New Hampshire DOT Turns to Micro Surfacing

Over the course of the past few years, the New Hampshire Department of Transportation (NHDOT) has honed its Pavement Management System to save money while also achieving environmental benefits.

In 2006, the NHDOT began researching the benefits of micro surfacing, a treatment that had never been used before in New Hampshire. Seven years later, after completing at least one micro surfacing project each year and achieving much success, the NHDOT has added micro surfacing to its Pavement Management System and has three micro surfacing projects scheduled for 2013.

Eric Thibodeau, NHDOT pavement management chief, first heard about the use of micro surfacing in surrounding states including Maine and Massachusetts. During his research he came across contractor Sealcoating Inc., located in Braintree, MA.

“They came in and did a presentation on the process, and we decided to do a test section,” Thibodeau says. “The test section was completed in 2006 and was 1.3 miles long.”

Initially, the maintenance district personnel had a positive response to the test section until it came time to plow the treated pavement.

“In the winter of 2006, we lost the painted centerlines and edge lines for that 1.3 miles because the roadway was in poor condition exhibiting extensive wheel path rutting and cracking prior to the micro surfacing,” Thibodeau says. “The cutting edge of the plows rode the high points on the road so that it shaved the micro down taking off the pavement markings.”

After experiencing these challenges with the surface during plowing, potential solutions were discussed internally to address some of the issues. Another test section was completed in 2007 on a one-mile-long section of Interstate that was in better condition compared to the first test section.

“We recognized the need to use the treatment on better conditioned roads,” Thibodeau says. “We also recessed or inlaid the pavement markings so the paint isn’t on top of the riding surface. It is recessed about 40 mils to allow the plows to ride up over it without losing the pavement markings.”

The 2007 Interstate test section showed that the surface held up well to heavy traffic, was easy to put down, and that it must be applied in good weather. “Our micro surfacing season is June through August,” Thibodeau says. “We do not allow micro to be placed before Memorial Day or after Labor Day because the temperature won’t support that kind of asphalt emulsion based operation.”

After successfully testing the micro surfacing application, NHDOT added the treatment to its Pavement Management System as a potential preservation treatment in 2008. Currently, NHDOT specifies a Type II gradation applied at a rate of 32 pounds/square yard.

Determining projects

As NHDOT continues to refine and improve its Pavement Management System, preservation applications are gaining momentum as a cost effective way of maintaining new construction or rehabilitated pavements.

“The standard practice used to involve performing rehabilitations on a 12 to 15 year cycle to restore pavements to new condition,” Thibodeau says. “The newer strategy is now to apply thin lift preservation type treatments like micro surfacing every 5 to 7 years to ‘keep good roads good’ and prevent the need the more costly rehabilitation.”

There are several aspects of a project that is considered before micro surfacing is selected as the pavement preservation method including traffic volume, climate and pavement condition.

“We look at traffic volume because some of our other pavement preservation treatments, like chip seals, are applicable to lower-to-medium volume roadways due to concerns for loose stones and flushing,” Thibodeau says. “Micro surfacing comes to mind with a higher traffic road.”

Climate also plays a determining role for the choice of where to use micro surfacing. “In this state we have everything from seacoast to mountains,” Thibodeau says. “We get more snow in the mountain areas so we have more plowing activity and snow removal in those locales. Micro surfacing is not considered for that type of environment knowing it will be plowed more aggressively than in the southern part of the state.”

Sealcoating Inc. completes micro surfacing

Dan Patenaude, of Sealcoating Inc., has spent the past 10 years partnering with dozens of agencies up and down the east coast helping them optimize their pavement management programs and assisting in developing and completing many micro surfacing projects.

Sealcoating Inc. completed three micro surfacing highway segments for the NHDOT this past season including NH 25 Tamworth-Ossipee, NH 9 Chesterfield and NH 12 Troy-Swanzey.

On the NH 12 Troy-Swanzey project, Sealcoating Inc. completed prep work including some isolated mill-and-fill spot repairs of areas too distressed to be suitable for micro surfacing.

Crews also cracksealed all existing cracks to prevent water from penetrating the surface. The existing thermoplastic and other built up pavement markings were removed so that the new micro surfacing would adhere directly to the existing asphalt.

Once the prep work was completed, crews readied themselves to complete the actual micro surfacing application.

“After the selection of suitable pavement candidates for micro surfacing the next step in the process is to develop a mix design based on suitable local aggregates,” Patenaude says. “The aggregates must be high quality and meet very specific gradation and durability requirements. They must also be compatible with the asphalt emulsion to be used on the job.”

Aside from the aggregates, additional components of the micro surfacing material mix include a polymer-modified asphalt emulsion, a mineral filler such as Portland cement, water to enhance mix consistency and workability, and a field additive to adjust the set time of the material as temperatures and other environmental conditions change throughout the project.

“Once a mix design is developed and the prep work of the existing asphalt surface is complete, crews and equipment are mobilized to a staging area near the project where the aggregate has been stockpiled,” Patenaude says. “There, the micro surfacing paver is calibrated for the local aggregate and other mixture ingredients to verify compliance with the mix design, and then the equipment and materials are moved out onto the roads where paving is to be done.”

The contractor used a continuous feed paver as well as support trucks to shuttle the aggregate, emulsion and water from the staging area out to the paver on the road. The asphalt emulsion was supplied by Ergon with the majority of the micro surfacing equipment manufactured by Bergkamp.

“The goal is to have enough of those feeder trucks available so that the paver can pave non-stop minimizing the transverse paving joints,” Patenaude says. “After traffic control patterns are established the tack coat is applied and projects include a two-course application of the micro surfacing materials.”

According to Patenaude, NHDOT typically specifies a total of 32 pounds per square yard with the first lift of material applied at about 16 pounds per square yard and the second coarse applied at about the same rate.

“We prefer to do the ‘scratch course’ [first lift] and the wearing course on successive days to ensure that the majority of the water in the bottom course material is evaporated and cured out before covering it with the second layer,” Patenaude says. “Both layers are usually cured adequately to put traffic on them within about 45 minutes of paving — depending upon the temperature and humidity at the time.

“After paving, temporary pavement markings are applied, and about a week later when the micro surfacing is near fully cured the permanent pavement markings are applied.”