Pattern-imprinted concrete–a unique aesthetic medium

Pattern-imprinted concrete–a unique aesthetic medium

Christie, Iain

lain Christie,

Roy Hatfield Ltd, outlines the manufacturing process and provides examples of the creative use of pattern-imprinted concrete

attern-imprinted concrete, also known as stamped concrete, is a method of transforming freshly placed concrete to replicate a diverse range of textures and patterns. The multiplicity of finishes that can be imparted to the concrete surface is limited only by the imagination of the designer and the skill of the contractor. The concept has been used to add beauty or create unusual effects on projects of all sizes all over the world, from theme parks to domestic drives.

The process normally involves the use of a dry-shake colour hardener, essentially a mini concrete mix in a dry powder form. The powder is broadcast on to the surface of freshly placed, screeded and floated concrete, at a standard rate of between 3 and 5 kg/ml. It draws water from the concrete below and, once floated into the surface, can be trowelled to a tight finish in a similar way to conventional concrete. The best colour hardeners provide three main benefits for the pattern-imprinter: a pseudo-plastic surface that reproduces the imprint pattern with the minimum number of stress cracks, a high abrasion resistance, and a uniform, streak-free colour. Once cured, quality colour hardeners become part of the concrete slab, resulting in a virtually permanent surface.

Once the concrete has been trowelled and has dried sufficiently to take the imprint, a release powder or polythene sheet may be applied to the surface. The release agent is a bond breaker, and is used in conjunction with imprinting mats to prevent them sticking to the concrete surface. Powder releases are usually coloured and may be used to impart a variegated colour. Polythene sheeting is normally used with open cutter tools. On most pattern-imprinting work, the pattern is achieved by using open cutting tools, imprinting mats or, occasionally, freehand with special tools. The pattern is achieved by pressing the imprinting tools into the concrete surface.

Open cutting tools – sometimes referred to as `cookie cutters’ – were the first to be used. They are generally metal castings and, as the name suggests, cut an open pattern in the surface. The detail is limited to the outline of the cut so the pattern tends to be restricted to cobbles, tiles or stones. To achieve a more realistic rounded cobble or stone effect, the process is sometimes carried out through a polythene sheet. More recently developed imprinting equipment consists of polymer mats, usually polyurethane. These are cast directly onto the texture to be replicated, or into purpose-made moulds. The mats fit together jigsaw fashion and can impart much more detail because the whole surface of the mat comes into contact with the concrete.

Better-quality mats fit together in multiple combinations, allowing greater distances before the pattern is repeated. There are normally several mats in a set, and a release agent must be used to prevent them adhering to the surface.

After the pattern has been applied, the concrete is allowed to begin curing. After two or three days, when the surface has developed enough strength, it is washed off to remove any excess release agent and allowed to dry. An acrylic sealer is then applied to enhance the colour and protect the surface from staining.

A good example of patternimprinted used to replicate traditional materials is the Supertram in Sheffield. Here, the Sheffield City Planners and Supertram Project Managers were anxious to reproduce the look of ironstone cobble Betts using modern, economical construction techniques, such as slip-forming. The cobble pattern was reproduced using Victoria Cobble imprinting mats with stone buff colour hardener and walnut flashings. For a more natural appearance, walnut and dark red release agents were used. As a precaution, the concrete was integrally coloured with stone buff to prevent the underlying grey concrete being revealed if damaged by snow ploughs.

A further example of how patternimprinted concrete may enhance the environment is the SGL car showroom (Figure 1). The original design included block paving, but the owner wanted something a little different. Designers also needed to allow for designated car parking spaces. Although the budget was restricted, the contractor was able to produce a unique appearance for the premises. Designated spaces for cars were defined by different coloured soldier courses and a blend of expertly emulated York stone and cobble contrasts. The effect complements the modern structure well. The imprinting mats were Yorkstone and Victoria Cobble, colour hardeners were a combination of sand beige, terracotta, graphite grey and walnut and the release agent was walnut.

Pattern-imprinted concrete can be used to develop a corporate image. By applying the same theme to any retail or fast food outlet, a consistent, easily recognisable image may be achieved. McDonalds needed to designate areas of their car parks as `drive-through’ facilities for customers. These had to be clearly defined, as they sometimes constituted part of the parking area.

To comply with local planning consent, patterns and colours were selected to reflect locally available materials. Many McDonalds Restaurants have been constructed in this fashion. In the illustrations, one has adopted terracotta colour hardener, pattern-imprinted with Old English Brick (running bond) mats. For the second, flint colour hardener was used, pattern-imprinted with Ashlar Slate mats. The release agent in each case was Graphite Grey.

In some sectors, pattern-imprinted concrete has had a somewhat chequered history. Despite this, with the constant development of new techniques and finishes, its use is becoming widespread. Many specialist contractors have had to turn into artists to satisfy the demands of some designers, and the material manufacturers are being asked to produce a more diverse range of colours to recognised quality standards. Provided that specifiers, contractors and manufacturers all work together to maintain the highest levels of quality, then pattern-imprinted concrete may come to be one of the most widely used paving techniques. I

Copyright The Concrete Society Nov/Dec 2000

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