RULES OF THUMB | LOW CARBON CONCRETE


CONCRETE FOR SHAFT AND TUNNEL LININGS


LOW CARBON


Low-carbon concretes are a welcome development but should be carefully used as some cases could see longer setting times, slower strength development and even increased embodied carbon, writes Charles Allen of OTB Concrete


Climate change and the need for humanity to reduce carbon emissions are currently major considerations in all industrial processes and in particular the construction industry. The COP 26 United Nations Climate Change


Conference is scheduled to be held in Glasgow (31 October–12 November 2021) and will provide the opportunity for the showcasing of global efforts to reduce carbon. Concrete is recognised as the second most widely


consumed commodity on the planet after water. It also contributes approximately 8% of global carbon emissions; the main source of these emissions is the manufacture of Ordinary Portland Cement (CEM I). Figures vary from country to country, but it is generally acknowledged that the production of one tonne of CEM I emits approximately one tonne of CO2 into the atmosphere, while 1m³ of C40/50 CEM I concrete in the UK contains approximately 350kg/m³ of embodied carbon; when you add 100kg/m³ of reinforcement, this figure rises to about 405kg/m³. A consequence of this is that there has been a focus


recently on using ‘low-carbon concretes’ and almost all the major ready-mix concrete suppliers now market low-carbon concretes. These generally comprise a range of concretes with CEM I replacements of up to 70% by ground-granulated blast-furnace slag (GGBS). This could be considered as an example of marketing ‘smoke and mirrors’, since GGBS and other cement replacement materials, such as fly ash and limestone fillers, have been used extensively in UK concretes over the past few decades and are specified in BS8500-1 to be used in below-ground concretes with certain exposure conditions, such as groundwater sulphates and chlorides.


28 | November 2021


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