RECYCLING/RECLAIMING: MARS roving

Recycling train makes successful passes across U.S.

Reclaiming / Recycling Article February 08, 2013
Printer-friendly version

Towanda, Kan.-based Dustrol Inc. continually strives to refine its hot in-place recycling (HIR) process to deliver the best possible product for the customer.


Founded in 1973 by current CEO Ted Dankert, the company initially specialized in selling emulsions to seal asphalt and control dust. Dustrol purchased its first cold planer in 1979, which began the company’s emphasis on asphalt recycling. Today, Dustrol travels a 16-state-market area in the heart of the country—from Idaho and Montana to Texas and Louisiana—cost-effectively upgrading road surfaces through its recycling services.


Nowadays, the company’s primary area of expertise is HIR. At first, the company solely used a heater scarification surface recycling process, a method still employed to this day. However, Dustrol wanted a process that could go deeper to remove surface issues such as cracking, rutting and raveling. This led to the development of its MARS (Mobile Asphalt Recycling System) process. Much of Dustrol’s MARS recycling train has been engineered, built and perfected by the company over the years.


The recycling train includes custom-made propane-fueled preheaters, tunnel heaters and grade-controlled mill heaters, which heat and mill off a predetermined depth of asphalt. Each mill is capable of milling up to 15 ft wide at a depth of 1 in.


“Our MARS train,” said Brian Hansen, vice president of Dustrol, “is capable of milling up to 3 in. deep,” although the typical application calls for a 2-in. recycling depth.


The last mill heater in the train includes an oil-metering system to inject a water-based emulsion as a rejuvenating agent for oxidized asphalt. Depending on job specifications, rock, sand or asphalt may be added to the milled material as well.


From there, the MARS recycling train includes conventional paving and compaction equipment to complete the process. A Terex pickup machine transfers the windrowed material into a conventional Terex tracked paver equipped with an electric screed. Crews use tandem vibratory rollers to compact the recycled mat prior to an overlay—a thin lift of fresh asphalt, chip seal, NovaChip, etc.—being applied for the final riding surface.


Dustrol can operate up to five MARS recycling trains at a time. Hansen mentioned that the company customizes the number of heaters and mill heaters in the train to meet jobsite and depth requirements.


“Typically, we can save our customers 10 to 30% versus a mill and inlay process,” he said.


Special remix
Occasionally, Dustrol’s field crews encounter a challenging application, where they meet specifications but the finished product does not attain the high expectations set by the company. A recent HIR project on Kansas Highway K-4, south of Valley Falls, was one such project. That is, until crews made a switch from the conventional slat paver to a paver equipped with in-hopper remixing capabilities.   


The K-4 rehabilitation project consisted of 30 miles of two-lane highway showing signs of cracking and minor rutting that had received heavy chip-seal overlays.
“The road had been chip-sealed twice before Dustrol’s crew recycled it,” said Bill Rieken, paver application specialist for Terex Roadbuilding. The 12-ft-wide lanes were recycled in place to a depth of 2 in. Job specifications called for a NovaChip overlay as the final riding surface.


Dustrol’s MARS recycling train measured nearly 1,000 ft long. Eight heater units, including four grade-controlled mill heaters, softened and milled 0.5 in. of the surface at a time, leaving the millings in a progressively larger windrow behind each subsequent mill. At the final mill stage, the windrowed material received an emulsion rejuvenator.


The challenge for Dustrol was the heavy chip-sealed surface.


“Chip seal sets up fast and doesn’t flow well,” explained Hansen. “We were seeing signs of shadowing and segregation under the center gearbox” driving the screed’s spreading augers. Additionally, the chip-seal aggregate exhibited some signs of stripping.


Dustrol’s crew paved the recycled and rejuvenated material using a standard Terex MS 5 pickup machine and Terex CR462 paver equipped with a Stretch 20 screed. The standard paver includes a conventional slat delivery system, which pulls material from the front to the rear of the hopper, and traditional center gearbox drive for the spread augers.


“We paved approximately 15 miles of one lane with the [conventional] paver,” said Hansen. The result was acceptable and met project specifications, but it was not as good a finished product as Hansen and Dustrol would have liked.


The K-4 application was the right opportunity to try a different type of paver to see if it would deliver a different result for Dustrol, which requested a demo with a remixing paver.
Working equally as well as either a material transfer vehicle or paver, the remixing paver was equipped as a paver and replaced the conventional paver on the K-4 project. One significant design advantage offered by the remixing paver was counter-rotating auger sets to replace the conventional slat-bar delivery system in the hopper. Two sets of two augers counter-rotate to uniformly pull and draw down material from all areas of the hopper, thoroughly remixing 100% of the asphalt prior to delivering it to the spread augers.


“The augers act like a pugmill to deliver a more homogeneous blend of material to the screed, virtually eliminating particle and thermal segregation,” mentioned Rieken.


After seeing the remixing paver in action on the K-4 project, Hansen agreed with Rieken and appreciated the benefits gained through the additional remixing of material.


“Our process adds emulsion in the last heater mill, and we get some reblending,” he said. “But the augers in the hopper give us more mixing time, which means additional coating and mixing of the aggregate with emulsion.”


Hansen also pointed out the elimination of the center drive gearbox for the spread augers as a significant advantage for the K-4 rehabilitation project. Rieken observed that prior to adding the remixing paver to the train, there were a number of workers busy with shovels to correct the centerline issues.


“After inserting the [remixing paver] into the recycling train, the mat had the same appearance as a mill-and-fill finish and the shovels were seldom used,” he said.


After switching to the new paver, production levels remained consistent to what crews were experiencing with the conventional paver.


Dustrol’s crew finished the remainder of the K-4 recycling job with the remixing paver. The mat behind the paver not only met job requirements, it also satisfied the high standards that Dustrol demands from its equipment. While Dustrol continues to rely on conventional pavers for many applications with its HIR recycling train, Hansen noted, “The new paver was definitely a game changer on this particular project. We purchased the demo paver and others for use on challenging applications such as this one, where we need the additional mixing and blending to give us a better finished product.” R&B
 

About the author: 
Zettler is president of Z-Comm, Cedar Rapids, Iowa.
Overlay Init