| October 04, 2011 |
Good Results in Bad Times
By Kirk Landers
“In times like these, the pavement manager’s most basic strategy is to keep sound pavements sound and keep bad pavements from becoming unsafe or unusable.”
So spoke Larry Galehouse, director of the National Center for Pavement Preservation, in an interview with Better Roads.
Indeed, Galehouse, one of the nation’s foremost experts on pavement preservation, sees the diminished road budgets of the Great Recession as a litmus test for pavement management strategies.
“The agencies that have pursued the traditional ‘worst first’ strategy, giving priority to rebuilding bad pavements, are feeling the budget shortfalls most acutely,” he notes. “Agencies that have given priority to prevention — to keep good pavements in good condition — are in much better shape.”
And that homily is Galehouse’s advice to pavement managers dealing with severely constrained budgets. “It costs a lot less to extend pavement life while the pavement is healthy than it does to rehabilitate or rebuild a pavement that has deteriorated too far,” he observes.
The key to executing a pavement preservation strategy is to bring the right prevention tool to the right pavement at the right time, he says. The challenge is to select the treatment with the greatest benefit for that particular pavement, and Galehouse notes that it takes a lot of up-front work to make that diagnosis.
“For example,” says Galehouse, “you need to identify subsurface failure spots, dig them out and repair them before applying a surface treatment.”
And Galehouse stresses that pavement preservation priorities apply to concrete and asphalt pavements alike.
One of the least-expensive asphalt pavement treatments that Galehouse often recommends today is the use of a rejuvenator shortly after the surface course is laid. A true rejuvenation of an asphalt surface requires the introduction of maltene fractions. Thus, rejuvenators containing maltenes – the oily, resinous component of asphalt – increase the asphalt binders’ resistance to oxidation by improving the chemistry and prolonging its flexibility.
Rejuvenator treatments can be repeated every few years to keep the surface pavement supple and weather-resistant, typically prolonging its life by two to three years. The maltene-based rejuvenator is clear and doesn’t affect paint lines.
“It’s important to get the true, maltene-based rejuvenator if a change in binder chemistry is desired,” says Galehouse, adding that there are many other products on the market. “I suggest getting references from other agencies about how the product worked on past projects,” he says.
Rejuvenators are often applied after a road pavement or airport landing strip has been retexturized, says Galehouse. “Today’s retexturizing technology is fast and inexpensive, and it improves pavement friction.”
The roughened surface accepts the rejuvenator treatment more efficiently and the process improves the tractive qualities of the aggregate. Galehouse warns that pavements with poor-quality aggregate will polish again relatively quickly, while good-quality aggregate will keep its texture for a long time.
As highway agencies have migrated to a prevention-first philosophy of road management, emphasis on crack treatment has grown. The process is inexpensive and has been shown to extend pavement life by two years and often more.
Galehouse considers crack treatment an important tool in the pavement manager’s toolbox.
“There are two approaches,” he says. “Crack sealing is a series of steps that first machines a reservoir in the crack, cleans the reservoir with compressed air, and then fills it with sealant. This approach can be very effective when used on the right pavement at the right time.” Galehouse estimates that crack sealing typically extends pavement life at least two to four years.
Crack filling, a process in which debris is blown out of the crack and the crack is filled with sealant, is used for nonworking cracks and wider cracks. Galehouse says crack filling typically adds about two years to the life of a pavement.
“Both of these processes are pothole preventers,” he says. “And the longer you can prevent potholes, the longer you can avoid more-expensive interventions like milling and overlays.”
Chip sealing has evolved as rapidly as any preservation technology over the past decade, says Galehouse, as contractors and suppliers have stepped up the quality of materials and application techniques. “It’s more of a science now,” says Galehouse, “though there are still people who don’t recognize it as such.”
Perhaps the most dramatic leap forward in chip sealing will be offered through the SHRP2 (Strategic Highway Research Project) Project R-26 in which chip seals with carefully selected aggregate and emulsion applied with precise construction technique and finished with a fog seal can be placed on high-volume roads. This will finally demonstrate that chip seals can perform excellently on high-volume roads when care is taken in design and construction, says Galehouse.
“A high-quality chip seal applied to a sound pavement prevents sunlight and water from destroying the pavement,” says Galehouse. “It also adds macro-texture to the road surface to enhance traction, and with a fog seal it provides more visibility to paint markings by improving contrast.”
Other surface treatment interventions include slurry sealing and microsurfacing. Slurry seals are a mixture of fine aggregate, emulsified asphalt, water and additives placed by special machines in a thin coat, one stone thick. The slurry seal fills hairline cracks and delays pavement oxidation, and is appropriate for urban/suburban neighborhood roads in good condition.
Microsurfacing is a slightly thicker intervention than the slurry seal, combining polymer-modified asphalt emulsion, crushed aggregate, water and other additives in a carefully specified mix design, and placed by specialized equipment. “Microsurfacing adds thickness to the pavement structure, so it can correct rutting and minor raveling, and improve friction,” says Galehouse. “It is also designed to stand up to high-traffic volumes and heavy loads.”
The next stop on the prevention continuum for asphalt pavements is the ultra-thin overlay — typically, 3/4-inch thick or less. “This is an intervention for a sound pavement,” says Galehouse. “With the advances in mix design and placement practices, it has become a very effective tool. It protects the surface of the original asphalt, fills minor imperfections, and improves ride quality.”
Ultra-thin overlays can also be designed to deliver other benefits. Use of a rubberized asphalt binder, for example, can mitigate traffic noise with great effectiveness. An open-graded friction course design can reduce spray during rain and enhance the quality of runoff water.
Thin overlays — up to 1.5 inches in thickness — cost more, but by virtue of their thickness can smooth out deeper imperfections, and achieve greater smoothness that improves the ride quality of the pavement.
Milling is the tool when the surface pavement has deteriorated beyond the point where lesser interventions can restore its condition. Milling is also employed in metropolitan areas where a simple overlay will not align properly with gutters.
“This is the Cadillac fix in the prevention tool box,” says Galehouse. “You mill off a thickness of deteriorated pavement and overlay with new asphalt to restore the ride quality. It’s a lot cheaper than waiting until you have to do a total structural rehabilitation, but for stretching budget dollars, you want to work as far up the deterioration curve as possible.”
In-place recycling technologies can also be very effective in treating aging pavements, says Galehouse. “It’s important to make sure the project you have in mind is a good fit for the technology, whether it’s hot-in-place or cold-in-place,” says Galehouse. “The best procedure is to get a good, reputable contractor to evaluate the project in terms of its appropriateness for (either).”
When in-place recycling technologies are viable, they bring a cost advantage to the project as well as environmental advantages, such as less energy consumption, lower CO2 emissions, and a 100-percent recycling of the existing resource.
Concrete Pavement Interventions
Contractors and pavement managers have developed an elaborate toolbox for concrete pavement prevention, notes Galehouse.
Joint resealing lies at the low end of the cost spectrum, followed by diamond grinding and partial and full-depth repairs.
“Cost analysis is key, especially with the more-expensive interventions” says Galehouse. “You have to weigh the cost of repair against the cost of replacement. So, for example, if you had to replace every other panel on a stretch of road, it would probably make much more sense to just replace that stretch of road.”
In many cases, says Galehouse, concrete pavements just need diamond grinding to remove surface imperfections and improve smoothness. Over a period of time, concrete slabs can settle due to movement of the road’s subbase. Most of the imperfections occur where the panels abut.
“If you have vertical displacement – called faulting – of the slabs, you might consider retrofitting dowel bars to stabilize the joint and improve the transfer of loading between slabs,” says Galehouse. “If they are tied together, just diamond grind it and seal the joint.”
While some pavement managers aren’t convinced that joint sealing improves concrete pavement performance, Galehouse does advocate the practice.
“The important thing is to keep the incompressibles out of the joint,” he says. Incompressibles include foreign objects that can clog joints and prevent the slabs from flexing as temperatures change and cause the panels to expand and contract. “By keeping joints sealed, you keep out the material that can cause blow-ups,” says Galehouse. “Seals also help protect the pavement from water seeping into the base and creating a ‘pumping’ action that forms voids in the subbase and cause cracks and even breaks in the panel.”
Joint seals typically last 10 to 12 years before leaks appear, says Galehouse.
Does prevention pay off with concrete roads? “If we take good care of our concrete roads with the tools we have today,” says Galehouse, “they will last far beyond what we have come to expect — over 50 years for good concrete.”
Coping with Our Times
There are still pavement managers in America who give their worst pavements first priority in budgeting, Galehouse notes, and their systems are suffering the most from the diminished budgets of the Great Recession.
“In good times or bad, the strategy that makes the most sense is to first keep your good pavements good — your dollars go further and your system stays stronger,” says Galehouse. “Then you keep your marginal pavements from deteriorating any further — to minimize safety concerns and the cost of the ultimate repair. And then you rehabilitate bad pavements as dollars allow, starting with safety concerns.”
Galehouse concedes that today’s tight budgets constrict everyone, but those pursuing sound management strategies that stress prevention will outperform the others, he says.
“Agencies that follow an asset management approach will come out of this cycle in good shape,” he concludes.
Larry Galehouse is a licensed professional engineer and a licensed professional surveyor. His experience includes tenure with an engineering consulting firm and with a large state DOT. In 2003, he helped found the National Center for Pavement Preservation located at Michigan State University in Lansing, Mich. Galehouse has been a leader in pavement preservation initiatives within AASHTO, NACE, FHWA and TRB. Contact and learn more about the National Center for Pavement Preservation at pavementpreservation.org.