Traditionally pavement preservation has been thought of as those activities that affect the upper 3 to 4 inches of the pavement structure. Once treatment activities approached 4 inches in depth, rehabilitation was needed, rather than preservation.
Typically pavement preservation has meant chip seals, microsurfacing and thin-lift, hot in-place recycling (HIPR) that addressed surface cracking and raveling.
Dustrol Inc. of Towanda, Kansas, has developed a technology that permits addressing pavement distresses to a depth of 3 inches. Dustrol refers to this process of deep-treatment, hot-in-place recycling as MARS—Mobile Asphalt Recycling System.
The MARS Process
“The types of distresses that lend themselves to the MARS process are in the top 2 to 3 inches of pavement—alligator or cold temperature cracking, oxidation and minor rutting, not roads with structural issues,” says Brian Hansen, Vice President of Dustrol. “The roads Dustrol treats usually include interstates, primaries and well-traveled secondary roads.”
A conventional HIPR process typically employs four or five heating units to raise the pavement surface temperature to between 250 and 300 degrees F. The MARS process requires the use of seven to eight heating units, depending on the ambient and pavement temperatures at the time of construction.
“We normally heat the pavement between 200 and 300 degrees F, depending on the material,” says Hansen. “But we don’t heat it so hot that we degrade the aggregate or existing asphalt.”
The first pair of heating units softens the upper one half inch of the pavement. A small hydraulic mill, its depth controlled by a sliding-shoe automatic control, is used to mill the heated material from the surface. The processed material is discharged in a single windrow from the central rear of the mixing chamber of the first heater/mixing unit.
Repeated Heating and Milling
This heated material is intercepted by a pair of centralizing plows that place the windrow at the entrance to the shroud of the third heating unit. In the third heating unit the material is separated into two windrows and moved through tunnel heaters to the outer edges of the heating shroud so the underlying pavement surface is exposed to the radiant heat. The fourth heating/mixing unit continues the heating process and mills an additional half inch of material.
The fifth and sixth heating and mixing units continue to move the windrow from side to side beneath the heating shrouds and heat the newly exposed pavement surface. An 8-foot traveling stringline-controlled milling head is used to remove the third half inch of the existing pavement. Due to the growing volume of processed material, a more powerful hydraulic mixing system is employed to process the material.
The process is repeated a fourth time beneath the seventh and eighth heating units. A traveling stringline is used again to control the cut depth of the final mill. Use of the traveling stringlines increases continuity and smoothness of the milled surface and the final pavement surface.
During the final milling and mixing process, a specially formulated rejuvenating material is added to the milled material at a rate of 0.2 to 0.4 gallons per square yard by injecting the rejuvenating agent into the milling and mixing chambers. The actual percentage of material added is determined by the asphalt supplier as a result of cores obtained from the pavement.
The rejuvenated material is then discharged in a single windrow and immediately lifted and deposited by a pickup machine into the hopper of a conventional paver. Dustrol’s paver is equipped with a hopper extension that is typically used with a material transfer vehicle. The hopper insert allows continuous operations without stopping while changing nurse trucks, which supply the rejuvenating agent to the process.
The paver extrudes the rejuvenated mixture in a uniform ribbon behind the paver. Automatic controls, consisting of two 30-foot-long fixed beams with three ultra-sonic sensing devices mounted on either side of the paver, control the thickness and smoothness of the extruded surface.
Double-Drum Compaction
A double-drum vibratory roller is used to compact the mix to the desired density. Pavement density is achieved in compliance with a rolling pattern that is checked twice daily. On one project, the Kansas DOT recorded a surface smoothness of 0.7 inches per mile.
“Typical field in-place densities are between 91 and 96 percent (of Rice or solid density),” says Hansen. “If we have a solid base to work with, we’ll get between 92 and 96 percent. Recently, on an I-40 project south of here, we got between 92 and 96 percent.”
Production Rate
“The process can move forward at a rate between 20 and 25 feet per minute or 1200 to 1500 feet per hour,” says Hansen. “During the summer heat, Dustrol can process as much as 3.0 miles per 10-hour day. Tonnage-wise this relates to a little less than 250 tons per hour, which is equal to the output of a small asphalt plant.”
Although the pavement may function satisfactorily without a surface layer, Dustrol recommends either a single-lift chip seal, microsurface, or hot mix overlay. “Of course, the choice of surface treatment depends on the volume of traffic. Typically, on roads with higher volumes of traffic, we will recommend the addition of a single-lift chip seal, such as Nova Chip, a microsurface or a 1.5-inch lift of HMA.”
Hansen says that the typical life of a MARS project with a microsurface or HMA surface is between 8 and 12 years. The average life of a MARS project with a chip seal is 8 to 10 years.