Eco-friendly Road Construction
Texas highways are built using many recycled materials, and CTR is proud to be part of the effort to benefit the Texas environment as well as Texas drivers. Three substantial and creative research initiatives undertaken by CTR on behalf of the Texas Department of Transportation (TxDOT) involve old tires, used copier and printer toner, and fly ash (a by-product of coal burning plants).
Old Tires Keep On Rolling
Texas faces two seemingly unrelated problems: an increasing stockpiles of waste tires and the growing need for highway construction backfill material (material used to replace and augment the soil excavated when a highway is built.) In a fascinating development, both problems might be solved by using old tires as a portion of—and as reinforcement for—the backfill material. CTR conducted research on the potential of this solution and found that the tire bale solution presents unique environmental, technical, and economic opportunities.
The problem of discarded tire storage is significant. TxDOT found in 2005 that Texans generate 24 million scrap tires each year, the same as the national average of over one tire for every person residing in the state—that’s 281 million tires being retired each year in addition to the discarded tires already stockpiled throughout the country. Further, the composition of vehicle tires is such that they cannot be recycled into the form of new tires, as tire rubber resists re-molding.
Fortunately, these old tires can still serve drivers on new highways. Waste tires can be compressed into bales, a process in which approximately 100 car tires with a volume of 20 cubic yards can be compressed into a 2-cubic-yard block.
Approximately 100 car tires with a volume of 20 cubic yards can be compressed into a 2-cubic-yard block
The bales are particularly useful as reinforcement for the much less stable backfill materials that cover them. Reinforcement is important because it adds an extra level of safety for steep highway embankments. The bales are relatively light compared to an equivalent volume of soil, and they pack together solidly, providing additional stability. The bales’ structure allows for drainage in the area, an essential feature of any backfill component.
In addition, tire bales involve a relatively inexpensive manufacturing process; each bale is fastened with galvanized or stainless steel baling wire. These bales are easy to install because they are compacted prior to delivery. As a consequence, tire bales have been reported to substantially reduce construction costs for large highway projects.
More information is available in CTR’s report 5-9023-01-1, Validation of the Effectiveness of Tire Bale Use in Slope Repairs and Embankment Construction (PDF, 648 KB).
Keeping Pavements Well Toned
Most people have had the common experience of changing a printer toner cartridge, or possibly calling for help when trying to add a new batch of messy copier toner to the office Xerox machine. Although many printer and copier toner cartridges are now recyclable, the spent toner inside is not, and must be deposited in landfills for lack of an alternate use. Another source of toner is also adding up in our landfills—the waste toner that is disposed of at the toner factories when a batch does not meet certain quality specifications. Waste toner has the potential to become a serious solid waste disposal problem. The most recent estimate available indicates that 25,000 tons of this below-grade toner is wasted per year.
Toner being placed in the mixer used in the Pharr, Laredo, and Bryan paving projects
At the request of TxDOT, CTR conducted research to discover whether some of this waste toner could be successfully integrated into asphalt pavement mixes. The researchers created test sections of pavement, in which one section had toner in its mix while the other was composed of the typical asphalt mix. The results were encouraging; the researchers concluded that waste toner can be successfully recycled into asphalt. The toner improves the strength of asphalt, helping to prevent ruts in the road. Also, this pavement is predicted to have a longer life than standard asphalt.
So encouraging were these results that TxDOT applied for a patent for toner-modified asphalt compositions, which they were awarded on September 5, 2000. CTR is proud of its contribution to research addressing the need for a better way to utilize waste toner.
For more information, read the CTR report 5-3933-01-2, The Toner-Modified Asphalt Demonstration Projects (PDF, 975 KB), and other related reports listed on the CTR Reports Online page.
Finding Strength in Ashes
Burning coal produces more than half the electricity generated in the U.S. The by-products of coal combustion represent a huge amount of material, some of which is actually put to use in construction applications and for snow and ice control. However, typically less than 20% is utilized rather than disposed of. One of these by-products is fly ash, tiny particles of non-combustible material that remain after coal burning.
CTR researchers have experimented with adding fly ash to concrete pavement, both to help keep fly ash out of landfills and to determine whether the addition of fly ash contributes to concrete’s strength or durability, with good results. In fact, concrete mixes containing more than 25 percent fly ash by weight has been dubbed “high performance concrete,” as it offers many benefits. It is stronger, less permeable, and more durable than standard concrete, nearly doubling the expected life of traditional concrete. These benefits are achieved because large amounts of fly ash in concrete will reduce the number of pores that are responsible for drying shrinkage, which reduces the overall shrinkage of the mixture. When shrinking is reduced, cracking is also reduced.
Concrete with fly ash mixed in can also
- improve workability
- improve resistance to sulfates and seawater
- enhance protection for reinforcing steel
- reduce costs
A more detailed account of the use of fly ash in concrete is found in CTR report 0-4098-4, Evaluation of Alternative Materials to Control Drying Shrinkage Cracking in Concrete Bridge Decks (PDF, 3.9 MB).