Feature

Precast concrete cladding

Concrete panels are an increasingly popular option for architectural and structural cladding, but they require close collaboration from the whole design team, writes Jaylina Rana

With the off-site manufacture of structural components becoming more common, the use of precast concrete cladding panels as the main building envelope has significantly increased over recent years. Precast concrete cladding panels primarily act as weather-proof envelopes, while fulfilling architectural, structural and other requirements in terms of fire resistance, thermal performance and sound insulation. They are used in a variety of structures, including in the residential, commercial, institutional and industrial sectors.

The adoption of the appropriate precast cladding system will be the result of an iterative decision-making process among project stakeholders including architects, designers, planners, contractors and manufacturers, who will have different priorities. For instance, the choice of finish is largely dependent on aesthetic preferences and local planning requirements, which in turn influences the architectural and structural considerations. Therefore, it calls for an integrated approach by all key stakeholders early in the design.

The structural design of precast cladding panels is significantly influenced by factors such as panel dimensions, location of the panel joints, number of apertures or openings, the number of storeys and floor-to-floor height of the building or structure.

       

Common precast cladding arrangements

The arrangement of the precast concrete panels is primarily influenced by the geometry and number of apertures or openings within the facade. The most common arrangements of precast cladding are punched cladding, spandrels and mullions, ribbon cladding and vertical band cladding (figure 1).

  • Punched cladding Panels are cast with an aperture into which a window or door assembly can be fitted off-site
  • Spandrels and mullions A series of horizontal (spandrel) and vertical (mullion) panels which span between the openings – for example, windows. The window assemblies are then fitted on site after the cladding has been erected
  • Ribbon cladding Horizontal panels which are joined at their ends, with windows fitted vertically in between
  • Vertical band cladding Vertical bands which transfer the cladding loads to the foundations or ground support system.

Types of precast cladding

Precast concrete panels can be designed to be structurally loadbearing or non-loadbearing.

  • Loadbearing cladding systems typically consist of an inner structural panel, an insulation layer and an outer non-structural panel. This type of cladding is mostly used instead of perimeter beams and columns, to transfer the vertical forces down the structure to the ground.
  • The most common type of loadbearing cladding system is sandwich panels. The structural concrete inner layer is designed to withstand the applied loadings from the floor and the structure above. The thinner outer layer acts as a weather- and airtight barrier and provides the desired architectural finish for the building envelope. An insulation layer is sandwiched between the inner and outer layers to provide the required U-value. The three layers are linked by connectors to ensure low thermal conductivity (figure 2).

ARCHITECTURAL CONSIDERATIONS

With recent developments in casting and manufacturing techniques, precast concrete facades are now available in an almost unlimited range of colours, textures and finishes.

Colour

Cladding can be manufactured in an extensive pallet of colours, and be designed to match the colours and textures of other materials such as stone, masonry and terracotta.

The colour of the concrete is dictated by the type, consistency, proportion and grading of the cement, fine aggregate, coarse aggregate and pigment. For instance, if the desired colour of the concrete is dark or light grey, then the most commonly available grey cement is used. For lighter shades of concrete such as natural stone, white cement is used. A wide range of naturally occurring aggregates is available in the UK, including limestone, granites and shingles. Fine aggregates, which form about 30-35% of typical concrete mixes, have a significant effect on the colour and can produce a variety of colour palettes from white and buff, to red, green, brown or dark grey. Coarse aggregates also influence colour, particularly when they are exposed in polished or exposed finishes.

Pigments are used in concrete to produce a wider variety of more intense and stronger colours, which cannot be achieved by the use of naturally occurring aggregates. Manufactured inorganic pigments are available in powder, liquid and granular forms. They must complement the type of cement and aggregate in order to achieve the required uniformity and intensity of colour. For example, terracotta-coloured concrete can be achieved by combining red pigment with off-white cements, sands and granite.

Texture and finish

Textures can be formed or cast, post-applied (for example, by etching, blasting or polishing), or cast monolithically with facings such as bricks or stones.

For geometric patterns, false joints, rebates and chamfers, the concrete is cast in special moulds incorporating the desired features. Materials such as stone, brick and terracotta can also be cast in to the concrete panels to replicate the facades of existing buildings. This approach is often used to satisfy planning requirements to ensure the new facade is in keeping with surrounding buildings. Fine textures in recon panels are usually applied to the concrete after curing and demoulding.

For finer textures, the surface of the concrete is etched to remove the top cement and fine aggregate. For coarser finishes, other techniques such as grit blasting can be used.

More information can be found in “Visual Concrete”, available to download from www.concretecentre.com

Loadbearing sandwich panels transfer the load from their self-weight (and the panels above) and the dead and imposed loads of the supported floors on to the foundation or supporting structure below. Additionally, these walls have to withstand lateral wind forces on the face of the cladding panels. Vertical continuity such as steel dowel bars or couplers are usually provided between cladding panels to resist the lateral forces applied. The structural layer of loadbearing cladding panels are also tied back to the floor using U-bars and horizontal tie bars.

  • Non-loadbearing cladding systems, such as solid or single-leaf precast cladding, have no structural function. These precast cladding panels act as weather and air barriers and provide the desired architectural finish to the building envelope. The windows, insulation and internal finishes are most commonly installed after the erection of the panels.
  • Non-loadbearing panels are either stacked off the foundations or ground beams, or supported by the main frame on or near column positions in order to avoid excessive mid-span deflections in slabs/beams.
  • Stacked system When stacked, loads are transferred vertically through the panels down to ground through dowelled connections (figure 3).
  • Supported system When supported, stainless steel brackets mounted on to the base of the panel transfer the vertical and lateral forces to the floor slabs – or secondary structure (see figure 4).
  • Restraint Restraint fixings are designed to hold panels back to the structure and transfer all horizontal forces (such as wind loading) to the structure. They should be designed to be adjustable to accommodate any differential movement between the structure and the cladding. Particular attention must also be taken of the allowable tolerances in the structure of the building and the manufacture of the precast units. Four restraints are commonly provided in average sized panels, typically at the corners of the panels (figure 5).

Other considerations

  • Transportation Precast concrete products are transported on trailers, which directly influences the size of cladding panels. The recommended maximum vehicle height on UK roads is 4.93m. There are many different trailer configurations that may be used. Typically, panels up to 4m high x 7m long, or 3m high x 13m long, can be readily transported to site. Beyond these parameters, early guidance should be taken from the manufacturer.
  • Lifting and handling Consideration must be given to handling and lifting at all stages, including production, transportation and erection, and also to the stresses induced by these operations. Handling must be reduced to a minimum to prevent damage to precast units, whether in the factory, during transport or on site. The strength of the panels at the time of these operations is fundamental in determining additional reinforcement needs. The precast units must also be stored so that delivery in accurate sequence for site fixing is possible.

Lifting methods must be clearly defined through close coordination early in the design process between the cladding manufacturer and site team.

 

Photos: Tom Harris Photography; Rasmus Hjortshoj; Stefan Muller

 

 

FURTHER INFORMATION

To see case studies of architectural precast, visit www.aspa-uk.org