact: David Walker.

A product association of British Precast