A wonderful world of coloured concrete

Concrete, Nov/Dec 2003 by Hatfield, Margaret

The concept of changing the way ordinary concrete looks by adding mm colour or texture has been around for over 50 years. However, the UK has been reticent to embrace the concept. After years in the wilderness, it seems that decorative and pigmented concrete is only now gaining general acceptance in the UK construction industry. Concrete can be coloured and textured to successfully emulate various natural materials and used to great effect in architecture as an art form. As manufacturers of colouring products, Roy Hatfield Ltd has been involved in many innovative decorative concrete projects.

Alnwick Castle Gardens, Northumberland

THE first phase of rejuvenating the castle gardens began in April 2000. The project included the construction of a grand water cascade sitting centrally in the axis of the 250-year-old Grade I listed garden. The design incorporated a sandstone-clad structure where water flows over a system of 36 weirs and pools. Initially, a black epoxy coating was proposed for the inner concrete face. This would provide a durable, waterproof system and disguise the underlying concrete. Subsequently, the idea of casting a coloured waterproof concrete against a flexible formwork liner was adopted as a far more aesthetically pleasing option. After discussions, a master formulier was made by pouring liquid elastomer onto a horizontal 'wall' of the original sandstone blocks. This was used to produce numerous liners, each faithfully replicating the existing stonework (see Figures 1 and 2).

Roy Hatfield Ltd was able to provide a bespoke colouring admixture, blended to match the natural local sandstone. Over 7000 tonnes of concrete provided by Tarmac Topmix Ltd were used in the cascade construction and colour variation had to be kept to a strict minimum. Hatcrete Colourmix was despatched in water-soluble bags, pre-weighed at the factory, to eliminate over- or underdosing of the batches. Colour accuracy was achieved by careful weighing of ingredients down to 0.1Og. Sandstone, by its very nature, has colour variations and flaws, and this was emulated by the application of Hatcrete Tekstain to the finished walls. Acid stains are composed of metallic salts dissolved in a weak acid solution that reacts with the lime in concrete to produce insoluble, abrasionresistant colour deposits that will not chip, crack or peel. This permanent stain was brushed onto areas of the concrete wall, creating a variegated effect akin to that of the natural stone (see Figure 3). The results were so successful that it was only with the greatest of difficulty that it was possible to identify the real stone from the concrete. The project was nominated, and subsequently won, the Robert Stephenson Award by main contractors Sir Robert McAlpine Ltd.

Selfridges, The Bullring, Birmingham

THE controversial, futuristic design of the new Selfridges store in the Bullring, Birmingham, is constructed with soft, curving walls in shades of deep blue underlying myriad metallic discs (see Figure 4). Wishing to continue the blue theme onto external balconies and walkways, the designers contacted Roy Hatfield Ltd to discuss the concept. Various options were available, but due to the vivid colour the designers were trying to achieve, a coloured ccmentitious topping was ultimately specified. This would provide the abrasion resistance and strength required for the highly trafficked pedestrian walkways. A bespoke colour hardener was formulated in a rich blue colour (see Figure 5). Slip resistance was essential, and sub-contractor Jon English Developments Ltd, a recognised expert in decorative concrete, laid several panels in various finishes before starting the project. A lightly textured roller finish was eventually chosen. To reduce the risk of early-age plastic shrinkage due to rapid drying, a colour-matched liquid curing membrane was used. This also minimised the likelihood of efflorescence. In keeping with all decorative concrete, conventional curing methods (including hessian, sand, ponding and plastic sheeting) could not be used as they could affect the final concrete colour. This unique building, now open to the public, is a tribute to its designers and has transformed the area.

Concluding remarks

These three very different projects illustrate the versatility and beauty of concrete. Whilst performing a very functional duty, concrete can also be used as an art form in modern architecture by the application of specialist pigmentation and texturing techniques combined with a little imagination.

Margaret Hatfield, Roy Hatfield Ltd