SUSTAINABILITY
UK manufactured reinforcement is made from 100% recycled scrap metal. 90% of a building's carbon footprint results from the energy used to heat, cool and light it. Concrete construction can significantly reduced this operational energy use and resulted CO2 emissions.The environmental, social and political climate is changing and society can no longer consider the environment as a free resource. In recent years we have identified the importance of the 'embodied' impacts of the materials used in our built environment, and have developed methods for quantifying them. A harder task now faces us in assessing the 'whole-life' impacts from the way we live our lives; the 'in-use' impacts of our built environment and use of the associated infrastructure.
The steel used in reinforced concrete utilises 100 percent recycled scrap steel as feedstock. At the end of its life, all reinforcing steel can be recovered, recycled and used again. The embodied energy values of reinforcing steel are based on the energy used to melt and reform it - unlike those for structural steel which are mostly converted in a very energy-intensive process from iron ore. The energy input per tonne of reinforced steel is less than half of that for structural steel.
Aggregates, including sand, gravel and crushed rock account for approximately 80 percent 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 percent 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 percent of concrete from their waste streams returning as aggregate.
A further sustainable benefit of concrete is its inherent fire resistance and long-term performance. It requires no additional fire protective coverings, chemical preservatives or paint systems that may release Volatile Organic Compounds (VOCs), effecting internal air quality, and which can require ongoing maintenance.
Cement is a constituent of concrete (approximately 10-15 percent by volume) and it is acknowledged by the industry that the energy used in cement production is a noticeable component of the environmental cost of concrete manufacture due to the high kiln temperature required to produce the cement. However, this must be viewed in context. The operational use of buildings accounts from some 50 percent of the UK's carbon dioxide emissions. Transport accounts for a further 30 percent. Cement manufacture accounts for less than 2 percent of the UK's CO2 emissions.
The cement industry has been making significant investments to reduce its energy consumption and CO2 emissions achieving, by the end of 2004, a reduction of 21.2 percent for energy consumption and 24 percent for CO2 emissions against a 1990 baseline. It is also actively recovering the energy from wastes by increasing the use of non-fossil fuels such as used tyres, waste solvents, refuse derived fuel (RDF), certain unrecyclable paper and plastics, sewage pellet, and meat and bone meal. Using these secondary fuels not only reduce the need for landfill sites or disposal by incineration but also helps preserve our finite reserves of fossil fuels.
Used tyres, for example, are an ideal secondary fuel as they have a calorific value similar to coal, contain steel which reduces the amount of iron oxide needed and lowers the industry's emissions of SOx and NOx. This process of 'industrial ecology' enables a single cement kiln to recover energy from 1 million used tyres every year.
The embodied energy of construction materials pales into insignificance when compared to the energy used by a building during its life. Around 90 percent of the total energy used in buildings is for heating, cooling and lighting, and it is here that concrete comes into its own. By utilising concrete's high thermal capacity, designers have the opportunity to significantly reduce the in-use energy of a building.
The excellent thermal capacity (often referred to as thermal mass) of concrete enables it to absorb, store and later radiate heat, stabilising the internal temperature of the 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 40C or 50C. 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 percent.
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 by up to 16 percent.
The UK cement and concrete industry is investing heavily in sustainability research and development for practical and sustainable manufacturing and construction solutions. The sector has done much to improve its sustainability performance and will continue to examine ways of making further improvements.
