RULES OF THUMB | LOW CARBON CONCRETE
Figure 1:
Embodied CO2 contents of cementitious blends in kg per tonne of binder (CO2e value for EFC is based on Australian values provided by Wagner)
By tonne of binder CEM I CEM IIA CEM IIB-S
GGBS - kg/t CEM I - kg/t
1 tonne binder 0 860 200 1000 800 703.9
300 700
625.9 CEM IIIA
400 600
547.8
500 500
600 400
CEM IIIB
700 300
469.8 391.8 313.7
800 200
235.7
EFC geopolymer
GGBS/PFA + Activator
117.6
Figure 2:
MPA declared embodied CO2 contents (2020)
Mineral Products Association declared values 2020 – to factory gate Material CEM1 GGBS PFA
CO2
Limestone Aggregate Rebar
0.860 0.0796 0.0001 0.0080 0.0026 0.4120
e kg/kg
Over the past 12 years, the Australian construction
materials company, Wagners, has developed a geopolymer concrete it has named Earth Friendly Concrete (EFC) that can achieve characteristic compressive strengths up to C50/60 and has been used structurally in permanent works’ applications both in Australia and the UK. It comprises approximately 75% GGBS and 25% fly ash with alkali activators and special super-plasticising admixtures. Figure 1 shows the embodied carbon values of
cementitious blends per tonne of binder. The use of low-carbon concretes to reduce a project’s
embodied carbon should also be complemented by innovative design solutions that will probably involve the design assisted by testing and can reduce the thickness of elements and hence the volume of concrete in a structure. The replacement of conventional reinforcement with steel or synthetic fibres should also be adopted wherever possible.
TUNNELS AND EMBODIED CARBON In a tunnelling project, it is generally considered that 60% to 70% of embodied carbon is contained in the concrete linings of the shafts and tunnels. It is paramount, therefore that the tunnelling industry does its utmost to significantly reduce or eliminate its use of cement in all applications – segmental linings, in-situ linings, sprayed concrete and annulus grouts. In-situ shaft linings generally use high volumes of fly
ash or GGBS. Piles and diaphragm walls often contain high volumes of GGBS in order to achieve the required
resistance to any deleterious materials in the ground and groundwater, and also to reduce peak heat of hydration temperatures in thick sections. Similarly, high replacement levels are used in the casting of in-situ base slabs in shafts not only for exposure conditions but to reduce or eliminate the possibility of early-age thermal cracking in the slab. Replacement levels of up to 95% GGBS with only a 5% CEM I content are allowed in the latest version of EN206, but such mixes require careful design and construction planning due to slow setting and strength development characteristics. Therefore, the only option to further reduce the embodied carbon content of shaft linings and base slabs from their current levels might be to adopt the use of a geopolymer concrete. The five shafts of the Lea Tunnel project several years
ago were the deepest ever built in London. They were constructed by diaphragm walls using 70% GGBS content and then fully lined with a steel fibre-reinforced concrete containing 50% GGBS and 30kg/m³ of steel fibres that was slipformed. It is estimated that approximately 1,800 tonnes of reinforcement steel were eliminated from the shaft linings, reducing their embodied CO2 by more than 740t – achieved by innovative design and extensive pre- construction testing(2)
. However, the use of high volumes of replacement
materials is difficult in concrete used for precast segments and sprayed concrete, both of which require a very rapid strength development. Sprayed concrete (or shotcrete) use in shafts and tunnels has been rapidly increasing since the widespread
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