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Concrete evidence

JCDecaux's London HQ challenged the architects to design a functional site while respecting the original listed building

The recently completed UK headquarters for JC Decaux, a leading supplier of street furniture, was designed by Foster and Partners. It has three distinct parts - a refurbished Grade II-listed 1930s office building, a new warehouse and a covered street linking the two.

Situated on the Great West Road in Brentford, west London, this simple and effective structure has won an award for excellence in construction in the Concrete Society Awards.

As the white render on the existing listed office building had to be retained, a material was sought for the warehouse that would respect its predecessor but express its own originality and create a clean, light and efficient image.The client and architect selected precast concrete, based on its aesthetic quality, its accuracy of finish and its ease of installation.

The scheme was constructed in a precast reinforced concrete frame in sharp white concrete, which spans 15m at 9m centres.This internal grid is clad with 1,530m 2of white precast insulated concrete sandwich panels, providing a structurally solid appearance as well as providing speed and efficiency in construction.

The shell of the 3,000m 2building was completed in just one and a half weeks.

The patented structural insulation panel system is called hardwall cladding.The panels are made up in factory conditions and comprise 75mm thick concrete slabs with connection bars built in. Insulation board is laid over the top so that the connection bars protrude through and then another layer of concrete is cast, trapping the insulation within the structural panel. It uses unique 'Thermomass' glass fibre and polymer composite bars to tie the wall together.The panels are left to cure in the horizontal position and then stored vertically for monitoring, before being delivered to site.

The composite bars are stronger, less thermally conductive, less corrosive and more elastic than traditional steel connectors.The usual type of connection in structural panelling - such as stainless steel pins - risk acting as 'thermal bridges'; draining heat energy from a structure.

The small area and low conductivity of these Thermomass bars, however, enable 99 per cent of the polystyrene layer's insulation value to be maintained, and as a result the client can reap the benefits in terms of reduced heating and cooling costs over the lifetime of the building.

The design of these connection bars also lets the outer 'leaf 'of the wall - which is more subject to solar variations, move independently of the inner 'leaf ' - which remains at a stable temperature on the internal face of the insulation.

The insulation extends to the edges of the panel to maintain maximum thermal protection. Overall, the thermal properties of this system provide a Uvalue better than originally specified for the job.

Trent Concrete in Nottingham, which manufactured the structural insulated panels, produced a white reconstructed stone mix incorporating Spanish dolomite for additional 'sparkle'. The interior walls were hand-trowelled to produce a quality, blemishfree finish, while the exterior surfaces were treated with a light acid etch. Smooth surfaces were an important client requirement; they require no further treatment and little ongoing maintenance.

The panels are self-finished, and since the structure was to have no internal lining, all fixings also had to be hidden from view. The cladding fixings were made as small as possible and recessed into the inner leaf of the sandwich, directly behind the columns.Similarly, column and roof beam connections were designed to fit into the top of the beams, keeping them out of sight.Computer simulations were set up to ensure that all the fixings were outside sight lines.

The absence of sheeting rails and sag rods also keeps the inner face clear and easy to clean. A clean appearance was also achieved externally by casting the rainwater pipes into the columns. This further enhances the crisp lines of the warehouse's simple rectangular design and makes it a fitting partner for the refurbished office building.

Precasting the sandwich panels meant production in a quality-controlled factory environment, with just-in-time delivery to site for immediate erection.

The design was optimized so that the smallest number of large units could be used.The cladding panels measured 9 x 3m; the main roof beams 900 x 400mm; external columns 600 x 400mm; and internal columns 400mm square. This enabled rapid construction of the 63m-long, 45m-wide and 9.65m-high warehouse, with the entire frame and cladding to the two long elevations being erected in just 12 days.This panelized articulation of the warehouse building also means the structure's large dimensions do not dominate the overall premises.

From a structural viewpoint, the building is designed to be braced in the longitudinal direction, using the precast sandwich panels as the structure.

The panels were dowelled at the horizontal joints to transfer the shear forces, and robust mechanical connections were installed to bolt the panels to the columns.

The roof structure and cladding act as a diaphragm to transfer wind forces into the braced frame. In the transverse direction the structure is designed as a sway frame, taking advantage of the 11.35m-high cantilever columns.

If the structural cladding panel had not been laid horizontally (that is, long side across), but had been laid vertically, the concrete grid frame could possibly have been omitted.The panels are of sufficient structural integrity to span from floor to eaves and to take conventional roof support structure loads.

This would have had a very different impact on the look and feel of the building.In so doing, the protuberances, which are the internal concrete columns, would have been avoided, but the rainwater pipes would have been visible.

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