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How to specify lower carbon concrete

Beyond the binder: understanding strength and structure is key to reducing concrete’s footprint, writes Jenny Burridge

In October 2020, the UK concrete and cement industry launched its Roadmap to Beyond Net Zero. The roadmap shows how the industry can continue its decarbonisation journey, with the aim of providing net-zero concrete by 2050. The industry has already taken considerable early action, and due to investment in fuel switching, changes in product formulation and energy efficiency, its direct and indirect emissions are 53% lower than 1990. The roadmap shows that the industry could become net negative by 2050, without offsetting, removing more carbon dioxide from the atmosphere than it emits each year.

Although a net-zero concrete is not yet available, significant amounts of carbon can be saved by looking at how we specify concrete and adopting the most appropriate solution for each project. This is not simply a case of selecting the most “low-carbon” mix, but understanding the properties of cement replacements and additions, factoring in considerations such as strength gain, and using materials as efficiently as possible. 

Cement replacements

If we consider the different constituents of concrete, around 85-90% of the mix is represented by aggregates and water. These have very low embodied carbon, with locally sourced primary aggregates responsible for about 4kgCO2/tonne. It is the cement, forming the remaining 10-15%, that leaves the biggest footprint. A critical means of reducing concrete’s embodied carbon is therefore to specify low-carbon mixes using cement replacements, or low-carbon cements.

All concretes to British Standard BS 8500 are based on Portland cement, or CEMI, but most contain secondary cementitious materials (SCMs) or additions, such as ground granulated blast-furnace slag (GGBS), fly ash, silica fume, limestone powder and pozzalana. These SCMs have a much lower embodied carbon than CEMI (see table 1).

Since the most recent version of BS 8500, ternary blends of cements have been allowed. Ternary refers to CEMI with two additions, normally limestone fines with either fly ash or GGBS. All of these cements are based on CEMI, but there are also geopolymers or alkali-activated cementitious materials (AACMs) that can be specified using PAS 8820, a publicly available specification produced by standards body BSI. These are normally based on GGBS, activated by a chemical that is added to the mix.

Image above: 
At 12,000m2, the Christie Proton Beam Center in Manchester is the largest treatment facility of its kind in the world. Its dense heavyweight structure, containing some 17,000m3 of concrete, is vital for shielding staff and visitors from high levels of radiation. The mix used by contractor Interserve contained 70% GGBS cement replacement, drastically reducing the embodied carbon of the structure and also minimising heat gains and thermal cracking during curing.

TABLE 1: EMBODIED CO2 OF UK CONCRETES*

Broad designation of cement type in concrete Percentage of addition Embodied CO2
kgCO2/m3 of concrete
CEMI 0% 283
IIA 6-20% 228-277
IIB 21-35% 186-236
IIIA 36-65% GGBS 120-198
IIIB 66-80% GGBS 82-123
IVB 36-65% fly ash or pozzalana 130-188

* Based on a cement content of 320kg/m3 of concrete

Strength gain

One of the things to note with the use of low-carbon concrete is that the higher the proportion of additions, the slower the strength gain. This might not influence the construction programme if the concrete does not need to be struck quickly or to 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 CEMIIIB cement will still be gaining strength at that stage and may be a further 40% stronger when it has gained full strength. The designer could take advantage of this by specifying a 56-day strength.

For foundations, cement replacement of up to 80% GGBS may be possible. Elements that need to have a faster strength gain, such as suspended or post-tensioned slabs, can still use additions and do not need to be restricted to using CEMI. There have been several projects that have used CEMIIIB for a post-tensioned suspended slab, and concrete producers can add an accelerant admixture to improve the setting time.

Material efficiency

Designers should also be aware that, even where a larger proportion of cement is needed for higher-strength concrete, in some instances this can actually reduce the embodied carbon of a structure, as a smaller volume of concrete is required overall. Alternatively, as the water-cement ratio is key to the strength of concrete, use of superplasticiser admixtures reduces the cement content in the same strength concrete by reducing the water.

Structurally efficient sections such as rib or voided slabs, or post-tensioned structures also use less concrete, while foundations can be made more efficient by avoiding standardised sizes across the site. If in doubt, the simplest way forward, at the moment, is to specify designated or designed concretes with a reduced range of cements. There is also a range of proprietary low-carbon concretes available. It is worth talking to your concrete supplier as early as possible to find out what can be achieved for the location and needs of the project.

Reducing a structure’s carbon footprint takes a lot more than simply specifying a material. There is currently no single structural material that can be considered “lowest carbon” across all projects. Instead, we must look closely at its constituents and performance, and its impact on the building as a whole, and specify the most appropriate solution in conjunction with factors such as ground conditions, building height, climatic conditions, floor loading, longevity and opportunities to dematerialize generally. And once material choices have been made, we must redouble our efforts to use them as efficiently as possible.

Images from top to bottom

  • Nithurst Farm in Kent by Adam Richards Architects also uses 50% GGBS. This both reduces the building’s embodied carbon and realises the architects’ desired pearl-grey finish.
  • At Nicholas Hare’s UCL Student Centre in London, all of the in-situ concrete and some of the precast elements use 50% GGBS cement replacement, as well as high levels of recycled aggregate.

REFERENCES AND FURTHER READING

1. Specifying Sustainable Concrete, MPA The Concrete Centre, 2020

2. UK Concrete and Cement Industry Roadmap to Beyond Net Zero, MPA UK Concrete, 2020

3. BS 8500-1:2015 + A2:2019: Concrete – Complementary British Standard to BS EN 206, Part 1: Method of specifying and guidance for the specifier. BSI, 2019

4. BS EN 1992-1-1:2004+A1 2014. Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings. BSI, 2015

5. PAS8820:2016: Construction materials. Alkali-activated cementitious material and concrete. Specification. BSI, 2016

Insights: Using 56-day concrete strengths

Tony Jones explains why specifiers should consider slower class cements

Concrete strength is usually specified at 28 days after casting. Striking of formwork normally occurs significantly before this as, in the temporary condition, the full strength is not required. All concretes will gain strength after 28 days but the amount depends on the type of cement used.

European Standard EN 1992-1-1 considers three different cement classes: R, N and S (see table 2). Concrete with Class R gains strength the quickest, Class S the slowest. However, concretes with slower class cements typically gain more strength overall –  is not accounted for if 28 days is used as the cut-off. Figure 1 compares three Class S concretes, specified to reach strength at different ages, with a Class R specified at 28 days. The Class S specified at 28 days is 15% stronger after 300 days than the Class R specified at the same time.

TABLE 2: CEMENT CLASSES

EN 1992-1-1 class Example cement type
R CEM I
N GGBS > 35% or fly ash > 20%
S GGBS > 65% or fly ash > 35%

Note: percentages relate to total cement content
Source: CIRIA 766. Control of cracking caused by restrained deformation in concrete. CIRIA, 2018

The Class S specified at 56 days is about 10% weaker than the Class R at 28 days but reaches the same strength at about 180 days, while the Class S specified at 90 days remains weaker than the Class R at 350 days. All concretes show some increase beyond the strength of the Class R at 28 days. Structural design codes normally rely on some increase to offset the fact that concrete strength is known to be lower under sustained loads. The default “recommended values” in EN 1992-1-1 imply an increase of between 13 and 18%.

Although no precise value is stated in the UK National Annex, historically the long-term strength gain of Class R concrete specified at 28 days has been shown to be adequate. Therefore if, at the time of loading, a concrete is stronger than a 28-day Class R equivalent of the same age, it will be acceptable. As figure 1 shows, the 56-day Class S was stronger than the Class R after about 180 days. Specifying concrete at 56 days provides an opportunity to reduce cement content and therefore embodied carbon.

Although savings will depend on the exact mix used, it is estimated to be up to 10kg/m3, based on the typical embodied carbon of cement – see Specifying Sustainable Concrete. British Standard BS 8500 permits the specification of concrete strength at 56 days, but it is important to discuss the mix with the concrete supplier to ensure that the aim of reducing cement content is met.

Class S concrete will not be appropriate where high early strengths are required. Its need to continue to cure beyond 28 days may be problematic on thin elements such as slabs subject to drying. However, Class S is normally well suited to use in foundations, retaining walls, larger columns and transfer slabs. In such instances, specifying strength at 56 days can be a very effective way of reducing embodied carbon.

Tony Jones is principal structural engineer at The Concrete Centre

Photos Brotherton Lock; Alan Williams Photography