'GREEN' CONCRETE OPTIONS
by Per Sauer
More than any other material, concrete forms an integral part of building today’s urban environment and infrastructure; in fact it is often quoted as being the second most consumed product on Earth behind water. It has great qualities, and constant technical innovation means that concrete forms are only limited by one’s imagination and the laws of physics. The main ingredients of water, sand, aggregate and limestone (used to make cement) are cheap and widely available and herein lies the dilemma—because it is an extremely useful, versatile and cheap building product it is subsequently used in vast quantities and is responsible for 5 to 8 percent of global CO2 emissions alone. In acknowledging this, there is huge potential for the producers and users of cement and concrete products to play their part in the reduction of greenhouse gas emissions and the mitigation of climate change. With this in mind many concrete suppliers, have for some time, offered greener alternatives to normal OPC (Ordinary Portland Cement) based concrete.
Design and Embodied CO2
There are many ways to minimise the impact of using concrete. The first of course is to minimise the quantities used through efficient design. Good designers can reduce the volumes of concrete used without affecting the safety or serviceability of the structure. Although using higher-strength concrete gives the benefit of reduced volumes, bear in mind that cement accounts for around 90 percent of the embodied CO2 in concrete and higher strengths are typically achieved by using higher cement ratios, meaning the actual CO2 reduction may
be less than expected.
The minimisation of material weight has a knock-on effect, particularly in high-rise structures, with less material required at lower levels and in the foundations. Both high-strength and lightweight concrete can be used to minimise the weight of concrete in buildings and lightweight concrete is particularly useful when making additions to existing structures.
It is not easy to predict the end result of embodied carbon calculations and by the same logic lightweight steel structures can offer more (or less) of a benefit depending on the situation. The best way to minimise embodied CO2 is to compare design options. Collection of data on embodied CO2 for various construction types by designers can make this relatively quick and easy. For example a study carried out by Arup, commissioned by The Concrete Centre3, compared various standard concrete and steel structural framing solutions and found that they had very similar embodied CO2 levels; this also suggests that an advantage can be gained by using concrete with lower embodied CO2.
Although not widely accepted in multi- and high-rise construction it is interesting to note that timber performs significantly better than both by this measure and the recent construction of a 10-storey apartment building in Melbourne (Forte) using cross-laminated timber demonstrates that it is possible. For further information on embodied CO2 a good source is the ICE inventory (Inventory of Carbon and Energy) produced by Bath University in the UK.
A key aspect of efficient use is the longevity of the structure, linked strongly to its durability and flexibility of use. If the life of a structure can be extended and reused rather than be demolished then huge amounts of materials and emissions can be saved. This is often a severely neglected factor in the face of pressure to redevelop and maximise returns on land. Flexibility can be increased by considering column spacing, layouts, plan depths and ceiling-to-floor heights appropriate for different uses and designing in ways to allow the plot
ratio or height of the building to be extended in the future if desired.