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Building a Rainbow JOHN GRANT, PICS LTD. Publication: Concrete The days when concrete had to be uniformly grey are long past and now clients and architects can choose from a wide range of colours and tones - indeed, chemists are continuing to add to the available pallette.
Although as pedestrians we probably walk every day on coloured block paving or precast concrete flagstones, pigmented concrete is not solely the preserve of paving. It can be used just as effectively with textured form liners to produce textured concrete retaining walls for bridges and underpasses as well as for cladding for buildings. In California, the state highway authority has installed many miles of concrete sound reduction barriers along the major highways around Los Angeles. These were not precast units as would probably be used in the UK but were poured in-situ using a coloured mix and textured form liners within the shuttering. The result is a reinforced concrete structure that looks like natural stone walling - and at an acceptable cost. The use of pigment in concrete has moved on significantly from precast slabs and similar products. We now add colour to the surface and not necessarily throughout the concrete mix. This reduces the need for washing out the ready-mixed concrete suppliers' trucks and at the same time gives the contractor and designer a wider range of more vivid colours from which to choose. Pigmented colour surface hardeners or dry shakes, as they are often called - have been used to create coloured and highly wear-resistant floors for projects including Honda's production complex in Swindon, Toyota in Derbyshire and BNFL's Sellafield plant. The colour surface hardener is power-floated into the fresh concrete at a rate up to 7kgfm' to create a hardened monolithic surface with a permanent colour. This technique is particularly useful in landscape design, where architects can combine it with pattern imprinting to produce hundreds of different combinations and effects. Originally developed in New Zealand and Australia, pattern imprinting was then adopted extensively in the USA before entering the UK market in the 1980s. The system involves the use of a modified version of the industrial colour surface hardener together with special mould release agents and three dimensional texturing tools tu recreate both traditional and modern paving materials and effects. The system can reproduce cobblestone, slate, brick and even timber surfaces that are both aesthetically attractive and extremely practical in terms of both initial and maintenance costs, compared to the 'natural' products. When the technique arrived in the UK, it was initially marketed direct to homeowners but it rapidly spread into the commercial sector, panicularly for restaurants, hotels, garages, theme parks and local authority pedestrian schemes. However, pattern imprinted concrete should not be regarded as a replacement for natural materials but as a means of expanding the area of paving that can be produced within a budget hut using natural materials as key features. For example, Derek Lovejoy & Partners have recently specified pattern imprinted concrete on a prestige hotel development where the requirement for natural stone could not be met within budget in all areas. The practical and cost-effective alternative was therefore chosen. The textured and pattern imprinted concrete market grew dramatically throughout the late 1980s when most of today's successful contractors became aware of the system. Initially It was marketed through a number of franchise organisations attracting building contractors who could see a bright future for an innovative product. The original methodology for the system was somewhat basic and had design limitations. Concrete was placed and the colour surface hardener was hand-applied to the surface of the fresh concrete and trowelled to a uniform density. A flexible 25 micron polythene membrane was then laid across the surface of the coloured hardened concrete ensuring no air was trapped. Next, a series of interlocking aluminium imprinting moulds were placed on the polythene membrane and impressed into the face of the concrete leaving a cobblestone, brick or similar impression. Once the concrete had hardened sufficiently, the polythene was removed and several days later an acrylic sealer was applied to complete the process. This system offered a limited number of designs and lacked the 'natural look' of stone and other similar natural materials. Over the past nine years, the range of designs has expanded massively to more than 120. This has been made possible by the introduction of specialist polyurethane imprinting mats that introduce texture to the surface in varying depths. The polythene membrane has been replaced by a range of coloured powder mould release agents. A highlight or antique effect is created as the pigment dement of the release powder is absorbed into the surface of the colour hardened concrete as the imprinting mat is pressed into the surface. A whole new range of natural finishes are available including York stone, natural slate, crazy paving and timber decking.
Another highly personal monument is the concrete luggage sculpture produced by John King for the junction of Mount Street and Hope Street in Liverpool where acid stains were used to add colour. This technique is also gaining popularity with designers seeking to create highly specialised designs for shop floors, public houses and nightclubs. The method can also be used in conjunction with pattern imprinted concrete. From these few examples, it is clear that concrete has acquired a new lease of life in the paving field and seems set (no pun intended) to continue.
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The introduction of colour into concrete has breathed new life into what was for decades a functional building and paving medium. However, adding pigment to concrete is not new. Indeed, it was used to great effect during World War II to disguise jungle airstrips. Among early problems was that the colours tended to fade. However, modern pigments can be produced that are stable to ultraviolet light and ensure both uniformity of colour through the mix and colour fastness with time. 
Practical paving and construction are not the only preserves of coloured concrete as more artists become aware of its potential. A good example of this is the concrete roller 'Walk On LA' designed by Carl Cheng and constructed in 1988 at Venice Beach, California. Here, a textured design is used within coloured concrete to ensure an individual method of beach grooming.