Concrete or wood? Railroads around the world face this decision daily as they replace millions of deteriorating cross ties used as a base for railroad tracks. In addition, new high-speed passenger rail lines under construction in the U.S. Northeast Corridor, Spain, China and other countries require use of precast concrete ties to obtain desired train speeds.
Advantages of using reinforced concrete over wood include longer service life, greater strength (thus requiring fewer ties per mile of track) and lower maintenance costs. However, while the performance and financial benefits of using reinforced concrete ties have been evaluated, little information is available on the preferred choice from an environmental perspective. Until now.
A new report from Robert Crawford of the University of Melbourne (Australia) supplies information that helps fill this environmental information gap. Crawford is a life cycle analysis expert who studies environmental impacts such as greenhouse emissions associated with various types of construction. He concludes that production, placement and lifespan of concrete ties produces as little as one-sixth of the greenhouse gas emissions as that associated with timber ties.
Previously, critics of concrete railroad ties have charged that their manufacture increases greenhouse gas emissions compared with wood ties because concrete tie production requires more energy than wood tie production. Crawford’s life cycle analysis points out that more than just tie manufacturing needs to be considered to accurately compare the environmental impact of the two types of railroad ties.
Crawford studied the greenhouse gas emissions of reinforced concrete ties and wooden ties based on one kilometer (0.62 miles) length of track over a 100-year life cycle that includes mining of raw minerals to make concrete versus timber harvesting, tie manufacture, tie installation and use and finally disposal at the end of the ties’ useful life. On this life cycle basis, Crawford’s results indicate total greenhouse gas emissions from reinforced concrete ties can be from one-half to as little as one-sixth (17%) of that from wood ties.
Of course, tie service life can affect the environmental impact of each type of tie. The more frequently ties must be replaced, the greater the environmental impact of the ties used on a mile of track over a long period of time such as the 100 years of Crawford’s study. Tie service life can vary depending on the weight of the tie and both the weight and speed of the trains riding over the ties.
The concrete tie service life Crawford used in his various life cycle analyses ranged from 30 to 50 years and 20 to 30 years for wood ties. He cited the lower value for heavy haul routes and the higher service life for lines with moderate traffic. These numbers seem reasonable given estimates by representatives of two U.S. precast concrete tie manufacturers.
According to Al Smith, a manager with Rocla Concrete Tie Inc., “Average concrete tie life is 55 to 60 years. Average life of hardwood ties is 20 to 25 years.”
Kevin Haugh, president of CXT Inc., provides somewhat shorter estimates of tie service life: about 40 years for concrete versus a range for wood tie life of from 8-10 years up to 15-25 years dependent on the climate and wood type. Most U.S. wood ties are hardwood, although less durable softwood ties were once used.
OTHER ENVIRONMENTAL FACTORS
Crawford suggests that greenhouse gas emissions per mile of track using concrete ties could be further reduced. Suggestions he offers include “displacing cement with up to 50 percent fly ash.”
Concrete ties form a more stable track bed resulting in a smoother ride leading to longer track and equipment life, lower fuel consumption and lower maintenance costs for both locomotives and rolling stock. While these factors are ordinarily considered economic and performance ones favoring precast concrete tie use, they also provide environmental advantages over using wood ties.
OUTLOOK FOR CONCRETE TIES
As of January 2008, the approximate market share in North America for traditional wood ties was 91.5 percent, according to Railway Tracks & Structures Magazine. Concrete tie use is substantially greater in some Asian countries, while in Europe, most new ties are precast concrete. The shorter European shipping distances improves relative economics of the heavier concrete ties versus lighter wood ones, Smith says.
With the exception of some commuter lines, environmental concerns are not high on the list of factors driving increased U.S. use of precast concrete railroad ties, notes Haugh. Currently economic and performance factors are leading to increased use of concrete ties in the U.S. However, “in considering choice of ties, environmental factors on are on the radar and becoming more significant,” says Smith. There is no doubt that environmental concerns are a stronger factor in the choice of ties elsewhere, mainly Europe.
The bulk of new business in precast concrete railroad ties is coming from Class I Railroads, which are replacing wood ties with precast concrete on heavy haul routes — those handling 100 million gross ton miles per year. Heavy haul comprises about 20 percent of Class I route miles.
The better rail support provided by concrete ties is becoming ever more necessary because the weight of freight trains is increasing. For example, notes Smith, 100-ton hoppers standard for unit coal and grain trains are gradually being replaced by 125-ton cars. In addition, some single track mainlines are being double-tracked to reduce freight train transit times and are using more precast ties. Burlington Northern Santa Fe, for example, is double tracking its mainline from the West Coast to Chicago.
New light rail systems are under construction in several large U.S. cities as well, with additional systems and expansions planned. “About 90 percent of the ties in new light rail systems are concrete ties,” says Smith. “In replacing worn wood ties in existing light rail and commuter train systems, most transit systems are switching to concrete ties.” Systems using precast concrete ties include those in Dallas, Portland, Sacramento, Los Angeles and San Diego, reports Smith. The Utah Transit Authority’s Salt Lake City system is being extended to Provo while the New Mexico Railrunner from Santa Fe to Albuquerque is being extended south to Belen. Both are using precast concrete ties.
One reason commuter lines prefer concrete ties is to reduce commuting delays caused by track maintenance operations. “System operators can almost ‘set it and forget it’ using concrete ties,” notes Smith. Some commuter systems such as the Chicago Transit Authority cite environmental concerns as being an important factor in switching to concrete ties, he reports.
Haugh believes that the $13 billion in President Obama’s economic stimulus package for high-speed rail will drive increased use of concrete ties. However, “it is difficult to say which of the proposed routes will actually be built. The good news is that there is actually money on the table for high-speed rail.”
So, returning to the original question, precast concrete or wood ties? It appears that the pendulum is swinging toward increased use of concrete ties for both environmental and economic reasons.
John Borchardt is a freelance writer who covers science, technology , career and business. His Oxford University Press book “Career Management for Scientists and Engineers” was a Science Book Club Alternate Selection.