Ecodesign in Practice is a series of sustainable construction design solutions that can be applied at scale. The solutions are also integrated as data sources in One Click LCA for Buildings and Carbon Designer 3D. We publish an article and associated solutions once every six weeks. Enjoy reading!
Low carbon concrete solutions
A quick guide for design teams and contractors |
A checklist for reducing embodied carbon when building with concrete
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Why is concrete so carbon intensive?
Figure 1: The typical composition of traditional concrete |
Figure 2: The cement manufacturing process: Cylinder lime-burning kiln |
What is low-carbon concrete?
There is no globally agreed definition for low-carbon concrete. What is commonly perceived as a low carbon concrete in the construction industry, is a concrete mix that results in lower embodied carbon compared to an average concrete mix.
However, there is no agreed benchmark in most regions and no agreed percentage reduction of embodied carbon in order for the concrete mix to be classified as low-carbon. Local attempts at doing this include the Concrete Sustainability Council (Germany), Lavkarbonbetong standard (Norway) and the UK’s Low Carbon Concrete Code.
This results in the term being used in any case where concrete has lower embodied carbon than a typical Portland cement mix, even if the reduction is minimal and at times even nominal. In most cases, the reduction of embodied carbon will be the result of cement substitution with more traditional alternative binders like Fly Ash, Ground Granulated Blast Furnace Slag (GGBS), calcined clays and in some limited cases natural pozzolans, and with innovative new solutions. This list is not an exhaustive list, many other solutions exist as well.
The problem with most of the low-carbon concrete made today is that clinker (the key ingredient of cement) is substituted with secondary materials from fossil fuel-based processes. While this is a useful transition mechanism, it cannot scale as we decarbonize power generation and other industries.
Reducing cement clinker will reduce concrete’s embodied carbon
Concrete is used in foundations, slabs and the structural frame of most buildings across the world. With the majority of concrete’s embodied carbon coming from the production of cement the key to reducing concrete’s embodied carbon is to reduce the total amount of cement used. This must be done via:
- Avoid overdesign of structural elements. This will reduce the amount of concrete and cement being used.
- Rationalization of live loads consideration during design. Live loads are often overestimated. Load overestimation can be avoided by detailed structural design without affecting any future adaptability of the building.
- Design for material efficiency by optimizing the span of the structural grid and incorporating more material efficient concrete elements like hollow core slabs, and composite decks.
- Avoid over specifying concrete’s compressive strength. Concrete mixes can be optimized for specific parts of the building; there’s no need to use standard strength throughout.
- Avoid over specifying concrete’s compressive strength in the early days after pouring (7, 14 and 28 days). This will allow the use of concrete with alternative binders that in most cases have longer curing times.
- The use of alternative binders like Fly-Ash, GGBS, calcined clays etc.
- The use of other innovative concrete solutions, where present, that are not necessarily related to cement reduction.
A key benefit of optimizing concrete and cement clinker quantity is that it saves capital costs.
Alternative binders to cement
The most commonly used alternative binders to cement are:
- Fly Ash / Pulverized Fuel Ash
- Ground Granulated Blast Furnace Slag
- Calcined Clays
Although some are widely used by the construction industry, their availability depends on fossil fuel related processes and is expected to be limited in the future when it is hoped that fossil fuel use will decrease. As a result, the use of such binders is important today but cannot be a significant part of concrete’s zero carbon future. Although the binders listed here will reduce concrete’s embodied carbon, they are often viewed with skepticism by contractors since they affect the curing rate of concrete. This means that concrete mixes with such binders will need more time to reach a set level of strength compared to traditional Portland cement mixes. A concrete mix with 50% GGBS for example will have half the strength of a Portland cement mix in the 7 days after pouring but should reach the same strength within 28 days. The slower curing time can hold construction back by a few days for each building floor added which can result in bigger delays and cost implications in taller building constructions. Contractors will have to plan for the longer curing time of low carbon concrete where possible to mitigate the delays and associated cost implications.
Other binders
Other solutions
Carbon capture technologies
Carbon Dioxide injection
Biotechnology
Plant based solutions
How could concrete become more circular?
The role of the specifier in low carbon concrete
The role of contractors in low carbon concrete
Comparing concrete mixes and how to model low carbon concrete in One Click LCA
- Industry average EPDs from manufacturer associations (e.g. NRMCA)
- One Click LCA generic datapoints for Portland cement mixes and concrete grades ranging from C12/15 (1700/2200 PSI) to C60/75(8700/10900 PSI)
- One Click LCA generic datapoints for concrete grades as above with cement substituted with PFA at a range from 10% to 50%.
- One Click LCA generic datapoints for concrete grades as above with cement substituted with GGBS at a range from 10% to 75%.
- Portland cement
- Ground Granulated Blast Furnace Slag
- Pulverised Fly Ash
- Silica Fume
- Metakaolin
- Standardised cement types like CEM I, CEM II, CEM III and CEM IV
- Virgin and recycled concrete aggregates at various densities
Find out more about how to create private datasets and private constructions with One Click LCA via these helpdesk articles.
How to compare different concrete mixes
A quick and simple way to compare different concrete mixes using One Click LCA, is to add the datapoints of interest to the “compare data” feature. This will automatically generate a graph showing the carbon emissions per life cycle module for each of the compared mixes for the required functional unit. The feature is available for both generic datapoints and manufacturer specific EPDs and can be used for any material type.
Find out more about how to use the Advanced Material Comparision feature.
Sustainable concrete reinforcement
Concrete, especially in buildings, will typically be used together with steel reinforcement. Reinforcement is necessary in order to allow concrete elements to carry tensile forces which otherwise would not be possible due to concrete’s low tensile strength. Depending on the region, the reinforcement rate and the recycled content of the steel reinforcement bars, reinforcement can be responsible for up to 50% of the reinforced concrete’s embodied carbon. (100% Portland cement mix with 60% recycled rebars and 200kg/m3 reinforcement rate). To reduce the impact of reinforcement in concrete, there are various alternative reinforcement types that can be used. None of them can be as versatile as the traditional steel reinforcement bars, however in many cases, using one of the following alternative reinforcement types could result in significant embodied carbon savings. The alternative reinforcements can be categorised into organic fibres like hemp fibres, steel fibres which are used widely in various applications like industrial floors, and other mineral fibres or bars made for example from glass and basalt. The following generic datapoints for alternative reinforcement can be found in the One Click LCA database in addition to the various EPDs developed by manufacturers.
- Bi-component polyester fibre, 100% recycled content
- Hemp fibres, straw and shives
- Basalt fibres
- Steel fibre for concrete reinforcement, 0% and 100% recycled content
- Glass fibre for concrete reinforcement
- Polypropylene fibre for concrete reinforcement, ranging 0% – 100% recycled content
- Flax fibre
- Jute fibre
- Kenaf fibre
- Basalt rebar for concrete reinforcement
How to find a lower carbon concrete mix in your region
When the time comes to specify the exact concrete manufacturer from where concrete will be procured – usually at detailed design stage or construction stage – how do you find lower impact concrete mixes and manufacturers? One Click LCA’s green material benchmark feature enables you to identify all plant specific mixes that have lower embodied carbon than the one you have already selected. The feature is available either directly from the material query through a material’s data card or via the results page where you can ask One Click LCA to give you a list of more sustainable alternatives to your currently chosen materials.
Find out more about green material benchmarks.
Who can supply low carbon concrete solutions?
Europe | North America | Middle East and North Africa |
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What is the future of low carbon concrete?
Low or lower carbon concrete is currently made available mostly by replacing cement with alternative binders like GGBS, PFA and silica fume. With the construction industry urgently needing to decarbonise as soon as possible, the demand for such alternative binders is continuously increasing and will increase more in the future. At the same time, the supply of some binders like Fly Ash which is a by-product of coal combustion is set to decline due to the reduced demand of the primary product (e.g. coal based electricity).
Concrete must be made available at even lower embodied carbon than typically achieved now with binders like GGBS and PFA and it must be produced with new innovative manufacturing processes and binders.
In its UK concrete and cement industry roadmap to beyond net zero, the UK Concrete Centre has identified a potential to reduce the embodied carbon of concrete by 39% by 2050 due to the decarbonisation of the electricity grid and transportation, lower carbon production of cement and other binders and the switch of the main fuel used in cement production to a renewable fuel. The remaining 61% reduction required for concrete to become a zero carbon material must come via carbon capture technologies which will address mostly the calcination related emissions.
Heidelberg Materials is already building the first Carbon Capture and Storage facility in a cement production plant in Norway and is planning to do the same in another one in Sweden by 2030. Other companies like CarbonCure and Carbonaide as mentioned above reduce the embodied carbon of concrete by using CO2 in the concrete mix itself while companies like Prometheus Materials and Biozeroc are looking into using biotechnology to eliminate the use of cement in concrete to allow reaching a zero or negative carbon building material similar to concrete.
Affordable EPDs with One Click LCA Concrete EPD Generator
Learn more about the One Click LCA Concrete EPD generator or watch the recording of our recent webinar on how to create fast concrete EPDs.