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Publication Number: FHWA-RD-01-067
Date: September 2001
The billions of dollars that our Nation invests in highway infrastructure each year has given us one of the best transportation systems in the world. But how can we maximize that investment? And how can we improve the performance of pavements so that they last longer? To answer these questions, the highway community initiated the Long-Term Pavement Performance (LTPP) program more than a decade ago.
Planned as a 20-year investigation of in-service pavements, the LTPP program's goal was to provide the data, information, and products needed to extend pavement life. Since the start of the LTPP pavement monitoring in 1989, data have been collected for nearly 2,300 LTPP test sections in the United States and Canada. This data has been used to develop a range of products and procedures that are helping States meet their need for longer lasting roads and maximize their highway investment.
The software tool LTPPBind and the temperature prediction equations upon which it is based, for example, allow engineers to more accurately select the correct Superpave asphalt binder for their specific environmental conditions. The improved reliability of the LTPPBind equations is significant because it reduces the need for modified binders, which can drive up construction costs. A national comparison of the asphalt binders selected using LTPPBind and those selected using the original Superpave system showed an annual construction cost savings of $50 million for the highway agencies using LTPPBind.
States have also benefitted from falling weight deflectometer (FWD) calibration procedures developed by the LTPP program. FWDs are used to evaluate the structural condition of a pavement, making accurate FWD data critical to States that use that data to make decisions about performing pavement rehabilitation. The LTPP FWD calibration procedures and the four FWD calibration centers established in cooperation with the State highway agencies in Minnesota, Nevada, Pennsylvania, and Texas, help to ensure that the FWD data collected by States and the LTPP program are as accurate as possible.
"FWD data plays a key role in developing our rehabilitation strategy, the design of our rehabilitation, and the resulting life-cycle cost analyses," says Gary Hoffman, Chief Engineer of the Pennsylvania Department of Transportation (DOT). "Pennsylvania's FWD calibration center has proved to be an excellent investment. With how much we spend each year on rehabilitating our roads, we really need the most accurate data we can get. Having the calibration center assures us of that."
Another cost-cutting tool developed by LTPP researchers is the Rigid Pavement Design software. The software helps highway engineers implement improved guidelines for the design of portland cement concrete pavements that were developed in 1998 by the National Cooperative Highway Research Program (NCHRP). Using the software, an engineer can tailor the rigid pavement design to site-specific conditions, materials, and traffic situations, resulting in a design that is more cost-effective and reliable. It is estimated that using the new guidelines reduces the life-cycle costs of pavements by 30 percent compared to current procedures. This means a potential savings of $52 million a year for U.S. highway agencies.
The LTPP research also promises to yield future benefits. For example, LTPP data has been used in the development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures. This publication, if adopted by the American Association of State Highway and Transportation Officials (AASHTO), is intended to replace the 1993 AASHTO Guide for the Design of Pavement Structures, which is recognized as being no longer adequate to meet the design challenges faced by today's highway agencies. LTPP data is helping to address some of the limitations of the current design guide. "The LTPP database is providing a wealth of data that is being translated into useable knowledge," says Matt Witczak of Arizona State University. For example, the LTPP database contains performance data on rehabilitated pavements and covers all climatic conditions in the United States, while the American Association of State Highway Officials (AASHO) road test, on which the current guide is based, was conducted at only one location and did not consider rehabilitated pavements. The LTPP test sections also cover a wide range of subgrade materials, in contrast to the one type of subgrade used for all the pavement sections in the AASHO road test.
Through products such as LTPPBind, the FWD calibration procedures, and the 2002 Design Guide, States are benefitting from the investment in the LTPP program and the results of the LTPP research. And with traffic volume having increased 68 percent between 1980 and 1997, while capacity resulting from new roads only grew 4 percent during the same time period, States' need for longer lasting roads is more pressing than ever. "LTPP is a valuable research program for everybody in America-Those who use our roads directly and those who are the beneficiaries of travel on our roadway systems," says Francis B. Francois, former Executive Director of AASHTO.
Looking ahead, funding remains a critical issue. The LTPP program's budget was reduced by about one-third with the passage of the 1998 Transportation Equity Act for the 21st Century (TEA-21). NCHRP funding approved by States has helped to fill the gap over the past 3 years. For the LTPP program to fulfill its full potential, however, a larger investment will be required than that provided by TEA-21. This investment will provide developers of the next generation of pavement design procedures with information on the impact of pavement drainage, climatic factors, traffic, and new pavement design features not available today. "To me it is very, very important that LTPP continues even beyond its intended life right now and that its funding be increased," says Witczak. "I think it's that valuable."
LTPPBind can be downloaded from the LTPP Web site at http://www.fhwa.dot.gov/pavement/ltpp/bind/order.cfm
To order the Rigid Pavement Design software, go to http://www.fhwa.dot.gov/pavement/ltpp/rigidorder.cfm
A videotape produced by FHWA through a cooperative agreement with the Minnesota Department of Transportation (DOT) explains the FWD calibration procedures in more detail. To obtain a copy, contact Katherine Petros at FHWA, 202-493-3154 (email: email@example.com). Following are a list of contacts for the four regional FWD calibration centers:
Dave Bullock, Minnesota DOT – Phone: 651-779-5542; Email: firstname.lastname@example.org
Doug Chalman, Texas DOT – Phone: 512-465-7653; Email: email@example.com
Tom Leitzel, Pennsylvania DOT – Phone: 717-783-6857; Email: firstname.lastname@example.org
Patty Polish, Nevada DOT – Phone: 775-888-7173; Email: email@example.com
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