Carbon footprint of concrete buildings seen in life cycle perspective

Article by Claus V. Nielsen and Mette Glavind, Danish Technological Institute
Featured at fib 2009 Symposium, June 2009, London, UK

Abstract

The Danish Technological Institute has worked with concrete and its role for the environment for the past 15 years. We are involved in international networks spreading the knowledge on how to design and produce green concrete structures that are sustainable. We hope this type of dissemination may help material manufacturers, designers and building owners to apply the knowledge with benefit to the environment.

Concrete may seem to have a negative environmental impact in the eyes of many decision makers. This is due to the fact that concrete and ordinary Portland cement is closely linked. However, the environmental benefits from using well-designed concrete properly are numerous. For many applications concrete is the only plausible choice for reasons of durability and strength. It is the scope of this article to present and highlight some of these environmental benefits in an understandable manner based on a sound scientific foundation. Furthermore, the importance of including all phases of the life cycle is demonstrated through a basic calculated example

The primary environmental indicator for concrete is the CO2 footprint. However, even though concrete is known to have a relatively high CO2 emission during production it is of paramount importance to include the service life of buildings as well as the secondary life after demolition and recycling. A slight difference in the energy performance of a building design may tip the balance from an environmentally friendly design to the direct opposite. These issues are demonstrated via the CO2 footprint as environmental indicator. Through a simple example the methodology of calculating the CO2 footprint of concrete is demonstrated.

The paper illustrates the benefits of heavy building materials over light weight materials. The higher thermal mass of concrete may be used to improve the energy performance of buildings. This means improved thermal comfort as well as improved energy performance during the many decades of service life. Typically a heavy weight material building design will suffer from a higher initial CO2 footprint than light weight materials. However, if the thermal mass is utilised to improve the building energy performance in terms of reduced energy need for heating and cooling this initial difference will slowly diminish during service life. After say 20 to 30 years the balance reverses and the accumulated CO2 footprint of the heavy weight building becomes the smaller of the two.

Another obvious environmental benefit from concrete is its ability to recy-cle back into construction after demolition. In that manner natural re-sources are preserved and land filling is avoided. After end of service life concrete is typically crushed into smaller fractions, which again may be utilised as back fill material in road constructions or as recycled aggregates in new concrete. When concrete is crushed its specific surface area is increased significantly and being exposed to atmospheric air means that the concrete rubble absorbs CO2 during the carbonation process. This process actually improves the initial carbon footprint of concrete over its full life-cycle even though the energy required for the demolition, sorting and crushing is included in the overall CO2 calculations. The article presents data to support this statement.

Keywords: greeen concrete, CO2 footprint, carbonation, recycling, thermal mass