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Precast facades have the potential to be made very cheaply using highvolume, standardised components, or made bespoke and shaped by unique profiles at very high cost. So it is not surprising to find it is standardised precast panels that are the dominant form of construction for housing schemes and for medium- and low-rise commercial buildings in Northern Europe.

Precast concrete as an engineered and decorative material offers wide-ranging possibilities of expression, application and performance. The choice and range of colours and raw materials, combined with surface texturing and profiling, gives scope for designing with freedom and imagination. It continues to be the dominant form of construction in Northern Europe because the precast units are made in large volumes to standardised profiles to maintain their price-competitiveness.

There are numerous examples of multi-storey apartment blocks in Scandinavia specified with structural load-bearing panels of up to 12 tonnes that are fully insulated and architecturally finished. There are also many examples of lightweight precast units like GRC (glass-reinforced concrete) that can dramatically reduce dead load and integrate precast with high-tech curtain wall systems. The panels are easy to handle and do not require heavy craneage - they can be installed using a cradle system. They are resilient and a popular choice in Austria and Germany.

The economic advantages, the method of manufacture and the aesthetic quality of these two mass-produced facade options are highlighted in this review. Some of the research for it has been taken from the book The Art of Precast Concrete Architecture published by Birkhäuser and available from The Concrete Centre ( www. concretecentre. com).

SANDWICH PANEL CONSTRUCTION This is the most structurally-efficient concrete building option and the dominant method of constructing residential buildings in Scandinavia. The sandwich panel is a storey-high precast unit that can be up to 8m long with an outer skin 60-80mm thick, a layer of insulation and then a backing leaf of load-bearing or self-bearing precast concrete 90-140mm thick. (Self bearing means it only supports its own weight. ) When it is load bearing it can support the structural floor and the facade above it and is the more efficient and popular choice of panel.

The structural floors are usually precast hollow-core planks that are stitched to the top of the inner load-bearing panel.

Load-bearing sandwich panels offer many advantages - for example they are fast to erect; they eliminate the need for columns and wet trades; they are self finished; and they are extremely competitively priced.

The two skins of the precast panels are interconnected by steel ladder reinforcement which acts as wind and shear connection. The thermal bridging through these steel connectors is minimal. The system has the advantage of providing structural integrity without placing any reliance on the insulation for load transfer. It can be configured so that an elevation can have solid panels or long continuous spandrels, also known as band systems, or slender rib systems with built-in columns for highly glazed facades. The cassette modular system (see SID Building, page 48) developed by precasters offers scope for even more architecturally challenging designs using sandwich panel construction.

PRECAST LIGHT - GRC The trend in modern building construction towards lighter weight 'high-tech' facades using glass curtain walling, resin-coated aluminium and steel fascias has some advantages over the heavier precast and reconstructed stone cladding unit. Typically, the skin thicknesses of GRC panels is 13-20mm, making them as much as 80 per cent lighter than the corresponding precast concrete unit.

Weight reduction of this magnitude offers substantial savings in transportation, structure, handling and site erection cost.

GRC is composed of a mortar mix of cement, selected crushed aggregates, sand, fillers, admixtures, water and alkaliresistant glass -bre strands. The glass -bre is typically 6-51mm long and 10-30 microns in diameter. It obtains its alkali resistance from a coating applied over the glass strands in the manufacturing process. For sprayed GRC it is recommended that 5 per cent of -bre by weight of the total cement mortar should be contained in the mix, to optimise on the tensile strength. Combinations of -bre lengths and the production process will ensure that adequate bond strength develops between -bres and the cement matrix and encourages a quasi-ductile failure by fracture of the -bres.

The various techniques used to manufacture GRC products - manual and mechanised spray methods and linear at bed wet casting - enable the material to be formed in a wide variety of shapes and profiles. It can be moulded easily to suit Classical or Modern architectural expression using thin at sheets, curved pieces with embossing or angular surface profiles. Being cement-based with no metal reinforcement, it also has inherently good durability and chemical resistances. It is non-combustible and produces no toxic smoke emissions and has high impact strength. It is not susceptible to rust staining or corrosion, and can be used in combination with insulating material and sound-proofing. Constraining factors in performance are generally due to its relatively large thermal and moisture movement and low ductility.

The need for GRC cladding to be exibly mounted on the supporting structure to accommodate thermal and moisture movement is therefore important. Many of the problems associated with GRC have resulted from the lack of mobility in fixing design, from errors of installations or as a result of introducing some other restraint to panel movement. Wherever possible, design GRC panels as independent skins to allow maximum freedom to shape, curve and profile panels. Good detailing of panel size; reducing horizontal at surface areas like window sills which may collect surface water and create high moisture gradients in the panel; and avoiding panel shapes that wrap around a building corner causing large thermal movements, will ensure a longer service life.

The following three examples feature precast sandwich panel construction:

RASTIPUISTO APARTMENT BLOCK At Rastila, Helsinki, Finland, by Helamaa and Pulkkinen Architects.

Precast by Parma Oy, Nummela The ribbed profile finish to the lower panels has been achieved using a set retarder, and water jetting has removed the surface laitance to reveal the small aggregates. To form the ridged horizontal bands plastic strips are fixed to the film-faced plywood in the moulds. Chemical retarder was brush-applied to the formwork and the profiled plastic strips before the concrete was cast. (For more on precast finishing, see pages 56-57. ) For the upper levels of the building, brick slips were placed carefully in the mould between the timber battens that form the recessed mortar joints. When the concrete panel was removed from the mould it was transported to the water-jetting area where it was cleaned under high pressure to remove all the retarder and the unset cement paste, revealing the fine aggregate finish. Using high-pressure washing avoids any cement fines streaking over the surface and leaving a dry, crusty film.

In the production of the sandwich panel, the exterior panel is compacted by an integrated shock-compacter which is built into the tilting table mould. The interior load-bearing element is compacted with a poker vibrator. Demoulding takes 12-14 hours.

SID BUILDING At Aarhus, Denmark, by 3XNielsen Architects.

Precast by Dalton Precast, Aarhus When pigmented precast panels are specified, customers are advised that colour consistency cannot be guaranteed. To achieve a very consistent black concrete, the only option is to use single-sized black aggregates and expose it on the surface. This can be done by retarding the concrete set in the mould and then water-jetting the surface to remove the cement paste to expose the coarse aggregates.

The panels on the SID Building were cast in five sizes - they were all 3.5m high, and either 1.2m, 1.5m, 1.8m, 2.2m or 2.7m wide. Special corner units were made which formed part of the returns for both elevations to avoid a vertical joint line at the edge, which the architect felt would not be as good. The edge was given a 10 x 10mm rebate to emphasise the corner line.

MUSTAKIVI SCHOOL & COMMUNITY CENTRE At Vuosaari, Helsinki, Finland, by Ark-House Architects. Precast by Rajaville Precast Company, Oulu The panels on the exterior of the school building have an outer skin 75mm thick, then 145mm of insulation and a 160mm inner skin of load-bearing concrete. To produce the stain effect of terracotta and green on the panels, a white concrete panel was cast on the outer skin and allowed to harden. Iron oxide chemical stain in a dilute hydrochloric acid solution was applied over the surface for the brown-red colour and copper sulphate solution for the green effect. The iron reacts with the calcium hydroxides of lime in the concrete to create the colour on the surface. This finish will appear streaky because of the direction of brushing and patination owing to the varying absorbency of the surface.

For the white panels, white cement was used, plus white limestone fines and whitish sand; there were no pigments. For the black panels, grey concrete was used with black rock fines, 3 per cent black pigment and special black 0-8mm gabro aggregates.

The surface set was lightly retarded and the water jetted to expose the black aggregates to ensure colour consistency.

The following two projects illustrate the use of GRC panels:

STADTVILLA APARTMENTS At UnterneuStadt, Kassel, Germany, by Alexander Reichel Architects The in-situ concrete load-bearing structure was clad with insulated GRC elements, the GRC just 30mm thick to prevent thermal bridging. Each precast component was coated with a hydrophobic fluid to produce a consistent, water-repellent outer surface.

This gave the surface a 'milky' two-toned shade.

The GRC panels were made by hand-spraying the concrete mix and the chopped fibres into the moulds.

The unreinforced face mix of 5mm was colour-matched to the grey concrete of the frame. This was overcoated with a reinforced backing mix which contained chopped fibres and was added at a dosage of 2 per cent of the concrete volume.

The backing mix was applied in five layers each 5mm thick and compacted by rolling to build up the panel thickness of 30mm.

PRIVATE HOUSE At Ulrichsberg, Austria, by Habringer-Landerl Architects, Linz.

GRC by Rieder, Salzburg This private house is clad in thin sheets of smooth, colour-stable GRC panels. Rieder has been manufacturing GRC panels for over 20 years and recently invested in sophisticated automatic production methods for making 13mm flat sheets of GRC of exceptional quality using a continuous fibre strand feed and wet casting on a moving metal mould bed. The mould bed is 1,250mm wide which fixes the sheet width, but it can be as long as 5.3m.

The external face was cast face down on a layer of stretched polythene placed on the metal bed to give a very smooth marble-like finish to the surface. Each 6.5mm layer of concrete is poured through grouting tubes with the glass fibre strand then stretched over the top and chopped to size to sink into the wet concrete. The wet layer is lightly rolled by the machine before the 6.5mm backing layer is placed in exactly the same way.

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