SHAPE, PATTERN, AND TEXTURE
Factory-produced precast concrete can be shaped straight or curved using either custom-made steel, concrete or timber moulds, or mould (or form) liners, which also create a range of patterns and textures.
Off-form concrete elements can be produced using grey or off-white cement. In this case, the Classes and tonal variations specified in AS 3610 Formwork for concrete, apply.
To enhance a grey off-form finish, shapes, patterns, textures, and colours can be incorporated into the off-form finish. Refer also to this Standard for specification guidance for coloured precast or precast with special applied finishes.
CUSTOM SHAPES AND 3D SURFACES
Elements can be custom-designed and manufactured with 3D surface finishes to create unique textures and profiles. This can be done using custom-made moulds, form-liners, tiles, bricks, stone, void-formers, or other techniques such as bush-hammered or hammered-nib applications.
Etching is a process that removes the fines from a concrete surface after manufacture. The result is a light, flat, sand-textured surface that resembles limestone or sandstone.
GRIT (OR SAND) BLASTING
Grit (or sand) blasting uses compressed air or water to propel particles such as granulated furnace slag, aluminium oxide, or glass beads onto a concrete surface. The depth of the grit blasting can be varied to produce a smooth texture, through to a coarse texture.
An exposed aggregate finish removes the top layer of the concrete matrix, revealing the aggregates in the concrete mix. Chemical retarders are often applied to the surface to etch away the concrete matrix, leaving the aggregate intact. Alternatively, a water washing technique may be used.
HONING AND POLISHING
Honing and polishing are surface treatments that can be combined as a two-step process to create a superior finish.
Honing involves the removal of the outer surface of concrete with a grinder, exposing the aggregate in the concrete and giving it a matt level surface. Polishing takes the finish to the next level using finer abrasive materials to take a finer cut from the concrete.
COLOURS: TRANSLUCENT, OPAQUE AND METALLICS
During sampling colours can be finessed by changing sands, cements, aggregates and pigments. Painting and staining are also options to achieve the perfect colour.
CEMENTS, SANDS AND AGGREGATES
Mechanical or chemical treatments of the precast surface can be used to expose the colour of sand and stone aggregates. Doing so can impart an overall colour to the concrete. Using coloured sands and off-white cement, lighter shades of brown, red, or yellow can be achieved. Darker grey tones can result from using dark sands and grey cement.
Where the desired colour cannot be achieved by the cement and aggregates alone, colour pigments can be incorporated into the concrete mix. These can be obtained through commercially blended hues.
Stains absorbed into the concrete surface result in a translucent or opaque finishes in flat or metallic colours and are often combined with the use of form liners to achieve a variety of aesthetic results.
Amazing Architecture (National Precast) and the Precast Concrete Handbook (National Precast) are both available for purchase at www.nationalprecast.com.au.■
Sarah Bachmann, National Precast CEO