Tier 1 CO2 uptake models for concrete
Tier 1 provides a simplified calculation method for estimating the annual uptake of CO2 in existing concrete structures on a national basis. The model should be used primarily as a first estimate or in cases where resources are missing to perform more accurate calculations according to the calculation methods in Tier 2 or higher, since the uncertainty is relatively high for Tier 1 as this is a simplified calculation method based on general data.
The data used for Tier 1 is based on previous CO2 uptake studies performed as scientific studies to calculate national CO2 uptake in concrete for some specific countries and regions. These uptake studies are performed using relatively advanced uptake calculation methods. The countries used for this Tier 1 model is at present the main data available and represent the following countries: Ireland, Netherlands, Norway, Spain, Sweden, Switzerland and a study with a global perspective.
The national annual CO2 uptake in concrete; in the use stage (existing structures), in end-of-life stage (demolition, crushing, stockpiling), and in secondary use, can be estimated according to this simplified method. The uptake values are related to the reported calcination emissions from the consumed clinker (produced-export+import) in the corresponding country. Note that the nationally calculated emission values may need to be adjusted for exports and imports of cement/clinker.
Uptake in the use stage of the primary concrete products
The annual uptake in the use stage can be estimated as 0.20x(the reported emission from calcination of consumed cement clinker). If the mortar for rendering applications, in total, amount to more than 10% but less than 30% of the cement consumption, the annual uptake factor in the use stage can be estimated at 0.20 + 0.0115(MR - 10), where MR is the mortar percentage for rendering of the clinker use. If MR is below 10% use MR=10 and if MR is above 30% use MR=30.
Uptake in end-of-life stage and secondary use stage
In the simplified methodology of Tier 1, we introduced a conservative “default” value for this phase based on the present normal handling procedures of demolished and crushed concrete. These procedures include normally storage in large unsheltered piles, during a rather short period of time. Moreover, the recycling rate (the annual amount of demolished and crushed concrete in relation to the annual production) is normally low in most countries. The volumes can however be anticipated to increase in the future, as more concrete structures reach the end of their service life. It is therefore important to base CO2 uptake calculations on the actual amounts of concrete that goes to end-of-life handling.
Annual uptake in the end-of-life stage and secondary use can be estimated as (0.02 + 0.01)x(the reported emission from calcination of consumed cement clinker), where the value 0.02 represents uptake for end-of-life processes and 0.01 represents uptake for secondary use.
Alternatively, the following estimation can be done for the end-of-life stage and the secondary use.
- If the annual amount of concrete being taken out of service and processed at a recycling plant is known, the CO2 uptake in the end-of-life stage can be calculated as 10 kg CO2/m3 concrete.
- If the annual amount of crushed concrete, entering secondary use as unbound material, is known, the uptake can be calculated to 10 kg CO2/m3 concrete.