How to specify lower carbon concrete

Author: Jenny Burridge

Date published

6 August 2020

Price
Free
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How to specify lower carbon concrete

Date published

6 August 2020

Author

Jenny Burridge

Price

Free

Author

Jenny Burridge

Jenny Burridge of the Concrete Centre discusses a more sustainable approach to concrete.

Globally, concrete is the second most used material after water. In the UK we produce about 109Mt of ready mixed concrete and precast concrete products annually (2017 figures). As structural engineers we therefore specify a lot of concrete every year. 

More of us are now trying to lower our carbon footprint and produce lower carbon intensive projects. One of the ways we can do this is to look at how we specify the concrete we use.


Concrete mix and embodied CO2

Concrete is made from aggregate, cement, and water.  Admixtures can be (and normally are) included in the mix.  In terms of embodied carbon for the different elements, aggregates and water have very low embodied carbon. Locally sourced primary aggregates have an embodied carbon of about 4kgCO2/tonne. It is the cement, forming about 10-15% of the mix, which holds most of the embodied carbon.

All concretes to BS85001 are based on Portland cement, or CEM1, but mostly contain additions, or other cementitious materials. These include:

  • Ground granulated blast-furnace slag (GGBS)
  • Fly ash
  • Silica fume
  • Limestone powder
  • Pozzalana

These additions have a much lower embodied carbon than CEM1. Significant savings can be made to the embodied carbon of concrete by specifying the cements that include additions.

The table below gives an indication of the savings that can be achieved by specifying the different cements.


Ternary blends of cement

Since the latest version of BS8500 was published last year, ternary blends of cements have been allowed. These use CEM1 with two types of additions, normally limestone fines with either fly ash or GGBS.

All these cements are based on CEM1, but there are also geopolymers or alkali activated cementitious materials (AACMs) that can be specified using PAS 88202. This is a publicly available specification produced by BSI.  These are normally based on GGBS, activated by a chemical which is added to the mix.


Additions and strength gain

An important point to note is that the higher the proportions of additions within the concrete, the slower the strength gain of the concrete. This might not influence the programme if the concrete does not need to be struck quickly or support load shortly after being cast.

For example, foundations are frequently cast against the ground and the load is applied only slowly as the project progresses.  Although the standard concrete strength is specified at 28 days, a concrete made with IIIB cement will still be gaining strength at that stage. The strength at 56 days may be 40% greater than that gained by 28 days.

The designer could take advantage of this by specifying a 56 day strength. The design needs to be modified slightly to take account of this change, but there would be a definite saving in embodied CO2 to be gained.

Elements which need to have a faster strength gain, such as suspended slabs or post-tensioned elements, cannot use this benefit. However, they can still use additions and do not need to be restricted to using CEM1. There have been several projects which have used a CEMIIIB for a post-tensioned suspended slab. Concrete producers can add an accelerant admixture to the concrete to improve the setting time.


Reducing the cement required for concrete

The cementitious element of concrete is responsible for the majority of the embodied carbon in the concrete. This means it is worth concentrating on reducing the cement required for the concrete.  Higher strengths of concrete require a larger proportion of cement and therefore the embodied carbon in higher strength concrete is greater per cubic metre than a lower strength concrete.

However, where the use of higher strength concrete allows the volume of concrete to be reduced, studies have shown that the higher strength concrete solution can have a lower embodied carbon overall. The use of structurally efficient sections such as rib or voided slabs also reduces the volume of concrete and hence the embodied carbon.

Superplasticiser admixtures can also help reduce the embodied carbon by reducing the water/cement ratio. This provides a stronger concrete for the same quantity of cement.

Most of the larger concrete producers have low carbon proprietary concretes. These are formulated to keep the embodied carbon down to a given level. They are happy to provide advice on what can be achieved for the location and needs of the project.

Tips to lower the embodied carbon of concrete:

  • Reduce the use of CEM1 by allowing additions
  • Do not use small aggregate unless necessary, keep to 20mm maximum
  • Use the appropriate concrete strength
  • Think of specifying 56 day or 90 day strengths where appropriate
  • Allow the use of admixtures

Further reading


Specifying Sustainable Concrete, MPA The Concrete Centre.

 

References


1 British Standards Institution. BS8500-1:2015 + A2:2019: Concrete – Complementary British Standard to BS EN 206. BSI, 2019

2 British Standards Institution. PAS8820:2016: Construction materials. Alkali-activated cementitious material and concrete. Specification. BSI 2016

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