Is concrete or steel more environmentally friendly?

Although many commercially sponsored studies promote one type of structural material as more sustainable than another, the results in all. To create an environmentally friendly building, the whole life cycle environmental impacts must be understood. Life cycle analysis (LCA), which assesses the impacts of all phases of the life cycle, from "cradle to grave", is the best method to achieve this goal. In this article, LCA is used to quantify the energy use and environmental emissions during the construction phase of two typical office buildings, one steel-framed and one cast-in-place concrete, and then put into the perspective of the total lifetime of each building.

Structural concrete frame construction leads to higher energy use, CO2, CO, NO2, particulate matter, SO2 and hydrocarbon emissions due to the higher number of formworks used, higher transport impact due to a higher mass of materials and higher equipment use due to a longer installation process. In contrast, the construction of steel structures has higher emissions of volatile organic compounds (VOCs) and heavy metals (Cr, Ni, Mn) due to painting, torch cutting and welding of steel elements. The energy use and environmental emissions of the two buildings are comparable if the total impacts of material manufacture, construction, transport, use, maintenance and demolition are taken into account. The energy use and environmental emissions of office buildings can be reduced by careful selection of materials and built-in and temporary construction equipment.

Of all the metals used in construction, steel is one of the most environmentally friendly. It has a lower embodied carbon impact than concrete and generates less waste. Proponents of the three main building materials claim certain environmental merits. Steel can be infinitely recycled and reused without degrading.

Trees grow back and wood used in construction stores carbon dioxide. And new admixtures have the potential to drastically reduce concrete's staggering carbon footprint. Our blue and green world is getting greyer. By one estimate, we may have already passed the point where concrete exceeds the combined carbon mass of all the trees, shrubs and bushes on the planet.

In these terms, our built environment is outstripping the natural one. However, unlike the natural world, it does not actually grow. Instead, its main quality is to harden and then degrade, extremely slowly. For hundreds of years, mankind has been willing to accept this environmental inconvenience in exchange for the undoubted benefits of concrete.

Engineers claim that these 12-metre-high concrete walls will stop or at least slow future tsunamis, but locals have heard such promises before. It was a time of extraordinarily expensive bridges to sparsely inhabited regions, of multi-lane roads between tiny rural communities, of cementing the few remaining natural riverbanks and pouring ever-larger volumes of concrete into the dikes that were to protect Japan's coastline. A wise man once said not to build your house on sand, and while this advice has been widely ignored by many, it is something that the concrete industry, among others, is having to come to terms with. Environmental scientist Vaclav Smil estimates that replacing mud floors with concrete in the world's poorest housing could reduce parasitic diseases by almost 80 per cent.

Brazilian operators boast that the 12.3 million cubic metres of concrete would be enough to fill 210 Maracana stadiums. There has also been a drive to design concrete to be as efficient as possible, for example, the use of post-tensioning to reduce the volume of concrete and steel required for slab construction. Concrete points out that it is non-flammable and the industry is making progress in reducing its emissions and input footprint. Due to their different manufacturing processes, the use of wood products produces far fewer carbon emissions than steel or concrete.

If we look around us, we can say that the most commonly used building materials today are concrete and steel. In cities, concrete also contributes to the heat island effect by absorbing the sun's heat and trapping exhaust fumes from cars and air conditioners, but at least it is better than dark asphalt. Although most of the ingredients in concrete are manufactured products or mined materials, it is the cement in concrete that has the highest embodied energy (Environmental Life Cycle Inventory of Portland Cement Concrete, Portland Cement Association). Carl Obst, director of the Institute for the Development of Environmental Accounting and former head of the National Accounting Branch of the Australian Bureau of Statistics, is recognised as a world expert in the development of accounting methods that integrate natural capital and environmental services.

The impacts of wood during this phase are relatively low compared to concrete and steel, which are made from substances that must be extracted and heated to extremely high temperatures. Every major city has a scale model of urban development plans that must be constantly updated as small white plastic models are converted into mega-malls, housing complexes and concrete towers.

Chloe Robinson
Chloe Robinson

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