Article pubs.acs.org/est
Calculating CO2 Uptake for Existing Concrete Structures during and after Service Life Ronny Andersson,*,† Katja Fridh,‡ Håkan Stripple,§ and Martin Hag̈ lund∥ †
Division of Structural Engineering, Lund University, Box 118, 221 00 Lund, Sweden Division of Building Materials, Lund University, Box 118, 221 00 Lund, Sweden § IVL Swedish Environmental Research Institute, Box 5302, 400 14 Göteborg, Sweden ∥ Bryggaregatan 3, 241 30 Eslöv, Sweden ‡
S Supporting Information *
ABSTRACT: This paper presents a model that can calculate the uptake of CO2 in all existing concrete structures, including its uptake after service life. This is important for the calculation of the total CO2 uptake in the society and its time dependence. The model uses the well-documented cement use and knowledge of how the investments are distributed throughout the building sector to estimate the stock of concrete applications in a country. The depth of carbonation of these applications is estimated using two models, one theoretical and one based on field measurements. The maximum theoretical uptake potential is defined as the amount of CO2 that is emitted during calcination at the production of Portland cement, but the model can also, with some adjustments, be used for the other cement types. The model has been applied on data from Sweden and the results show a CO2 uptake in 2011 in all existing structures of about 300 000 tonnes, which corresponds to about 17% of the total emissions (calcination and fuel) from the production of new cement for use in Sweden in the same year. The study also shows that in the years 2030 and 2050, an increase in the uptake in crushed concrete, from 12 000 tonnes today to 200 000 and 500 000 tonnes of CO2, respectively, could be possible if the waste handling is redesigned.
1. INTRODUCTION A fundamental prerequisite for assessing the contribution of concrete to greenhouse gas emissions is to properly describe the net balance of greenhouse gases in the use of concrete as a building material. A greenhouse gas model for concrete structures from a lifecycle perspective has been developed.1 This means that emissions and uptake during the entire product chain must be considered. When such a model is used to illustrate the environmental performance of concrete as a building material, or when the net emissions of CO2 from concrete use in a country during a specific year are required, the model should be used together with the CO2 uptake of the already existing concrete structures. Every year that concrete is used in the society there are CO2 emissions from its production and CO2 uptake in practically all the concrete that has been produced throughout the years until most of the CO2 that was driven off from the raw meal in the cement kiln is taken up by the carbonation process. To be able to handle the time aspect of these emissions in a correct way, which is important from a climate perspective, a model is needed which can calculate the total uptake of CO2 in the concrete stock every year in, for example, a country. In this article, such an uptake model with an application example is presented. This uptake figure, in combination with the yearly emissions, gives a measure of the actual greenhouse gas contribution from using concrete in the © 2013 American Chemical Society
society. This approach is parallel to the models established for the forestry industry where forests in balance on a national level are considered “carbon neutral” since their uptake and emissions are considered equal in size.2,3 The aim of the presented work has been to develop a model that makes the calculation of the CO2 cycle possible for existing concrete structures (houses, infrastructure, etc.), based on published physiochemical mechanisms and data on a national basis. This model therefore mainly covers the uptake of CO2 in primary concrete products (e.g., bridges, house frames) and the uptake in secondary products, which can be obtained through the demolition and crushing of primary products and used, for example as road base materials or aggregates in new concrete. Unless stated otherwise, the estimated service life of the products used in the model is 100 years and 100 years for the crushed primary products. The ultimate goal for the proposed model is to be a general, internationally applicable model for which input data is commonly available. The specific CO2 uptake for a concrete surface varies with climate, cement type (mostly regarding the amount of CaO, but also the influence of Received: Revised: Accepted: Published: 11625
April 23, 2013 September 5, 2013 September 5, 2013 September 5, 2013 dx.doi.org/10.1021/es401775w | Environ. Sci. Technol. 2013, 47, 11625−11633
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supplementary cementitious materials such as fly ash and slag) and concrete product. A future goal is that in the future the model will be developed and refined further, depending on the needs, preferences and available data in each country. The areas of application of the model presented include the international reporting of greenhouse gas emissions, the scientific validation of simplified calculation methods and the different LCA-based declarations such as the ISO 14020 series (e.g., eco-labeling, Environmental Product Declarations). The model can also form the basis for an open discussion on the components of the CO2 cycle and their potential for improvement. The entire environmental impact of a product is heavily dependent on its service life, including for instance the heating and cooling of a house where several factors affect the house’s overall environmental and climate performance in a complex way. The model developed here does not address the overall picture of such applications, but focuses only on the properties of the concrete structure. The model is still valid and applicable for the actual concrete material even in a complex structure. 1.1. The CO2 Cycle for Concrete Applications. The balance of greenhouse gases is normally expressed as the global warming potential (GWP), which is the weighted sum of various greenhouse gases. For concrete, CO2 is clearly dominant since the amounts of CH4 and N2O are insignificant compared to CO2. During cement production, limestone is burned (and CO2 is emitted) using fuel such as coal and polymer waste and the raw material then also emits CO2 (calcination). During the lifetime of the concrete product, CO2 in the air will react with the material in the concrete and become bound. Emissions of CO2 from the raw material during production can thus be said to come from a chemically stable form in the limestone, passing into the atmosphere for a period of time, and then return to a stable, chemically bound form in the concrete. The uptake of CO2 in the concrete is permanent and can therefore be considered as a real sink of CO2. Only reburning can cause the CO2 to be emitted again. An important difference between emission and uptake is the time aspect. The emissions are almost instantaneous for most processes related to the production of cement. The uptake of CO2 in concrete is a slow process that may, however, be accelerated, for example, by an increase in the surface area of the concrete. This occurs when concrete that has reached the end of its service life is crushed and then used or stored in an application where new surfaces of the crushed material can be exposed to the CO2 in the air. CO2 uptake during end-of-life or as a secondary product (e.g., crushed aggregates) may extend over several hundred years and exceeds the normal service life of the primary product. Demolished concrete also continues to absorb CO2 until the concrete has reached its maximum level of bounding. The theoretical uptake potential of CO2 in the concrete can, for concrete produced with Portland cement, be set equal to the amount of CO2 expelled from the raw materials during calcination. This amount of CO2 is well-known for such cements in the world. It is not completely clear whether all the CO2 from the calcination can be bound to the concrete again. The level of bonding is expressed as the degree of carbonation, and it has been found that, in practice, the degree of carbonation varies in the range of 30−90% depending on the degree of hydration and the relative humidity.4−6 In this paper, this degree of carbonation is referred to as the maximum degree of carbonation practically possible. For cements with other compositions, especially blended cement with high amounts of
additives, the uptake potential of CO2 in concrete has to be adjusted depending on other raw material in the cement.
2. MATERIALS AND METHODS 2.1. Calculation of CO2 Uptake in Existing Concrete Structures. In this paper, a model based on historically known statistics for cement production and cement use in various concrete applications and details of the rate of carbonation for different environments are used to calculate the uptake. In a test calculation, the model has been applied on Swedish data and the carbonation rates representing Northern Europe climates and Swedish building technology have been used. In Section 2.2 it will be explained how the depths of carbonation are determined for different applications and environments. In Section 2.3 it will be explained how the cement use is recalculated to a stock of concrete structures. In Section 2.4 the equations for the calculations are given together with an example of a subcalculation for an application. In Section 2.5 an estimation of the uptake in concrete after service life is presented. 2.2. Calculation of Carbonation Depths and Degree of Carbonation. The carbonation of concrete has attracted significant attention because of its role in the corrosion of reinforced concrete in humid environments. When the carbonation front reaches the steel reinforcement, the original high pH-value is lowered to levels that break the passivation of the steel bars and corrosion is possible. The carbonated concrete as such is, however, hard and stable, and carbonation will not endanger the stability of the concrete itself.4 A simple measuring technique used worldwide is to measure the pHchange by applying phenolphthalein to a newly cut surface. If the surface remains uncoloured, the concrete is regarded as carbonated and if the surface turns pink/purple, the concrete is regarded as uncarbonated (pH > 8−9). The thickness of the uncoloured layer is then referred to as the depth of carbonation. Typical results of this kind of measurement on outdoor reinforced concrete structures can be found in several investigations.5−7 How the depth of carbonation varies with the surrounding climate and concrete quality is also documented, and different kinds of carbonation models have been presented.8−11 This information is sufficient when corrosion is solved by the concrete regulations through requiring a sufficient concrete cover with proper concrete quality. A relatively new question is whether it is possible to estimate how much CO2 will be taken up by the concrete during its service life.4,12−16 Results1 will make it possible to perform a more complete lifecycle analysis of the carbon footprint of concrete and concrete structures. In this study, the carbonation depths and degrees of carbonation are needed. The degree of carbonation is defined as the ratio between carbonated and available CaO in the cement. Different reaction products in the cement paste will be more or less likely to carbonate in different environments and studies have shown that the degree of carbonation is between 30 and 90%4,12,18 the lower values being for very dry indoor climates. In this work, two models are used to evaluate the rate of carbonation depending on concrete use, quality and environment. One is based on field measurements4 on concrete while the second is a theoretical model.17 The carbonation process on a concrete surface is often simplified to a moving boundary according to Fick’s law. The depth of carbonation, XCO2, is then generally described as 11626
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Table 1. Exposed Surfaces, Strength and Ratio Area to Volume of Seven Swedish Concrete Applications; the Strength Is Obtained on Dried Cubes type of application
bridge
residential
offices
roof tiles
concrete pavement
shotcrete
sleepers
(unit)
per bridge
per apartment
per GFAa
per tile
per paving stone
per m2
per sleepers
m2
MPa
without surface cover painted with tiles/clinkers with plastic/linoleum with parquet/laminate
93 247 20 0.3 68
20−25 20−25 20−25 20−25 20−25
with mineral wool with polystyrene without insulation, coarse gravel without insulation, with sand/gravel
0.1 18 12
30−45 30−45 30−45
0.26 0.01
30−45 30−45
22
30−45
0.32
30−45
38 8 526.4 67.42 0.205
30−45 30−45
0.06 0 2.85 0.29 0.367
surfaces
exposed to rain sheltered from rain total area A (m2/unit) volume V (m3/unit) relative distribution between applicationsb j ratio area to volume, strength and exposure assumes also to represent a
m2
170.8
43.8 422 636.6 277 0.153
MPa
>45
>45 >45
general repair, stabilization/ piling
one-family houses
m2
MPa
0.84 0.61 0.08 0.64 0.03 Slab
m2
MPa
m2
MPa
m2
MPa
m2
MPa
1.61 0.694 2.304 0.119 0.068
>45 >45
Indoors 30−45 30−45 30−45 30−45 30−45 on Ground
Outdoors 30−45 30−45
other buildings
0.2 >45 0.15 >45 0.35 0.001703 0.025
1
30−45
1 1.19 3.19 0.05 0.110
30−45 30−45
cast concrete pavement
1 30−45 1 0.04 0.072 tunnels
water & sewer, wind power
Gross Floor Area. bfor year 2010.
When the degree of carbonation and carbonation depths are determined then an estimate of the different amounts of surfaces is needed. 2.3. Estimating a Stock of Concrete Structures in a Country. To be able to calculate the CO2 uptake in concrete structures in a country (in this paper, Sweden) it is required to estimate the amount of applications and what different kind of surfaces/situations/climates (of the 11 previously defined) these applications have. This is done with knowledge of the cement use. A dynamic flow macro-model is used to describe the accumulation of used cement in buildings and infrastructure in the U.S. between the years 1900−2005.19 That paper states that cement consumption correlates to the “value of construction” and that the method provides valid numbers. As a leading supplier of cement in Sweden, Cementa AB has been documenting national statistics for cement used in Sweden (including national statistics concerning import) since 1893. This data is shown in Figure S2, SI. Cementa AB also has detailed knowledge of the corresponding distribution of the cement across various applications. Up until 2000, the majority of the cement used in Sweden was CEM I with a calcination of 494 tonnes of CO2/ktonne cement. Since 2000, a substantial amount of CEM II has been produced lowering the calcination emissions to 455 tonnes of CO2/ktonne cement, see Figure S2, SI. It is then possible to calculate the uptake using historically known statistics for cement production and cement use in various concrete applications, combined with details of the rate of carbonation for different environments, see Table S1 and S2, SI. In order to be able to describe what surfaces these applications have, the cement usage was divided into three categories based on consumption data from Cementa AB; civil
XCO2 = k t
where k is a rate factor, which is dependent on several environmental and material parameters. However, this is not valid for surfaces with a cover material. For the theoretical model,17 an adjusted polynomial is therefore used and for the field measurements,4 a factor 100 years
0.077
70
10
0.00542
66.7
30
comments
within 100 years
share of total CO2 uptake (%)
void spaceb (%)
0.00813 11%
a
Calculated as an average between concrete for bridges (0.0911 kg CO2/kg concrete) and concrete for house frames (0.0638 kg CO2/kg concrete).1 The theoretic uptake is calculated from the chemical composition of the raw meal to the clinker kiln. bdefined as VV/VT, where VV is the volume of void-space and VT is the total or bulk volume.
AU (kg) at the end of year t1 for constructions of type j cast in the year t0 to t1 is t1
AU jt0→ t1 =
∑ kV ,j(min(t1 − t + t = t0
where Pj(t ) = K (t ) ·T (t ) ·uj(t )
k V,2011(11.5) =
= 0.646
1 , Lj)) ·Pj(t ) 2
The potential uptake P(2000) is with a calcination of 0.494 ktonne CO2/ktonne cement then P(2000) = 0.494·1510·0.025 = 18.6 ktonne CO2 which represent the total potential uptake during the entire service life of the roof tiles. And the final uptake for the roof tiles is analogously to eq 3 then
(3)
and L (years) is the expected service life. The term +1/2 is introduced to account for the assumption of an equally distributed production over the year and thus July first is chosen as the computational production day for all cement. Finally, the total uptake equals the sum of the individual contributions:
(k V,2011 − k V,2010) ·P = (0.646 − 0.616) ·18.6 = 0.558 ktonnes CO2
m t 0 → t1 AUtotal =
∑ AU jt → t 0
j=1
1 0.8·0.9·0.2 + 1.5·0.8·0.15 · · 11.5 0.0017 1000m/mm
1
where kv,2010 uses tcalc = 10.5 years. The uptake of CO2 during 2011 in roof tiles produced in 2000 is about 558 tonnes. 2.5. CO2 Uptake in Concrete after the End of Service Life. The uptake of carbon dioxide continues after the primary service life of a structure is over (provided that the structure is not already fully carbonated). The aged concrete may be crushed and reused for other purposes, for example as aggregate in new concrete or as a substitute for gravel. In the demolition and recycling phase of the lifecycle of concrete, the rate of the uptake is particle size and time-dependent. Demolished large concrete blocks still have a small surface to volume ratio, but when the used concrete is crushed and/or ground into smaller fractions, the uptake of CO2 will be more efficient and both the maximum practical uptake (since the reused concrete might be used in a more favorable environment for carbonation) and the uptake rate can be increased. In order to create a complete model that shows how much concrete is carbonated annually, it is necessary to know how much concrete is taken out of service and demolished each year. It is also important to know how the concrete is handled at end-of-life since the CO2 uptake in this stage may be significant and must be included in the model. The information on quantities and handling for recycling and other secondary uses of concrete is limited in Sweden and therefore a simplified and theoretical calculation has been made. Today’s concrete end-of-life management system has not been developed and adapted for CO2 uptake. This means that
(4)
where m is the number of construction types. Based on needs, preferences and available data, each underlying submodel or formula can be developed at different levels of detail in different countries and/or for different purposes. Example: What is the uptake in 2011 in roof tiles produced in 2000? The use of cement for roof tiles represents 2.5% (see Table 1) of the total cement use in Sweden that year, which was 1510 ktonnes (see SI Figure S2). Each roof tile has a surface area of 0.15 m2 that is sheltered and 0.2 m2 that is exposed to rain and the volume of a tile is 0.0017 m3, see Table 1. Using the model in ref 17, the carbonation depth at different times and degree of carbonation can be found in Table S2 SI; (1), top and sides: outside exposed to rain, → k-factor 0.8 and degree of carbonation 0.90 and (2) bottom: outside sheltered from rain → k-factor 1.5 and degree of carbonation 0.80. In all the calculations it is assumed that all cement is produced on July first and that is also the day that the structures are ready to absorb CO2. The time for the tiles produced in 2000 to absorb CO2 in 2011 is then tcalc = 11.5 years. Equation 2 gives 11629
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prevents air (CO2) circulation. An indication of the possibility for air circulation in the material can be obtained by studying the void volume in the crushed material. The effect of very fine particles is probably not only related to the void volume per se, but also to the size of the very narrow channels formed that effectively prevent gas circulation, but they are eventually also filled by moisture adsorbed on the inner surface of the very fine channels. Any future end-of-life management system must be improved in terms of the CO2 uptake in the end-of-life phase of the concrete. In the demolition phase, the CO2 uptake rate is slow in the large concrete pieces. A shorter time for crushing and storage in crushed form can increase the overall uptake. If the crushed material is then also sieved into specific narrow size fractions, a less compact material will be formed that allows air to circulate. Further information about this process is needed to confirm the involved mechanisms of the expected CO2 uptake. Preliminary estimates indicate that about 80% of the CO2 that was driven off from the raw material during calcination could be taken up during the first 20 years with a developed concrete end-of-life management system.1 The model is also used to discuss the possibility of developing new procedures for crushed concrete which could increase CO2 uptake by about 8 times. With an assumed continued service life for concrete structures of 80 years, the amounts of crushed concrete would rapidly increase at the same rate as the historical cement use. With such conditions, a procedure for crushed concrete would in 2050 be equivalent to an annual additional uptake of about 500 000 tonnes of CO2 per year. Already in 2030 the annual additional uptake could be about 200 000 tonnes of CO2 per year, Figure 2.
there is a potential for an increased uptake of CO2 in the concrete end-of-life phase. A starting point for an analysis of the CO2 uptake in end-oflife concrete is to analyze current management systems and the uptake of CO2 in those systems. The main current concrete end-of-life management system in Sweden can be divided into the following steps: 1. Demolition of used concrete products. 2. Intermediate stockpiling of demolished concrete. 3. Crushing of demolition products resulting in a mixed size fraction. Steel rebar recycling. 4. Intermediate stockpiling of mixed crushed concrete fraction. Small size fractions can easily carbonate, but the stockpile has low air circulation (CO2 access). 5. Use of the mixed concrete fraction in construction applications. Current examples are construction land filling, coarse road base or building foundations. Long future exposure time, but low CO2 access. In the demolition storage, the CO2 uptake occurs mainly at the fresh concrete surfaces. The amount of fresh surfaces can vary greatly and can therefore be difficult to estimate. 4.8 m2 of fresh concrete surfaces has been assumed for 1.2 m3 of concrete.1 A carbonation depth of 5 mm has been assumed. A 1% additional carbonation share has been added for carbonation in small crushed stones and concrete meal. This gives a total carbonation share of 2%+1% = 3%. The crushed concrete mix is assumed to be stored in piles with a height of 5 m. Carbonation can only be detected to a depth of 50 mm into the piles which corresponds to approximately 2% carbonation in the pile. The use phase of the crushed concrete is assumed to be 100 years and the degree of carbonation and the carbonated volume share of concrete are estimated and can be seen as an example, see Table 2. Table 2 shows a rough estimate of the CO2 uptake in the various concrete fractions for the Swedish concrete end-of-life management system of today. The CO2 uptake is calculated per amount of concrete and shows the maximum theoretical CO2 uptake in that amount of concrete. The carbonated share of that concrete together with the corresponding degree of carbonation for the carbonated share gives the actual CO2 uptake in the concrete. The share of total CO2 uptake is given for each demolition stage and the total uptake share of maximum theoretical uptake is given in the raw below the Total raw. In addition to this CO2 uptake, there has also been a CO2 uptake prior to this in the use phase of the product. Built-in concrete sooner or later also generates a corresponding amount of crushed concrete from demolition. This crushed concrete can be used or processed in different ways, which will affect its potential uptake of additional CO2. The amount of concrete that is available for crushing in Sweden today is roughly estimated by the authors to be about 1.5 million tonnes annually. With an assumed cement content of 13% by weight, this concrete contains about 200 000 tonnes of cement. As indicated in Table 2, only 11% of the maximum theoretical CO2 uptake occurs today in the end-of-life phase. A number in the same order of magnitude as other studies.16 Based on this, the uptake in 2011 in crushed concrete is therefore calculated to be about 12 195 tonnes of CO2 (total uptake 0.00813 kg CO2/kg concrete). The reason for this rather low number is the low air circulation in the crushed concrete material, both when stockpiled and in use, and can be explained in that different size fractions are mixed, leading to a compact material that
Figure 2. Calculation of a future scenario for CO2 uptake in Sweden over the next 50 years. The results are based on the theoretical model.17 It is assumed that the production rate as well as all other factors will be the same in the future as they were for the year 2011 (80 year service life is assumed).
The results show that the concrete will continue to take up large amounts of CO2 for a very long time without any changes in the production of cement and concrete structures. Improvements in cement production to reduce CO2 emissions from calcination will continue and this will obviously also decrease the potential for CO2 uptake in new structures. But because of the long service life of concrete structures, CO2 uptake will however persist at significant levels and perhaps also increase with time. The result is that the annual emissions minus the annual uptake will be smaller. 11630
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Figure 3. Annual uptake of CO2 from existing Swedish concrete structures during the years 2001−2011. Depth of carbonation according to theoretical model17 and model based on field measurements.4
Figure 4. The influence of concrete strength and thus rate of carbonation on the amount of CO2 taken up in Sweden during 2001−2011. The main result is based on the market shares of 2010 and the strengths shown in Table 1. Three calculations for the cases where all applications have strengths of 25 MPa, 30−45 MPa or >45 MPa are also presented. The results are based on the assumption that the distribution of the market shares over the years are 40%, 37%, and 23% (except for the fourth sensitivity calculation with a varying market share according to SI Figure S1).
3. RESULTS AND DISCUSSIONS
about 125 kg of CO2/tonne of cement. This main result is defined by distribution of investments between different segments historically has been the same as in 2010, strengths of the concrete and surface areas of each application all according to Table 1. 3.1. The Influence in the Uptake Due to Different Preconditions. The main results assume that the distribution of investments between different segments historically has been the same as in 2010, Figure 4. To investigate the effect of this assumption, a separate comparative calculation has been performed based on the actual historical official investment statistic variations between 1950 and 2010 between the three construction groups residential, commercial and civil engineering structures as presented in Figure S1, SI. The results in Table 3 show that the model on a national level produces similar results regardless of the two historical variations used in concrete use.
By accumulating the annual CO2 uptake of the seven different applications from the first year of construction, the accumulated CO2 uptake for existing Swedish concrete structures can be obtained. The total uptake in a given year is obtained by subtracting the accumulated uptake for the year before from the accumulated uptake for the given year. The uptake of CO2 will continue after the service life, and will depend on how the concrete is handled after the end of its service life. Figure 3 show the uptake calculated with carbonation depths either from the theoretical model17 or from field measurements.4 The difference between the two results is small. The results show that about 300 000 tonnes of CO2 were taken up in Sweden in 2011 by the already existing applications during its service lives. The contribution of the uptake in the crushed concrete is not included. This represents 17% of the total emissions from producing cement used in Sweden in 2011, or 11631
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data is directly proportional to the error in the output from the model. The carbonation rate in concrete for different concrete surfaces is also well-known, but needs to be adapted, for example for climates different from Northern Europe as well as for the use of other cement such as blended cements. Previously published works4,12,20 attempting to quantify CO2 uptake in existing concrete structures highlighted the problem of the low quality of the historical data. Consequently, the model described in this paper has been designed to be robust to historical variations by encompassing, for example, the following aspects: • By defining the maximum theoretical CO2 uptake potential as the amount of emitted CO2 during calcination, the uptake will not be overestimated. The uptake for a specific year could, on the other hand, be overestimated or underestimated, but the uptake in the long run will not. Some self-correction exists since if one concrete application is overestimated, the others are underestimated. • Calculating the carbonation based on known market shares and typical applications makes it possible to directly calculate annual carbonation based on the volumes of these applications. From the application, it is easier to estimate a reasonable concrete strength and thereby a cement content. • The model’s robustness is due to carbonation mechanisms being well-known, as well as how the carbonation rate diminishes as a function of time, which means any errors in the input data also diminish over time. Instead, the quality of the results is determined by the quality of the input data and its statistical distribution, for the most recent year. The required inputs are likely to be available for many countries. 3.3. Comparisons to Other Studies. The results confirm that durable reinforced concrete structures also have a very low CO2 uptake during their service lives. For example, in bridges, CO2 uptake only occurs in a few percent of the structure, namely in the concrete cover layer. On the other hand, a roof tile will be completely carbonated during its service life due to its small thickness. An important input to the model and decisive for the results are the carbonation depths and CO2 uptake being applied for the studied structures. The CO2 uptake given in relation to the total emissions from the production of the cement used for the products varies according to published literature. A survey according to four different studies can be found in ref 20 Other studies presenting figures for CO2 uptake are found in refs 21−23T.he differences in the results of those studies and the uptake presented in this paper are mainly attributed to: • too conservative assumptions that only the CH phase of the cement contributes to CO2 uptake. In studies4,12 used in this paper, the conclusion was that all hydrated phases carbonate, but of course the contribution of CH is the largest. • different cement types. In Sweden, CEM I was the major cement quality until 2000. • different concrete qualities. The presented study includes concrete of all strengths, separated into three categories; 45 MPa. • different climates and exposure. • different building types being studied. This study includes all kinds of concrete structures.
Table 3. Total CO2 Uptake in Existing Concrete Structures in Sweden 2011 for Different Scenarios (Service Life Is 100 Years unless Otherwise Stated) ktonnes depths of carbonation from field measurements depths of carbonation from theoretical model17
287 310
varying market shares 1950−2011 depths of carbonation from theoretical model17
307
4
different service life for the applications depths of carbonation from theoretical model17 (90 years service life) depths of carbonation from theoretical model17 (80 years service life) depths of carbonation from theoretical model17 (70 years service life)
308 304 298
effects of overlapping surfaces for concrete pavement (have been subtracted in all calculations) depths of carbonation from theoretical model17
45 MPa) depths of carbonation from theoretical model17
169
all applications have at all times been produced in a lower quality concrete (fc< 25 MPa) depths of carbonation from theoretical model17
477
The main result is obtained by using the distribution of the concrete applications valid for the year of 2010 and the concrete strengths of the applications according to Table 1. If the concrete strength of all the applications was set to be the highest value (strength >45 MPa) or the lowest value (strength
