Hidden Ecological Potentials in Concrete Pavements

Florian Gschösser, Michael Bösch, Holger Wallbaum

Conference Paper for the

CIB World Congress 2010,

May 10th - 13th, the Lowry, Salford Quays, United Kingdom

Task Group: TG62 Built Environment Complexity

Hidden Ecological Potentials in Concrete Pavements

Florian Gschösser Institute for Construction Engineering and Management, ETH Zurich, Switzerland

gschoesser@ibb.baug.ethz.ch Michael Bösch

Institute of Environmental Engineering, ETH Zurich, Switzerland boesch@ifu.baug.ethz.ch

Holger Wallbaum Institute for Construction Engineering and Management, ETH Zurich, Switzerland

wallbaum@ibb.baug.ethz.ch

Abstract

In recent years, sustainability has become a major concern in the field of infrastructure. Several studies with the goal of determining the environmental potential of road constructions in general, as well as of concrete roads specifically, have been or are about to be carried out. These studies are focused on the comparison of environmental impacts concerning different types of road constructions using materials produced in their standard way, without looking deeper into the material production processes and their ecological optimisation potential. This paper focuses on the environmental potentials concerning greenhouse gas emissions in CO2–equivalence emissions hidden in concrete pavements, i.e. potentials of the cement and concrete production. These possible ecological advantages are investigated by performing several cradle-to-gate life cycle assessments (LCA) including all processes from the raw material extraction to the moment the material leaves the production plant, for different production configurations. Thereby, concrete compositions and cement types which can be used as alternatives in road pavements, as well as the production technology and the use of alternative resources for the fabrication of the three materials (clinker, cement, concrete), can be used as “set screws” to influence the environmental impacts resulting from the concrete pavement.

For Switzerland, realistic improvement potentials are identified and compared to the present situation. Potential production changes analysed in this study comprise the utilisation of CEM II instead of CEM I cement and the use of increased amounts of recycled aggregates. Results show reductions in greenhouse gas emissions (kg CO2-eq) of up to 25%. Furthermore, the ecological impacts of the production of cement could be reduced by increasing the thermal substitution rate of waste, the share of biogenic wastes and the energy efficiency of the cement kiln systems. However, these potentials are more difficult to achieve. Changes in the production infrastructure are highly cost intensive and will only occur in the long-term. The fuel substitution rate and the type of co-processed wastes depend on waste availability, market prices, waste regulations, and may also require costly infrastructure modifications. Therefore, average data for the cement production was used. Finally, based on the LCA-calculations the potentials for the Swiss and the Central and Eastern European (concrete) motorway network are

determined. In Switzerland within the next 15 years 180 km of motorways will be built. Assuming the percentage of concrete pavements remains at 14%, the application of the concrete with the lowest environmental impact would contribute 0.2% of the aspired CO2 emission reduction of Switzerland by 2012. For the annexation of the EU-12 countries motorways with a total length of 14’000 km will be built in Central and Eastern Europe until 2015. Assuming a concrete pavement percentage of 25%, the application of the “best practice” would here contribute 0.35% of the aspired carbon emission reduction of the total EU by 2012.

Keywords: road construction, concrete pavements, life cycle assessment, ecological potentials, carbon emissions

1 Introduction

The reduction of carbon emissions is one of the main aims of a sustainable development. Therefore, the European Union aspires to reduce the CO2 emissions by 20% until 2020 in comparison to the emissions of 1990 (European Communities, 2008). In 2006, transport caused (without taking the infrastructure into account) 24.6% of the carbon emissions of the EU-27 countries (European Communities, 2009), whereby fuel consumption causes the biggest part of the transport emissions. Thus, research focuses on energy efficient transport vehicles. However, the infrastructure can also contribute to a sustainable development, by developing resource-conserving road constructions with lower maintenance frequencies, less rolling resistance and lower noise emissions.

At the moment 14%, i.e. 247 km, of the Swiss road network is paved with a concrete layer (Werner, 2004). However, the Swiss Federal Roads Office ASTRA pursues the policy not to construct national roads with concrete pavements in the near future for several, partly out-dated, reasons: expensive damages, complex construction process, delayed opening for traffic (Werner, 2004). Therefore, Swiss cement and concrete producers pursue the goal of becoming “part of the game” again by determining and improving the performance of their products. Several life cycle assessments were or are about to be carried out in order to demonstrate the ecological usability and properties of concrete itself and furthermore of concrete pavements (Birgisdóttir, 2005; Holldorb & Meisenzahl, 2003; Mroueh, et al., 2000; Conway-Schempf, 1999). Most of these LCAs compare ecological impacts of different road construction types over the whole life cycle of production, use and disposal applying standard processes.

This paper attempts to quantify the ecological improvement potentials in production processes of concrete pavements concerning greenhouse gas emissions. Previous LCA studies indentified the production characteristics with large ecological potentials: clinker production, type of cement used, use of recycled aggregates (Bösch, et al., 2009; Jeske, et al., 2004; Kytzia & Seyler, 2009). The aim of this paper is to combine the potential of these three “set screws to a realisable “best practice” and to calculate the total potential for the Swiss national road and Central and Eastern European motorway network.

2 Concrete Road Pavements

2.1 Design of Concrete Road Pavements

Concrete pavements have many advantages concerning roads with high traffic volume and a high proportion of slow moving heavy vehicles, i.e. concerning motorways or as they are called in Switzerland national roads. Thus, this paper analyses the possible contribution to greenhouse gas emission reduction of concrete pavements of the highest traffic load class T61

In standard road construction, a concrete paving layer consists of 5m x 5m unreinforced plates connected to each other by anchors every 50 cm on all sides. The appearing joints between the plates should be realised in a waterproof manner (Holcim Schweiz AG, 2008). Since the beginning of the 1990s concrete pavements with an exposed aggregate layer on top have been used in Central Europe (Pertl, 2000) and can now be seen as state of the art for concrete road pavements in Switzerland as well. However, they are not yet mentioned in the VSS

(Vereinigung Schweizerischer Strassenfachleute, 1997).

2 standard concerning concrete pavements; this is planned for the upcoming version of the standard. A concrete road pavement with an exposed aggregate layer for the traffic load class T6 can be seen in figure 1.

Figure 1: Cross-section of a concrete pavement

The effects of putting an exposed aggregate layer on top of the pavement are noise reduction and a higher road grip (Holcim Schweiz AG, 2008).

The comparison of concrete types in this study is based on the production of 1m2 concrete pavement. Based on the cross-section shown in figure 1 the needed concrete for 1m2 can be quantified with 0,05m3 of exposed aggregate concrete and 0,19m3 of bottom concrete.

1 Daily Equivalent Traffic Load > 3 000 … 10 000 average daily passages of equivalent single axle load on one lane during a significant period under observation

2 Swiss Association of Road and Transportation Experts (Schweizerischer Verband der Strassen- und Verkehrsfachleute)

2.2 Concrete Compositions

Concrete generally consist of cement, mineral aggregates, water and optional additives. The required properties of concrete used in a concrete pavement (Vereinigung Schweizerischer Strassenfachleute, 2008) can be seen in table 1.

Table 1: Required properties for concrete pavements (Vereinigung Schweizerischer Strassenfachleute, 2008)

32mm 16mm 8mm 32mm 22mm 16mm

+14%3 ... 7 3.5 ... 7.5 4 ... 8 340 ... 450 +7%

≥ 5.5 ≤ 0.45C30/ 37

Effective water cement ratio

[-]

Air void content depending maximum aggregate size [Vol.-%]

Percentage of aggregate <0.25 mm depending on maximum aggregate size

[kg/m3]

≥ 340

Exposure classes

XF4, XC4, XD3

Compressive strength class

Bending tensile strength

after 28 days[N/mm2]

Content of cement [kg/m3]

Each concrete composition depends on the intended use of the concrete. Therefore, different compositions for the bottom concrete and the exposed aggregate concrete are implemented. For simplicity, in this study aggregates are grouped into “sand”, “gravel” and “gravel, crushed”.

Before the invention of the exposed aggregate layer, concrete pavement was constructed as one total layer using a composition, which typically only fulfilled the requirements of the VSS standard (Vereinigung Schweizerischer Strassenfachleute, 2008). Presently, the bottom concrete layer consists of the same type of concrete as the former total layer. For the production of bottom concrete primary aggregates and recycled aggregates can be used. The bottom concrete composition with primary aggregates applied in this study represents a common composition used in practice (table 2).

Table 2: Composition of bottom concrete with primary aggregates (Holcim Schweiz AG, 2008)

Material [kg/m 3 ]

Primary sand 650

Primary gravel 1262

Cement 343

Water 144

Plasticiser 3.4

Air entering agent 2.4

Total 2404.8

The effective water cement ratio of this composition is 0.43. The amount of plasticiser and air entering agent, which is intended to give the concrete a high resistance against freeze-thaw attacks, will be the same in all concrete compositions analysed. The applicable cement types are described below in detail.

The mixture composition for exposed aggregate concrete must fulfil further requirements, compared to the bottom layer because of its additional task to reduce the generation of noise.

Therefore, no recycled aggregates and only crushed gravel of high quality can be used. Furthermore, a higher portion of cement paste is needed (table 3). As previously stated, concrete pavements with an exposed aggregate concrete layer are not yet mentioned in the VSS standard and the level of experience in Switzerland is rather low in comparison with other Central European countries like Germany. For this reason, the composition of an exposed aggregate concrete used in practice at a motorway section in Saxony-Anhalt (Hemrich, 2008), which fulfils the requirements of the German regulations (Forschungsgesellschaft für Strassen- und Verkehrswesen, 2008a, b) was analysed (table 3). The effective water cement ratio of this composition is 0.41.

Table 3: Composition of exposed aggregate concrete with primary aggregates (Hemrich, 2008) Material [kg/m 3 ]

Primary sand 420

Primary crushed gravel 1325

Cement 420

Water 170

Plasticiser 3.4

Air entering agent 2.4

Total 2340.8

Hofmann and Jacobs (2007) state that the VSS standard for recycling (Vereinigung Schweizerischer Strassenfachleute, 1998a), as well as the SIA3

Table 4: Compositions for bottom concrete using recycled mineral aggregates

standard for concrete (Schweizerischer Ingenieur- und Architektenverein, 2003) allows a percentage of 100% recycled aggregates in bottom concretes. Furthermore, it is mentioned that only recycled concrete granulates can be used as aggregates within recycling bottom concrete, as opposed to aggregates generated from concrete mixed with other components. Four compositions using different portions of recycled aggregates will be analysed. The use of recycled aggregates requires a higher portion of cement paste in the concrete (Hofmann & Jacobs, 2007). The compositions can be seen in table 4. All four compositions have the same effective water cement ratio of 0.44, which fulfils the requirement of a ratio lower than 0.45 (Vereinigung Schweizerischer Strassenfachleute, 1998b).

Percentage of recycled aggregate [%] 100% 75% 50% 25%

Material [kg/m 3 ] [kg/m 3 ] [kg/m 3 ] [kg/m 3 ]

Recycling sand 578 433.5 289 144.5

Recycling crushed 1122 841.5 561 280.5

Primary sand 0 162.5 325 487.5

Primary gravel 0 205.5 631 901.5

Cement 420 395 360 340

Water 220 210 190 176

Plasticiser 3.4 3.4 3.4 3.4

Air entering agent 2.4 2.4 2.4 2.4

Total 2345.8 2253.8 2361.8 2335.8

3 Swiss Society of Engineers and Architects (Schweizerischer Ingenieur- und Architektenverein)

2.3 Cement types

The type of cement used within a concrete composition has a big influence on its environmental impact (Bilgeri, et al., 2007). At the moment CEM I 42,5N, which consists of 95-100% clinker (European Committee for Standardization, 2000) is used as a standard cement type for concrete pavements in Switzerland (Vereinigung Schweizerischer Strassenfachleute, 2008). Several LCA studies have already shown that clinker production causes the greatest amount of environmental impacts of total cement and concrete production (Holldorb & Meisenzahl, 2003; Mroueh, et al., 2000; Conway-Schempf, 1999). Hence, the percentage of clinker within the cement and concrete should be reduced. For this reason, the use of cements with other main constituents besides clinker (e.g. cement types CEM II or CEM III) for exposed aggregate concrete, as well as for bottom concrete, is recommended. The usage of concretes with CEM II or CEM III at test sections on German motorways demonstrated that concretes with cements containing reduced clinker contents offer the same or even higher quality results, and equal or better properties than a concrete with the standard CEM I cement (Bilgeri, et al., 2007). Holcim Schweiz AG (2008) names three cements of the type CEM II, which can be used as an alternative to the standard cement. According to the standard EN 197-1 (European Committee for Standardization, 2000), it has been assumed that all cements contain 5% gypsum and 2.5% minor additional constituents, i.e. the portions of the cement constituents stated in the standard (table 5 – without gypsum) have to be allocated to 92.5% of the total mass (table 5 – with gypsum). In this study, the averages of the stated ranges for main constituents are applied.

Table 5: Cements according to the EN 197-1 (2000) and compositions applied in this study

Cement type

Cement clinker

Other main constituents

Other main constituent

type

Minor components

Cement clinker

Other main constituents

Other main constituent

type

Minor components Gypsum

CEM I Portland cement CEM I 95-100 0 0-5 92.5 0 2.5 5

Portland blast furnace cement

CEM II/A-S 80-94 6-20 Slag sand 0-5 80.5 12 Slag sand 2.5 5

Portland limestone cement

CEM II/A-LL 80-94 6-20 Limestone 0-5 80.5 12 Limestone 2.5 5

Portland shale cement

CEM II/B-T 65-79 21-35 Oil shale 0-5 66.5 26 Oil shale 2.5 5

CEM II

Cement name Composition in mass percent(portions stated in without gypsum)

Composition in mass percent(with gypsum)

In total, for all six concrete compositions using the four different cements, 24 life cycle assessments have been performed (figure 3).

3 Life Cycle Assessments

A life cycle assessment is a method to analyse environmental impacts of products and services (International Standard Office, 2006). This study applies a cradle-to-gate approach which comprises all resource consumptions and emissions from the primary resource extraction to the finished product. The use phase and final disposal of the concretes are not assessed. It is assumed that the use and disposal is equal for all assessed concrete types and hence does not influence the result.

3.1 Scope and Functional Unit

Life cycle assessments (LCA) are generally structured around a functional unit. All inputs and outputs in the life cycle inventory (LCI) and consequently the life cycle impact assessment (LCIA) profile are related to the functional unit (International Standard Office, 2006). The functional unit of this study is 1m2 of concrete pavement. As previously mentioned, the amount of concrete needed for 1m2 of concrete pavement can be quantified as 0.05m3 of exposed aggregate concrete and 0.19m3 of bottom concrete. To begin, 24 LCAs for 1m3 of the different concrete compositions have been performed (four for exposed aggregate concrete, 20 for bottom concrete, see figure 3). The results of these calculations will later be combined according to the amount of exposed aggregate and bottom concrete needed for 1m2 of concrete pavement. The type and amount of anchors and joint bands needed for concrete pavement is not influenced by the concrete composition applied. Therefore, they will be excluded from the system boundaries of this LCA study. The system boundaries for this life cycle assessment can be seen in figure 2.

Figure 2: System boundaries of the concrete production adapted from Jeske, et al. (2004)

3.2 Input for assessment

In this study, actual industry data, LCA data from the ecoinvent database v2.0 (ecoinvent center, 2007) and data from relevant literature were combined. In general, the clinker and cement production would offer a high potential for the optimisation of the ecological performance of the total concrete production. Although the ecological standard in Switzerland is quite high, the green house gas emissions of cement and clinker production could be further reduced by increasing the thermal substitution rate of waste, the share of biogenic waste, and improving energy efficiency. This “set screw” is challenging to “adjust”. Changes in the production infrastructure are highly cost intensive and will only occur in the long-term. The fuel substitution rate and the type of co-processed wastes depend on waste availability, market prices, waste regulations, and may also require costly infrastructure modifications. For these reasons, averaged data for the complete Swiss cement and clinker production was used (Cemsuisse, 2008). For the clinker production the heat requirement was quantified with 3’450 MJ, the thermal substitution rate of waste with 46.5%. The amounts and the characteristics of

the fuel and waste types were specified as given by Cemsuisse (2008). In general, the grinding of blended cements, as for example CEM II, requires more electricity than CEM I, because finer grinding is required. However, total electricity consumption is slightly reduced because of the lower clinker content. In this study, the electricity requirement for the clinker production and the cement grinding, i.e. the total electricity that is consumed at a cement plant, is assumed as 99.7 kWh/t for all cement types (Verein deutscher Zementwerke, 2007). Regarding the CO2 intensity of the used electricity, UCTE electricity mix was applied which represents the European situation. The life cycle of the recycled aggregate production starts after the transport of the reclaimed material to the production plant. For the LCAs of the recycled concretes the ecoinvent process “gravel, crushed” was modified, or more precisely, all processes concerning mining and the transport to the plant were subtracted. Therefore, the energy needed for the crushing and classification of the recycled concrete were assumed to be the same as those of the primary aggregates. The processes given in ecoinvent are based on a survey of the complete Swiss concrete industry, thus they implement average values for the concrete production, for example transport distances or mixing energy. Regarding the waste fuels used in clinker production, only waste preparation is taken into account but not waste production. The environmental impact of waste production is generally not attributed to the waste treatment industry, but to the industry responsible for its generation.

3.3 Life Cycle Inventory Analysis and Impact Assessment

Based on the given and collected input data, the 24 life cycle inventory analyses and life cycle impact assessments for the production of 1m3 of the several concrete compositions were performed. As previously mentioned, the focus of this study is on greenhouse gas emissions of concrete pavements. Therefore, the “IPCC 2001 climate change” indicator is implemented in the LCIAs, the results of which are the CO2-eq emissions of the different concretes (figure 3).

Figure 3: Greenhouse gas emissions of the different concrete types

As shown by the results, the use of cement with a lower amount of clinker in general causes lower greenhouse gas emissions from concrete. Figure 3 also demonstrates how using a high amount of recycled aggregates for the bottom concrete causes higher greenhouse gas emissions than concretes using primary aggregates. This can be explained by the need for more cement paste in the recycling concrete. This study determines a concrete composition containing 25%

recycled aggregates as a best practice composition, as it allows substantial concrete recycling with only marginal additional cement usage. However, as this study does not include concrete disposal processes, no definitive statement can be made on the optimal concrete recycling rate. In the next step, combinations of bottom concrete and exposed aggregate concrete were analysed. Three different combinations were used for this analysis: the standard combination using primary aggregates and CEM I in both layers, the best practice combination using 25% recycled aggregates in the bottom concrete and CEM II/ B-T in both layers and the worst case combination using 100% recycling material and CEM I in both layers. The results can be seen in figure 4.

Figure 4: Comparison of greenhouse gas emissions of concrete pavements

The comparison of the standard combination and the best practice combination demonstrates that the application of the best practice would result in a reduction of 19.5 kg CO2-eq or 25% per square metre concrete pavement. Thus, assuming that the policy of the Swiss Federal Roads Office would still allow concrete pavements for Swiss national roads and the percentage of concrete pavements would stay the same, the application of the best practice would bring a reduction of 7’435 tons CO2-eq for the 382’123 m2 of concrete pavement, i.e. in total 25 km (Bundesamt für Strassen, 2009) that would be built in the next 15 years. This would be 0.2% of the aspired CO2 emission reduction of Switzerland by 2012 (European Communities, 2009; Bundesamt für Umwelt, 2008) or the CO2 emissions caused by 3’150 cars, calculated with an average consumption of 7l/ 100km and an annual distance of 15’000 km driven (Green orange, 2009). The European Union Road Federation (2004) states that for the annexation of the EU-12 countries 14’000 km of new motorways need to be built in Central and Eastern Europe by 2015. In this study, the Swiss cement production data as described above is applied to estimate the potential in the EU-12, since no production data from Central and Eastern Europe was available. It should be noted that the actual benefits are probably larger than estimated in this study, since due to the less efficient cement production in EU-12 compared to Switzerland, a reduction of the clinker content in cement leads to a larger reduction of environmental impacts. Thus, with an assumed concrete pavement percentage of 25% the total reduction for this 14'000 km of motorways would be 1’242’145 tons CO2-eq. This would be 0.35% of the aspired CO2 emission reduction of the EU by 2012 (European Communities, 2009) or the CO2 emissions caused by 526’333 cars (7l/ 100km) over 15’000 km (Green orange, 2009).

4 Discussion and Conclusions

This paper examines the ecological potentials in production processes of concrete pavements concerning greenhouse gas emissions. It has been shown that the use of cement types with a lower percentage of clinker offers the opportunity to reduce the greenhouse gas emissions of 1m2 of concrete pavement by up to 25%. Furthermore, it is demonstrated that the use of a high amount of recycled concrete aggregates causes higher CO2-eq emissions. Up to 25% of recycling concrete can be substituted without increasing the CO2 emissions compared to primary concrete. This value has been chosen as ideal substitution rate in this study as it allows recycling a fraction of end-of-life concrete and saves primary resources without increasing the CO2 emissions compared to primary concrete. For a more comprehensive analysis, the actual impact of concrete disposal and resource depletion should be taken into account. The data used for this study can be seen as authentic for the case study Switzerland because the ecoinvent database, recent data from the Association of the Swiss Cement Industry and current Swiss as well as European standards were used as a basis. Large uncertainties are present in the estimation of the potential of Eastern and Central Europe, since Swiss cement production data is applied. The benefits from reducing the clinker content in cement (i.e. using CEM II instead of CEM I) will probably be larger in this case study, as clinker production can be assumed to be less energy efficient than in Switzerland. Further uncertainties exist in the assumption that the crushing of recycled aggregates consumes the same amount of energy as the crushing of primary material, but since the greenhouse gas emissions of the crushing processes compared to the emissions of the clinker production are disproportionally low, there is no need for a particular process analysis at this stage. As a final conclusion, it may be observed that by using cements containing a low percentage of clinker, which causes the greatest portion of emissions, the national (0.2%) and Central-Eastern-European concrete road systems (0.35%) may be small a means to achieving the common goal of carbon emissions reduction, which can be realised easily. The effectiveness of this means could be improved with an even more ecologically efficient cement and clinker production, i.e. the minimisation of the ecological impacts of cement and clinker. Since the aim of the larger research project behind this study is to compare complete road pavements using different paving materials, the next steps should be the calculation of the greenhouse gas emissions for the Eastern and Central European situation and the analysis of the ecological potentials hidden in the asphalt production to allow for a comparison of different paving materials. Finally, a tool for the life cycle wide ecological and economic comparison of different road construction types will be developed.

Acknowledgment

We would like to thank the Holcim Foundation for Sustainable Construction for its financial support. We also would like to thank cemsuisse for offering us their data. Furthermore, we want to thank Prof. Dr. Stefanie Hellweg and her team from the Institute of Environmental Engineering for the good collaboration.

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