CIR a Hit in L.A.
Better Roads Staff | July 8, 2013
Cold asphalt recycling offers “a lot of positives” for L.A. County Department of Public Works
By Daniel C. Brown, Contributing Editor
On seven projects performed in just the past two years, the Los Angeles County Department of Public Works has used cold asphalt recycling methods to save $3.15 million in rebuilding its asphalt streets, compared to the hot mix asphalt process. Cold asphalt recycling is a “taxpayer-friendly” and sustainable pavement rehabilitation solution.
“I think cold recycling is an approach that agencies should really consider in maintaining their road network,” says Greg Kelley, assistant deputy director for the L.A. County Department of Public Works. “ It has a lot of positives including lower costs and reduced greenhouse gas emissions compared to hot mix. On our seven cold asphalt recycling projects we completed in the last year and a half, we calculated that we reduced the greenhouse gas emissions by 80 percent compared to hot mix treatments.”
Five of the seven projects were done with cold-in-place recycling (CIR), which uses mobile asphalt recycling trains that move down the road and recycle the material in place; two projects were completed with cold central plant recycling (CCPR), which uses a stationary plant located in close proximity to the job site to recycle the asphalt.
In the spring of 2012, L.A. County performed a three-quarter-mile CIR project on Altadena Drive. Pavement Recycling Systems (PRS) was the subcontractor for the recycling portion of the project, and Hardy and Harper was the general contractor. “That was our first test with an urban setting,” says Kelley. “We were a little bit concerned about what impacts the cooler springtime weather would have on the CIR treatment, but it performed very well.”
At Altadena Drive, the county first milled off 1.5 inches of asphalt and then performed a 3-inch CIR treatment on the remaining pavement. The top 1.5 inches was removed because the roadway had existing curb and gutter and the county wanted to keep the same grade, and add a 1.5-inch overlay at the end. The CIR process began with a Caterpillar PR 1000 milling machine. PRS milled and recycled Altadena Drive to a depth of 3 inches. The 1,200-horsepower mill conveys recycled asphalt pavement back to a self-contained, closed-circuit asphalt recycling plant, which is towed by the milling machine. For this project, PRS used a CRMX 2 cold recycling plant made by Nesbitt Contracting of Arizona.
In California, all recycled asphalt is required to meet a specific gradation requirement of 100-percent passing a 1-inch screen. The recycling plant has a screen deck that vibrates as the RAP passes over the screen. Oversize material goes through a vertical impact crusher and is returned to the top of the screen deck where the material is re-screened. The process continues until all of the RAP is sized to meet the required gradation, says James Emerson, project developer with Pavement Recycling Systems. Once through the screen, the RAP loads onto a conveyor belt equipped with a weigh bridge that continuously weighs the material and streams the weight information to a computer.
From there, the RAP travels through a dual-shaft pugmill for mixing with an engineered emulsion. Using software developed by Nesbitt, the recycling plant computer meters the application of the emulsion based on the weight of the RAP delivered to the pugmill to rejuvenate the asphalt. In the case of Altadena Drive, the CRMX 2 recycling unit added a designed percentage of PASS-R emulsion made by Western Emulsions.
The now recycled asphalt is discharged from the pugmill into a windrow underneath the recycling unit. A windrow pickup machine and a Caterpillar AP 1055 tracked asphalt paver follow close behind to pick up and place the recycled asphalt in a 3- to 4-inch pavement mat. The lay-down operation is followed by compaction with a 12-ton Caterpillar double steel drum asphalt roller and a 25-ton BOMAG pneumatic roller operating in series. To cap the recycled asphalt at Altadena Drive, L.A. County elected to place a 1.5-inch-thick layer of rubberized hot mix asphalt as a wearing course.
Kelley estimates that the CIR process at Altadena Drive – not a major project (approximately 10,000 square yards) – saved the county $109,000, compared to conventional hot mix. At Upper Big Tujunga Canyon Drive, a much larger project (approximately 135,000 square yards), the county estimates that the CIR treatment saved $1.2 million compared to hot mix asphalt.
Two of L.A. County’s cold recycling projects used cold central plant recycling (CCPR); on those projects, PRS used a Wirtgen KMA 220 portable recycling plant, serving as a stationary unit at an on-site recycling station. With the cold central plant process, the asphalt is first milled, and then hauled to the nearby recycling plant where the material is crushed, sized and re-engineered with asphalt emulsion. The recycled asphalt is then hauled back to the road for placement through a conventional asphalt paving process and a paver spreads it back into place. For those two projects, the county estimated that it racked up savings of $650,000 compared to conventional hot mix asphalt. What’s more, the greenhouse gas emissions savings on the two projects reached 360 tons of carbon dioxide equivalency.
“One of the huge benefits of CIR – besides obviously the greenhouse gas reductions – is the cost savings compared to conventional hot mix asphalt,” says Kelley. “In addition, the truck traffic required to haul the RAP away from the project site and the truck traffic needed to import the hot mix asphalt is eliminated reducing the amount of time that you impact traffic by one-third of the time that it takes you to do hot mix applications. Because the county has a lot of roads in remote locations, significant savings can be achieved by eliminating hot-mix hauling.
“The train just keeps going all day, and you to open the road to traffic within two to three hours resulting in less disruption to the motoring public,” says Kelley. “The overall benefit that cold asphalt recycling provides makes this treatment process an integral strategy in maintaining our road network in LA County.”
HIR Hot in Georgia
For the right road with the right problem, hot-in-place recycling works.
Not every road that is failing is failing at the same rate and in the same way.
For some of those roads that are not prime arteries and not collapsing hot in-place recycling (HIR) offers a valuable alternative choice to road managers.
“Hot in-place recycling is a low-cost maintenance strategy that we were able to implement,” says Brian Frix, traffic/transportation engineer for Rockdale County, Georgia. “It was obviously eco-efficient. And mainly it was less expensive to surface-recycle our roads than to mill and inlay them.”
HIR heats, scarifies, and rejuvenates asphalt pavements.
Frix says the county used HIR on 68,000 square yards of roads last year, and is looking at another 200,000 square yards this year. The savings with HIR run about 15 to 20 percent, Frix figures.
To mill the pavements, haul the RAP to the plant, remix it, and inlay the surface back at 1.5 inches deep would have cost $9.00 per square yard, Frix says. By contrast, the surface recycling – plus a 1-inch overlay – cost the county $7.50 per square yard. “The savings are very attractive,” says Frix.
Gallagher Asphalt of Thornton, Illinois, performed the HIR process on four roads for the county last year. Frix says the roadways had experienced some environmental cracking, and had some structural deficiencies, which the county patched and repaired before doing the HIR.
“So basically what was left was some cracking, some minor rutting and some low areas,” says Frix. “The existing roads were full-depth asphalt, with anywhere from 3 to 5 inches of hot mix. Some of them may have been on graded aggregate base, others might have been on soil cement, and some may have been on dirt.”
Patrick Faster, national sales director for Gallagher Asphalt, says he recommends HIR for structurally sound asphalt roads that have the typical seven- to ten-year distresses, such as longitudinal and transverse cracking, reflective cracking, rutting, oxidation, and edge raveling. By towing a series of two or three ovens over the old road, Gallagher heats the surface to approximately 300 degrees F. Propane burners heat fire bricks in the oven, which in turn reflect the heat into the pavement.
Once the surface is hot and pliable, Gallagher applies a set of flexible tines, working 1.5 inches deep, to scarify the pavement. Faster says the HIR works to the same depth as the last lift of pavement placed. Next the recycling train introduces a liquid polymer-modified rejuvenating agent at the rate of 0.10 to 0.20 gallons per square yard. Then a set of reversible augers mixes the rejuvenating agent into the asphalt. “The augers pick up the material and feed it back to the paving screed,” says Faster.
Compaction comes next, with a double-drum vibratory roller. “After the roller passes, the road can be reopened to traffic,” says Faster. “We don’t have lane closures, we have lane restrictions.”
Depending on the volume of traffic carried by the road, the owning agency typically chooses an overlay treatment to cover the recycled surface. That can range from a chip seal or a slurry seal to an ultra-thin layer of hot mix to a conventional 1.5-inch layer of hot mix asphalt.
“If you do HIR and an overlay, your savings will typically be 30 to 35 percent compared to a 3-inch mill and fill process,” says Faster. “And the process can be done in about half the time it takes to mill-c and-fill.”
Says Frix: “We actually completed the project under budget and well in advance of the project completion date.”
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