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Publication Number: FHWA-SA-97-020
Date: February 1997
As a result of its participation in the long-term pavement performance (LTPP) program, the Kansas Department of Transportation (DOT) has confirmed the importance of preventive maintenance, improved its preventive maintenance techniques, and expanded its range of preventive maintenance treatments. The result-a highly effective and economical strategy for keeping asphalt concrete pavements in top condition.
In 1990, Kansas decided to expand its knowledge of preventive maintenance treatments by participating in the LTPP program's Specific Pavement Study (SPS) 3, which examines the effectiveness of preventive maintenance treatments for asphalt concrete pavements. Each SPS-3 project has five test sections: a control section that has received no preventive maintenance, and four sections that have each received a different preventive maintenance treatment (an overlay, a chip seal, a slurry seal, or crack seals). This arrangement allows engineers to make side-by-side comparisons of the effectiveness of the different treatments. Kansas constructed two SPS-3 projects-one on Route 68 near Ottawa, and a second on U.S. 400 near Ford, about 27 km (17 mi) from Dodge City.
In 1995, Ronald Shuberg from the Kansas DOT joined colleagues from Iowa, Kentucky, Michigan, South Dakota, and Manitoba in a project to evaluate the 19 SPS-3 projects in the north-central LTPP region. The team's mission was to compare the treatments' effectiveness and their potential for use in a preventive maintenance strategy.
The team found that thin hot-mix asphalt overlays and chip seals were extremely effective. Of the test sections with overlays, almost 80 percent were in better condition than the control sections. Overlays also improved ride quality.
Two-thirds of the pavements treated with chip seals were in better condition than the control sections. Chip seals are made up of a thin coating of asphalt binder that is then covered with uniformly sized aggregate to waterproof the pavement and improve surface friction.
Slurry seals were a disappointment-less than half of the sections with slurry seals were in better condition than the control sections.
The real surprise was the value of crack seals, which proved much more effective than the team had predicted when they had evaluated the SPS-3 projects in 1993. Almost two-thirds of pavements where cracks had been sealed were in better condition than the control sections-even though many States hadn't kept new and existing cracks sealed over the course of the experiment. Sealing cracks helps preserve pavement by preventing water and incompressible material from getting into the cracks and causing more damage.
As a result of the 1995 evaluation of SPS-3 projects, Kansas DOT has changed its preventive maintenance practices to incorporate lessons learned from the experiment. The DOT has increased the amount of crack sealing it performs and has adopted the heat lance for cleaning, heating, and drying routed cracks, a technique other States in the north-central region have found effective. Although other States in the north-central LTPP region used wide, shallow reservoirs for crack sealant at their SPS-3 projects, Kansas DOT is staying with easily cut narrow reservoirs (approximately 12.5 mm [0.5 in] wide).
"We're getting lifespans of 4 to 5 years with the current shape, so we haven't tried to change our practice," says Dean Steward, assistant director of the bureau of maintenance and construction at Kansas DOT. "It's more important to get the cracks sealed and for maintenance personnel to understand the concept." The DOT does encourage experimentation on crack seal shape, so this policy may change over time.
The evaluation team's findings-particularly on crack seals-reaffirms the value of what Kansas DOT had long practiced. Prior to the SPS-3 experiments, the highway agency had set up a committee to review research and other States' experiences with crack seals and to update the agency's policy on crack seals accordingly. The committee included representatives of sealant and equipment suppliers, which Steward notes are an important part of the training process. Although the DOT was confident that crack seals were valuable, there was little hard data on the subject.
"The crack seal committee had annual meetings with district maintenance engineers to talk about the proper ways to seal cracks and to encourage crews to seal cracks soon after they've appeared," says Steward. "That's been confirmed by SHRP findings."
Steward has little doubt that chip seals and crack seals are effective and economical. "In my estimation, early crack sealing extends pavement life, and preventive maintenance definitely saves money," he says. "Take our site on Route 68 in Ottawa. The portion we chip sealed and crack sealed is still one of our best-performing pavements. It's evident that it's performing extremely well, and the treatments were very inexpensive. And now we know that it's a statistically valid strategy."
Crack and chip seals are particularly important for the open-graded and semi-open-graded asphalt concrete mixes some highway agencies are using for pavements and overlays, Steward says. These mixes are durable, but some in Kansas are also susceptible to stripping as a result of their greater permeability. This increases the importance of sealing cracks soon after they appear.
The timing of preventive maintenance treatments is critical to good performance and cost-effectiveness. Based on Kansas DOT's experience, "If you crack seal one year and chip seal the next, it's an ideal situation," Steward says.
"My primary objective is to promote early and effective crack sealing and chip seals where applicable," says Steward. He tours all 25 construction and maintenance areas in the State every year and discusses preventive maintenance with everyone from district engineers to front-line supervisors. As a result of the SPS-3 experiment and the work of suppliers, top management, and the committee on crack seals, Steward says, "I've seen an increase in crack sealing, and maintenance personnel now know what works and why. We are meeting with FHWA and suppliers to explore training seminars for field employees."
For more information on Kansas DOT's findings, contact Dean Steward (telephone: 913-296-3576; fax: 913-296-6944). For more information on the analysis of SPS-3 data, contact Bill Bellinger at FHWA (telephone: 703-285-2530; fax: 703-285-2767; email: firstname.lastname@example.org).
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