Concrete is a material that has suffered through indiscriminate use, however the 90’s have seen a renewed architectural interest in the material partly due to the Jubilee Line Extension. This appears to have sharpened the industries’ attention to the subject of finishes. Alongside this, the trade contractors have made significant technological progress in the areas of flooring, flat slab, framing and large span construction.
It still requires careful research to stand any chance of producing reasonable in situ finishes. Three significant issues, from a finishes point of view, are the wider availability of GGBS, a cementitious material that, when carefully used, can produce a lighter and warmer grey concrete; the availability of medium density overlay formwork materials that can help to produce a more uniform matt surface material and the more open-minded approach of the ready mix suppliers to using special aggregates selected for their effect on appearance and finishes. There are a number of basic principles for producing concrete where appearance is important and these are set out in a series of publications by BCA titled ‘Appearance Matters’. The Concrete Society provides a very useful advisory service that can be used during the design and construction phase.
Points to watch
Almost too numerous to mention!
It is essential to understand the difference between in situ and pre-cast concrete in terms of specification and detailing and the site inspection effort that will be necessary to achieve a good in situ finish. It is also important to understand the difference between pre-cast floor planks and other forms of pre-cast concrete in respect of finishes.
In situ concrete finishes (and to a lesser extent pre-cast concrete) are highly dependent on workmanship. It is essential to confirm the practicality of all proposals with competent trade contractors and/or the concrete advisory service. This exercise should be carried out at an early stage in the design.
It is beneficial to identify existing examples of concrete to form the basis of any discussion about finishes with a trade contractor.
It is important to use a specification method appropriate to the project. These are discussed in the Concrete Society Technical Report 52 titled ‘Plain Formed Concrete Finishes’.
There has been a considerable advance in concreting technology over the last 10 years and it is important to investigate this when faced with the steel vs. concrete argument. Currently, it is interesting to note that the multi-storey housing sector has moved strongly in favour of concrete.
The entire below ground structure is concrete forming a hierarchy of both structure and surface finish. The outer watertight enclosure is formed by diaphragm walls. They are horizontally braced by reinforced concrete open trusses supported vertically by the diaphragm walls and freestanding columns. Two freestanding pairs of blade wall are located between the trusses. The blade walls contain escape stairs and ventilation plants as well as assisting the base slab to resist uplift from water pressure. All the visible concrete within the station is based on a GGBS/OPC concrete mix. The blade walls incorporate Leemoor sand that provides a slight surface sheen. Further detailed information can be found in the Concrete Society Technical Report 52 titled ‘Plain Formed Concrete Finishes’ available from the Concrete Society.
An undulating concrete wall encloses the vent intake and extracts and the shaft entry/exit point. The concrete is a GGBS/OPC mix formed against sealed plywood boards orientated at 45° and in turn fixed to the main formwork support system. Further detailed information can be found in Concrete Society Technical Report 52 titled ‘Plain Formed Concrete Finishes’ available from the Concrete Society.
These three shafts utilise a concrete mix comprising GGBS/OPC and black basalt aggregate. The formed surfaces have been surface ground with diamond grinders to reveal the aggregate. The nature of the in situ material results in a random appearance of the aggregate which reveals interesting detail the closer the shaft is viewed.
A concrete blade wall and two in situ concrete floors and columns provide the main structure of the building. The concrete is a GGBS/OPC mix. The external blade wall has been treated with an anti-graffiti coating which has not affected the warm light grey appearance of the concrete.
© Ian Ritchie Architects (G.T.)