Asphalt Recycling Section
| July 07, 2011 |
By Dan Brown, Contributing Editor
How do you cold-recycle the top 6 inches of a 9-mile stretch of rural asphalt roadway for just $5 million? In several stages, says the Utah DOT. And they did just that this spring on US 191 near Bluff.
The first stage involved milling 3 inches of asphalt and stockpiling the reclaimed asphalt pavement (RAP) at a central plant location. In the second stage, Coughlin Co, St. George, Utah, cold-recycled – in place – the second 3-inch lift and added lime slurry and emulsion in the process. The third stage entailed rejuvenating the stockpiled RAP with emulsion and lime at the central cold plant, then paving it back. A double chip seal completed the process.
By comparison, just overlaying the pavement with 6 inches of hot mix would cost $5 million, says Kirk Thornock, asset management engineer for UDOT’s Region 4. That would not include any milling or trucking.
The thought process
Block cracking on this section of US 191 was severe. Over the years, cracks had grown wider and deeper. Some of them reached up to 10 to 12 inches wide and extended to the full depth of the 12-inch-thick asphalt, says Thornock. Fortunately for traffic, most of the cracks were longitudinal. “It was a very rough road for the traveling public,” says Thornock.
A few years ago, the Utah DOT named a budget figure of $5 million to fix the road. Several companies had tried to fill the cracks with some type of mastic or cement. “Every one of them failed,” says Thornock. “Plus, the prices for milling and filling those areas with mastic or some type of asphalt got out of hand, even for that simple type of work.”
Meanwhile, Utah has had success with cold-in-place recycling (CIR). “So I started thinking about cold recycling to help bridge these cracks, knowing this would not be an end-all solution, but a very good alternative,” says Thornock. “I noticed that the Nevada DOT had incorporated central (cold) plant recycling into their spec, and the Utah DOT had not tried that yet.”
Thornock wanted to rehabilitate the pavement as deeply as possible – a minimum of 6 inches. Limited funds prevented milling 6 inches deep, then filling back with a hot-mix overlay. And cold-in-place recycling could only go 4 inches deep in one step.
Initially UDOT considered milling 2 inches off and then doing a 4-inch CIR. But UDOT officials discussed that with contractors and other team members. They decided that to mill off 3 inches, then cold-recycle 3 inches would be a superior solution. “One, we would get better compaction on 3 inches,” says Thornock. “And, secondly, the 3-inch lift would help to get the moisture out of the cold-in-place material faster.”
Frehner Construction won the bid to handle the CIR and repaving process. Frehner selected Coughlin for the cold recycling and opted to self-perform the paving work. Coughlin used two Roadtec RX-900 milling machines for the initial 3-inch milling. Once that was complete, the CIR could begin, says Darren Coughlin, owner of the company.
How the train works
Two milling machines led off the CIR train. The first was a Caterpillar PR-450 that upcut a pass 7 feet wide and left RAP in a windrow for the second milling machine, a Roadtec RX-900 that cut a pass 12.5 feet wide. By overlapping a bit, the two machines could cut an 18-foot-wide pass. Quicklime slurry, at the rate of 1.5 percent, was added in the cutting chamber of the Roadtec mill.
Working in a down-cutting mode, the RX-900 feeds RAP directly into a Roadtec RT-500 mixing trailer. The mill’s conveyor places the RAP onto a JCI double-deck screen that measures 5 feet by 14 feet in size. Fully 100 percent of the material is screened to 1.25 inches minus. Oversize RAP runs through a Telsmith impact crusher aboard the mixing trailer. A return circuit runs RAP from the crusher back over the screen.
Material that passes the screen drops onto the 42-inch-wide belt with a weigh bridge on it. The weigh bridge sends a signal to the blending computer that adjusts the flow of emulsion – at a rate of 2 percent – to the pugmill mixer located in front of the machine. After a full mixing cycle, the rejuvenated RAP is discharged onto the roadway in a windrow. Roadtec says the belt scale system provides accuracy to within plus or minus 1 percent.
The construction team used Central Utah Testing and Inspection for mix design. The solventless emulsion used came from Ergon Asphalt and Emulsions.
With a windrow pickup machine, Frehner picked up the recycled asphalt and paved it back down with a Blaw Knox paver working 18 feet wide. Compaction followed, with two double-drum rollers and a pneumatic tired machine. “It turned out to be a really great project,” says Coughlin.
Once Coughlin finished the in-place recycling, the company adapted the Roadtec RT 500 mixing trailer for central plant operations. “Basically we added a mini-hopper and a couple of more belts so we could feed it with our wheel loader,” says Coughlin. The cold recycling plant added lime slurry and solventless emulsion to the RAP – in the same amounts as for the in-place recycling. Belly-dump trucks hauled the material back to the road, and Frehner paved and compacted it. Following compaction, Frehner applied a fog seal to the recycled mat. Construction started in early April and was complete by early May.
It’s a low-volume road, Thornock says. “We should see a 20-year design life,” he notes. “The only caution is that we still don’t have a great base under the road. So the cracks can reappear, and the key for us is to treat those cracks faster than we did in the past. That was the main culprit before; the cracks were not treated quickly enough.”
Take the Stockpile to the Jobsite
Traditionally, contractors applying surface treatments on pavements require multiple pavers, which often act as haul trucks for more than 70 percent of the job. The truck-and-paver rigs must make multiple trips between the off-site material stockpile and the jobsite to reload with material.
But now Bergkamp Inc., Salina, KS, has introduced the Mobile Stockpile, a fully-mobile material transfer trailer that increases the time pavers spend on paving instead of hauling. The unit reduces costs, simplifies job management and minimizes overweight truck citation risks. It eliminates the need to find off-site stockpiles — one of the biggest challenges that pavement preservation contractors face.
Standard trucks bring aggregate and emulsion directly from the supplier and load the Mobile Stockpile, which is located on the jobsite. Then, truck-mounted slurry seal or microsurfacing pavers can easily connect to it and be fully replenished with material on-site in less than 10 minutes.
Because this static stockpile is on site, multiple long-distance trips are eliminated. That way, fewer pavers can do more work in less time. The results: better allocation of workers, fewer pavers per job, less paver idle time and reduced fuel costs.
By using standard dump trucks to load the Mobile Stockpile with aggregate, and tankers for emulsion, contractors avoid the risk of receiving overweight citations due to overloaded pavers driving to the jobsite from an off-site stockpile location. Water is replenished via a tanker or hydrant through the Mobile Stockpile’s 50-foot hose reel transfer. The unit is designed to use a minimal footprint, taking just one lane-width of roadway for an entire static stockpile operation. The Mobile Stockpile is compatible with most equipment manufacturers’ slurry seal and microsurfacing pavers.
Aggregate and emulsion are delivered through an efficient process. Aggregate is dumped into the inlet hopper located at the rear of the unit. The material is then transferred to the 21.5-cu. yd. main hopper on an enclosed 32-inch inlet conveyor at a speed of up to six tons per minute. The main hopper can either store the material or simultaneously transfer it during filling via a 24-inch discharge conveyor. The discharge conveyor moves the aggregate up a 28-degree angle before leveling out horizontally toward the discharge end to make a smooth aggregate transition to the slinger conveyor. The conveyor extends and throws the material into the truck-mounted paver at a rate of three tons per minute, retracting and slowing down as the truck evenly fills up to avoid over filling or spilling of the aggregate. Asphalt emulsion is loaded from a tanker evenly into four separate tanks on the Mobile Stockpile that together hold 6,000 gallons of asphalt emulsion. It is then pumped into the truck-mounted paver’s asphalt emulsion tank.
The Mobile Stockpile weighs 40,650 pounds empty and 147,900 pounds when fully loaded. A semi-truck or truck-mounted paver, capable of towing 42,000 pounds with a 6,000-pound vertical load, can pull the unit to the desired location by attaching to a pintle hitch on the end of a sliding tow bar.
New Modified Slurry Seal Making Inroads
TRMSS Scores a Hit in Southern California
Tire Rubber Modified Slurry Seal (TRMSS), a new type of slurry seal, is catching on in Southern California. Last year, contractors placed some 16,000 tons of Type I, II, and III TRMSS in the greater Los Angeles area. More is expected to go down this year, says Lance Allan, Manager of Roy Allan Slurry Seal, a contractor based in Santa Fe Springs, Calif.
To make TRMSS, tire rubber is first digested into liquid asphalt at the terminal in amounts of 5-, 10- or 15 percent rubber. Then that rubberized asphalt is emulsified to make the emulsion used in the slurry seal. It is relatively easy to emulsify the 5 percent rubberized asphalt, and more difficult to emulsify the 10 or 15 percent material, Allan says. But recently, Pacific Emulsions Inc., a Roy Allan affiliate, has been able to emulsify 15 percent rubberized asphalt.
“Rubber products have two benefits,” says Nelson Nelson, assistant director of public works for the City of Corona, Calif. “One is that it stays black longer than a conventional slurry seal. Secondly, it is more flexible under traffic. The conventional seal is more brittle, flakes off, and doesn’t last as long. I have been very happy with TRMSS.”
What’s more, Allan says, TRMSS can be applied with the same slurry machines as conventional slurry requires. Other rubberized slurry seals require modified machinery to place, he says. Typically a slurry machine, such as a Bergkamp, fits onto the rear of a tandem axle truck.
TRMSS differs from other rubberized slurry seals because it is based upon a cationic quick set emulsion. Cationic, (positively charged) emulsions cure chemically, not through evaporation like anionic (negatively charged) emulsions. That fact allows cationic emulsions to be used in cooler temperatures and still cure successfully. By their nature, cationic emulsions set quickly and create a natural bond to negatively charged aggregates.
Recently Roy Allan Slurry Seal placed 3,200 tons of Type II TRMSS for the City of Thousand Oaks, Calif. The 3,200 tons covered a little more than 4.3 million square feet for a cost of 14.4 cents per square foot, says Lance Allan. By comparison, he says a conventional slurry seal would cost about 12 cents per square foot.
Another competing product in the Los Angeles area is REAS, or Rubberized Emulsion Aggregate Slurry. Allan says REAS typically costs about 50 percent more than conventional slurry – about 18 to 19 cents per square foot. REAS is an anionic slurry seal, which means it cures by dehydration. “And with REAS, the rubber is put into a finished emulsion,” says Allan. “And a finished emulsion is basically at ambient temperature, so there is no real interaction between the rubber and the asphalt.”
Nelson says TRMSS typically is bid at around 14 cents per square foot. REAS comes in at anywhere from 21 to 31 cents per square foot, he says. “When I do bidding with competition between the two, the price of REAS comes down as well,” says Nelson.
He recently let for bids a slurry project in which he allowed TRMSS as an alternate to REAS, and the TRMSS came in as the low bid. He usually specifies TRMSS as a Type II slurry, which means it takes 12 to 15 pounds to cover a square yard. Type I covers at a rate of 8 to 10 pounds per square yard.
On one TRMSS project, Nelson found that the slurry was unraveling a bit. So he modified the specifications to require rubber-tired rolling after the slurry cured and before it opens to traffic. “That solved the problem,” he says. “You put that rubber-tired roller on it and I did not have that unraveling any more.”
Nelson says he has learned to wait until later in the year to apply REAS, because it requires a warmer ground temperature to cure properly. “The ambient temperature of the air might be fine, but if the ground is too cold, it won’t set up properly and will unravel on you,” he says. “And I am finding that I don’t experience that problem with TRMSS.
Nelson does not place TRMSS in the winter, just because the days are shorter and he wants to complete projects in as few days as possible. “So I still do TRMSS in the summer, but it allows a longer time frame in which to place it,” he says.
The streets for which Nelson specifies TRMSS are in good structural condition with some cracking but no alligator cracks. He seals the cracks before applying slurry. And areas that need to be removed and replaced are patched before the slurry goes down. “We expect no less than seven more years of life with TRMSS,” Nelson says.
City Streets Improve with Hot In-Place Recycling
Saving 30 percent compared to mill and fill claimed
You could say that Chandler, Ariz., is celebrating its tenth anniversary with hot in place Recycling (HIR) this year. Since 2001, the city has repaved nearly 1.3 million square yards of asphalt roadways, or 30 centerline miles, with the HIR process. The majority of those roads are four- and six-lane urban arterials that carry heavy traffic volumes.
“The arterial roads for which we use HIR are structurally sound,” says Rex Hartmann, street superintendent in Chandler’s Transportation and Development Department. “But on our HIR candidate roads we see longitudinal and transverse cracking, some minor alligator cracking, some surface raveling and some shoving.” On a scale of 0 to 100, with 100 being perfect, Hartmann says roads run in the range of 49 or so prior to getting the HIR treatment.
Hartmann says the city has learned that HIR is not a cure-all. Some roads still need milling and filling. But the HIR process costs in the range of 30-percent less than a 2-inch mill-and-fill, Hartmann estimates. Based on a 2011 project, the cost of HIR runs $12.70 per square yard. That includes everything – edge milling, the virgin hot-mix overlay, utility adjustments, striping, and street light replacements.
“Our city council and senior management have always been supportive of cost-effective and the newer green-technology recycling projects,” says Hartmann. “And the HIR is a very fast-moving, single process that limits the time contractors spend on the road.” Traffic can return to an HIR road within a half-hour to forty minutes of being compacted. Plus, the HIR process permits the contractor to adjust the crown on a roadway.
Chandler’s HIR work has all been performed by Cutler Repaving Inc., Lawrence, Kansas. Cutler has been doing HIR continuously since 1965. “They have always won the low-bid contract with us,” says Hartmann.
Cutler’s process begins with a preheating unit that softens the asphalt. That is followed by a mobile repaving unit that continues to heat the pavement, taking it up to 350 to 375 degrees F. The same machine scarifies the pavement to a depth of 1 inch, adds a recycling agent, remixes the scarified asphalt in-place, and redistributes the material, with a screed, as a 1-inch leveling course. While that material remains at a minimum temperature of 225 F, the same repaving unit places a virgin hot-mix asphalt overlay over the leveling course. Compaction completes the process.
Road users benefit from the Cutler process because there is no time delay between the time the pavement is recycled and the time the overlay is placed. The result is a safer – one-pass – work zone for workers and for user traffic. And because the hot virgin mix is placed over the heated, recycled leveling course, the process achieves an interlocking bond between the two layers.
“From an engineering point of view, there is no delamination between the recycled layer and the new overlay,” says John Rathbun, vice president of sales for Cutler Repaving. “That’s very important in predicting life cycle performance. The same heat that is used to take the road apart is used to put it back together, and the two layers are effectively compacted into one lift.”
The amount of recycling agent needed is determined by lab tests on the existing asphalt. Rathbun says the recycling agent is mostly composed of a high-float medium-set emulsion with a polymer modifier. It replaces both the asphaltenes and the maltenes in the existing asphalt. The benefit of heat, says Rathbun, is that you don’t break the existing aggregates up, so you don’t change their gradation from the original design.
In 2010, Cutler performed HIR on nearly 112,000 square yards of Chandler’s streets — or 4 centerline miles. This year, Cutler is doing HIR on a 4.8-mile project that covers nearly 182,000 square yards.
“We have found that the HIR process works best on arterial roads or streets, or collectors with a thicker cross-section,” says Hartmann. “What works best for us is a cross section of around 3 inches or thicker.
“We have also found that it doesn’t pay to go too thin on that final lift,” Hartmann continues. “On our first year with HIR, we did a 1-inch overlay, and we had just a bit of an issue with that. Ever since then, we have been going to a minimum of 1.5 inches. We will do 1.5 to 2 inches and that really seems to cure the problem. We have not seen the delamination that we saw before.”
That the overlay thickness is flexible is an advantage, says Rathbun. “We can lay it as thin as one inch,” he says. “It depends on the pavement and the traffic load. If you have a high traffic load, which they have in Chandler, and you want to lay it a little thicker, our machine is capable of doing that. The paving screed has electronic grade and slope control, just like you have on any paver.”
Hartmann estimates that HIR will extend a Chandler arterial’s life by 20 years or more – with additional regular maintenance. By maintenance he refers to slurry seals, microsurfacing and the like. And now Chandler has begun to use Tire Rubber Modified Slurry Seals (see related story on Page 31).
While Chandler’s Street Division has used rubberized asphalt on overlays and HIR projects, Hartmann says he has not seen overwhelming improvements in roadway life cycles or in noise reductions – not sufficient to regularly justify the increased costs, especially during challenging economic periods. He says the decibel levels of traffic noise are not perceptibly different between regular asphalt and rubberized asphalt for inner city applications. And for city streets, he says rubberized asphalt has its place, but may not be the most cost-effective globally-exercised application.
Hartmann stresses that a road agency should not force the HIR process onto a road that really needs a more aggressive treatment. “I think we got a little excited on a couple of our first roads and thought this was a cure-all,” he says. “And we were in a learning curve and have certainly come to better understand its limits. And HIR has them. But it has turned out to be a great product and a great application for us.”