The sustainability of concrete is determined by much more than just the CO₂ emissions of cement and with the advances in technology, the case for using concrete in green buildings is even stronger today than before.
Concrete is the second-most used commodity in the world, after water, and while the worldwide average consumption of concrete is about one ton per year per person, despite its extensive use, the global cement industry only accounts for about 5% of man-made carbon dioxide emissions.
“People often refer to the high CO₂ emissions of cement, but one must keep in mind that structures are not built out of cement; it is but one ingredient of the concrete used for construction,” states Bryan Perrie, managing director of the Concrete Institute.
“In addition, cement companies are continuously working to reduce their carbon footprints in the manufacturing of cement and concrete by using alternative fuels in cement kilns and reducing the clinker ratio in cement, using cement extenders like ground granulated blastfurnace slag (GGBS), fly-ash and limestone,” he says.
Johan van Wyk, a concrete technologist and general manager of the Southern Africa Readymix Association (SARMA), explains that extenders only start working once the cement has been hydrated, but with the advances in the development of admixtures, the time for activation is shortened, which means that more extenders can be added.
“On top of this, even more fly-ash and ggbs are added at readymix plants. And since fly-ash and ggbs are secondary products from the power generation and iron industries, these products are used rather than ending up in a landfill”, he says.
“Similarly with cement bags, you can nowadays use less cement to make more concrete thanks to extenders and admixtures. The strength of the concrete is determined by the cement-to-water ratio, so being able to add less of these, with stone making up the volume, concrete is becoming a more environmentally-friendly product,” Van Wyk adds.
A few years ago, the Cement and Concrete Institute commissioned a study to determine the values of CO₂ emissions for all the ingredients of concrete, including cement, ground granulated blastfurnace slag (GGBS), fly-ash (FA), aggregates, water and admixtures, and assessed the production of both ready-mixed and precast concrete.
While an accepted international ratio of CO₂ per ton of cement is 1 000kg, it was found that by using cement extenders, the overall CO₂ emissions can be reduced to averages ranging from about 500kg to 900kg.
“This is because the extenders, aggregates and water play a very small role in adding to carbon emissions,” Perrie explains. “So when you consider the reduced amount of cement in a cubic metre of concrete, the emissions per ton of concrete have come down drastically in recent years.”
According to the Concrete Institute’s publication, Sustainable Concrete, for aggregates, the average CO₂e per ton of cement is 5kg and if recycled concrete is used, it further reduces this value while at the same time reducing the depletion of natural resources and the dumping of concrete in landfills. In addition, the average value for admixtures is 220kg CO₂e per ton and for water 1kg CO₂e per ton.
Compared to 1972, today it takes 37% less energy to produce a ton of cement, enough to power 2,3-million homes a year. This is according to the Concrete Joint Sustainability Initiative, a coalition of concrete associations in America which campaigns for the responsible use of concrete in sustainable development.
A holistic view
Van Wyk points out that sustainability does not only refer to the CO₂ emissions of cement, but to all the environmental concerns that go with the management thereof for its full lifecycle. “If we think about sustainability, we have to think about people, jobs, good quality concrete, no rebuilds, no failures, responsible waste management and recycling,” he says.
Readymix plants, for example, must adhere to legislation regarding environmental issues, health and safety and also road transport. These include the use and reuse of water and the management and recycling of waste water, as well as dust management and the responsible handling of dump concrete. When using site mix, it is the project team’s responsibility to ensure that sustainable practices are followed.
“Quality, however, is not law,” Van Wyk states. “In the Eastern Cape last year, R600 million was paid for substandard construction and as part of foundations, substandard concrete can lead to walls cracking an ultimately collapsing.
“When something goes wrong with the concrete in a building, it endangers many people at once.” He refers to a building that collapsed in Meyersdal last year, killing seven people, and a wall that collapsed at a Durban Hotel last month, where two men were seriously injured.
“Anyone can make concrete and therefore the engineers or consultants working on a project must plan to test the standard of the concrete and ensure that these tests are done at a laboratory accredited for the tests. People who try to save costs, compromise on the quality and also health and safety concerns,” he advises.
Local vs imported cement
Van Wyk explains that imported cement, for example, results in a loss of local jobs and may impact negatively on the quality of concrete. Usually it is offered at a cheaper price when there is an oversupply in countries such as Pakistan, India and sometimes China.
“While the cement might have been manufactured in exactly the same way as local cement and may even carry the SABS stamp, one has to stay cognisant of what happened to the cement since. Just as fruit can rot, cement is also a perishable product with a shelf life. And cement that is transported by ship is exposed to high humidity, both while at sea and at the harbour. I’ve seen contractors sifting cement on site to get lumps out of what is clearly not suitable cement anymore,” he says.
According to Van Wyk, there is a possibility of a tax being added to imported cement, which will level the playing field a bit more.
When a building fails, the investigation aims to determine whether the material has failed or whether the testing was incorrect. “For a structure of 200 000m³ of concrete, all decisions are made on a one litre, 100mm x 100mm test cube, therefore it is imperative to make and test the cube correctly and also to ensure that the test methods being used are not out of date,” says Van Wyk.
“When engineers specify concrete, they have to make sure they know exactly what they require. It is not enough to only specify concrete strength and then you get something cheap that, although it adheres to the specification, is difficult to work with. Specification gives responsibility to the supplier, contractor and engineer. When a dispute ends in court, it is asked what was specified and how it was contravened,” he explains.
“We in the built environment must take a stand against delivering bad quality to the end-user and we have to start with good materials,” he states.
Concrete makes sense in green building
Perrie points out that apart from the many advances in the manufacturing process, concrete has an excellent ecological profile compared to some other construction materials. “Properties such as exceptional durability, thermal mass and recyclability add to the sustainability of concrete use in green buildings,” he says.
“Architects need to explore the inherent advantages of concrete and apply many different strategies to ensure the sustainability of a structure in terms of its environmental and social impact, to minimise the use of energy, to minimise the use of water, and the generation of waste. All of this can only be assessed by carrying out a full lifecycle assessment of the structure,” Perrie concludes.
Full thanks and acknowledgement are given to the Concrete Institute and SARMA for the information given to write this article.
The many benefits of concrete:
– Locally produced.
– Labour-intensive, which leads to job creation.
– Design flexibility.
– Variety of finishes and new innovations.
– Extreme durability.
– Cost-effective because of durability and low maintenance.
– Structural integrity.
– Thermal mass – critical for passive solar design of buildings.
– Fire resistance.
– Recyclability and reusability.