by Michael L. Starn, Editor
It may be a marriage of convenience, but it is also one that will probably last. The marriage is between solid waste and roads. There is ample give-and-take in this relationship, with plenty of reusable solid waste being generated and a strong affinity for these materials in the road construction industry.
In Maine, state and local transportation officials, and those in the road construction business, have taken their recycling vows and are finding creative ways to reuse, rather than bury or burn, certain types of solid waste materials. Scrap tires are being chipped and used as fill and the rubber is being added to asphalt mix; glass is being crushed and used as a gravel substitute or mixed with asphalt to make "glasphalt"; roofing shingles are being converted into cold mix (and to a lesser extent hot mix) for pothole patching; and old asphalt, which used to be land-filled, is now considered a valuable resource by state and local transportation officials and paving contractors who are reusing it in all aspects of road construction.
"The recycling of waste material in pavements by most accounts is here to stay," says Larry Flynn in an article for Roads & Bridges magazine (October,1992). Two questions, says Flynn, must still be answered: "Putting waste materials in our roads may benefit society, but to what extent? And what impact will it have on the quality of our roads in coming years?"
Nationally, approximately 180 million tons of solid waste are generated in each year. Scrap tires make up just under 3.6 million tons, which translates into about 240 million tires; however, there is a huge stockpile of existing tires estimated to number two to three billion. Glass accounts for about 15 million tons of solid waste generated each year. And, approximately seven million tons of old roofing shingles are placed in landfills each years, with an additional 900,000 tons of waste produced by shingle manufacturers.
In Maine, about 1.25 million tons of municipal solid waste is generated in a year. Scrap-tires account for about 12,000 tons; glass totals 65,000 tons; and while the amount of asphalt roofing shingles discarded each year is not known, Tom Kane from Commercial Recycling Systems in Scarborough, the only DEP licensed facility in the state to accept them, says the volume is significant.
Crumb rubber as an additive or modifier in asphalt pavements is one of the two biggest potential markets for the use of scrap tires, according to the Environmental Protection Agency. This statement along with an increasing environmental concern over the stockpiling of scrap tires in this country encouraged Congress to throw its support behind the use of rubberized asphalt when it passed the Intermodal Surface Transportation Efficiency Act (ISTEA) in 1991. That Act requires that states use scrap tire rubber in asphalt pavement projects receiving federal funds beginning in 1994 (however, this has been put on hold until 1996).
This federal mandate does not sit well with some state officials and some roadbuilding contractors.
Most of the concerns are over cost. Asphalt-rubber mixes can be 40 to 100% higher than that of standard asphalt mixtures. Additional amounts of asphalt cement are required in the wet process because the rubber absorbs a portion of the asphalt cement during the blending process.
Crumb rubber can be added to asphalt pavement in one of two ways: the wet process or the dry process.
The wet process melts and then blends the crumb rubber with the asphalt cement prior to incorporating cement with the aggregate. Asphalt cement is the black, sticky, petroleum-based substance that binds aggregate together to create the asphalt pavement. In general, rubber comprises 15 to 20 percent, by weight, of the asphalt cement when used.
The dry process differs from the wet process in that crumb rubber is mixed with the aggregate before the mixture is charged with the asphalt cement. In this process, dry crumb rubber is used as a substitute for some of the aggregate in the asphalt mix.
Warren Foster, Director of Technical Services for MDOT, says the state is doing a lot with scrap tire, but he isn't real keen on rubberized asphalt.
Mostly the state is using chopped rubber tires as light weight fill. Over 100,000 tires were used in a North Yarmouth project and another 200,000 in a project on Route 9. A project in Topsham (the Brunswick-Topsham By-Pass) is expected to use upwards of 350,000 tires.
"Tires are taking the place of common borrow (regular fill)," says Foster. "As a light-weight fill, tires are very cost effective," he adds.
Chipped at 6" minus for fill, the tires are costing the state about 25 cents a piece. This is significantly cheaper than the $20 to $25 apiece that the DOT spent for 6,000 tires blended into the asphalt (wet process) in an experimental project in Old Town.
Foster says the Federal Highway Administration (FHWA) appears to be backing off the rubberized apshalt mandate. "There are other ways to meet the requirements," he says. "If we use 300,000 tires in fill, it should satisfy the objective."
When the state does use rubber in asphalt, Foster says DOT is leaning toward the dry process. "It performs well if you don't put too much (rubber) in," he says.
MDOT is also finding a use for scrap tire as backfill for retaining walls.
At the local level, public works officials have had limited opportunities, because of limited resources, to use scrap tires. The foremost local project to date has been in the Town of Richmond where a professor at the University of Maine and the U.S Corps of Engineers teamed up to lay down tire chips as an insulating layer—12 to 24 inches thick—under a 750 foot section of (gravel) road. Approximately 20,000 tires were used in the Richmond project.
Yarmouth Public Works Director Bill Shane saw that Regional Waste Systems had a problem that he might be able to help them with.
RWS collects about 600 to 1,000 tons of glass a year from its member communities. The recycling markets for glass are limited, says Shane. Clear glass has some marketability, but colored glass does not. RWS's problem was that separating the clear from the colored glass was too labor intensive and landfilling it was too expensive (about $45 a ton). So, the solution was to hire a contractor (RJ.Grondin in Gorham) to crush the 600 tons (for $10 a ton) and use the product to make glasphalt.
The towns of Yarmouth and Falmouth decided to give glasphalt a try. They hired Blue Rock Industries of Westbrook to get the crushed glass (1/4" minus) from Grondin and mix it with the asphalt (in Yarmouth used as the lower binder course). The glass was crushed into very small pieces so that it would be a substitute for the sand portion of the aggregate in the asphalt mixture.
Shane says that there was only a $1 per ton difference between glasphalt and conventional hot mix asphalt. He thinks that in the future, with more competition and more experience with the technology, the price differential will disappear.
Yarmouth used a 10 percent mix of glass. Shane says, "We did it because it's a way to get rid of a waste product without having to landfill it; to avoid disposal costs."
South Portland experimented with glasphalt almost three years ago. Public Works Director Arvin Erskine says the "glasphalt has been excellent" and "I want to do more of it." He adds that the price differential was minimal, only 54 cents per ton more for the glasphalt.
In South Portland, glasphalt was used in the top layer of pavement. Erskine says a portion of the "glasphalt" project runs in front of his sister's house where at night under the streetlight, "you can see it glitter."
That's why for his next project with glasphalt, Erskine would like to use it in curbing.
One of the concerns over using glasphalt has been the potential for stripping (meaning that the glass particles tear away from the asphalt cement). To help avoid this problem, in South Portland the crushed glass was encapsulated (coated) with asphalt cement before it was mixed. Neither South Portland or Yarmouth has seen any evidence of stripping.
A potentially "hot" item in the recycling marketplace is roofing shingles. Asphalt roofing contains high grade materials that can be reclaimed and used in both cold mix and hot mix asphalt.
Cold mix is the more established and readily used of the two reclaimed asphalt roofing waste material products. The most common use of this product for municipalities is to patch potholes.
The City of South Portland has tried the cold mix with asphalt shingles and Public Works Director Erskine is very happy with the results. He has been particularly pleased with the way the patch has held up through rain and melting snow. "We've had very good luck with it," he says.
"l think it's a good way to get rid of shingles, and patch potholes at the same time," says Erskine.
Reclaiming (or reusing) asphalt is a widely accepted practice in Maine; however, the reclamation of roads—instead of overlays or reconstruction—has yet to gain full acceptance by municipalities or the MDOT.
There are two basic reclamation processes. One (-the most common in Maine) uses a rotary drum machine that functionally resembles a rototiller by digging into the asphalt and base, churning it up into the drum, mixing and grinding it thereand redepositing it as a base material. The other is a Hammermill process that pulverizes pavement and base into small chunks, without mixing. This process is used primarily when there is a large thickness of pavement or oversized rocks are in the base material.
"It (reclamation) is an area where I think towns could be doing a lot more," says Peter Coughlan, director of the Maine Local Roads Center.
"Thin overlays are great from a political standpoint," says Coughlan, "but they are short-term solutions."
Yarmouth's Bill Shane agrees. "I've been preaching reclamation for six or seven years," he says. "It's truly the way to go, otherwise you're throwing money away (by overlaying roads that have inadequate base material)."
"A lot of money is being spent by towns in the wrong places," says Shane. He adds that town officials need first to understand that drainage is the most important thing, followed by an adequate road base.
Shane points out that reclamation is not the answer to everything. The road's situation has to be taken into account. "Reclamation doesn't solve drainage problems," he says.
In addition to reclamation, there are a number of other ways to reuse asphalt. Milling, planing and grinding are processes where asphalt is reclaimed.
"Nothing is ever wasted" in state road construction projects, says ,MDOT's Warren Foster. Foster says either asphalt surface material is reused in the job or the paving contractor takes it back to the asphalt plant to he reclaimed.
South Portland's Erskine has done some planing, followed by stockpiling the asphalt and then using it on a hot summer day when it can be broken into pieces by a front end loader. Erskine says planing can take off three-quarters to an inch of road surface (where several overlays have built up the surface). "We use a front end loader to take it out (from the stockpile), lay it down and then run over it with a dozer to flatten it, and then finally grade it," says Erskine.
OTHER RECYCLED MATERIALS
Plastics, bricks, concrete, and oil containing soil can also be recycled but their use in road construction has been limited to date.
The use of polymers (plastic) to modify the characteristics of the asphalt cement binder in hot mix asphalt mixtures is an accepted practice in the highway construction industry. Properties inherent in polymer additives enhance the performance characteristics of asphalt cement binder. Recycled plastics have also been used in highway appurtenances, such as sign posts, snow fences, manhole covers, and traffic cones.
Bricks and concrete from building demolition projects can be crushed and used as road fill, base or shoulder material.
Soil that has been contaminated by underground tanks that have leaked can be remediated and put back into use as road base, road shoulder, recycled hot mix or "hot sand" (a de-icer). After remediation, it can even be used on lawns or as landfill cover.