a remarkable material
It is cheap, simple and everywhere. In fact, given its strength and durability, concrete offers an incredibly economic solution to many construction challenges. It’s also incredibly easy to produce. All that is needed is aggregate (sand and gravel), cement and water, and away you go. Globally, we produce around 40 billion tonnes of concrete products per year: enough concrete to wrap an eight-lane motorway around the world, fifty times, every year.
The United Nations has predicted that globally, we will construct another 230 billion m² of floor area in the next 40 years, double the current floor area in the world’s buildings. The vast majority of this will be in Africa and Asia, however there is still expected to be significant growth in Europe, with 25 billion m² to be added by 2060. In many situations there is currently no practical alternative to concrete, and it will have an important role in meeting the demand for new buildings and infrastructure.
Concrete is a low-carbon material, by weight. Per kilogram, concrete has only about 7% of the embodied carbon of steel. However, in terms of the material’s density, a cubic metre of concrete has an embodied carbon of between 250kg-500kg eCO2/m3, a similar quantity as that of a barrel of crude oil.
How then will we reconcile the need for new development with the need to reduce our emissions to net zero in just 30 years?
This comes down to finding a way to continue using this material without the emissions, requiring a focus on the constituents of concrete.
Cement is the magic ingredient. Without it, we would simply have a pile of wet stones. However, once the water has started to hydrate the cement, the reaction results in the cement binding the sands and gravel together into a strong and durable material. It will start to stiffen within hours, achieving most of its strength in days and continues to strengthen for months to come. Cement is the key, but also the problem.
Cement represents around 10-20% of concrete by weight, yet accounts for up to 90% of the embodied carbon. As a result, cement production is responsible for up to 8% of global carbon dioxide emissions. This is where we must focus our attention if we are to have any chance of reconciling using this material in the future with meeting our targets.
By utilising geometry, it is possible to adjust the form to keep more of the concrete in compression. Concrete structures, which necessitate complex formwork, have fallen out of favour in the last half century due to the labour costs and programme implications.
Reducing the amount of concrete we use is a relatively easy step, and one we should already be acting on. Concrete is rarely utilised to its full potential; in most applications it is being used in a low grade, low stress situation. Even in high-rise buildings, there will be areas of concrete that are either not stressed at all or are discounted in the design. Therefore, easy wins consist of only using concrete when necessary, cutting out over-design and optimising the use of structural elements.
A University of Cambridge report has highlighted that structural elements are typically only designed to utilise 80% of their capacity – this is after all safety factors have been applied. Beyond this, there are more interesting opportunities that would allow us to have a renaissance in stunning concrete structures, utilising concrete in its most efficient application – compression.
Concrete is strong in compression but poor in tension, with a tensile strength of around 10% of its compressive strength. To deal with this, we embed steel reinforcement. However, after cement, steel reinforcement adds the next largest component of embodied carbon, at around 10-20% – so we must be careful to avoid unnecessary reinforcement, too.
By utilising geometry, it is possible to adjust the form to keep more of the concrete in compression. Concrete structures, which necessitate complex formwork, have fallen out of favour in the last half century due to the labour costs and programme implications. 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 structures using innovative forming techniques to cut out unnecessary concrete. This can result in visually stunning as well as materially efficient structures.
Once we have learned to use concrete judiciously, we must also minimise the amount of cement in the concrete. Frequently, more cement is used than is needed to meet the requirements. This is due to conservatism, a desire for rapid strength gain, and a lack of focus on carbon. In a study by Ramboll of over 90 concrete mixes used in UK projects, it was found that the amount of cement in the concrete varied from 300 kg/m3 to 525 kg/m3, even for the same specified strength. There can be technical reasons for using more cement, but they do not justify this degree of variation.
The main type of cement used globally is Portland cement. It is created by heating limestone and clay to a high temperature to chemically change the structure and make it reactive with water. The heating of the cement is responsible for around 40% of the carbon in cement, 10% is associated with the electricity use of the processing equipment and the remaining 50% is related to carbon dioxide, which is directly emitted by the chemical process as the limestone is heated.
There are, however, alternatives to pure Portland cement. Portland cement can be blended with supplementary cementitious materials (SCM), typically waste products from other industries. This has already been done in some instances, but it could be done more frequently, as it presents an easy way for us to reduce the carbon in the cement. It is also possible to replace up to 80% of Portland cement with SCMs under current technical codes, though it may not always be appropriate to do so. The two most common SCMs are Ground Granulated Blast Slag and Fly Ash, produced by steel blast furnaces and coal fired powered stations respectively. The life of these SCMs is thus limited by our need to also address the carbon intensity of those industries.
Nevertheless, we have adequate supply currently and there is also work ongoing to access the many millions of tons of landfilled Fly Ash which has accumulated over the course of the industrial revolution.
Beyond these SCMs, there are novel cements using different processes in their manufacture and different reactions when acting as a binder. There are even cements that use carbon dioxide in the air as the reactant, locking in carbon as they cure. These nascent alternatives offer promising future opportunities; though there is much work to be done before they are commercially viable compared to traditional Portland cement.
There is also the potential for carbon capture and storage at the Portland cement producing factories, potentially allowing the carbon generated to be offset when Portland cement is produced, but this would require a significant development in technology and investment at existing facilities.
The challenge to eliminate carbon dioxide emissions associated with concrete in just 30 years is daunting, but it is achievable with collective action from policymakers, clients, contractors, engineers and industry. For the UK government to achieve its legally binding target of net zero by 2050, it is inevitable that embodied carbon will need to be regulated (in fact, a study into how embodied carbon could be regulated in the Building Regulations was commission by the Climate Change Commission last year).
In many cases construction businesses have even more ambitious targets than the government, seeking a zero-carbon position in late 2030s or 2040s. As such, their aspirations may well drive change faster than the regulations. Collaboration across the whole industry could see an increase in demand for low-carbon and carbon neutral concrete. We can tighten and focus our specifications around carbon, assess how construction issues can be overcome and develop new cements, additives and processes to cut carbon. I believe the carbon constraint will not stifle but will catalyse innovation, and we may find that the solutions are far better than their carbon intense predecessors.