| November 12, 2011 |
Do the Right Thin
Thin asphalt overlays: Valuable versatility
For a host of reasons, thin asphalt overlays are proving to be one of the most useful road treatments in the tool boxes of state and local transportation departments. Budget restraints have pressured agencies to make their road dollars cover more square yards, and thin overlays can do that. Plus, agencies want to add the maximum life to a pavement – preserve it for as many years as possible.
Again, thin overlays answer the bell.
“We’ve been pretty steady on thin asphalt overlays,” says Aric Morse, a pavement engineer with the Ohio DOT. “Smoothseal is a tool that our districts like to use very much.” He is referring to a trademarked thin overlay product that comes in two varieties, Type A and Type B. Both of them go down at about 1-inch thick; sometimes Type B is placed a bit thicker, at 1.25 inches. The Type A overlay has 8.5-percent polymer-modified binder and is typically placed on low- and medium-truck-volume roads. The Type B overlay has 6.4-percent polymer-modified binder and is placed on medium- to high-truck-volume roads.
“Thin overlays give us a new surface course; and the Smoothseal overlays are a little richer in binder content,” says Morse. “They should last 12 to 15 years, if they’re placed on the right pavement.
“You get some economy by placing these overlays a little bit thinner. But they cost somewhat more per ton because there is more asphalt in them.”
Ohio’s Smoothseal Type B requires 100-percent two-faced crushed coarse aggregate for mixes used in heavy traffic conditions. The crushed aggregate provides internal friction, leading to greater stability. “We use PG 76-22, and we also allow a blend of that binder with 5-percent latex rubber,” says Cliff Ursich, executive director of Flexible Pavements of Ohio. “The synergy of using crushed aggregate and a polymer-modified binder results in durability superior to conventional fine-graded hot-mix asphalt.”
Many districts in Ohio place a 3/4-inch leveling course of asphalt topped by a 1.25-inch surface course. “That’s been a strategy of many of our districts for years and years,” says Morse. “They have performed very well. And they have been cost-effective for us.”
At the National Asphalt Pavement Association (NAPA), Kent Hansen, P.E., director of engineering, points out that thin asphalt overlays offer:
• some structural improvement;
• improved ride quality;
• the ability to maintain grade and slope with minimal drainage impact;
• an engineered approach to materials selection and design;
• no loose stones after initial construction;
• very little or no dust generation during construction;
• no curing time to delay opening;
• low-noise generation under traffic;
• no binder runoff; and
• the ability to recycle the material.
Thin overlays are superior to surface treatments such as microsurfacing, says Christie Barbee, executive director of the Carolina Asphalt Pavement Association. “A thin asphalt overlay gives an agency more structure than you get with microsurfacing,” says Barbee. “With a thin overlay, you have the opportunity to level and smooth the road surface, which you don’t get with slurry seals.”
Barbee sums it up by saying, “You’re going to get a longer life out of a thin overlay than you will from a surface treatment like microsurfacing.”
New Jersey’s Tool Box
The New Jersey DOT has five thin overlay mixes available to preserve pavements:
• high-performance thin overlay (HPTO);
• ultra-thin friction course;
• stone-matrix asphalt (SMA) 9.5-mm surface course;
• modified open-graded surface course (MOGFC); and
• asphalt rubber open-graded friction course.
Which one is used the most? “The 9.5-mm SMA, placed 1.5 inches thick, is our most popular thin overlay,” says Robert Blight, project engineer in the Bureau of Materials, New Jersey DOT. “For the cost, that mix gives you the most bang for the buck. We’re not using the two open-graded mixes as much because we’ve had some issues with snowplows gouging them.”
The ultra-thin friction course is placed with a special spray paver that applies the polymer-modified emulsion tack coat directly from the paver before the mix is placed. The top-size aggregate is 9.5 mm and most aggregate is larger than 4.75 mm, so it is basically chips. The mix has a binder content of 4.5 to 5.7 percent and no air void requirement.
“We paved a project with the ultra-thin friction course this past year and it seems to be performing very well,” says Blight. “We put it down in the spring, and so far so good. We’ll see how it goes through the winter.”
Blight says New Jersey did not place any HPTO in 2011. “Right now we’re looking at HPTO, the Ultra-thin and the SMA, and we’re evaluating all three of them for performance and cost-effectiveness,” he says.
Blight estimates that in 2011 the DOT placed on the order of 10 thin overlay projects with the 9.5-mm SMA mix. “A lot of them are mill-and-inlay jobs where we place a 1.5-inch surface course,” he says.
HMA Ultra-Thin in Michigan
In Michigan, the DOT uses an HMA ultra-thin surface treatment that is typically placed at 83 pounds per square yard, or approximately 3/4-inch thick, says Kevin Kennedy, a preventive maintenance engineer with the Michigan DOT.
Kennedy says an agency needs to take care in selecting roads for a thin overlay. “The existing pavement should exhibit a good base condition and a uniform cross-section,” he says. “The visible surface distress may include slight raveling, minor surface irregularities and a slightly polished surface. The cross-sections should be free of ruts or distortions.”
Michigan’s HMA ultra-thin surface treatment protects the remaining pavement structure, slows the rate of deterioration, corrects minor surface irregularities, improves skid resistance and improves ride quality, Kennedy says.
Michigan does impose a crushed aggregate requirement for the HMA ultra-thin overlays. For high-volume roads, the minimum crushed content is 95 percent; for medium-volume roads, the requirement is 75 percent; and for low-volume roads, it is 50 percent. The top-size aggregate is 3/8 inch. Similarly, the binder grades vary, depending on traffic volumes. For low-volume roads, Michigan specifies a PG 64-22; for medium volumes, it’s PG 64-28P; and for high-volume roads, the requirement is PG 70-22P.
Thin overlays are succeeding in this country because they work and they’re cost-effective. A road manager just needs to take care in selecting which roads get thin overlays, and choose a well-proven mix design.
The economics of recycling roof shingles into asphalt pavement
Everyone wants to be green until they see red, meaning we all want to recycle until it costs us money,” says Phil Hutchens, chairman of Missouri-based Hutchens Construction. “Recycling with roofing shingles in asphalt saves in two ways – it saves a product that would otherwise go to a landfill and it saves money.”
Since shingle recycling for pavements got started in the late 1990s, asphalt producers have increased the amount of recycled shingles to about a million tons per year, according to estimates by the National Asphalt Pavement Association. That’s the good news. The not-so-good news is that we still send 10 million tons of roofing shingles to landfills every year. A ton of shingles contains approximately 25-percent liquid asphalt. If you do the math, that means we waste 100,000 transport loads of asphalt binder every year.
When the costs of processing and handling shingles are considered, it often turns out that the liquid asphalt cement (AC) in those shingles is a bargain. Recycling a ton of shingles costs Hutchens’ company about $30. That covers handling, asbestos testing, grinding and so forth. It takes about four tons of shingles to make one ton of liquid AC. So if you multiply four tons times $30, you can acquire liquid AC for just $120 per ton. (That is an operating cost only.) Virgin AC can cost $500 a ton. “Plus you get 3 tons of sand from the shingles,” says Hutchens.
“Really our driving force when we started was more of an environmental effort than it was economic.”
– Phil Hutchens, Chairman
“We started running shingles about five years ago, in 2006,” Hutchens says. “We first started thinking about it in about 1996; that was around the time NAPA wrote its first publication on shingle recycling. At that time it was not very feasible, because there was no one available in this area to grind shingles.
“Once some people began getting into the shingle-grinding business, we kicked it off and decided to incorporate some shingles into some of our mixes. We were running shingles for selected commercial jobs before MoDOT approved them. When we first started doing it, liquid asphalt was considerably less expensive than it is currently. Recycling was not as big a factor as it is right now. So really our driving force when we started was more of an environmental effort than it was economic.”
Shingles can save roughly one percent of liquid in a mix. If shingles were free and AC cost $500 per ton, that’s a savings of $5 per ton of asphalt mix. “But then you have some expenses in the shingles,” says Hutchens. “I would say you can expect it to save you about $3 to $3.50 per ton of mix. You do have a significant investment to run shingles. You need another additive bin and some equipment. You have to be able to stockpile shingles, and somebody has to test them for asbestos.”
Hutchens receives shingles at four locations and at some quarries for the convenience of customers. All of Hutchens’ shingle grinding is subcontracted to Swift Recycling, Neosho, Mo. Swift mobilizes a shingle grinder and brings it to Hutchens’ shingle recycling centers, and some grinding is done at Hutchens’ asphalt plants.
A Statewide Effort
In Wisconsin, asphalt producer Payne and Dolan, based in Waukesha, recycles roofing shingles from across the state and northern Illinois, says John Bartoszek, a regional manager for the company. “Our primary sources are post-consumer tear-off shingles that go to the various transfer facilities,” says Bartoszek. “The transfer facilities bring them to our facilities, and when they come to us they have been pre-cleaned. That means they have been picked from various debris, but we do a post-cleaning and grind the shingles.”
A company called Green 46 shreds shingles for Payne and Dolan, and the asphalt producer grinds some of them as well. Shingles must be ground to 100 percent passing the 3/8-inch sieve. Shredding costs vary, says Bartoszek; the range is from $15 to $20 per ton.
A typical asphalt mix for the Wisconsin DOT might consist of 5-percent shingles and up to 10-percent RAP (reclaimed asphalt pavement), Bartoszek says. Just as in Missouri, the effective asphalt content of shingles is approximately 22 to 25 percent. So, it takes about four tons of shingles to make a ton of liquid AC.
In Wisconsin, the cost to recycle shingles varies widely, Bartoszek says. “By the time you figure in all of your costs, the value of the AC in shingles may be anywhere from $275 to $300 per ton,” he says. By contrast, the cost of virgin AC in Wisconsin runs about $550 per ton.
To mobilize a shingle grinder and send it to a location in northern Wisconsin is expensive, compared to a location that has a stationary grinder. “When we mobilize to a northern location, it can cost twice as much to set up a temporary grinding operation,” says Bartoszek. “On average, the savings from shingles can run anywhere from 50 cents to a couple of dollars per ton of asphalt mix.”
Shingles are good for asphalt quality, say asphalt producers. Shingles contain cellulose fibers and in some cases they have a very hard igneous aggregate that is good for a pavement’s durability. The cellulose can prevent drain-down of the AC in asphalt – and it may even help with resistance to cold-weather cracking. “In a lot of cases, the quality of asphalt made with shingles can be equal to or better than a virgin mix,” Bartoszek says.
Payne and Dolan began running shingles about four years ago, some two years before the state of Wisconsin approved them for paving mixes. “We went through a research and development phase, you could call it,” says Bartoszek. “There are a lot of best management practices associated with running shingles. You have to grind them to the right size, make sure they are clean, add them at the right percentages and understand your mix volumetrics.”
St. Louis-based Pace Construction began recycling roofing shingles in 2004, says Phil Hocher, president. Subcontractors grind shingles to 3/8-inch-minus for Pace. The cost of grinding runs about $15 to $25 per ton of shingles, Hocher says. Pace does not do any asbestos testing in-house; the shingle supplier does that.
Hocher says mixes made with shingles are typically a bit stiffer than conventional asphalt mixes. Shingle mixes can be easier to compact in some cases, but Pace has never cut back on compactive effort for such mixes.
We asked Bartoszek why Payne and Dolan started recycling shingles. “We are proactive in the sustainability of our operations,” says Bartoszek. “And we saw the opportunity for recycling the liquid in shingles. We could cut down virgin AC demand. It is an economic as well as an environmental benefit for us.” Indeed, Bartoszek says the company began recycling shingles at about the same time liquid binder shot upward in price.
Daniel C. Brown is the owner of TechniComm, a communications business based in Des Plaines, Ill.