LOW CARBON CONCRETE | RULES OF THUMB


introduction of wet-mix steel fibre-reinforced shotcrete and robotic spraying in Europe in the 1980s and early 1990s. The development of alkali-free set accelerators that are added at the spraying nozzle has mostly replaced the highly alkaline aluminate and silicate accelerators in major projects, so that the in-place early strength requirements of the order of 1MPa at two hours of age may be achieved with a very small reduction of ultimate strength compared to the previous types of accelerators available. Sprayed concrete mixes also tend to incorporate very high CEM I contents. In a recent UK project, the CEM I content used by the supplier was in excess of 500kg/ m³. This would give an embodied carbon content of the CEM I alone that is more than 430kg/m³. Varying levels of success have been experienced with replacements of CEM I with fly ash or GGBS – or both together – but CEM I contents are still generally more than 350kg/m³ and often in the region of 440kg/m³. If rebound, wastage and overspray of shotcrete are taken into account, then a further 10%–15% would have to be added to the total embodied carbon per cubic metre in place. It would therefore appear that the largest reductions


in embodied carbon per cubic metre of concrete in shaft and tunnel linings are available in the shotcrete sector. But it will be very difficult to achieve with current practice and technology, and the development of AACMs and geopolymer concretes for sprayed applications needs to be looked at rapidly. Similarly, the replacement of CEM I with a geopolymer binder in two-component annulus grouts has significant potential, and this type of grout is currently being developed in Germany. In segmental lining production, heat curing is often


applied to the freshly-cast segments in order to achieve a demoulding strength of approximately 15MPa within 6–8 hours from casting. In a major project, this can mean operating a curing chamber temperature of around 50ºC on a 24/7 basis for months or even years. This will also add to the carbon footprint of the segmental lining and is dependent on many factors, such as type of fuel, heat losses from the chamber and size of chamber. Most segmental lining production in the UK


incorporates a percentage of CEM I replacement materials, such as GGBS in the concrete mix, but normally not more than 40%. Should higher GGBS quantities, such as 70% or 80%, be included in the concrete then, due to the slower strength development of the mix, the heating regime may need to be increased by raising the temperature or length of time in the curing chamber (or both), thereby significantly increasing the cost and carbon footprint of segment manufacture.


A combination of cement replacements with fly ash


and/or GGBS combined with the use of steel fibres can give dramatic reductions in the embodied carbon of tunnel lining segments when compared with traditional segments used in major projects. This was highlighted by Edvardsen et al(3)


and is shown graphically in figure 3.


It can be seen that even further reductions in embodied CO2 could be achieved with the adoption of the use of a geopolymer concrete. It has been estimated(4)


that if EFC geopolymer


concrete replaced a normal concrete containing 30% GGBS in the 400mm-thick steel fibre-reinforced segmental lining of a 7.5m ID rail tunnel, then the embodied carbon of the tunnel would be reduced by approximately 1,840t per kilometre of tunnel. In a major rail project comprising 100km of tunnel, embodied carbon could be reduced by 184,000t. The results of tests conducted by Wagners in


Germany and Australia on steel fibre-reinforced EFC geopolymer concrete in recent years indicate exceptional performance in terms of strength and durability. The concrete has excellent physical properties in terms of compressive, flexural and tensile strengths, all of which are of the same or greater magnitudes as those of normal concretes. Water permeability, chloride penetrability and sulphate resistance are better than normal concretes. It also has three other extremely good properties that are very applicable to tunnels and shafts: (i) a very low heat of reaction (ii) good resistance to hydrocarbon fires and (iii) excellent resistance to biogenic corrosion in sewer tunnels.


Geopolymer concrete has a very low heat of reaction and several ‘hot box’ trials have been conducted on


ton CO2-eq. per m3 concrete


0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0


reinforcement OPC + trad. reinforcement


OPC + fly ash + trad.


reinforcement


Blast furnace slag cement + trad.


OPC + steel fibres


OPC + fly ash + GGBS + steel fibres


Steel fibres Reinforcement Concrete


Figure 3:


Comparison of embodied CO2 for different types of binder and steel


reinforcement used for various major


infrastructure projects (Edvardsen et al)


OPC + GGBS + steel fibres


OPC + GGBS + steel fibres


November 2021


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Copenhagen Metro, DK


Westernschelde Tunnel, NL


Green Heart Tunnel, NL


District Heating Tunnel, DK


STEP, UAE


Abu Hamour Tunnel, UAE


Doha Metro RLNU, DA


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