Get talking – Structural concrete in a zero-carbon future

Materials World magazine
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3 Jun 2020

The drive towards carbon neutrality can catalyse innovation in concrete production and application, says Paul Astle, Principal Structural Engineer at Ramboll UK.

The United Nations predicts that, globally, we will construct another 230bln m² of floor area in the next 40 years, double the current floor area in the world’s buildings. At the same time, we need to reach carbon neutrality. However, in many situations, there is currently no practical alternative to concrete. How will we reconcile these conflicting futures?

While concrete is a low-carbon material by weight, with only about 7% of the embodied carbon of steel, we use it in such vast quantities that our concrete carbon footprint is huge. A cubic metre of typical structural concrete has an embodied carbon of between 250kg-500kg carbon dioxide equivalent. Portland cement is responsible for the greatest proportion of this. In concrete that uses a pure Portland cement binder, it represents only 10-20% of the concrete by weight, yet accounts for up to 90% of the embodied carbon. 

Materially efficient

Reducing the amount of concrete we use is an obvious first step. Concrete is rarely used to its full potential – in many applications it is being used in a low-stress condition. A University of Cambridge, UK, report highlights that structural elements are typically only designed to use 60-80% of their capacity – this is after all safety factors have been applied. It could therefore be argued that a key reason for this degree of overdesign is due to uncertainty in the material’s performance. 

Concrete exhibits complex non-linear behaviour under laboratory conditions. When the additional uncertainties of batching consistency, aggregate variation, ambient temperature, as well as construction skill are added, it is perhaps understandable that designers exercise caution. One avenue that is being explored is better real-time understanding of concrete’s behaviour to improve quality assurance and reduce over-design.

There are also interesting opportunities that may allow a renaissance in stunning concrete structures. By sculpting concrete, we can cut out material and place as much of it as possible in a compressive state. While these ideas are not new, the design and construction challenges have limited their use. However, we are now at a point in time when we can tackle these challenges with computational power, advanced digital manufacturing and a drive for low-carbon structures. 

Researchers at ETH Zurich have been working on how to apply computational design to create some compression-only concrete elements, using innovative forming techniques that cut out unnecessary concrete. This can result in visually stunning, as well as materially efficient, structures. 

Adding to the mix

We must also minimise the amount of cement in concrete. Frequently, more cement is used than is needed to meet the requirements. In a study by Ramboll of over 90 concrete mixes used in UK projects, it has been found that the total quantity of cement (binder) varied from 300kg/m3 to 525kg/m³, even for the same specified strength. There can be technical reasons for an increased cement content, but the range measured is far in excess of the minimum code requirements. 

Direct carbon dioxide emissions from the calcination of limestone during Portland cement production are responsible for 50-60% of the emissions associated with cement. Even assuming we can substitute the remaining emissions with a carbon neutral source, it would still be necessary to use costly carbon capture and storage technology to offset these process emissions. 

Increased replacement of Portland cement with supplementary cementitious materials (SCM) allows us to reduce its carbon intensity. The two most common SCMs – ground granulated blast slag and fly ash – are co-products of steel blast furnaces and coal-fired powered stations, respectively. However, the life of these SCMs is limited by our need to also address the carbon intensity of those industries. 

Towards mid-century, these SCMs will be in short supply. It will be necessary to combine multiple SCMs in the future, most likely using calcined clays and ground limestone. Furthermore, it has been proposed that, by 2050, we will need to be recycling 20% of cement paste from demolition waste as new feedstock for cement kilns. Recycled paste would not release the same process emissions as limestone alone. 

Simple steps can now be taken to reduce unnecessary concrete, cement and carbon through a better understanding of behaviour, optimised design, carbon-focused specifications and an increased use of SCMs. Longer term, we need to develop and use new concrete technology with new mixes, designs and forms of construction. I believe the carbon constraint will not stifle, but in fact catalyse innovation, and we may find that the solutions are far better than their carbon-intense predecessors.