Looking to improve the "mileage" of your town's shrinking road maintenance budget? Microsurfacing might be just the additive you've been hoping to find.
A technology that is relatively new to the U.S. market, developed in Germany in the 1960s, microsurfacing is not only a cost saver, but also gets very high marks for sustainability. One of the most versatile tools in the road maintenance arsenal, microsurfacing is a polymer-modified, cold mix paving system that can remedy a broad range of problems on today’s streets, highways and airfields.
According to BASF Corporation’s recent “Microsurfacing Eco-Efficiency Analysis (EEA),” verified by the independent NSF International, microsurfacing results in the following environmental benefits:
• 50 percent fewer resources consumed - Over a 40-year life cycle, a road treated with pavement preservation consumes less aggregate and binder, even when considering using recycled asphalt pavement (RAP) in mill and fill.
• 40 percent less energy used - Since pavement preservation uses cooler production and application rates and less asphalt and aggregate than some traditional pavement repair methods, it consumes about 40 percent less primary energy.
• 45 percent fewer greenhouse gases (GHG) emitted - Pavement preservation requires half the amount of material to be shipped to and from manufacturing and job sites, reducing emissions. It also uses a cooler process, which means roads are drivable within a short period after application, reducing traffic delays and their associated GHG emissions.
According to Carter Dabney, a director of the International Slurry Sealing Assn. (ISSA), Annapolis, Md., and vice president of Slurry Pavers, Richmond, Va., “Government agencies are saving millions each year and preserving hundreds of extra lane miles by choosing pavement preservation over traditional pavement reactive maintenance and repair methods.
“Using the right pavement preservation application, on the right road, at the right time can mean a savings of up to $400,000 to $500,000 per mile of two-lane road over a 50-year period. Considering today’s shrinking budgets and the number of miles in road networks, it is clear that municipalities cannot afford to ignore the value of microsurfacing for pavement preservation," Dabney said.
He stressed that while some traditional pavement repair methods alone will accomplish the same goal as pavement preservation in extending the pavement’s life, BASF’s analysis found that the life-cycle cost over a 40-year period is 25 percent higher. “By consistently devoting money to pavement preservation each year, a town is able to preserve four to five times the amount of lane miles than if it only repaired its worst roads first.”
Dabney said the pavement preservation processes promoted by ISSA - slurry and microsurfacing, chip sealing and crack treatment - stretch budgets, while using fewer natural resources, consuming less energy and reducing overall GHG emissions. “Saving money and resources allows communities to balance the needs of people, nature and the economy, while in turn addressing a national demand for sustainable living.”
Microsurfacing, as defined by the ISSA, begins as a mixture of dense-graded aggregate, asphalt, emulsion, water, and mineral fillers with advanced polymers and other modern additives. The mixture is made and applied to existing pavements by a specialized machine that carries all components, mixes them onsite, and spreads the mixture onto the road surface. Materials are continuously and accurately measured and then thoroughly combined in the machine's mixer.
As the machine moves forward, the mixture is continuously fed into a full-width surfacing box, which spreads the it across the width of the traffic lane in a single pass. Edges of the microsurface are automatically feathered. The new surface is a dark brown color and changes to a finished black surface as the water is chemically ejected and the surface cures.
Traffic is permitted within one hour in most cases. Continuous-load machines use support units that bring the materials to the job site and load the machine while it is working, thus maximizing production and minimizing transverse joints.
The Martin Asphalt Company of Houston reported that microsurfacing and slurry sealing offer numerous benefits:
• Both techniques seal small cracks and surface imperfections, waterproof the surface, and protect the pavement structure of both asphalt and concrete pavements.
• Their new surfaces improve skid resistance.
• They provide attractive, smooth black surfaces that aid in lane delineation.
• Their very thin surface is ideal where curbs and overhead clearances (i.e., under bridges) need to be preserved.
• The quick-setting microsurfacing emulsion reduces user delay by allowing traffic in about an hour after re-surfacing.
• The quick-setting emulsion makes microsurfacing suitable for night application on heavy-traffic streets, highways and airfields.
• Microsurfacing fills depressions, small cracks and ruts, and provides some surface leveling.
• Both are cost-effective pavement preservation techniques.
Steve Olsen, area manager for the Sacramento, Calif. office of Intermountain Slurry Seal, Inc., said microsurfacing differs from slurry in some key ways: It has a higher polymer content in its emulsion; it has a higher asphalt residual content; its fast-setting chemicals allow a faster break; it contains higher quality aggregate; and it requires higher quality control.
Olsen said based on the standard Pavement Condition Index (PCI) (See Figure 1), the estimated service life of microsurfacing and slurry seal treatments is seven to 10 years for a PCI of 80 (good condition), five to seven years for a PCI of 60 (fair condition) and only two to five years for a PCI of 40 (poor condition).
Patrick Sullivan, a representative of Asphalt Paving Systems, Mechanicsburg, Pa., underscored the cost-effectiveness and sustainability advantages of microsurfacing at a recent American Public Works Association (AWPA) Sustainability in Public Works Conference in Pittsburgh.
“Microsurfacing is a great product and process," Sullivan said. "It allows us to use resources without destroying the ecological balance of an area and leads to many savings — cost, materials, time, economics, environmental, public impact and safety. Typically, microsurfacing costs about $2.50 a square yard versus $12 for traditional mill and fill.”
For Sullivan, roads are an asset and an investment. “Microsurfacing can offer slow sustainable growth through low-cost spending and preserving our road systems with a safe process,” he explained. “You need to take care of any road before it gets to its bad period.”
Sullivan said one of the sustainability issues in road maintenance is the shrinking availability of aggregate in the U.S., one of the reasons why microsurfacing was developed in Europe where even less aggregate is available.
“Our quarries are running out of aggregate," he said. "Traditional hot asphalt mill and fill requires about 120 lbs. of aggregate per square yard. Microsurfacing only requires 16 to 36 lbs per square yard for single and double applications. And it contains much smaller No. 10 stone dust aggregate.”
Microsurfacing treatments use three different sizes of aggregate — Type I, Type II and Type III — for higher and lower volume pavement and rut filling. It is usually applied in thicknesses of one-quarter to three-eighths inches. Traditional asphalt repairs are about two inches thick.
“The key is that using microsurfacing for appropriate applications gives communities an ability to design the material for their roads and the particular needs for that road,” Sullivan added.
Microsurfacing is especially well suited for bridges. “It is a great low-cost protector of decks because it is almost impervious to moisture. It seals decks from salt and brine. On older, weaker bridges that might be unable to accept the heavy weight of a mill and fill, microsurfacing can be effective since it weighs much less,” said Sullivan.
ISSA’s Dabney said, “Not only can you save up to 40 percent of the cost of mill and fill, but you use a lot less virgin material. Microsurfacing requires about half of the time of mills and fills. Plus, it doesn’t make sense to do complete reconstruction on a road paved only seven years ago. Use microsurfacing to extend its life.”
Microsurfacing does have its limitations. For example, it can’t be used on roads that are badly deteriorated. “Depending on the road’s condition, you do have to do a certain amount of repair — patching alligator cracks, for example — before microsurfacing. It won’t work on a 30-year-old road with lots of potholes.”
Dabney said microsurfacing works best when it is integrated into an existing maintenance program. “Again, it needs to be used at the right time on the right road. Many municipalities tend to fix the worst first, but often by the time they do that, other roads have deteriorated beyond pavement preservation with microsurfacing.”
Microsurfacing, which Dabney said is excellent for filling in ruts, also works well over a chip seal (stone and hot oil) to form a cape seal.
While Sullivan said microsurfacing is not as well known throughout the U.S. as it should be to help solve today’s economic and environmental challenges, Dabney said he is finding that more and more contractors over the last five years are trying it and the consensus is very positive.
According to an article in the Winter 2011 Pavement Preservation Journal, the Tennessee DOT began using microsurfacing in the late 1990s. Special Projects Engineer Jay Norris said, “We saw success with it and we’re at a point in our program that we’re very comfortable with it and feel it’s a lasting tool for us.” He said his DOT also uses chip seal, crack treating, shoulder sealing and single joint stabilization for preservation.
Overland Park, Kan. has been using pavement preservation techniques for the past 50 years. Civil Engineer Matt Laipple said, “Mill-and-overlay, removing the top two inches of asphalt, on a 10- to 15-year cycle has historically been used by many communities to provide a new wearing surface on asphalt pavement.
“It’s a great surface, but it’s also expensive in terms of the pavement’s life-cycle cost. We can preserve nearly five times more lane miles of pavement with microsurfacing or chip seal than we can with the same funds used for mill-and-overlay,” explained Laipple, adding that without some form of preventative maintenance program, it’s not uncommon for an asphalt pavement to need complete reconstruction after 20 to 30 years.
Do the math, said Sullivan. “It costs about $30,000 to microsurface a 22-foot-wide mile of low-volume road, which gets a 10-year life for a cost of only $3,000 per year. And it is virtually maintenance free. Compare that to spending about $155,000 per mile to put on 1½ inches of material. That’s five times the cost!”