Railroad ties around the world are up for replacement. The crossties that hold railroad tracks together are traditionally made of wood, but now, stronger and longer lasting -- but more energy intensive to produce -- concrete ties are available. Which tie material is greener - wood or concrete?
Railroad Tie Replacement
Rail is an efficient method of transporting goods and people long distances. With a single gallon of diesel fuel, a train can move one ton of materials over 450 miles (Judge, 2010). However, just how green railroad transport is depends in part on the material used to replace old ties. Railroad ties, also called crossties or sleepers, are the crosspieces that hold railroad tracks together. Traditionally, they are made of timber, but reinforced concrete ties are now an option. All crossties deteriorate over time and will need replacing. The material choice for replacement ties affects the environment, mainly from carbon emissions associated with the life cycle of the ties. Which material is the greener choice, wood or concrete, is up for debate.
Timber Railroad Ties
A benefit of wood crossties is that the majority of the energy requirement for their production is solar energy consumed during tree growth. The ties themselves also sequester carbon in their biomass, although live trees provide more benefit by actively converting carbon dioxideto oxygen.
The main concern over wood ties is that they require harvesting of large amounts of timber. Additionally, as wood ties age they release carbon dioxide back into the environment. Another concern is creosote treatment of wood ties. This preservative poses an environmental concern because it can leach into ground water. On the positive side, creosote- a waste product itself from the coal smelting process of steel production- is biodegradable and lengthens sleeper life by eight to ten times, reducing the number of trees needed.
Concrete Railroad Ties
Reinforced concrete sleepers are stronger and last longer than timber ones, lasting about 30 to 50 years, as opposed to the 20 to 30 year life span of wood ties. Due to their strength, they can support greater loads or have wider spacing than wood ties, thus cutting down on material requirements. Additionally, reinforced concrete ties make stiffer tracks with less resistance than wood, increasing the fuel efficiency of trains by 1.2 to 4.5 percent.
The environmental concern regarding concrete ties is the large amount of fuel required during their manufacture. Although there is some fuel consumption involved in manufacturing wood ties it is much greater for concrete. Mining raw materials for concrete requires yet more fuel and damages local ecosystems.
Railroad Tie Life Cycle
Every part of a railroad tie’s life cycle, from raw material acquisition, to manufacturing, to transport and end of life, factors in to its overall sustainability. Transportation of material to the site is a significant source of carbon dioxide emissions associated with railroad ties, no matter their material. Therefore, the locations of timber mills, concrete plants, raw material mines and timber farms are all important factors to consider when choosing between wood and concrete ties.
A study published in Environmental Science and Technology assessed the entire life cycles of wood and concrete ties, based on one kilometer of track over 100 years in Australia. Results show that despite their more energy intensive production process, concrete ties produce one-half to one-sixth less carbon dioxide over their life than timber ties do. In this study, concrete ties came out on top, but results may differ by location and by assumptions made during life cycle analysis.
Reduce Carbon Emissions
No matter what material they are, there are ways to improve the sustainability of railroad ties. At end of life, combustion of wood sleepers can provide thermal energy, and offset the burning of fossil fuels for heat and electricity. This process does return sequestered carbon to the atmosphere, but this occurs regardless as ties decompose.
Making use of recycled materials, particularly recycled steel as concrete reinforcement, reduces carbon emissions associated with concrete tie manufacture. In addition, an innovation in concrete mixes incorporates fly ash, a toxic byproduct of coal-fired power plant operation. In railroad ties, replacement of up to 50 percent of the cement with fly ash is possible. Not only does this reduce the use of a non-renewable, expensive resource- Portland cement- it also safely sequesters fly ash that otherwise must be disposed of as a hazardous material.
Which is Greener?
Which railroad tie material is greener- wood or concrete- is not simple to determine. Based on previous crosstie life cycle research, the answer is tentatively concrete. However, multiple factors are at play, and each railroad site and situation is different. At a site with local, sustainably harvested timber available, wood may be the more sustainable solution, especially when concrete ties must travel long distances to reach the site.
Railroad companies are wise to consider which material has the least environmental impact when railroad ties come up for replacement. Choosing carefully will provide positive publicity for the company and can reduce lifecycle carbon emissions associated with railway transport.
Berryman, Charles W., et al. "Utilization of Fly Ash in Reinforced Concrete Transportation Applications" University of Nebraska-Lincoln, College of Engineering and Technology
Choi, Charles Q. "Working on the Railroad" Scientific American. July 2009. v301 i1 p30.
Crawford, RH. "Greenhouse Gas Emissions Embodied in Reinforced Concrete and Timber Railway Sleepers" Environmental Science & Technology. May 2009. v43 i10 pp3885-3890.
Fountain, Henry. "Environmental Edge For Concrete Rail Ties Over Wooden Ones" The New York Times. May 19, 2009.
Judge, Tom. "Crossties: Color Them Green" Railway Age. March 2010 v211 i3 p23(3).
Related - Bright Hub: Recycling Concrete
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