Precast concrete cladding provides the designer with substantial freedom in the choice of form, texture, and appearance because concrete’s flexibility allows for limitless scope in expression and character, says Bryan Perrie, MD of The Concrete Institute.

Precast concrete panels are reinforced concrete units available in a variety of mixes, colours and finishes, such as formed, acid-etched, smooth or coarse ground, rubbed, polished, grit or sand-blasted. Designers revel in the fact that complex and innovative designs can be incorporated using an infinitely mouldable concrete mix.

In specifying precast concrete as cladding material, the designer gives the building contractor the opportunity to have the building “envelope” moved off site to a precast facility where a skilled workforce can pay full attention to the demands for accuracy and quality of workmanship in the finish.

One of the main distinctions between precast concrete cladding and tilt-up concrete panels is that the precast elements are usually cast in a specialised manufacturing plant, away from their final location. Tilt-up panels are usually cast horizontally at or near their final destination, and then lifted into place. 

The precast panels are often cast face-downwards in purpose-moulds, normally made of steel, often with rubber, timber or fibre glass inserts forming a pattern. Prior to the concrete being placed, a steel reinforcement cage is placed in the mould. Through the insertion of mats or rods in the base of the mould, different designs and effects can be created.

Precast cladding offers a wide range of finishes. In off-the-form finishes, the mould detail is mirrored in the concrete surface and can produce, for example, board-marked finishes, ribbed finishes, or rope-patterned finishes. Exposed aggregate finishes, on the other hand, reflect the natural colour of the aggregate. Other finishes available include polished reconstructed stone finishes, or panels can be given a polished stone or ceramic tile finish.

Architectural concrete claddings generally fall into two categories:

• Claddings that purely serve as enclosures: in this application the claddings are applied as self-finished panels or as backing components to other façade materials, such as brickwork; or
• Claddings that form an integral part of the framework of a building and therefore perform an enclosing as well as a structural function.

Cladding panels can be manufactured in a variety of shapes and sizes but are mainly applied as storey-height or spandrel panels: the storey-height units span from floor to floor; and the spandrel version forms a sill wall beneath fenestration strips.

Storey-height panels can be manufactured with apertures for windows and doors later installed on site. Window frames can also be attached in the cladding manufacturer’s plant and applied paint finishes can be carried out before the panels are delivered to site.

Apart from its major cost benefits, precast cladding offers property owners long term durability and minimum maintenance without the need for frequent cleaning.

The inherent capabilities of concrete also help to make precast cladding a popular choice as a result of:
• Faster programme times – unaffected by weather or labour shortages;
• The early enclosure of the dry envelope enables follow-on trades to commence their work sooner;
• A high standard of workmanship in factory conditions and reduces the potential for accidents;
• Exceptional fire resistance;
• Superior acoustic quality that will help keep traffic and industrial noise from the office environment;
• Control of radiation and conduction of heat and prevents air leakage; and
• The use of concrete sandwich panels with incorporated thermal insulation in colder climates keeps the office environment warm.

Harsh coastal environments may threaten the lifespan of some cladding materials but decades-old concrete structures (such as the Port Elizabeth Post Office building, north of the CBD and virtually next to the corrosive influences of the ocean) bear testimony to concrete’s exceptional durability. The façade of the main administrative building and Council Chamber wing of the Johannesburg Civic Centre (a landmark near the top of Rissik Street) still looks attractive after more than four decades since it was originally clad. Large precast concrete panels, 8,7m high and 2,1m wide and weighing 6 800kg, were used as the walls of the Council Chamber.

The Concrete Institute recommends that input is obtained from an experienced precast specialist in the early decision-making stages of a cladding project.

(Information supplied by The Concrete Institute, www.theconcreteinstitute.org.za

Caption:

Much of the iconic Sydney Opera House is covered with ceramic tiles but underneath the tiles are about 2 000 precast concrete segments.