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Frequently Asked Questions

Here are the answers to some common questions about CO2 concrete uptake.

To obtain as complete a picture of the climate impact (CO2 balance) of the use of concrete as possible, and to find and quantify effective ways to increase the uptake with concrete technology related measures. Current life cycle assessments (LCA) of construction materials do not typically account for CO2 uptake by concrete.

CO2 emissions from cement production emanate from both the combustion of different fuels in the cement kiln and from the thermal decomposition of the raw materials, mainly due to various incoming carbonates. This latter process is usually referred to as calcination and can be exemplified by the following chemical reaction for limestone:
CaCO3 → CaO + CO2

Carbonation is a slow chemical process that starts at the surface of concrete and can last for many years. The reaction is often presented by the simplified formula:

Ca(OH)2 + CO2 -> CaCO3 + H2O

CO2 is permanently absorbed into the concrete by carbonation. In principle, the same amount of CO2 that is driven off during calcination of raw materials in the cement kiln can be taken up (CO2 uptake), permanently, in the concrete by carbonation. However, the amount of CO2 that will be taken up by carbonation in a practical time scale depends on several factors.

Carbonated concrete is chemically stable and actually increases the strength of concrete with cements based on Portland clinker. Carbonation is however associated with a lowering of the pH in the concrete, thus reducing the chemical corrosion protection for steel reinforcement. Internationally, therefore, construction codes have design rules that protect steel reinforcement by a concrete layer of sufficient strength and thickness to reduce the reinforcement corrosion potential for the designed service life. Several factors , for example, climate exposure and material properties are considered. In addition, corrosion of steel embedded in carbonated concrete also requires certain levels of humidity. Indoor reinforced concrete protected from rain and exposed to RH <75% have a negligible corrosion.

The CO2 uptake per unit area (m2) of concrete is determined by the cement type and content in the concrete, the concrete strength and permeability, the exposure conditions (wet, dry, covered surfaces) and time. The CO2 uptake per volume (m3) of concrete is also dependent of how much of each m3 of concrete is exposed to the air, often called specific surface area (m2/m3).

The CO2 uptake can be quantified according to the standard CEN/EN 16757:2017 (Sustainability of construction works – Environmental product declarations - Product Category Rules for concrete and concrete elements). Annex BB in this standard provides a method to assess CO2 uptake through carbonation in different life cycle stages. Other calculation methods may be used if transparently documented. Further information can be found in the Technical Report CEN/TR 17310:2019 (Carbonation and CO2 uptake in concrete).

The strong environmental impact of CO2 (global warming) makes it important to find ways to increase carbonation and CO2 uptake of concrete, where it can be done without risk of reinforcement corrosion. This can be done by material, production, and structural design alterations in the production and use stages. In the end-of-life stage, the possibility of increasing the uptake is very large if suitable handling and storage of crushed concrete is provided.

IPCC is short for Intergovernmental Panel on Climate Change. Today, emissions of greenhouse gases from the different countries are reported, which in turn are used to support different climate strategies. Reporting takes place nationally to national authorities and internationally to the United Nations Framework Convention on Climate Change (UNFCCC). Guidelines for the emission calculation are developed and kept updated by the IPCC. The current version, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, covers greenhouse gas emissions from cement and concrete processes. Both CO2 emissions from fossil fuel combustion and emissions from the raw materials (calcination) are included. However (apart from being noted as an area for future work), no consideration is given to the carbonation of concrete. This may be considered a shortcoming in these calculations, which can lead to less accurate results.

UNFCCC is an abbreviation of United Nations Framework Convention on Climate Change. It was established during the Rio Conference in 1992 as a framework for international cooperation with the aim to combat climate change by limiting average global temperature increase. Annual national reporting of greenhouse gas emissions and removals takes place to national authorities and internationally to the UNFCCC.

NIR is an abbreviation of National Inventory Report, that is a report to the national emissions reporting system. A first report of CO2 uptake in concrete in Sweden according to the Tier 1-method was included in the 2020 Swedish NIR.

An EPD (Environmental Product Declaration) is a type III environmental declaration according to ISO 14025 and provides quantified environmental information for e.g. a construction product or service on a harmonized and scientific basis. The purpose of an EPD in the construction sector is to provide the basis for assessing buildings and other construction works, and identifying those, which cause less impact to the environment. For the construction sector, the EPD system has been developed based on the standards EN 15804:2012 and ISO 21930:2017. EPDs are increasingly used as a criterion in the various existing environmental certification systems.

The European standard CEN/EN 16757:2017 (Sustainability of construction works – Environmental product declarations - Product Category Rules for concrete and concrete elements) provides additional rules specifically for concrete and concrete elements. Annex BB in this standard provides a possible method to assess CO2 uptake through carbonation in different life cycle stages. Other calculation methods may be used if transparently documented. Further information can be found in the Technical Report CEN/TR 17310:2019 (Carbonation and CO2 uptake in concrete).

Preferably through product EPDs comprising all modules. Additional information may also be of importance for the assessment of concrete as a material. For further guidance see CEN/EN 16757, Annex BB, presently under revision.

Preferably through annual reporting via the National Inventory Reporting (NIR) to UNFCCC/IPCC of country specific uptake, calculated according to a report from IVL Swedish Environmental Research Institute, B 2309, CO2 uptake in cement-containing products. Background and calculation models for IPCC implementation. Models are available in spreadsheet form for use in CO2 uptake calculations and can be downloaded from this website.

Although the carbonation process is the same, the calculation is different, so the numerical results may be different and cannot be directly compared.

When reporting CO2 uptake for a concrete product, the calculation of CO2 uptake is carried out for a single product but over all the life cycle stages included in the EPD. Therefore, the timescale could be 60 to 100 years or more. The calculation is carried out only once for each product. The emissions and other impacts are related to the concrete products included their different life cycle stages and thus not to the product’s geographic location.

When reporting annual CO2 uptake of all existing concrete structures in a country, the calculation includes all concrete products in the country, but over a much shorter timescale of one year. The emission and uptakes are related only to those activities that occurs in the specific country of reporting. The calculation for inclusion in National Inventory Reporting to the UNFCCC/IPCC of country specific uptake is carried out every year.

Tiers 1 - 3 are levels of calculation methods defined by IPCC and presented in the report from IVL Swedish Environmental Research Institute, to calculate annual CO2 uptake in the use stage and in end-of-life stages. Tier 1 is the easiest method to use, but also the least accurate. Tier 3 is the most complex and accurate method.

Carbonation and CO2 uptake is a scientifically established natural process. In the carbonation process, CO2 is permanently absorbed into the concrete by chemical reactions. By using calculation methods according to the CEN/EN 16757 and the report B2309 from IVL Swedish Environmental Research Institute a balanced and realistic estimation of the CO2 uptake can be performed.

Uncertainty can generally be described as lack of knowledge of the true value of a variable. The uncertainty depends on the quality and quantity of applicable data as well as knowledge of underlying processes.

Carbonation and CO2 uptake is a scientifically established natural process. In the carbonation process, CO2 is permanently absorbed into the concrete by chemical reactions. By using calculation methods according to the CEN/EN 16757 and the IVL report B2309 a balanced and realistic estimation of the CO2 uptake can be performed.

The methodology for the calculation of CO2 uptake in a concrete surface is presented in CEN/EN 16757. The uncertainty of the method and calculations is not discussed in the standard, but some information can be found in the technical report Carbonation and CO2 uptake in concrete CEN/TR 17310:2019.

Uncertainty can normally be decreased by increasing the amount and precision of the relevant data. In the case of determining the annual CO2 uptake in the existing building stock, such data include cement and concrete statistics, construction knowledge, carbonation models, and the calculations involved. One can easily understand that finding the true value is a challenging task. Consequently, it is also difficult to give a precise value on the uncertainty. It has to be a mixture of statistical evaluation and expert estimation.

The IPCC generally recommendation is to neither over- nor underestimate the value. The results of the Tier calculations are therefore mean values. This means that a national reported CO2 uptake value can be somewhat smaller but also somewhat larger than the “true” value. From a global point of view, with all reports summarized, this problem will even out.

The IPCC general recommended way to decrease uncertainty is to move to a higher Tier. The uncertainty is largest with the Tier 1 method and smallest with the Tier 3 method.

The uncertainty of the Tier 1 method is mainly dependant on the uncertainty of the factor (0.23) given by the method to multiply the emission with. This uncertainty cannot be improved by national reporting.

The uncertainty of Tier 2 is more influenced by national reporting. The inventory of the use of cement and concrete, and design and exposure of the existing building structures, can be done more or less thoroughly. These input data have however good availability and reliability in most countries.

Uncertainty in the Tier 1 and Tier 2 methods are discussed in the report from IVL Swedish Environmental Research Institute and in the Guidance documents for the methods.

Tier 3 method has the possibility of being most accurate and to have the least uncertainty.

Additions are materials other than portland cement clinker used in concrete such as limestone, silica fume, fly ash or/and ground granulated blast furnace slag (GGBS) that can influence carbonation. These additional cement constituents, or additions to the concrete as fly ash and slag, contribute to CO2 uptake in addition to the clinker, although to a lesser extent due to their lesser amount of calcium. Proper quantification of this uptake is yet not possible to do with the present state of the art. However, carbonation rate compensation factors for additions are available in the Tier 2 model (from CEN/EN 16757). Estimation of the CO2 uptake according to the CEN/EN 16757 and the report from IVL Swedish Environmental Research Institute are thus done, only considering the clinker content and an adjusted carbonation rate using the rate constant (ki) compensation factors. The most advanced method for calculation of annual CO2 uptake (Tier 3) accepts however that uptake of additions can be considered if scientific references can be presented.

Mortar is a cement-based material consisting mainly of cement, sand and water. Sometimes, slaked lime, Ca(OH)2, is also added. In these calculations, only the cement clinker part is considered. Mortar is normally used in thin structures of relatively low strength. This means that carbonation of the whole volume is relatively fast. If high quantities of mortar are part of the cement consumption in a country, this will increase the uptake rate compared to normal consumption in concrete. Two different ways are proposed in Tier 1 and Tier 2 to correct for the higher uptake rate. Tier 1 has an increasing uptake factor if cement consumption for mortar is between 10 and 30%. Tier 2 considers all mortar to be completely carbonated in a few years.

At the end of the use stage (service life), a concrete structure is normally demolished, and the concrete debris enters the end-of-life stage. This stage can be divided into three parts:

Demolition, crushing, and storage, (CEN/EN 16757). Suitably stored small concrete pieces can take up considerable amounts of CO2 thanks to the large specific surface (m2/m3 concrete) that was not previously exposed to CO2 in the air.

CEN-EN 16757, Sustainability of construction works - Environmental product declarations - Product Category Rules for concrete and concrete elements, provides information on how to estimate CO2 uptake in concrete products in different life stages.

CEN-TR 17310:2019, Carbonation and CO2 uptake in concrete, is a Technical Report, which provides detailed guidance on carbonation and CO2 uptake. The guidance is complementary to that given in CEN/EN 16757.