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WITH SELF FINISHES, THE PRECASTER HAS A LIBRARY OF SAMPLES

CONCRETE QUARTERLY/ SUMMER 06 - PRECAST CONCRETE / FINISHES

When it comes to concrete, the precast cladding industry is now able to produce virtually any finish and effect called for by architects. Essentially, precast cladding panels to BS 8297 have two generic types of surface fi nish that, combined with the facility to design robust 3D shapes and profiles, offer the widest choice of facade design and construction to the architect.

These generic types are 'self finishes', where the concrete surface is on view, and 'applied finishes', where other materials are applied to and supported by the precast concrete.

SELF FINISHES Self finishes exploit the ability of the precast manufacturer to design concrete mixes by selecting the cement (white or grey) and the aggregate with the option of adding a range of stable purposemade pigments. Exposed surfaces are generally textured to produce a range of effects, typically: acid etching/surface retarding; rubbing; grit blasting; bush hammering; and grinding and polishing.

False joints and other profiles may be formed by timber ribs in the mould. This gives a natural effect on acid-etched panels and improves weathering properties. Rubber mould liners can produce many textures and profiles. Techniques used are:

acid etching and surface retarding. Etching with dilute hydrochloric acid removes surface laitence to expose the sand and tips of the main aggregate in the mix, producing a stone-like texture. The depth of exposure can be varied to produce from a relatively fine texture to a deeper etch giving more pronounced exposure of the aggregate.

However, not all surfaces are suitable for such treatment.

Vertical cast surfaces may have blow holes, which need to be filled with a paste of cement and fine aggregate in the same proportions as the design mix of the main body of the panel. A final acid etch can then produce a consistent surface. Casting face down or vertically will produce subtle differences in the finished surface.

Coarser textured surfaces can be obtained by applying a retarder to the surface of the mould. After demoulding, the retarded surface is brushed and washed to expose the main aggregate.

Surfaces cast face up do not require a retarder; instead, the surface is sprayed with a fine water jet to expose the main aggregate;

rubbing. Surface laitance may be removed with a hand stone or orbital sander;

grit blasting. This uses air or water as the propellant and various grades of fineness/coarseness of grit, which will determine the depth of aggregate exposure. The effect is similar to acid etching but, being more vigorous, the depth of exposure of the aggregate is greater. In addition, the aggregates are partially abraded;

bush hammering. More vigorous than grit blasting, this involves roughening the panel surface using a mechanical pneumatic/ electric hand-held machine equipped with a variety of tools from needles to various chisel star-points. The finish produced is visibly more aggressive owing to the exposure and shattering of the surface of the main aggregate;

grinding and polishing. Although expensive and time-consuming, this can produce spectacular results. Grinding machines section the aggregate. Final polish varies according to the hardness of the aggregate, ranging from a honed effect to a high polish.

APPLIED FINISHES Applied finishes include bricks, brick slips, tiles, terracotta, slate, granite or limestone - preassembled in the mould before casting.

This can realistically replace on-site masonry-laying.

With bricks, a machine-made brick with three perforation holes is typically used. To produce a dovetail key, these bricks are cut longitudinally on the centre line and laid to a grid in the mould. Special bricks are cut or formed for different shapes and keys. Grout seal strips are inserted into the brick joints and the concrete is cast. Panels are finished by removing the strips and gun-pointing the joints with a colour-matched proprietary mortar.

Tiles and brick slips are treated similarly, cast face-down in a mould in a grid of timber strips or on a rubber mat with a grid.

Proprietary terracotta cladding tiles with extrusion holes are split to give very effective bonding grooves on the inside surface. The terracotta tiles are set in a gridded mould and, when cast with open joints, the precast panel acts as a rainscreen.

Typical stone facings used are granite, limestone and slate.

Hard stone such as granite is applied as a veneer at least 30mm thick, while other stones such as Portland limestone are applied no thinner than 50mm.

Stone panels are drilled in the back for bonding in 60mm stainless steel dowels. These are set at 60infinity to the stone face and alternate rows of dowels are reversed in angle. A flexible grommet is fitted to the dowel at the stone face and the rear face is treated with a de-bonding agent. These enable the stone to move relative to the backing concrete panel to accommodate thermal movement.

Stone-to-stone joints are a minimum of 5mm wide (as BS 8298:1994, Table 10) and are grout-sealed in the mould with waterproof tape to prevent penetration by the backing concrete.

SAMPLES AND MOCK-UPS With self finishes, the precaster will have a library of samples, which can be the starting point for developing a unique finish.

A panel at least 1m 2 should be cast to production standards, and its casting must replicate actual manufacturing conditions. For applied finishes, an approved sample of the stone, etc, must be permanently displayed in the factory.

On large contracts, a mock-up enables finishes and details to be viewed at full scale. The viewing distance of finished units should ideally be not less than 3m. On larger projects, and particularly for complex, tight-tolerance units, a full-scale premanufacture prototype should be considered. Generally, the architect should inspect regularly during manufacturing.

Varying conditions of lighting and weather, and time since casting, will cause different colour shading. Time must be allowed for the units to mature to a consistent appearance.

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