1. Introduction
Of the many environmental issues facing humanity, two of the more concerning are climate change and plastic pollution. These two environmental challenges have far reaching consequences for human and ecosystem health, and solutions to these problems will need to come from across the spectrum of human endeavour [
1]. One way to help resolve both of these issues is to use new products that have lower embodied emissions while also finding new ways to reuse plastic waste. The recycling and reuse of plastic is urgent as, in 2018 alone 359 million tonnes of plastics were produced, an increase of 3% on the previous year [
2]. If plastic is not recycled and reused, it accumulates in local and global ecosystems causing harm to natural habitats. Given that transport is one of the largest contributors to global greenhouse gas emissions [
3], one way to address plastic pollution would be to find new ways to use reclaimed plastic that improve transport networks while also reducing greenhouse gas emissions [
4].
To help reduce transport emissions, the increasing use of rail transport is a necessity because it is one of the lowest emission forms of transport [
5]. However, rail is not completely emissions free, with one of the largest sources of rail emissions coming from rail infrastructure, especially the use of railway sleepers [
6]. Traditionally, sleepers were produced with treated hardwood timber, with timber sleepers first used for rail infrastructure in the 1800’s. However, due to a scarcity of wood around the 1880’s, steel sleepers became more common, although timber sleepers continued to be deployed worldwide [
7]. Although at first glance timber sleepers might seem to be an environmentally friendly option, the harvesting and processing of the timber generates substantial waste. Moreover, because timber sleepers are chemically treated prior to installation when they reach their end-of-life stage it is often not possible to reuse them and they are difficult to discard [
8]. These reusing and recycling challenges, coupled with the short life span of timber sleepers, which last only ten to twenty years, means that timber sleepers can have a large negative environmental impact [
9].
One of the most common timber sleeper replacements in use today are concrete sleepers. Concrete sleepers were introduced in Britain, France, and Germany as early as 1943 due to the scarcity of timber during World War II, but prestressed concrete sleepers were only adopted in the United States in 1966 [
10]. Concrete sleepers have since become one of the most common types of railway sleepers installed worldwide owing to their increased life span and strong structural performance. However, the production of concrete sleepers may release in the range of 10 to 200 times more carbon dioxide equivalent (CO
2e) than treated hardwood sleepers [
11,
12]. Hence, while concrete sleepers offer some conveniences over timber sleepers, they are exacerbating the climate crisis through their high carbon emissions and, as such, they reduce the environmental benefits of rail as a transport option.
A possible solution to this problem, as well as that of plastic pollution, is the development of composite sleepers assembled from recycled waste plastics [
13]. Recycled composite materials have been used for the development of railway sleepers in Japan since the 1980’s [
14], although the use of this material has not been considered a realistic alternative in many countries due to perceived limitations of the material [
7]. However, technological advancements coupled with decades of research have resulted in improved products that mitigate many of those limitations, making this option potentially more appealing to railway industries.
The global market for composite sleepers is currently expanding at a rapid pace as research into these technologies created opportunities to improve upon the limitations of traditional sleepers, yet in Australia there are still few suppliers of these products due to a lack of demand. The lack of uptake in Australia is causing limited production, which in turn can cause the unit price of these products to be higher than traditional sleepers. However, at this stage, no study has investigated the relative emissions benefits of a shift to composite sleepers, nor whether such sleepers are cost competitive in Australia. Thus, it is unclear where the products might fit into the Australian market and whether they represent a viable method for reducing transport emissions and the generation of plastic pollution.
The purpose of this study is to address this problem by assessing the life cycle emissions and costs of composite and traditional sleeper technologies and determining whether a shift to composite sleepers is warranted and financially viable. To achieve these goals, the specific objectives of this study are to:
- (1)
conduct a Life Cycle Assessment (LCA) on the emissions of traditional and composite sleepers,
- (2)
conduct cost analyses of traditional and composite sleepers that considers the purchase, installation and life span of these products, and
- (3)
determine whether composite sleepers are more environmentally friendly than traditional sleepers and whether they can be cost competitive.
4. Discussion
The purpose of this study was to identify which type of railway sleepers have the lowest emissions and which have the lowest cost over their life cycles. To this end, the study included two sets of life cycle analyses, one focussed on emissions and the other on cost. The impetus for the study is an assessment of the potential of short and long fibre composite sleepers, which have the potential to help reduce plastic pollution by finding an alternative uses for recycled plastic that are long lived and hard wearing. Whether these sleepers are viable, however, also depends on whether they can contribute to meeting net zero goals and whether they are cost competitive with other types of railway sleepers.
The results of this study show that the total embodied emissions over a 100-year period favour short fibre composite sleepers with the lowest overall embodied emissions at 24.85 (or 19.17 with 50% reclamation) tonnes of CO2 equivalent, which is approximately 45% of the emissions of concrete sleepers, the most common sleeper type in use today. In contrast, the long fibre composite sleepers, which are more suited to high-speed rail applications than short fibre composites, had the highest emissions of any sleeper types with emissions more than three times higher than concrete sleepers. These results suggest that there is promise in using short fibre composite sleepers to help reduce the global warming impact of rail infrastructure, but the same is not true for long fibre composite sleepers.
In terms of the calculated emissions for the different types of sleepers, there were a few findings that may challenge conventional thinking. First, it is known that concrete is generally considered to be a large source of CO2 emissions with cement responsible for 8% of global emissions. This would suggest that concrete sleepers might be amongst the worst sleeper types in terms of emissions. However, this study showed that they were in fact the second lowest, and perhaps surprisingly, had much lower total emissions than timber sleepers. The high emissions identified from timber sleepers was another unexpected finding from this study. However, when one considers the energy intensive nature of the processes that go into creating timber sleepers, the result is more obvious. The high emissions for timber sleepers are largely the result of treating timber to make it a stable material to work with. The process of drying the timber often takes place in kilns that are a major source of CO2 emissions. Kiln drying is the fastest process whereas air drying the timber often costs manufacturers more because air drying takes up a lot of valuable space and more time. Another reason for the high timber emissions is their comparatively short design life and the need to use new fasteners during reinstallation (as it is not possible to achieve a 100% reuse rate in fasteners due to deterioration and damage over time). These factors combine to make timber sleepers one of the less attractive sleeper options from an emissions standpoint.
In terms of the transport emissions component, this study identified that, while all sleepers are roughly the same size, regardless of type, there are significant differences in weight that impact transport costs. Generally speaking, concrete sleepers and long fibre composite sleepers are much heavier than the other sleeper types, so transporting these sleepers to their installation site results in higher emissions than the other, lighter, sleeper types (timber and short fibre composite sleepers). The transportation emissions are proportional to the weight of the sleeper, with the short fibre variant having a weight of approximately 76 kg and the wooden sleeper weighing approximately 65 kg. This equates to emissions of 0.68 tonnes of CO2 equivalents and 1.47 tonnes of CO2 equivalents, respectively (note that timber sleeper emissions are higher owing to the need to replace them more frequently). The long fibre and concrete sleepers are far heavier and their transport emissions are 2.89 tonnes and 7.30 tonnes of CO2 equivalents, respectively.
For the purposes of this study, the installation and removal of sleepers were taken as a single process, because installing a sleeper on an existing track requires the removal of an old sleeper. Although sleepers with a high workability share the same method of installation, the design life of the sleepers ultimately means that the timber sleepers emit 15.25 tonnes of CO2 equivalents, which is significantly higher than that for any of the other sleeper types. The concrete sleepers had the lowest emissions attributed to installation and removal as the method of installation is more specialised and there are less sleepers per 100 m of track, therefore installation is faster and uses less fuel. The emissions associated with the short and long fibre variables are 6.10 tonnes of CO2 equivalent and the emissions associated with installation and removal of concrete sleepers is 5.81 tonnes of CO2 equivalent.
Next, the study analysed life cycle costs to assess whether the more environmentally friendly sleeper options were cost competitive, with industry rarely changing for environmental reasons alone. For example, the shift from timber sleepers to concrete sleepers was largely driven by lower costs [
11,
26,
27]. This price differential was confirmed in this study with, using the design considerations previously described, the total inflation adjusted life cycle cost of timber sleepers is
$436,929.72 compared to prestressed concrete, which costs
$141,888.44, about one third the cost of the timber they replaced. In addition, although the initial purchase price of individual timber sleepers is already higher than concrete sleepers, the need to replace timber sleepers more often results in an even higher cost. The analyses above also show that there was an unintentional reduction in greenhouse gas emissions at the same time, suggesting that the shift from timber to concrete sleepers was beneficial in several ways [
21,
26]. However, the global focus on climate change and pollution means that there is now more concern than ever about the impacts of the products and materials we use. So, the question is whether there is a viable replacement product for concrete sleepers that has an even lower environmental impact for a similar economic cost.
This study looked at two such products, short and long fibre composite sleepers. These two products have different uses, with long fibre composites able to replace concrete sleepers for high speed and heavy load tracks and short fibre composite able to replace concrete sleepers for lower speed and load tracks. The emissions analyses suggest that there is no imperative to switch concrete for long fibre composite sleepers, because their emissions are higher than that of concrete sleepers. Moreover, the cost analysis shows that this sleeper type is also the second most expensive, nearly as expensive as timber sleepers, at a life cycle cost of $427,233.66. Therefore, it seems clear that the long fibre sleeper is neither financially viable nor environmentally desirable and in the absence of future improvements that change this equation, these sleepers are not recommended for extensive use in Australia or elsewhere. However, long fibre composites do have a niche role to play in the market.
This study considered the use of long fibre composites within mainline use and has concluded that they are not viable for that use. However, where they may be used is as bridge transoms that distribute train axle loads directly to bridge girders or where low ballast is a requirement. These specific cases require materials with unique properties, which long fibre composites fulfill better than any other material types considered in this study. Due to the specialised nature of long fibre composites they should not be considered for mainline use but they will likely continue to be used in these special circumstances.
In contrast, short fibre composite sleeper costs in the base case are more like those of concrete, although still somewhat higher at
$186,518.12. These results suggest that cost conscious railway industries are likely to shun this product even though they are clearly superior from an environmental standpoint. However, this initial calculation is based on assumptions of no material reuse. Although the short fibre sleeper industry is still in its infancy, and as such there is currently no market for the reuse of these materials, if this product were to become more common, such an industry would clearly develop and short fibre composite sleeper manufacturers suggest that they would be able to reclaim up to 100% of the material for reuse [
16]. This study adopted a more conservative approach, assuming that 50% of the material could be reclaimed. Adopting this conservative approach, the analyses found that short fibre composites life cycle costs were reduced 23% to
$142,701.76, which is just above the life cycle costs for concrete sleepers (more expensive by
$813.32). That is, the short fibre composite sleepers are financially viable and their environmental benefits are substantial, suggesting that a shift to short fibre composite sleepers is warranted anywhere conditions for their use are met (i.e., lower speeds and loads). However, there is likely to be some initial resistance to using these products from the industry, which may resist change due to safety considerations and because the benefits of the resilience and reuse dimensions of these alternative products will not be felt for many years. Research such as this, however, should help to promote the viability of short fibre composite sleepers.
In terms of the breakdown of costs for each sleeper type over its life cycle, transportation costs are relatively high for concrete sleepers, amounting to just over 5% of the life costs for these sleepers and to less than 1% of total costs for all other sleeper types. However, the study did assume that all sleeper types were transported to the site by truck, and if rail transport was used instead, the costs of transport would decrease for all sleeper types. This would obviously impact most on the calculation of total costs for concrete as its starting costs for transport are higher. For fastenings, the highest costs were observed for timber sleepers (8.6% of the total cost of these sleepers), owing to their shorter lifespans, whereas concrete sleepers included no specific fastener cost as their fasteners are included in the purchase price of the sleepers themselves.
This study assumed that the same equipment, machinery and labour would be used for the installation and removal of each sleeper type. Excluded from the calculation of installation/removal costs was the purchase of the equipment or machinery, so the costs reflect the diesel fuel consumed during these processes and are dependant on current diesel fuel prices. The price of diesel fuel in Australia at the time of the analyses was $1.53 per litre. Installation and removal costs were the second largest contributor to life cycle costs for all sleeper types. The cost for installation and removal of a single sleeper was calculated at $121.75, which was the same for all types. Of the $121.75, $110 was from labour expenses for a combination of labourers, machine operators, supervisors, and safety coordinators. The other $11.75 came from fuel consumption. The fuel consumption was calculated from the use of a Caterpillar M315F excavator and Caterpillar 950GC front end loader working for eight hours a day. The M315F excavator was used as it is a wheeled excavator rather than a tracked excavator and these excavators are often preferred because they can easily be equipped with a hi-rail attachment giving them the ability to drive on train tracks, while the 950GC front end loader is a typical medium sized wheeled machine.
The purchase prices for individual sleepers showed that timber was the cheapest at $90 plus fastenings, followed by concrete at $140 including fastenings, short fibre composite at $203 plus fastenings and the long fibre composite at $650 plus fastenings with the fastening systems costing approximately $20 per sleeper. These prices were obtained via personal communication with representatives from the respective sleeper producers or suppliers. The price of standard gauge short fibre composites is held in commercial confidence so the price of $203 has been estimated using the available price of narrow-gauge sleepers produced from the same material. To extrapolate this price, the six-tenths rule was used with the volume of the two products. Further, while the price of long fibre composites was publicly available it was provided in dollars per cubic metre so this was adjusted based on the assumption that the long fibre composite sleeper would have the same volume as a timber sleeper, due to their similar physical properties. As expected, the price of the individual sleeper was the largest share of the life cycle costs for all sleeper types amounting to 38.65% of the cost of timber, 50.63% of the cost of concrete, 58.66% of the cost of short fibre composite and 82% of the expenditure for the long fibre composite sleepers. Given the importance of purchase price for all sleeper types, any changes to these over time will significantly impact their total costs. Of the sleeper types discussed in this paper, only short fibre composite sleepers have the potential to significantly reduce their costs because this industry is still in its infancy and has yet to benefit from economies of scale, experience curves and competition forces that would likely lead to future cost reductions. Meanwhile, the other sleeper types have been on the market for a long time and their costs are only sensitive to fluctuations in material prices or changes in government legislation or regulation.
Beyond production costs, the conservative parameters used in the analyses above suggest that there are several likely changes in the future that could make short fibre sleepers more cost competitive. If we consider the comparison between concrete and short fibre composite sleeper costs in more detail, for the zero reuse or material/fastener reclamation case, there is a difference in life cycle costs of $44,629.68 in favour of concrete if the sleepers achieve their design life. However, this gap is erased when the 50% reuse case is considered. However, these costs do not include the likely future price reductions that are expected to occur for short fibre composite sleepers, as the total value of any such reductions is unknown. Another possible driver of future price reductions for short fibre composite sleepers would be the broader use of a local (Australian) or global carbon price. As previously discussed, short fibre composite sleepers have significantly lower emissions than their concrete equivalents. A notable carbon price would result in future price reductions relative to concrete sleepers, which would need to pay a higher carbon cost.
Carbon prices have been established in many parts of the world to stimulate a shift to more environmentally friendly alternatives [
28]. Carbon prices are a set unit price that liable companies are required to pay per tonne of carbon dioxide that they produce as a result of operations [
29]. Australian Carbon Credit Units hold momentary value and can be awarded to companies that undertake projects that either store or avoid carbon dioxide emissions in units of one tonne [
30]. Currently the Australian Carbon Credit Unit spot price is
$37 [
31]. This study has shown that concrete sleepers produce 43.55 tonnes CO
2e while short fibre composites produce 24.85 tonnes CO
2e, over the same time and for the same length of railway track. This results in a difference of 18.7 tonnes CO
2e, meaning that if a rail authority chose to use short fibre composites over concrete sleepers, they may be eligible for Australian Carbon Credit Units to the value of
$691.93 per 100 metres of track. Meanwhile, the European Union’s price of carbon is
$78.06 [
32]. If carbon prices within Australia where to increase to this level, rail authorities would be eligible for
$1459.78 per 100 m of track (more than the
$813.32 higher price for short fibre relative to concrete sleepers). This would make short fibre sleepers more cost effective than concrete sleepers and although the total financial benefit of just over
$600 per 100 m of track may seem trivial relative to the total life cycle costs for each sleeper type, they represent significant costs (or costs savings) over long lengths of track and make short fibre composites clear winners over concrete sleepers financially as well as environmentally.
The combined focus of this study on both embodied emissions and cost implications allows for a more holistic evaluation of the sleeper products. The results clearly highlight the environmental advantages of short fibre plastic composites and based on environmental performance alone this study suggests that these products are the most advantageous. Including a consideration for cost makes the evaluation more challenging. Currently, the reclamation of composite sleeper material is not practiced due to the current product life cycle and the limited available volumes of the product. When considering the life cycle cost for this case, the short fibre composite is outperformed only by the concrete variant. However, calculating the life cycle costs with the assumption of 50% reclamation of materials, the cost advantage of concrete is completely removed. Furthermore, as short fibre composites are completely recyclable it would theoretically be possible for the nominal 50% figure to be surpassed and which would tip the cost advantage towards the short fibre composites.
Although this study provides a relatively comprehensive assessment of the four sleeper types, there were some limitations to the work. These include the heavy reliance on existing literature for the collection of data. Each of the considered studies will have drawn on data that may have had their own limitations in sourcing information. In addition, as composite sleepers are a new product, there is very limited published work available for them. Hence, several assumptions were required about their utility and longevity and only time will tell whether these have been reasonable or not.