Greenhouse Gas Emissions Embodied in Reinforced Concrete and

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Environ. Sci. Technol. 2009, 43, 3885–3890

Greenhouse Gas Emissions Embodied in Reinforced Concrete and Timber Railway Sleepers ROBERT H. CRAWFORD* Faculty of Architecture, Building and Planning, The University of Melbourne, Parkville, Victoria 3010, Australia

Received August 25, 2008. Revised manuscript received March 9, 2009. Accepted March 26, 2009.

In Australia, there are currently two main materials used for railway sleepers: timber (river red gum, a species of eucalypt) and reinforced concrete. Within the state of Victoria alone, there are currently seven million railway sleepers that make up the rail network. It is estimated that around two million sleepers, or 29%, are presently required to replace timber sleepers and upgrade the entire network, for which there are significant environmental implications, such as the emission of greenhouse gases. These emissions are mainly as a result of the energy and other resources required or “embodied” through the sleeper manufacture, including those associated with harvesting timber and mining raw materials for manufacturing cement. Where alternatives are readily available, it is important that the environmental impacts of the various choices are assessed, ensuring that these impacts are minimized. This study aimed to assess the life cycle greenhouse gas emissions associated with timber and reinforced concrete railway sleepers and showed that the life cycle emissions of reinforced concrete sleepers were up to six times less than the emissions associated with timber sleepers. Taking the potential errors associated with this type of assessment into account, there appears to be a significant advantage in using reinforced concrete sleepers, in terms of reducing the life cycle emissions associated with the provision of railway sleepers.

To make an informed decision on which may be the preferred option from an environmental perspective, either reinforced concrete or timber sleepers, a life cycle analysis of both alternatives is essential. Previous researchers have attempted to conduct such an assessment, presenting a variety of, sometimes conflicting, findings. However, none of these studies can be considered conclusive as they are based on system boundaries of unknown completeness. The aim of this paper is to investigate the life cycle greenhouse gas emissions associated with timber (river red gum) and reinforced concrete railway sleepers to determine the sleeper type resulting in the lowest life cycle greenhouse gas emissions.

Life Cycle Greenhouse Gas Emissions of Railway Sleepers Railway infrastructure represents a significant long-term investment and choices made today can have long-term financial and environmental implications. For some railway infrastructure components, such as rails, alternative materials are limited. However, for components such as railway sleepers, there are a number of alternative material choices available. Traditionally in Australia, Eucalyptus Camaldulensis, or River Red Gum, has been used to manufacture railway sleepers. More recently, these timber sleepers are being replaced with sleepers made from reinforced concrete. The advantages of using reinforced concrete over timber include longer anticipated life, greater strength (thus fewer sleepers are required per length of track) and lower maintenance costs. While the physical and financial benefits of using reinforced concrete sleepers have been studied, there is very little information available on the preferred choice from an environmental perspective. The manufacture of materials, such as timber and concrete, requires significant quantities of natural resources and produces sometimes large quantities of waste and emissions. In Victoria alone, the two million sleepers currently required to upgrade existing tracks equates to approximately 178 000 m3 of timber or 570 000 t of concrete plus approximately 20 000 t of steel in fastenings and reinforcement. This replacement is ongoing as sleepers reach the end of their service-life or fail under excessive loads.

Introduction

Review of Previous Studies and Methods

In the past, it has been shown that the manufacture of railway sleepers has been responsible for significant environmental impacts, particularly resulting from the harvesting of timber. The relatively short life and large number of sleepers required have also been contributing factors. More recently, reinforced concrete sleepers have been used to replace timber sleepers, providing improved strength, durability, and cost savings. However, this substitution has been criticized for supposedly contributing to an increase in greenhouse gas emissions, mainly from sleeper manufacture. The main resource requirement for railway sleepers is in the fuels, materials, and other goods and services consumed during their manufacture, including the significant waste generated through the timber harvesting process. Greenhouse emissions result from the supply of these fuels and materials. These emissions are known as the indirect embodied emissions. Emissions are also released during the transportation of the final product as well as during installation, which are known as direct emissions.

A number of studies have been performed in the past assessing the greenhouse emissions associated with timber and reinforced concrete railway sleepers in an attempt to inform environmental decision-making. A report produced for the Australian Rail Track Corporation by Energy Strategies (1) contends that timber sleepers are responsible for the emission of up to five times more CO2 emissions than reinforced concrete sleepers over the life of the sleepers. In contrast, the Australian Environment Foundation (AEF) (2) argues that concrete products (not specifically sleepers) produce six times more greenhouse emissions than timber because of the processes involved in mining, quarrying, and manufacturing. A study by Werner (3) comparing the life cycle impacts of a range of sleeper types concludes that timber sleepers result in considerably less life cycle greenhouse emissions than concrete sleepers. Also, the Australian timber industry (4) has argued that harvesting and processing trees for timber sleeper production produces fewer CO2 emissions than reinforced concrete sleeper production because of the carbon stored in the stumps and roots of harvested trees and in the range of wood products, including sleepers that are produced.

* Corresponding author phone: +61 3 8344 8745; fax: +61 3 8344 0328; e-mail: [email protected]. 10.1021/es8023836 CCC: $40.75

Published on Web 04/17/2009

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FIGURE 1. Upstream, downstream, and sideways truncation errors in the concrete sleeper system boundary. Apart from the differences in argument concerning the treatment of various forestry activities and phenomenon (such as carbon sequestration and timber decay), a possible explanation for these discrepancies is differences in the consistency and comprehensiveness of the system boundaries selected for the analysis. The studies by Energy Strategies (1) and Werner (3) are based on a method known as process analysis to quantify the emissions embodied in railway sleepers and components. There is also another method called input-output analysis, and hybridizations of these two, but process analysis is explained in detail first. Process Analysis. Process analysis uses a combination of process-, product-, and location-specific data to calculate environmental impacts, such as those resulting from the consumption of energy or release of CO2-e emissions. Process data is generally seen to be more accurate and relevant to the product being analyzed, but on the other hand, its collection can be labor- and time-intensive. Moreover, it suffers from a systemic incompleteness, which is caused by the delineation of the assessed system by the finite boundary and the omission of contributions outside this boundary. This issue of system boundary incompleteness in process analyses is not a problem that brute force can solve, even with practically unlimited resources. As a result, life cycle assessment (LCA) based on process analysis does not usually cover the input system of the functional unit to a sufficient degree. Input-Output Analysis. Input-output analysis uses sector-based financial data to trace resource requirements and pollutant releases between industry sectors. Generalized input-output frameworks have been applied extensively to environmental analysis since the late 1960s (see for example, refs 5 and 6). An introduction to the input-output method and its application to environmental problems can be found in papers by Leontief and Ford (6), Proops (7), and Dixon (8). Generally, input-output analysis covers completely the economic system defined by the national or regional statistics. These models treat the whole economy as a system and any number of inputs from other sectors are included, an almost limitless number of potential transactions upstream through the supply chain. An input-output model comprises the direct requirements and the indirect requirements of inputs from all other sectors. The sum of the indirect requirements can be deduced by subtracting the direct requirements from the total requirements. The single biggest problem with input-output analysis is the inability to model discrete, mutually exclusive indirect requirements. The system boundary of input-output analysis is economic, such that if a sector pays for any product, the inputs to that product are counted. Any environmental or other parameter can be analyzed using input-output analysis. It is this power that is the main benefit of input-output analysis. However, the same features that give the technique this power 3886

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also contribute to its main limitations. Aggregation of commodities and establishments into sectors reduces the relevance of the results to any particular product or region. The economic system boundary that gives input-output analysis its depth and breadth can reduce the reliability of results when these financial flows are attributed to physical quantities of materials or environmental impacts. Lenzen (9) has used Monte Carlo simulation to show that the uncertainties associated with input-output analysis are lower than the truncation errors of a process analysis. Also, the results obtained from an input-output analysis are often quite close to the results obtained from process analyses (10), although they are often higher because of the comprehensive system boundary. Hybrid Analysis. Because of the quite specific limitations with process analysis and input-output analysis, some have sought to combine the best of both methods. A number of researchers have suggested and demonstrated hybrid approaches (inter alia 9, 11-13). In a tiered hybrid LCA (13, 14), the direct and downstream requirements (for manufacture, use, and end-of-life), and some important lower-order upstream requirements of the functional unit are examined in a detailed process analysis, while remaining higher-order (upstream) requirements (for materials extraction and manufacturing) are covered by input-output analysis. In this way, advantages of both methods, completeness and specificity, are combined. Moreover, the selection of a boundary for the production system becomes obsolete. Upstream truncation error is eliminated for the inputs for which process data has been collected. But even with this tiered hybrid technique, it is still virtually impossible to identify all individual requirements of goods and services, relying on the consultant to decide which processes are important and require analysis. This can only resolve the upstream truncation error for items that the user decides are relevant.Because the supply chain is disaggregated to allow the integration of process data, the potential exists for sideways and downstream truncation error (Figure 1). Sideways truncation includes inputs that are generally quantified in financial terms, such as advertising, as well as other items thought insignificant. Downstream truncation can include the processes associated with turning basic materials into more complex products, sometimes difficult to quantify using physical data. The magnitude of these truncation errors depends on the type of product or activity, but can be in the order of 50-80% (see, for example, refs 15-19). Lenzen (9) and Treloar (12) have shown that within conventional process-type LCA that this error is not usually reducible to an acceptable level by extending the system boundary because of the complexity of the supply chain that would have to be investigated. The hybrid LCA model developed by Treloar (12) (known as input-output-based hybrid analysis) addresses many of

TABLE 1. Reinforced Concrete and Timber Sleeper Properties timber sleeper: timber species River red gum, Eucalyptus Camaldulensis air dry density 900 kg/m3 dimensions 125 × 250 × 2 850 (broad gauge) (0.09m3) weight 80.2 kg per sleeper sleeper cost A$45 per sleeper (excludes fastenings) quantity of sleepers 1460 (685 mm spacing) per km of track reinforced concrete sleeper: concrete weight 285 kg per sleeper (0.12m3 at 2400 kg/m3) concrete strength 50 MPa, based on AS1085.14 sleeper cost A$100 each, including fastenings fastening details 10 kg/sleeper (based on Pandrol E clips) pretensioning steel 8dia. × 2700 × 4 per sleeper tendon wire quantity of sleepers 1400 (714 mm spacing) per km of track

TABLE 2. Reinforced Concrete and Timber Railway Sleeper Scenarios scenario

service-life

fastenings reuse

C1 C2 T1 T2 T3 T4 T5 T6

30 years 50 years 20 years 30 years 20 years 30 years 20 years 30 years

0% 0% 0% 0% 50% 50% 96% 96%

these problems by starting with a disaggregated input-output model to which available process data is integrated. This can avoid the possibility for sideways and downstream truncation errors discussed above, in addition to upstream truncation. Using Treloar’s approach, this study extends on similar previous studies by providing a more comprehensive assessment of the greenhouse gas emissions associated with railway sleepers (resolving substantially the issue of system boundary incompleteness) over a range of life cycle scenarios.

Methodology This study considered the life cycle greenhouse gas emissions (in carbon dioxide equivalent, CO2-e) associated with two railway sleeper materials: • River Red Gum (Eucalyptus Camaldulensis), untreated • Reinforced concrete. This section outlines the properties of the sleepers, the scenarios considered and the methods used to assess the life cycle CO2-e emissions of the sleepers. Sleeper Properties. The properties of the two alternative sleeper types are outlined in Table 1. The details of the fastenings used for the timber sleepers are provided in Supporting Information Table S1. Description of Scenarios. A total of eight scenarios were considered in this study. These looked at two scenarios for reinforced concrete sleepers (as seen in Table 2) including two service-life options and the consideration of decay emissions of replaced timber sleepers. Six scenarios were considered for the timber sleeper alternative (as shown in Table 2). These looked at two different service-life options, the consideration of decay emissions from new and replaced timber sleepers and three options for the reuse of steel fastenings. Effective Service-Life. The service-life of reinforced concrete sleepers with properly maintained rail, pads, ballast, and subgrade is typically up to 50 years for lines with moderate axle loads. This can be as low as 30 years for lines overstressed

or continually pushed to design limits (e.g., on heavy haul lines). Both service-life options have been considered in this study. For timber sleepers, the service-life varies considerably depending upon timber species, traffic levels, and environmental conditions. In addition, as the number of older red gum trees declines, younger and lower quality trees are harvested, potentially lowering the possible service-life of the railway sleepers (20). Two service-life options have been considered in this study: 20 years and 30 years (based on ref 21). In the calculation of the life cycle CO2-e emissions of the different timber sleeper scenarios, it was assumed that 29% of existing timber sleepers were replaced at the beginning of the study period, while the remaining sleepers were replaced progressively at a rate of 4.76% of the total number of sleepers per km, per annum (for 15 years). It was assumed that this cycle was repeated at the beginning of every 20 or 30 year interval (depending on the service-life option considered) within the 100-year study period. Fastenings Reuse. For timber sleepers, up to 96% of existing steel fastenings are reused (22). In practice, these rates may vary considerably, depending on the type of replacement sleeper and the quality of existing fastenings. This study has considered three options for the reuse of steel fastenings: (a) no reuse, (b) 50% reuse, and (c) 96% reuse. The reuse proportion of each fastening component for options b and c is shown in Supporting Information Table S2. Further assumptions and exclusions are detailed in the Supporting Information. Life Cycle CO2-e Emissions Assessment. The life cycle emissions associated with railway sleepers includes the net embodied emissions of the initial sleeper replacement as well as the emissions embodied in subsequent replacement sleepers (recurrent embodied emissions) and the emissions resulting from the gradual decay of existing and new timber sleepers. The life cycle emissions associated with both timber and reinforced concrete sleepers were calculated for a period of 100 years, on a per kilometer of track basis. Embodied CO2-e emissions. Embodied CO2-e accounts for the emissions resulting from the manufacture of products and materials, including the emissions resulting from the manufacture of goods and services used during this process. For example, the emissions embodied in timber products, typically comprise emissions from harvesting, transporting, and milling the lumber and from finishing and delivering the products. Other emissions are embodied in good and services, including capital, utilized during these processes, and so forth. There are many uncertainties and variabilities with respect to embodied emissions analysis, which need to be taken into consideration when interpreting the results of this study. Many factors (including technology, fuel supply structures, climate, region, product specification and embodied emissions analysis methods) can contribute to large ranges of variability in embodied emissions data. For example, if timber obtained from a forest local to the milling site was extracted using a minimum of energy efficient mechanical devices and cured naturally, then the embodied emissions figure could be significantly lower than an industrywide average figure. The recurrent embodied emissions have been calculated based on the number of required sleeper replacements over a 100-year period for the two service-life options considered for both sleeper types (see Supporting Information Table S3). This assessment is based on a functional unit of one kilometer length of track in Victoria, Australia, and was performed using an input-output-based hybrid analysis. Input-Output-Based Hybrid Analysis. This method is applied using an input-output model of Australian energy use, developed by Professor Manfred Lenzen, Department VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. CO2-e Intensity of Materials material

CO2-e intensity (t CO2-e/m3)

timber-hardwood concrete 50 MPa steel

2.94 0.54 45.9

of Physics, The University of Sydney. The base input-output data was taken from the Australian National Accounts (23) and combined with CO2-e emissions factors by fuel type. The combination of these two sources comprises the input-output model. The model includes the value of capital purchased in previous-years, and capital imported from other countries, amortized over the capital item’s life (as described and analyzed in ref 24). Capital refers to the equipment and machinery used to make products such as railway sleepers. The input-output model was used as a basis for the embodied emissions analysis of the sleepers. The appropriate allocation of emissions to the sleepers is achieved using the input-output model. Emissions flows (kg of CO2-e) are allocated based on the financial value of the sleepers and the total value of individual inputs from each and every other sector per $1000 of output from the target sector. The best available process data was incorporated for specific materials manufacturers as per the input-output-based hybrid method (12). Process specific data for the emissions from the manufacture of cement and concrete were obtained from the Cement Industry Federation (25). The remaining process data was obtained from the latest available Simapro Australian database (26). No other source of substantially better, more up-to-date public domain process data covering such a broad range of materials is known to be available in Australia. The calculation of the CO2-e emissions embodied in reinforced concrete sleepers was based on the CO2-e emissions intensities from Table 3, which includes the emissions from fossil fuel consumption and those resulting from the decomposition of limestone during the production of cement. The calculation of the CO2-e emissions embodied in timber sleepers was also based on the CO2-e emissions intensity factors from Table 3, and includes the following components: • direct and indirect emissions resulting from the energy required to produce timber sleepers and their steel fastenings (+ve) • CO2-e stored in timber sleepers (-ve) • CO2-e emissions from roundwood conversion (burning or decay of waste, +ve) See Supporting Information for a detailed description of the derivation of the timber-hardwood (Table S4-S5) and concrete 50 MPa (Table S8-S9) emissions intensities. The quantities of the materials used for the manufacture of the sleepers were multiplied by their respective CO2-e intensities. The sum of these results gave the total processbased hybrid embodied emissions for the different scenarios. Using the input-output model, the total emissions intensity value of the input-output pathways, for which physical quantity data was obtained, was then deducted from the total emissions intensity of the “plaster and other concrete products” or “sawmill products” sector (for the reinforced concrete and timber sleepers, respectively) to give the “remainder” (which in this case comprised only part of the input-output component of the hybrid result). The remainder thus corrects sideways and downstream truncation error (Figure 1), at least in terms of the Australian economic system as defined by the Australian Bureau of Statistics (23). The relevant process-based hybrid embodied emissions values for the sleepers were then added to the ‘remainder’ value, to complete the system boundary. 3888

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See Supporting Information for a detailed breakdown of the derivation of embodied emissions for the timber (Table S6-S7) and concrete (Table S10-S11) sleepers. Locked-up Carbon. When trees grow they absorb carbon from the atmosphere. This carbon remains locked-up in the structure of the timber until most of it is released back into the atmosphere when the tree decomposes or is burned, forming CO2. This locked-up carbon has been subtracted from the carbon in the CO2 emissions associated with the manufacture of the timber sleepers. Roundwood Conversion Emissions. The carbon lockedup in timber represents only one part of the nonenergy related emissions. When forests are harvested, underbrush is disrupted, bark, leaves and branches are stripped, and off-cuts and sawdust result. It is common practice to chip the roundwood conversion waste for use in other timber products, burn it on site, leave it where it lies, or burn it in the kilns used for drying timber or for other process heat (27). This process is generally termed roundwood conversion. Roundwood conversion emissions are those emissions that are released from the conversion of timber products from roundwood. It was assumed in this report that the carbon dioxide from the roundwood conversion waste enters the atmosphere as a greenhouse gas emission. For the purposes of this study, a factor of 40% has been assumed to represent the proportion of total biomass that is converted to sleepers. The Australian Greenhouse Office (28) suggests a roundwood conversion factor of 53%. These values can be compared to those found by Snowdon et al. (29), who found that the factor could vary from 17 to 80% depending on forest type, harvesting method, and available markets. A factor of 40% is considered to be a good overall estimate for red gum timber harvesting. The remaining 60% represents timber waste that is treated as a roundwood conversion greenhouse gas emission. This study assumed that the emissions resulting from these processes occur immediately (which is typically representative of what occurs when burning roundwood conversion waste, for example). Arguably, carbon locked up in waste reclaimed for other products such as wood chips could be attributed to those products. But in this case, if not for the decision to harvest these trees for sleepers, these wastes would not be available. If the decision to harvest the trees is made in part because of the opportunity to make these other products, then part of the total emissions should be attributable to them. Decay Emissions. After timber has been harvested, it ceases to sequest carbon dioxide and instead begins to decay, releasing carbon dioxide back into the atmosphere. For the purposes of this study, this decay is assumed to occur over a period of 100 years for timber sleepers (based on ref 4). Even after existing timber sleepers have been replaced, they continue to decay, releasing greenhouse emissions during the process. The emissions resulting from the decay of existing timber sleepers were included to balance the consideration of the locked up carbon. These decay emissions were determined based on the average remaining life of the existing sleepers. It was assumed that 29% of sleepers were at the end of their service-life, while the remaining sleepers (71%) were, on average, halfway through their service-life. For the reinforced concrete sleeper scenarios, the decay emissions of existing sleepers being replaced were based on the average remaining life of all sleepers, assuming complete replacement of sleepers. For the timber sleeper scenarios, the decay emissions of existing sleepers being replaced were based on the remaining life of the service-life-expired timber sleepers until full decay (i.e., period required for full decay (100 years) minus the expired service-life). The emissions resulting from the eventual decay of the new timber sleepers were calculated based on the estimated carbon mass content of 50% of hardwood timber (28). This

TABLE 4. Initial and Recurring Embodied CO2-e Emissions of Reinforced Concrete and Timber Sleepers, Per km of Track recurring embodied emissions (t CO2-e/km)a

initial embodied emissions (t CO2-e/km)

reinforced concrete timber, virgin fastenings timber, 50% reuse timber, 96% reuse a

328 812 687 572

20-year service-life

30-year service-life

50-year service-life

984 2244 1898 1579

328

3249 2749 2287

Recurring embodied emissions shows the total emissions embodied in replacement sleepers over a 100-year period.

of steel fastenings (2.8 kt CO2-e). Compared to the life cycle emissions for all timber sleeper scenarios, the emissions associated with the reinforced concrete sleepers are from 46 to 83% lower. For the timber sleeper scenarios, the reuse of the majority (96%) of the steel fastenings saves 25% (0.9 kt CO2-e) of emissions over a 100-year period, compared to using virgin steel fastenings. Figure S3 in the Supporting Information provides a more detailed breakdown of the life cycle emissions of the sleeper scenarios.

Discussion

FIGURE 2. Comparison of total life cycle CO2-e emissions for sleeper scenarios by km of track. equates to 147 kg of CO2 per sleeper resulting from complete decay over 100 years, based on a conversion factor of 3.67 kg CO2/kg of carbon and a mass of 80.2 kg of timber per sleeper. The rate of decay for both the new and existing sleepers was assumed to be uniform for the entire life of the sleepers and assumed to result in 100% decay.

Results This section presents the results of the life cycle CO2-e emissions assessment of the eight railway sleeper scenarios. First, net embodied emissions are presented, and second, life cycle emissions are presented. Embodied CO2-e Emissions. Table 4 shows the initial and recurring embodied CO2-e emissions of the reinforced concrete and timber sleepers over a 100-year period based on 30-year and 50-year service-life and 20-year and 30-year service-life options, respectively. See Supporting Information (Figure S1 and S2) for a further breakdown of initial embodied emissions of reinforced concrete and timber sleepers. Life-Cycle CO2-e Emissions Comparison. This section presents the life cycle CO2-e emissions from both reinforced concrete and timber sleepers based on the method described in the Life Cycle CO2-e Emissions Assessment section (see Supporting Information Table S12 for calculation of values). Taking a life cycle perspective at this point is important, particularly considering that reinforced concrete sleepers will typically last longer than timber sleepers and also because the method of including locked up carbon needs to be balanced against decay. The comparison of the total life cycle CO2-e emissions for all eight assessed scenarios is shown in Figure 2. This shows that in terms of CO2-e emissions, the use of reinforced concrete sleepers are the preferred option from a CO2-e emissions reduction perspective. The life cycle emissions of the reinforced concrete sleepers over a 100-year period (0.82 kt CO2-e) are up to 70% lower than the best timber scenario, the timber sleepers with a 30-year service-life and 96% reuse

The embodied emissions figures presented may vary depending on a number of factors, including: source of harvested timber, modes of transport, location of timber mills and concrete plants, variations in durability from the assumed scenarios, and potential emissions savings from the recyclability of the materials postuse and from the use of materials with recycled content. Potential errors include those derived from the assumptions made in the study noted above, as well as others relating to the use of the various CO2-e emissions data, as detailed elsewhere (10). For example, the use of input-output data aggregates the products, commodities, establishments, or region into sectors, reducing the relevance of the results to any particular product, commodity, establishment or region. In addition, the economic system boundary that gives input-output analysis its depth and breadth limits the reliability of results when these financial flows are attributed to physical quantities of materials or environmental impacts. This paper presents the net greenhouse emissions associated with timber (red gum) and reinforced concrete railway sleepers on the basis of one kilometer length of track over a 100-year life cycle. This study has found that the emissions resulting from the use of timber sleepers can be from two to six times more than the emissions associated with the use of reinforced concrete sleepers. The extent of the difference in life cycle emissions between reinforced concrete and timber sleepers is dependent on a number of factors, including, their effective service-life and the extent of fastening reuse. The results suggest strongly that reinforced concrete sleepers result in lower life cycle greenhouse emissions than timber sleepers. However, there are a number of options that exist for reducing the CO2-e emissions from the use of railway sleepers, including: reducing the size and material intensity of sleepers if possible without adversely affecting structural performance or durability (for example, by displacing cement with up to 50% fly ash), using steel with recycled content, further improving the efficiency of materials use through the design, manufacture, and disposal stages, and considering impacts associated with other associated goods and services. While not currently common practice in Australia, the combustion of end-of-life timber sleepers and the use of the resulting thermal energy, as a substitution for the combustion of fossil-fuels, has the potential to provide further CO2-e emissions savings. A comparison of material and sleeper emissions intensities with those previously published by Energy Strategies (1) is provided in the Supporting Information. Some consideration VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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of the life cycle environmental impacts associated with rolling stock and associated buildings of the rail transport sector is also warranted. See Railway industry emissionssfurther context in Supporting Information.

Acknowledgments The author thanks the late A/Prof. Graham Treloar, The University of Melbourne, for his feedback on an earlier version of this paper and his many years of encouragement and support, as well as Prof. Rod Keenan, School of Forest and Ecosystem Science at the University of Melbourne and A/Prof. Greg Nolan, Centre for Sustainable Architecture with Wood, at the University of Tasmania for their valuable comments and advice given in relation to an earlier version of the research presented in this paper. The author also acknowledges Prof. Manfred Lenzen, The University of Sydney, for his work in developing the base input-output model used in the study. This paper is based on a study that was commissioned by the Victorian Department of Infrastructure in response to community concerns about the issue and was supervised by Mr David Hill and Ms Kate Murphy in the Freight, Logistics and Marine Division who have given permission for these findings to be published. The author thanks the anonymous reviewers for their valuable comments, which greatly improved the paper.

Supporting Information Available Details of the fastenings used for timber sleepers, proportion of reuse by fastening type for timber sleeper fastening reuse options, details of the number of sleeper replacements required over a 100-year period, detailed calculation of timber and concrete emissions intensity factors, detailed breakdown of initial embodied emissions of sleepers, detailed calculation of life cycle emissions values presented in Figure 2, and further discussion on sleeper emissions in the broader context of emissions from the Australian railway industry. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Energy Strategies: Review of CO2-e emissions from concrete versus timber sleepers; Report prepared for the Australian Rail Track Corporation; Energy Strategies: Manuka, 2007; 5 p. (2) Australian Environment Foundation (AEF). Forest Campaigners Can’t See the Emissions for the Trees. Available at www. aefweb.info/media792.html (Accessed March 23, 2007). (3) Werner, F. Life Cycle Assessment (LCA) of Railway Sleeperss Comparison of Railway Sleepers Made from Concrete, Steel, Beech Wood, and Oak Wood; Society for Research on Wooden Railway Sleepers: Zurich, Switerland, 2008; 7 p. (4) NSW Forest Products Association. Timber and Concrete Sleepers Comparison of Carbon Balance; NSW Forest Products Association: Sydney, 2007, 4 p. (5) Isard, W.; Bassett, K.; Choguill, C.; Furtado, J.; Izumita, R.; Kissin, J.; Romanoff, E.; Seyfarth, R.; Tatlock, R. On the linkage of socioeconomic and ecologic systems. Pap. Proc. Reg. Sci. Assoc. 1967, 21, 79–99. (6) Leontief, W.; Ford, D. Environmental repercussions and the economic structure: An input-output approach. Rev. Economics Stat. 1970, 52, 262–271. (7) Proops, J. L. R. Input-output analysis and energy intensities: A comparison of methodologies. Appl. Math. Modell. 1977, 1 (March), 181–186.

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