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Pavement Preservation Compendium II
When Prevention Is the Cure
by Tom Kuennen
A current pavement inventory, identification of conditions, and correct timing of pavement preservation applications are the secrets to successful preservation of hot-mix asphalt driving surfaces, according to a growing body of experts and research.
Naturally, government agencies want to keep citizens happy by providing as large an inventory of smooth streets as possible. But state, city, and county road agencies alike have to remember that maintenance techniques applied to pavements that are completely deteriorated beyond a certain point are a waste of money.
A "worst-first" pavement maintenance philosophy tosses scarce public funding at pavements that should be allowed to fail first, then be reconstructed in an orderly, programmatic manner.
Asphalt pavements will perform well and deteriorate very slowly through the first eight to 10 years of their lives, then fail rapidly as an assortment of ills does them in.
The best way, experts say, to spend scarce maintenance funds is to determine where the pavement is in its life-cycle "curve," and apply preventive maintenance techniques just before the period of rapid deterioration sets in.
Unfortunately, the public agencies responsible for pavements ultimately must answer to elected officials. It takes guts for a public works supervisor to insist to a mayor or aldermen that the municipality will get the best use of funds by allowing a street to fall apart before rebuilding, despite what the voters say.
Support for preservation
Experts agree pavement preservation is best executed in the framework of a pavement management system that will enable a road agency to identify pavement condition throughout its road inventory. Not only will such a system help allocate where funds are best spent, but it will also provide a database to prove the long-term benefits of pavement preservation and justify additional preservation spending.
"The key is to have a good pavement management system that will allow you to track the condition of your pavements over time," says Mark Buncher, Ph.D., P.E., director of field engineering for The Asphalt Institute. "The best preventive maintenance practice will be indicated by the pavement history in a pavement management system."
Today, programmed pavement preservation is gaining major support from some heavy hitters in the road and bridge community.
A new way of thinking
The theme of asset management supporting pavement preservation is driving the new emphasis on preventive maintenance for the local and state transportation infrastructures of the 21st century.
"Pavement preservation is at the core of all future highway programs," says Bill Ballou, president of the Foundation for Pavement Preservation. "Without asset and system management, we can't maintain highway systems cost-effectively. We want pavement preservation to be a routine undertaking for road agencies."
"Pavement preservation is more than just a collective set of specific pavement maintenance techniques," said FHWA's Office of Asset Management Construction and System Preservation Team leader Jim Sorenson at last year's annual meeting of the International Slurry Surfacing Association. "It is a way of thinking and the guiding force behind an agency's financial planning and proper asset management."
And that leads to challenges in asphalt pavement preservation and maintenance, Sorenson said at an FP2 conference in 2000. "The vast majority (about 94%) of U.S. pavements are made of asphalt," he said. "The balance of the pavements in the high-service corridors are of portland cement concrete. We make choices, and we need the competition between the industries. But the mainstay of our pavements is black, and that's where we need to place much of our preservation focus."
Sorenson adheres to the classic definition of pavement preservation - application of the right treatment, to the right road, at the right time. "We have to get the best bang for the buck out of those asphalt pavements," Sorenson said. "That means we have to do at least something at the right time." And to be able to respond at the right time requires planning and budgeting, he said.
Since 2000, the FHWA has promoted transportation infrastructure asset management as a best practice for state and local road and bridge owners. Asset management is a philosophy borrowed from the private sector, now being applied to government agency-owned transportation infrastructure.
"Asset management is a business process that's being introduced into the highway community to allow agency top management - our policy decision makers - to be cognizant of the investments we're making in our highway system," Sorenson says.
"Highways are a big business, and every dollar that we invest in the highway system must get a high rate of return. And in the United States, in the postwar era, we can show those returns in the way our economy has stabilized and developed."
For more information and tools to use, visit the FHWA's Office of Asset Management.
Foundation for preservation
Research and educational support of long-lived pavements through preventive maintenance is being undertaken by the public/private-funded Foundation for Pavement Preservation.
Commonly known as FP2 ("FP squared"), the foundation is the nation's leading proponent of pavement preservation. Like FHWA's Sorenson, the foundation believes that proper pavement preservation means application of the right treatment, to the right road, at the right time, and communicates this principle to the top management levels of government agencies, as well as to field personnel.
FP2 funds and conducts research, coordinates development of educational courses and programs, sponsors symposia and workshops, issues external publications, and identifies research problem statements for future work.
Established in 1992, FP2 is facilitating change in the transportation infrastructure community, Sorenson says, by providing resources to advance knowledge for improved asset management for maintaining and preserving highway pavements.
"We are closely monitoring how reauthorization will support pavement preservation," says FP2's Ballou. "While we are not a lobbying group, we are acutely aware of the need to include pavement preservation research funding, as well as system preservation funding itself."
In 2001, FP2 began high-level talks with the FHWA and the American Association of State Highway & Transportation Officials to readdress pavement preservation research and policy needs, and to establish a focus for a national program. And last February, FP2 brought FHWA, AASHTO, and private sector stakeholders together into a "Strategic Partnership in Pavement Preservation Research" initiative to identify common ground as TEA-3 approaches. "We discussed reauthorization and how we could participate and what our best options were," Ballou says.
Ongoing research on surface sealants and rejuvenators being coordinated by FP2 likely will pay big dividends in the near and long term.
Sealers have been used in approximately two-thirds of the states, the foundation reports. Rejuvenators are formulated to penetrate into the pavement and enhance the properties of the asphalt binder of the existing pavement.
To explore which treatments will work best under which climatic conditions, the FHWA contracted with FP2 in February 2001 to evaluate the effectiveness of spray-applied emulsified sealer/binders and rejuvenators.
This was a unique approach as it brought together a partnership between industry and the FHWA. Although the FHWA provides the significant share of research funding, the pavement preservation industry also contributes to this effort through the foundation. FP2 conducts the research through its partners and contractors.
As part of this research, a workshop and field application of test sections was conducted in early autumn 2002 in southern Minnesota.
Preservation through maintenance
Preventive maintenance is a means to achieving pavement preservation.
"Preventive maintenance applies lower-cost treatments to retard a highway's deterioration, maintain or improve the functional condition, and extend the pavement's service life," says retired Michigan DOT pavement preservation engineer Larry Galehouse, P.E., L.S.
"With various short-term treatments, preventive maintenance can extend pavement life an average of 5 to 10 years," Galehouse says. "Applied to the right road, at the right time - when the pavements are mostly in good condition - preventive maintenance can improve the network condition significantly at a lower unit cost."
Beginning in 1992, Galehouse's work at the Michigan DOT provided hard evidence that preventive maintenance is a wise investment. According to a study undertaken by the DOT and confirmed by an independent consultant, in the long run Michigan's preventive maintenance strategy is more than six times as cost-effective as rehabilitation and reconstruction projects in providing acceptable driving surfaces.
In Michigan, surface treatments for flexible pavement surfaces include microsurfacing, chip seals, slurry seals, crack sealing, 0.75-inch overlays of ultrathin hot-mix asphalt, and 1.5-inch hot-mix asphalt overlays, Galehouse says.
Ills of asphalt pavements
Such procedures mitigate the degenerative problems that hot-mix asphalt pavements endure. Some asphalt pavement ills - like raveling - indicate the need for immediate pavement preventive maintenance. Others, like alligator cracking, indicate a pavement that has deteriorated too far, or a subbase that requires repair lest good money be thrown after bad.
New look at fatigue cracking
This spring a contract was pending for new research into fatigue cracking, as proposed by the National Cooperative Highway Research Program. Project 1-42, Top-Down Fatigue Cracking of Hot-Mix Asphalt Layers, would be a two-year, $400,000 project that will take another look at why fatigue cracking happens.
Until recently, load-associated fatigue cracking of hot-mix asphalt pavements that occur in the wheel path have been thought to always initiate at the bottom of the HMA layer and propagate to the surface, NCHRP says.
But new studies indicate that load-related fatigue cracks can also be initiated at the surface of the pavement and propagate downward through the HMA layer, NCHRP says. These studies say environmental conditions, tire-to-pavement interaction, mixture characteristics, pavement structure, and construction practices are among the factors that influence the occurrence of this cracking.
Research proposed by NCHRP would identify mechanisms that govern the initiation and propagation of top-down fatigue cracking, identify or develop methods of lab-testing HMA mixes for susceptibility to surface cracking, determine factors associated with top-down fatigue cracking, and identify predictive models.
A variety of techniques and materials can be used to preserve a pavement - if it is at the right stage in its life. These include chip seals, fog seals, cape seals, slurry seals, microsurfacing, and thin-lift hot-mix asphalt overlays. But before spending money on an application, the engineer must determine if the treatment will actually prolong pavement life.
"There are lots of different tools in the tool box to choose from," the Asphalt Institute's Buncher says. "The pavement owner will have to choose the best solution for the funds that are available."
For example, most fatigue or alligator cracking indicates a failed subgrade following years of weathering and traffic loads. The existing fatigued asphalt and base must be milled or dug out, then rebuilt prior to placement of a treatment or new lift of asphalt.
Rutted asphalt may be treated by a thin-lift HMA overlay, or by microsurfacing. But a chip seal or slurry seal will do little to improve the riding surface, much less prolong pavement life.
Simple raveling, on the other hand, may be treated by a number of surfacing solutions, including the family of surface seals.
"The application of slurry seal will significantly extend the life of existing pavements by protecting the under-surface from damage caused by water seepage," says the International Slurry Surfacing Association. "Improved surface performance is an added bonus. A pavement maintenance program using slurry seal will not only help to protect your pavement, it will help to protect your paving investment."
Simple pavement seals
There are four approaches to pavement sealing in common use today.
The classic chip seal is an inexpensive solution for an oxidized or raveling asphalt pavement. An asphalt chip seal, also referred to as a seal coat or a bituminous surface treatment, consists of sequential applications of asphalt and stone chips, applied either singly or in layers, to build up a structure that can approach 1-inch thick, according to the National Highway Institute's course of instruction, Techniques for Pavement Rehabilitation.
Rubberized asphalt chip seals are a special type of chip seal in which rubber (ground-rubber tires) is blended with the asphalt cement. "This application has been used both as a SAM (stress-absorbing membrane) and a SAMI (stress-absorbing membrane interlayer) to help reduce reflection cracking, but it has also been used without overlays," NHI says.
Fog seals are very light applications of an emulsion to the pavement surface with no aggregate, according to NHI. "These applications seal the surface and provide a small amount of rejuvenation, depending on the type of emulsion used and the condition of the existing pavement surface," NHI says.
Cape seals are a combination of chip seal and slurry surfacing or seal. "For paved roads, the chip seal is applied first and, between four and 10 days later, the slurry seal is applied," NHI says. For unsurfaced roads, an application of penetration oil (MC-70 or SC-70) is applied first as a prime coat, followed about two days later by a chip seal and about two weeks later by a slurry seal.
A slurry surfacing, also known as a slurry seal, is a mixture of aggregates dispersed in asphalt and applied in a slurry state.
"Slurry seal is a mixture of an asphaltic oil and water (emulsion) and crushed rock aggregate that is spread over the street at about one-fourth-inch thickness," says the City of Austin (Texas) Street and Bridge Division. "The slurry 'cures' when the water evaporates, leaving only the asphalt to coat the crushed rock."
Afterward, the asphalt acts as a binder to hold the slurry together and bond to the existing pavement. The slurry seal protects the existing street surface from the effects of aging and oxidizing and increases the skid resistance.
"Slurry seals typically include some crack sealing on cracks 0.1875-inch and larger," Austin says. "Prep work may also include a minor amount of level-up and surface replacement areas depending upon conditions; however, slurry seal is typically used on streets that are in good to excellent condition."
"Slurry surfacings are designed in a lab, are proportioned by the slurry machine, and laid down and cured so the asphalt-to-aggregate ratio is maintained at the optimum value to assure uniform aggregate coating and adhesion," says Jeff Reed, president, Valley Slurry Seal Co., Sacramento.
Such surfaces use very large fractions of fines material, giving a very high surface area and a lot of microstructure, leading to a sandpaper surface and a high skid resistance, while maintaining a smooth finish, Reed says.
Microsurfacing evolved from slurry
Microsurfacing is a more advanced extension of the slurry-surfacing concept. Microsurfacing is described as a polymer-modified, cold-mix paving system that can remedy a broad range of problems on today's streets, highways, and airfields.
Like slurry seal, microsurfacing begins as a mixture of dense-graded aggregate, asphalt emulsion, water, and mineral fillers, says the International Slurry Seal Association, but microsurfacing has added capabilities, thanks to the use of advanced polymers and other modern additives.
Introduced in the United States in 1980, microsurfacing is made and applied to existing pavements by a specialized machine which carries all components, mixes them on-site, and spreads the mixture onto the road surface. These materials are continuously and accurately measured, and then thoroughly combined in the microsurfacer's mixer.
As the machine moves forward, the mixture is continuously fed into a full-width "surfacing" box, which spreads the width of a traffic lane in a single pass, the ISSA says. Also, specially engineered "rut" boxes - designed to deliver the largest aggregate particles into the deepest part of the rut to give maximum stability in the wheel path - may be used to fill ruts. Edges of the microsurface mat are automatically feathered.
The next step up from microsurfacing is a full-blown hot-mix asphalt thin surfacing. This consists of a single layer of hot-mix asphalt (minimum of 1-inch thick, but often 2-inches thick) used to level, waterproof, and restore the original street shape and ride.
Thin overlays for pavement maintenance got a big boost this year when the results of a National Cooperative Highway Research Project were revealed at the 82nd meeting of the Transportation Research Board in Washington in January.
There, a paper on NCHRP Project 20-50 (03/04) - LTPP Data Analysis: Effectiveness of Maintenance and Rehabilitation Options - assessed the relative performance of different pavement maintenance and rehab treatments, including the influence of pretreatment condition and other factors on treatment effectiveness.
The study was executed by Kathleen T. Hall and Carlos E. Correa, both with ProTech Engineering, Inc., and Amy L. Simpson, now with PCS-Law. Data used in the study were drawn from the Long-Term Pavement Performance Studies SPS-3 experiment.
"In terms of roughness, rutting, and fatigue cracking, the most effective of the maintenance treatments in the SPS-3 core experiment has been the thin overlay treatment, followed by the chip seal treatment, and then the slurry seal treatment," they write. "The thin overlay treatment was the only one of the four SPS-3 maintenance treatments to produce an initial (albeit small) reduction in roughness, and the only one of the four to have a significant effect on long-term roughness, relative to the control sections."
For rougher pavements, however, there was some evidence that chip seals and slurry seals also had some effect on long-term roughness, rutting, and cracking, relative to the control sections. "Slurry seals and crack seals did not have any significant effect on long-term roughness, rutting, or fatigue cracking," the authors reported.
It should be kept in mind that the term effectiveness, as used in this paper, refers to the magnitude of effect on initial and long-term condition levels, and is not meant to imply anything with regard to the relative cost-effectiveness of the different treatments. "The most effective treatment is not always the most cost-effective treatment," the authors warn.
Options are vast
The options available to pavement engineers to help preserve pavements are vast, sophisticated, and are getting better. But perhaps the engineers' biggest challenge will be to move forward and apply true pavement preservation principles to the infrastructure under their control. Fortunately, support is building for pavement preservation, and they won't be alone.
Reprinted from Better Roads, June 2003. Better Roads can be visited online.