Choosing micromilling over paver-placed surface treatment

By Greg Harder, P.E.

As the old adage goes, “there is more than one way to skin a cat.” A similar saying with a less gruesome reference would be “there are more ways of drawing a cat out of a well than by a bucket.” But with either proverb, the meaning is clear – a problem generally has more than one solution or that one can find more than one way to reach a similar outcome.

This expression was recently realized on a New York State Department of Transportation (NYSDOT) resurfacing project on Interstate Route 81 in central New York. The prime contractor – Suit-Kote Corporation, a vertically integrated asphalt company based in Cortland, New York – submitted a Value Engineering Change Proposal (VECP) to NYSDOT on this project that illustrated an alternative method that would exceed all of the specified requirements without the state of New York incurring any additional costs.

Original project details

This project was located in NYSDOT Region 9 in Broome County, beginning near the Castle Creek exit and heading 7.6 miles to the north. This section of I-81 is a limited access four-lane divided highway (two lanes in each direction) with a traffic loading of greater than 30-million ESALs projected over the next 20 years.

The original project proposal was let in July 2012 and consisted of 7.6 miles of transverse joint repair followed by an entire resurfacing using a paver-placed surface treatment, with Suit-Kote being the apparent low bidder.

This section of highway was a composite pavement consisting of approximately 4 inches of hot mix asphalt placed over a 9-inch thick PCC layer. Any and all transverse joints that exhibited any distress at the surface were to be milled 4 feet wide across the entire lane width and to a depth approximately 7 inches, completely through the asphalt layers and partially into the PCC pavement. These trenches were then to be filled in two lifts with a 19mm hot mix asphalt mixture and compacted to within 1/8 inches of the existing lane height.

So why change?

Prior to commencement of construction, concerns arose regarding the final ride quality over these joint repairs with just a single paver-placed surface treatment as cover.
“With the number of joints that were in need of repair, we became concerned that motorists would feel each of those repairs through the single lift of paver-placed surface treatment after the completion of construction,” said Don Knickerbocker, construction manager for Suit-Kote. “We consequently met with NYSDOT to voice our concerns and began discussing alternatives.”

It is well known to the industry that the paver-placed surface treatment process is an acceptable method of pavement preservation for use on particular high volume roadways. However, in some cases it may not be as cost effective as other processes. Recognizing this, Suit-Kote proposed to NYSDOT in their value engineering change proposal to replace the paver-placed surface treatment with a micromilling process completed with a thin lift hot mix asphalt overlay.

According to the NYSDOT Comprehensive Pavement Design Manual, both processes appear to be very similar in producing the specified result. However, engineers believed that the proposed substitution would outperform the paver-placed surface treatment in correcting current profile issues that were present within the project limits.

“We identified an opportunity to marry some emerging technologies with a material with proven performance, while keeping the overall project cost neutral. Utilizing a micromilling process provided a smooth transition across the joint repairs while giving us a uniform surface to place our 6.3mm polymer modified thin lift overlay,” says Frank Suits, Jr., Suit-Kote President.

Adoption of the proposed changes in the VECP further enhanced the project by adding four thousand more tons of hot mix asphalt than was originally specified. After analysis, the group concluded that the proposed changes would not affect the construction schedule but may add the possibility of overnight lane closures to ensure the most efficient implementation of the milling and paving processes.

NYSDOT agrees to new scope of work

After receiving the blessing from NYSDOT, the micromilling process was accomplished using a Roadtec RX-70 full lane machine with a 4-wrap drum in downcut mode. The company operates all of their milling machines in a downcut direction so as to better control the chunk size of the grindings. At 7/8ths of an inch nominal milling depth, the 4-wrap drum provided a much tighter milling pattern (than a conventional 2-wrap drum) while allowing the grinder to operate at a ground speed of greater than 50 fpm (feet per minute) without walking out of the pattern. This process also produced a very fine RAP, eliminating the need for further processing back at the HMA plant.

“The micromilling process is like a leveling course in reverse. Instead of hauling material to the project to correct the profile deficiencies, we are removing a recyclable material from the job site while still making these corrections,” says Knickerbocker. “We are then left with an ideal surface to place the thin lift overlay on.”

Recognizing the importance of a good bond between thin lift overlays and the underlying layer, Suit-Kote proposed to use their premium trackless tack coat SKNTT-1HM. The use of “trackless tack coat” was used in response to past NYSDOT concerns of tracking or picking of the tack used with 6.3mm polymer modified HMA.

The thin lift overlay was a 6.3mm hot mix asphalt mixture using a PG 64-22P polymer modified binder. This particular mixture has demonstrated outstanding performance on this same section of I-81. This same section of highway was last overlaid in 2004 by Suit-Kote with a 12.5mm hot mix asphalt mixture using a PG 64-22 binder. However, at that time, they also placed at this same location a one-mile section of the 6.3mm hot mix asphalt polymer modified mixture in both southbound lanes – the same mixture that was proposed in the VECP.

According to Tony Jones, Suit-Kote’s project manager, “The 12.5mm sections exhibited distresses at the surface that required more than 50 joint repairs per lane mile while the 6.3mm section required less than 10 repairs. It was clearly visible that the polymer modified section outperformed the unmodified sections with regards to cracking.”

Working with the NYSDOT, Suit-Kote was able to find a way “to save the poor cat without the use of a bucket.”

Greg Harder is an Asphalt Institute Regional Engineer based in New York.