Visual cast in situ concrete
Ready-mixed concrete is mixed, or manufactured in a batching plant, not far from the site, for direct delivery to site to be cast in situ. Site batched concrete is not typically recommended for visual concrete due to the challenges of adequate measurement and mixing.
The concrete should be specified using BS EN 13670: Execution of Concrete Structures. The two standards of formed finishes appropriate for exposed visual concrete are ‘Plain’ or ‘Special’ depending upon the quality required. 'Ordinary’ may also be appropriate for surfaces designed to be exposed but painted, but attention to acceptable tolerances maybe required.
The National Structural Concrete Specification (NSCS v4) is a sound basis specification for concrete, especially when including visual concrete. It incorporates standards and guidance in one document and usefully includes a detailed description for each of the standards stipulated by BS EN 13670 for visual in-situ concrete.
It is recommended that the concrete be specified as a designed concrete, which allows the designer to specify minimum cement content and maximum water/cement ratios. Prescribed concrete, where the exact composition and constituent materials are specified, may also be used, although the strength of the concrete cannot be specified using this method. Some concrete producers have their own proprietary concretes, some of which are suitable for visual concrete (e.g. self-compacting concrete). In this case the specification should give the name of the proprietary concrete and the options required if offered by the producer.
The colour of the concrete is determined by the colour of the smallest particle in the concrete, as this is what forms the surface. The smallest particle is normally the cement; therefore the type of cement will affect the colour. White cement can be used, but is imported to this country and therefore is more expensive. The blended cements containing either ground granulated blast-furnace slag (GGBS) or fly ash can be used to modify the colour of the concrete – GGBS will produce a lighter concrete and fly ash a darker concrete. As the colour of concrete varies around the country, it is necessary to view examples of visual concrete in the locality of your project.
To produce a good finish on concrete a reasonably high cementitious content is required, most experts suggesting that a minimum of 325 - 350kg/m3 is specified. A structural engineer could therefore use the additional strength of concrete (RC40/50 has a minimum cement content of 340kg/m3) in their structural design.
Coloured concrete can also be produced by adding a colouring agent to the mix.
The water content will also change the colour of the concrete; a wetter concrete mix tending to be darker with more potential for surface cracking and grout runs. The water cement ratio of the concrete should therefore be <0.5 with an S3 consistence using super plasticiser and controlled as closely as possible.
The following ‘recipe’ is recognised as suitable for achieving visual concrete and forms the basis of a ‘designed’ mix specification.
||not more than twice the cement content
||not more than six times the cement content
||20mm max. Size not more than 20% to pass through a 10mm sieve
||125 to 150 slump S3
|Water / Cement ratio
|| 0.5 or less
Care should be taken with the grading of the sands and coarse aggregate to avoid blemishes on the formed surface.