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The CO2 in concrete varies with the type of concrete and additions (i.e. ggbs or fly ash). Figures for embodied CO2 include the CO2 embodied in the cement, reinforcement, admixtures, aggregates from the quarry, to their transport to the concrete manufacturer. Delivery transport to the customer is not included, but this will be negligible if it is sourced locally.
The sustainable concrete website has a table of CO2 emissions relevant to concrete mix designs.
The figures come from a study carried out by Arup for the BCA and The Concrete Centre in 2006.
Other references:The Concrete Centre