LTPP Program Charts a Path to Advanced Pavement Design

Just halfway through its 20-year life, the long-term pavement performance (LTPP) program is already well on its way to accomplishing a key objective—the development of improved design procedures for new and reconstructed asphalt and portland cement concrete pavements. Data from the LTPP studies have been used to prepare guidelines that will help highway agency staff design better-performing asphalt and portland cement concrete pavements, more accurately predict pavement performance, and gear up for the shift from today's empirical design procedures to advanced mechanistic design procedures.

The new guidelines for portland cement concrete pavements have been published as a supplement to the 1993 AASHTO Guide for Design of Pavement Structures. The guidelines will help reduce the likelihood of cracking or faulting in new or reconstructed pavements by providing tools to tailor pavement designs to the base course and underlying soil layers at the project site. To make it easier to implement the guidelines, the Federal Highway Administration (FHWA) is developing a Microsoft Excel spreadsheet that automates the design and analysis procedures. The spreadsheet, which is expected to be available later this month, has been tested by pavement designers in several States, the American Concrete Pavement Association, and the American Association of State Highway and Transportation Officials (AASHTO).

Will the improved performance increase the price of pavements? "Not necessarily," says FHWA's Cheryl Richter. "The guidelines may actually bring a decrease in costs. For example, until now, agencies haven't had a way to tailor joint spacing in portland cement concrete pavements to environmental conditions. The guidelines will enable them to optimize joint spacing, which in some cases will cut initial costs. And because the guidelines will improve pavement performance, they will reduce life-cycle costs."

The new guidelines for asphalt pavements are available in three pamphlets:

The pamphlets describe procedures engineers can use to obtain more reliable values for key pavement and subgrade properties for use with the AASHTO asphalt pavement design guidelines.

"In the past, designers have used broad assumptions and general relationships when determining critical input for the AASHTO design equations," says FHWA's Mark Swanlund. "Many correlations were tenuous at best. The asphalt pavement design pamphlets explain how to better characterize three of these inputs." Swanlund says the procedures will allow highway agencies to design longer-lasting asphalt pavements and more accurately predict how long they will remain serviceable.

The new design guidelines will also help highway agency staff prepare for the shift from empirical pavement design procedures to more advanced mechanistic design procedures (see sidebar below). "The new procedures from the LTPP program were developed for use with AASHTO's current design guidelines, but agencies that use the new procedures will be in a good position to implement AASHTO's 2002 design guidelines, which will use mechanistic procedures," says Richter.

To order the pamphlets on asphalt pavement design, contact FHWA's Research & Technology Report Center (phone: 301-577-0906; fax: 301-577-1421). To order the Supplement to the Guide for Design of Pavement (Publication No. GPDS-4S), contact AASHTO (phone: 800-231-3475; fax: 800-525-5562).

For more information on the design guidelines or the concrete pavement design spreadsheet, contact Mark Swanlund at FHWA (phone: 202-366-1323; fax: 202-366-9981; email: mark.swanlund@fhwa.dot.gov).

Empirical vs. Mechanistic Design

Current pavement design procedures—known as empirical procedures—are based on engineers' experience with and observations of the relationships between pavement performance, traffic load, and pavement thickness for a particular region and climate. Mechanistic design procedures are based on materials properties and how those properties relate to pavement performance. Mechanistic procedures allow engineers to tailor pavement designs to specific materials, environments, and traffic loads.

Although a handful of States now use mechanistic design procedures, there are no nationally accepted guidelines. To develop those guidelines, the National Cooperative Highway Research Program (NCHRP) recently launched a new project, "Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures: Phase II—Stage A" (NCHRP 1-37A). The project will use data from the more than 2,400 LTPP test sections and other pavements to validate and calibrate the procedures. For more information, visit the NCHRP Web site (www2.nas.edu/trbcrp/nchrp5.html).