Visual concrete

Visual concrete is once again being appreciated and used expressively by architects. Partly this is due to the wish to use the thermal mass of concrete in buildings to reduce the heating and cooling loads. The latest version of the National Structural Concrete Specification (NSCS) from Construct, the concrete frame contractors’ trade association, now makes the specification of visual concrete much easier. The previous specifications found in BS 8110 have now been replaced by four different classes of formed finish: basic, ordinary, plain and special. Normally either ‘plain’ finish or ‘special’ finish should be used for concrete which is to be visible during the lifetime of the building.

A plain finish is for use where visual quality is of some importance such as areas occasionally seen or to be directly painted. Joints between formwork panels will show and the step may be up to 3mm. Tie-bolt holes should ideally be recessed and panels and bolt holes should be in a regular pattern. The colour might change with the concrete delivered and the re-use of the forming material. A project example should be produced as one of the first areas of concrete poured on the project and used as the benchmark for the rest of the concrete.

To achieve visual concrete, a special finish should be specified where the visual quality is of great importance. Sample panels should be specified using the formwork system, the concrete and the typical reinforcement to be used on the project for producing the particular finish. The size and complexity of the sample should be agreed to test the project detail and confirm that the execution can produce the finish on a repetitive basis.

Where a project example or a sample panel is specified, the viewing distance should also be agreed. The normal viewing distance is assumed as 3m. The appearance of concrete is affected by the concrete mix, the formwork and the workmanship on site.