“As a stand-alone treatment, the bonded wearing course is only 3/4-inch thick, but it’s durable, and it’s more economical than an alternative like a 1-1/2-inch mill-and-overlay,” said Jim McGonagle, Commissioner of the Newton Public Works. McGonagle heads up a department with eight divisions staffed by 200 personnel. These municipal employees perform diverse tasks ranging from collecting trash; maintaining water pipe; and improving sewers and drainpipe; to preserving 280 centerline miles of roads.
The latter is facilitated under the City’s pavement maintenance program, which is managed by consultant, Conrad Leger of Environmental Partners. The program not only comprises the use of proven preventive maintenance and pavement preservation treatments, but encourages the testing of new applications or unusual ways of combining current treatments as well. The goal of the trials is finding economical, sustainable surface treatments to add to the city’s preventive maintenance toolbox. In managing this program, Leger has consulted with various industry representatives, among them Robert Betsold of All States Materials Group (ASMG).
ASMG has applied ultra-thin bonded wearing course to diverse Newton roads in recent years as part of the City’s search for innovative pavement maintenance treatments. The thin overlay is used to restore ride quality while sealing and protecting the underlying pavement. It can also be used to mitigate shallow (less than 1/2-inch) rutting and can help slow fatigue cracking. A non-structural layer, ASMG’s bonded wearing course is typically only 3/4 of an inch thick and uses a gap-graded asphalt mixture with a polymer modified asphalt emulsion tack coat.
“The City first utilized ultra-thin bonded wearing course treatment on its roadways in 2018,” said Betsold. “Following two seasons of successfully using the process as a standalone overlay application to preserve some of the higher volume roadways, the City explored additional opportunities to incorporate the treatment into their pavement maintenance program.
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“In 2020, working together with their consultant engineer and industry representatives, the City completed a trial project to preserve a section of deteriorating concrete roadway. This application involved a combination of several proven pavement maintenance and preservation treatments with the bonded wearing course as the final surface.
“Encouraged by the trial performance, the city increased the use of the new combination treatment on additional, higher volume concrete sections of road in 2021. Utilization of the bonded wearing course treatment expanded again in 2022, with applications as a standalone overlay, in conjunction with a hot in-place recycling project, and as the concrete road preservation treatment on a section of the famous Boston Marathon route,” Betsold said. (See "Various Applications of Bonded Wearing Course" for details of applications.)
The Bonded Wearing Course placed by ASMG consists of a warm polymer modified asphalt emulsion bond (tack) coat followed immediately by an ultra-thin, gap-graded hot mix asphalt wearing course. Both processes are applied by the same machine, a Vogele Super 1800-3i SprayJet, which can place thin asphalt overlays as well as conventional binder and surface courses. The machine usually sprays the bond coat at between 120- and 170 degrees Fahrenheit at the rate of 0.15 to 0.25 gallons per square yard, depending on the project specs.
Immediately after applying the bond coat, the self-priming spray paver applied the gap-graded hot asphalt mix overlay at about 330 degrees Fahrenheit and smoothed the surface of the material in one pass without any equipment driving on the bond coat. The coarse aggregate for the HMA were maximum 3/8-inch selected for high performance. The multi-tasking spray paver incorporates a receiving hopper, feed conveyor, insulated storage tank (for the emulsion), metered tack coat spray bar and a variable width, heated, ironing type screed.
A 12-ton Caterpillar CB64B Roller compacted the finished wearing course, which was only 3/4 inches thick. Following compaction, and allowing a few minutes for pavement cooling, ASMG crews re-opened the roadway for traffic.
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Newton has a few concrete roads built sometime in the 1950s. For the maintenance of these roads, ASMG applied a combination of the bonded wearing course and asphalt rubber stress absorbing membrane interlayer (ARSAMI). In this process, after the existing concrete surface had been prepared, the paving crew placed an ARSAMI layer, which consisted of an asphalt rubber chip seal, and followed with the bonded wearing course.
As part of ASMG’s asphalt rubber chip seal process, the crew hot sprays (between 325 and 400 degrees Fahrenheit) approximately 0.60 gallons per square yard of crumb rubber modified asphalt over the road. The asphalt rubber is made in their Deerfield, Massachusetts, terminal where performance grade liquid asphalt binder is blended with 20 percent crumb rubber formed from shredded recycled tires. This modified asphalt rubber is said to provide a flexible, durable seal. The 7/16-inch single sized stone chips are run through a hot mix plant to be heated and lightly coated with asphalt to assure it is clean and dry for the chip seal process.
An Etnyre chip spreader was used to distribute the coated stone over the hot-sprayed asphalt rubber binder at the rate of approximately 35 pounds per square yard, then rollers pressed the stone into the asphalt rubber binder. The final step was sweeping away any loose cover aggregate.
The ARSAMI layer was followed by the bonded wearing course.
Public Works Commissioner McGonagle was pleased with the placement and performance of the bonded wearing course.
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“This thin treatment extends the life of our roadways, and there is a cost benefit to that. This is the most economical treatment, and we don’t have to mill and overlay.
“We’ll be exploring other applications for this type of preventive maintenance,” McGonagle said.
- Mill keyways at project limits and intersecting roadways
- Crack seal (if needed)
- HMA patching / leveling (if needed)
- Adjustment of utility structures to finish grade
- Paving of bonded wearing course treatment
- Final striping
BWC as part of a combination of treatments to maintain concrete roads:
- Full width milling of existing roadway surface
- Crack seal / Mastic crack seal of joints and cracks > ¼”
- HMA patching (if needed)
- Adjustment of utility structures to finish grade
- Application of Asphalt Rubber Stress Absorbing Membrane Interlayer (ARSAMI)
- Paving of bonded wearing course treatment
- Final striping (temporary striping used after milling and ARSAMI)
BWC as a final overlay after Hot In-Place Recycling (HIP):
- HIP (separate project / contractor)
- Full length gutter milling (if needed to preserve curb reveal)
- Adjustment of utility structures to finish grade
- Paving of bonded wearing course treatment
- Final striping
