Technical Showcase | Perfecting Finishes With Precast

A vast arrange of design options are available using architectural precast. Here, we look at the different finishes available, and the best practice steps to follow to ensure the highest quality and most cost-effective outcome.

Architectural precast is enjoying a welcome renaissance. Advances in mixes, more sophisticated moulding and a greater variety of finishing techniques mean that exposed precast concrete can offer many aesthetic possibilities – as well as its other acknowledged benefits such as thermal mass, fire protection, low maintenance. durability, quality, safety and speed of installation of site.

High profile recent projects which have embraced architectural precast – external and internal – include Cornish Concrete’s new Student Centre at University College London, Creagh’s University of Roehampton’s library, Techrete’s John Lewis in Leeds, while FP McCann has employed a wide range of finishes across its hotel, housing and education portfolio.

There is a huge range of possibilities for designers and constructors looking to use architectural precast, but key to success, as The Concrete Centre’s senior architect Elaine Toogood explains, is “starting the conversation with the precast specialist as early as possible”.

An important early consideration is the aesthetic quality of the surface. The expertise and knowledge of the manufacturer is essential to help navigate the potential combinations of aggregate type and sizes and surface finishing techniques and therefore the final aesthetic quality of the concrete. “Precast suppliers will have a back catalogue of mixes and picking one they’ve already used before will likely save time,” says Toogood. “A new ‘recipe’ will require new samples, and not all aggregates can be finished in the same way.”

The heritage-based approach to development in the UK has led to many architectural precast examples used as a cost-effective alternative to stone – hence it’s common name of ‘reconstituted stone’ or ‘recon’. Natural sands provide the colour, with a matrix of white cement. “The colour of the concrete can be changed through finishing techniques which take off the outer surface, revealing more or less of the aggregates,” explains Toogood.

Pigments can also be added depending on the requirement. “Natural, earthy colours are most commonly used,” says Toogood, “Greens and blues are also available but tend to be more expensive.” 

Moulds are an important factor in terms of cost. “Making the mould is potentially one of most expensive parts of the process, so precast firms have developed manufacturing techniques to keep costs down, often by optimising their reuse,” says Toogood.

“Since most architectural precast concrete receives a post finishing treatment to change the surface of the concrete, the form facing material for the mould has little impact on the final aesthetic and is left to the precaster to suit their preferred manufacturing technique.”

Adjustable steel moulds for stairs and flat panels can be reused hundreds of times over, simply adapted in the factory to suit project requirements. Alternatively, fully bespoke moulds can also be created, often for more complicated, three-dimensional shapes. 

“There are lots of ways to create some wonderful forms,” she adds, pointing to the Dover Esplanade scheme, where Thorp Precast  created the wave-themed ramps, stairs and retaining walls using a variety of techniques. 

“For well designed and constructed architectural precast, thought will have been given to where the joints between the elements are located,” notes Toogood. “With flat façades, it’s practically impossible to conceal them and so should be embraced as part of the architecture. Alternatively, the surface pattern can help. At Fish Island Village in Hackney, London, the architect worked a ‘Herringbone’ pattern into the design of Cornish Concrete’s precast sections, the unusual angle effectively hiding the angled joints. “Dusting the sealant in the joint can also help,” she adds.

Architectural precast is usually cast into moulds horizontally, and the underside is always the best surface because of gravity. “Remember that architectural precast is designed to be seen, so the design and onstruction planning should consider which surfaces will be visual in the completed building,” says Toogood. “The unformed face is usually the one that is hidden.”

Beyond conventional moulds, it is also possible to use formwork liners to create more varied textures and patterns. "The surface relief that can be achieved is extraordinary,” says Toogood. 

Acid etching is a common post finishing technique for architectural precast. Mechanical abrasion, such as grit or shot blasting is also used in post finishing, says Toogood.

“As well as changing the texture, or creating patterns, it should be remembered that these techniques also change the light reflection and colour of the precast unit,” notes Toogood. “The more you remove fine particles from the surface, the more you change the colour, and the deeper you go, more aggregates will be revealed.”

Polishing of precast units can involve automatic machines. A ‘bridge polisher’, for example runs systematically over the large surface face in the factory. Techrete provided architectural precast panels with both polished and acid-etched surfaces for the complex ‘diagrid’ façade at the John Lewis in Victoria Gate, Leeds.

“Polishing small areas, or window reveals, is likely to be done by a hand held machine,” explains Toogood. “This is why it is key to have an understanding of all the processes involved in achieving the final result.”

Matching architectural precast with in situ poured concrete on the same project is difficult, though not impossible, as Cornish Concrete’s UCL project demonstrates. “It should be avoided ideally, but if it’s absolutely necessary, match the precast sample with in situ, not the other way around,” says Toogood.