First publishedin Aggregates Business Europe
Wolfsburg's Phaeno Centre science exhibit is one of a growing number of prestigious buildings in Europe being built in fair faced concrete
Concrete used to be seen as a functional construction material. Now the growing popularity of fair faced concrete is placing it at the forefront of architectural aesthetics. Claire Symes looks at the role of aggregates in this emerging market
Few can deny that the concrete used to create the sweeping and smooth lines of the Zaha Hadid-designed Phaeno Centre in Wolfsburg, Germany are dramatic. But it is not an experimental project - it is one of a growing number of high profile buildings in Europe that are using fair faced concrete to make an architectural statement.
Nonetheless, the simple aesthetics of these completed buildings belie the complexity and challenges involved in every stage of their construction. Successful use of fair faced concrete calls for careful aggregate and cement selection as well as tight quality control on site during placement.
"There is a growing trend towards fair faced concrete in Europe because of the modern and clean designs that can be achieved," said Doka product specialist Helmut Weissengruber. "Three dimensional imaging combined with new construction methods and the latest formwork solutions are helping to drive wider acceptance of the material.
"Fair faced concrete used to be the preserve of larger, prestigious buildings but we are beginning to see the material being more widely used in residential construction. The initial cost of fair faced concrete may be higher than conventional concrete but if the right controls are used, it offers much lower maintenance costs during the life of the building.
"The trend towards increased use of fair faced concrete is one that we expect to continue in Europe, but careful control at every stage will be essential to ensure the high quality finish that is key to fair faced concrete's current popularity." Officially, fair faced concrete is taken to mean concrete surfaces that fulfil the appearance requirements of DIN 18217 "Concrete surfaces and formwork surface". However, this standard neither mentions nor defines the term fair faced concrete, nor does it set out any precise rules or guidelines for it. According to the German cement industry association Bundesverband der Deutschen Zementindustrie
, the reason for this is that there are a number of influences that cannot be foreseen or controlled with absolute certainty in the course of manufacture and on site job execution.
Weissengruber added that many contractors in Europe do not understand the difference of working with fair faced concrete and conventional concrete and this knowledge can be make or break for the success of a project.
The smooth finish on fair faced concrete is achieved by using a higher fines content - up to 3 or 4% higher than is used in conventional concrete. This higher volume of fines carries the risk of bleeding (the loss of water) during transit or placement, which could result in the aggregate being exposed in the finished face.
Fines used in the mix need to be carefully selected from a source that can supply material throughout the project to ensure the concrete has a uniform colour. Most fair faced concrete projects call for a light finish, which is usually achieved using limestone dust.
Selecting a source that can supply the quantity needed is not the end of the story though - the material must be carefully analysed before use to check its iron content. If this is too high, the finished concrete could quickly end up with pink, orange or grey staining after exposure to the elements. Iron pryrites is also sometimes present in the limestone but staining from this can take two to three years to show.
One industry expert, who asked not to be named, said, "Washing can remove some of the iron content but it is hard to guarantee. Many aggregates companies refuse to supply limestone materials for fair faced concrete projects due to the tight specifications and the high risk of litigation.
"The sedimentary conditions under which these materials were laid down present a strong chance of contamination from other minerals. Igneous sources are much more uniform with less potential for contamination but such materials often do not offer the white finish that is so often desired." Finding a suitable source for fines local to the project is not always easy and the location of the project sometimes dictates what material is used. The coastal position of the Tate
Modern at St. Ives, UK meant that the risk of staining from iron contamination was too high and the project specification called for fines from an igneous source to be imported from Norway.
Reinforcement on the control tower at Santiago de Compestela in Spain was protected to reduce the risk of rust staining on the finished fair faced surface
Fair faced techniques now mean that concrete is now being used as an architectural feature in its own right rather than just a construction material