The 'Greening' of Concrete

The UK concrete industry is making determined efforts towards reducing the environmental impacts commonly associated with the material. Andrew Minson of 'The Concrete Centre' argues that from its constituent materials to its performance in use, concrete has a surprising number of green credentials.

Local transportation impacts

The raw materials for concrete are sourced from within the UK, unlike those for other construction materials which are often imported from thousands of miles away with all the resultant CO² transportation emissions. A key principle of sustainability is that a product should be consumed as near to the place of its production as possible in order to minimise the need for transport, support the local economy and prevent the export of raw materials from locations with potentially less stringent environmental and social protection legislation. The UK is highly self-sufficient in cement. Imports represented just 11.2% of the country’s total production in 2004. Aggregates are produced locally. On average, there is an off-site ready mix concrete plant within ten miles of every UK construction site. Consequently, the energy and CO² emissions associated with transportation are relatively low.

Cement manufacture

Since the beginning of the 1990s, the UK cement industry has reduced the energy used to make Portland cement by almost 25%. In turn, CO² emissions have been lowered by over three million tonnes, much of which is due to a significant investment in energy efficient technologies, along with the use of alternative fuels and the incorporation of materials such as slag, ash and limestone.

The cement industry is actively recovering the energy from wastes by increasing the use of non-fossil fuels such as waste solvents, refuse derived fuel (RDF), certain unrecyclable paper and plastics, sewage pellet, and meat and bone meal. Using these alternative fuels not only reduces the need for landfill sites or disposal by incineration but also helps preserve our finite reserves of fossil fuels.

Cements incorporating limestone, fly ash or slag

Cement combinations incorporating limestone, fly ash or ground granulated blast-furnace slag can be specified and, in some exposure conditions, may be more appropriate. Embodied impacts are also reduced, for example, cements incorporating 50% slag will reduce the embodied CO² of concrete by some 40% (compared to a CEM I cement). For guidance on specifying concrete with cement combinations download How to use BS 8500 with BS 8110 from www.concretecentre.com/publications (design aids

Recycled reinforcement

UK reinforcement utilises 100% recycled scrap steel sourced from the UK as feedstock. At the end of its life, all reinforcing steel can be recovered, recycled and used again. Contrary to trends in other manufacturing industries, the level of UK production of reinforcement is set to increase, with the opening of production facilities by CELSA and Thamesteel.

Recycled aggregate

Aggregates, including sand, gravel and crushed rock account for approximately 80% of a typical concrete mix. Here the concrete industry is actively pursuing a policy of recycling concrete in order to reduce the use of these natural resources. Concrete is 100% recyclable and concrete from a demolished building or infrastructure can be crushed and recycled as aggregate for new construction. Some construction companies are reporting a recycling rate of 70 to 90% of concrete from their waste streams returning as aggregate. There has been a 45% decrease in the production of primary aggregates between 1989 and 2001 and a 94% increase in the use of recycled and secondary aggregates. By 2011, 30% of all aggregates are expected to come from a non-primary source, equivalent to 70 million tonnes.

High thermal mass

The excellent thermal capacity or thermal mass of concrete enables it to absorb, store and later radiate heat, stabilising the internal temperature of a building. In all buildings, heat is generated by people, electrical equipment, computers, lighting and solar gain which means that buildings can overheat during the summer. Exposed concrete absorbs much of this heat, and can reduce daytime temperatures by up to 4°C or 5°C. Night-time ventilation is then used to cool the building, ready for the coming day. This approach can, in many cases, eliminate the need for air-conditioning entirely, or in buildings where it cannot be avoided it can reduce the associated CO² emissions by up to 50%. In winter, concrete’s thermal mass can be used to store heat which is released at night, thus sustaining warmer overnight temperatures and reducing the need for heating. Natural ventilation and good daylighting are features of many high thermal mass concrete buildings. Exposed concrete can reflect light far into a building's interior, helping reduce the need for artificial lighting. The high airflow rate provided by a natural ventilation system results in good air quality, and typically allows occupants control over their internal environment. These factors have been shown to improve productivity.