Perfecting Finishes With Precast

Transportation and site assembly

Once the unit is finished in the factory, architectural precast has to consider the logistical and construction issues in transporting the units and assembly on site.

When it comes to transportation. “This starts with moving the units into the yard, to be loaded onto the trailer,” says Moses. “We make sure the exposed face doesn’t sit on the bearers, but transportation is trickier with columns that will be exposed on all four sides.

Finally, there is the construction. “We always worry about site installation with any architectural precast panels, damage causes by ourselves or – more typically – other site operations,” says Moses.

Form liners

A wide variety of textured finishes and patterns, plus ornamental and sculptural features, are possible with architectural precast, though the use of flexible formwork, form liners and other creative innovations.
Reckli designs and manufactures elastomeric polyurethane form liners and moulds for patterned concrete. It offers numerous different finishing effects, ranging from conventional wood and stone textures, bespoke ornamental features, and even 3D illustrations and photography.  The form liners are supplied to precast factories to create  the desired finish.

This technique was used to create the lace pattern in the fa├žade at Nottingham Contemporary Arts Centre, where  Trent Concrete supplied the architectural precast units. 

The sculpted ‘Parthenon’ frieze of the Olympics Village in east London, is another example of the possibilities of flexible form liners. Here, Techrete replicated sections using architectural precast cast in to form liners made using a digital scan of  the originals. 

An alternative technique is illustrated in the new Milford-on-Sea beach huts in Hampshire, where the precast wall panels feature patterns of pebbles. These shapes were created by a form liner from Finland’s Graphic Concrete, which has patented a technique for printing a pattern or image in the paper form liner using a surface retarder, resulting in a pattern – of different texture and colour – being left in the surface of the precast concrete when it is taken out of the mould.

“One advantage of architectural precast concrete is the opportunity for creativity, through a huge range of forms, textures and patterns, with the potential to be replicated again and again,” says Toogood.

Acid etching

Approaches to acid-etching vary across the BPAS membership, though there are three common methods: dipping the entire unit into an acid bath, manual application of the acid wash, or a pressure washer, the method favoured by Cornish Concrete.

“The precast unit comes out of the mould and is taken to the acid etching bay in our factory,” explains Cornish Concrete director David Moses. “We regulate the ratio of hydrochloric acid to water in the washer, depending on the level we want to etch.”

The etching typically varies in depth depending on what the client wants, at the UCL Student Centre a heavier etch was used to begin exposing some of the stone.

After the acid treatment, the precast section is then washed to remove all the acid, and then ‘made good’. 
“Most units have a few small blow holes when they come out of  the mould, but acid etching exposes a few more,” explains Moses. “So, we fill them, then lightly etch it again. For a large unit, this will take two to three hours in total. 

Colin Richards, quality assurance materials manager at FP McCann, says that selection of the mix materials is crucial any exposed aggregate or acid-etched finish.

“Aggregates are selected for their colour, size, shape and resistance to acid attack,” he says. “Where corner returns are required, you would not select a flaky or elongated aggregate as the exposed colour and texture of the aggregate on the return face will look different to the flat face of the unit.

“Aggregate size is also important. If a customer wants us to produce a flat, exposed aggregate finish, our mix design will use a 20mm coarse aggregate with a reduced sand content to allow the coarser aggregate particles to sit tightly on the bottom of the unit as cast.” 

Richards warns that aggregates should be selected for their hard-wearing properties for acid-etched units, which FP McCann has used recently on Motel One in Manchester, the University Locks scheme in Birmingham and the new Holiday Inn Express in Bridgewater.

“You would not use a material that is likely to be dissolved by  the acid,” he says. “The mix design is also crucial, because if there is not enough fine material in the mix, you can undercut the  coarser aggregate particles and they may fall out after removal of the retarder.

“For a deep acid etched finish, you only want to see the tips of  the coarser aggregate particles, and on a fine acid etched finish,  just the fine aggregate particles.”

At FP McCann, the etching process is carried out manually. “We take the unit into the acid washing area as soon as practical after de-moulding,” says Richards. “Then, we firstly spray the unit with water, this controls the depth of etching with the acid.

“We then systematically apply the acid to the unit with a watering can, washing with clean water between coats. This ensures a  nice even etch. After the final wash, the unit is removed to a finishing area. 

“A bespoke finishing material is then used on any surface defects such as blow holes and allowed to harden for a few days. The unit is then brought back into the acid bay for a final etch, which removes any excess material from the face. Finally, it is washed thoroughly again and then removed to a stacking area.”