More than a decade ago, the highway community joined together to initiate the Long Term Pavement Performance (LTPP) Program. 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 LTPP pavement monitoring in 1989, data have been collected for nearly 2,300 LTPP test sections in the U.S. and Canada. Currently, data collection is ongoing for about 2,100 of those test sections. With 12 years worth of data gathering behind us, are the LTPP efforts paying off? What is the value of this research? And is the LTPP program still important today?
With traffic volume having increased 68% between 1980 and 1997, while capacity resulting from new roads only grew 4% during the same time period, the need for longer lasting roads is more pressing than ever.
The products developed by the LTPP program are helping to meet this need in a number of different ways. For example, the software tool LTPPBind and the temperature prediction equations on which it is based allow engineers to more accurately select the correct Superpave asphalt binder for their specific environmental conditions. The improved accuracy of the LTPPBind equations is significant because it reduces the need for modified binders, which can drive up the costs of construction. A national comparison of the asphalt binders selected using LTPPBind and those selected using the original Superpave system indicated an annual construction cost savings of $50 million for the highway agencies using LTPPBind.
Falling-weight deflectometer (FWD) calibration procedures developed by the LTPP program also have proven valuable to states. FWDs are used to evaluate the structural condition of a pavement. Accurate FWD data are critical to both the LTPP research effort and the states that use the 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 DOTs in Minnesota, Nevada, Pennsylvania and Texas help to ensure that the FWD data collected by both the LTPP program and states are as accurate as they can be.
The many reports produced by the LTPP Program also have proven useful to states. For example, Pennsylvania DOT decided to stop using skewed joints to control the development of pavement joint faulting after reviewing the results of an LTPP study. The study showed that perpendicular joints with adequate subdrainage are as effective at controlling faulting as skewed joints. This is significant because skewed joints are more difficult and costly to construct than perpendicular joints.
So what benefits will the LTPP program yield in the near future? One significant item is the use of the LTPP data in the development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures. This publication is intended to replace the 1993 American Association of State Highway & Transportation Officials (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 will help address many of the limitations of the current design guide. For example, the LTPP database contains performance data on rehabilitated pavements and covers all climatic conditions in the U.S., while the American Association of State Highway Officials (AASHO) road test that the current guide is based on was conducted at only one geographic 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.
The 2002 design guide is only the beginning. With continued full-scale operation of the LTPP program, the next generation of pavement designers will have information on pavement drainage, rehabilitation strategies, climatic factors, traffic, new pavement design features and life-cycle cost analysis procedures. As Francis B. Francois, former executive director of AASHTO, said, "LTPP is a major contributor toward assuring that we will have good pavements into the 21st century."
