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May/June 2002
Benefitting from LTPP—A State's Perspective
by Gary HoffmanPavements are where the proverbial rubber meets the road. For more than a decade now, the States, Federal Government, and the Canadian provinces have invested in the Long Term Pavement Performance Program (LTPP), a 20-year pavement research project. During its first 10 years, LTPP gathered, processed, and analyzed data describing the structure, service conditions, and performance of more than 2,500 pavement test sections in North America.
Why the investment in this program? What are the values and benefits of LTPP to date? At the Transportation Research Board's (TRB) 2002 Annual Meeting, I addressed these questions from a State's perspective. Following are highlights from my presentation to the TRB audience.
Benefits of LTPP to Date
In the long run, the most important benefit of LTPP is the database. It is an unprecedented resource that will provide the pavement performance data needed to answer all kinds of questions—even those we have not yet conceived.
The more visible LTPP products and findings that benefit Pennsylvania are in some sense old news, but their value and importance has not diminished. One benefit is calibration procedures for falling weight deflectometers (FWDs) and FWD calibration centers.
Calibrating FWDs
Falling weight deflectometers measure the structural condition of a pavement, and most States have been using the equipment for some time. It was not until the late 1980s, however, that LTPP developed FWD calibration procedures. These procedures are the only nationally accepted means of verifying that FWD information is accurate. Subsequently, LTPP opened four FWD calibration centers in cooperation with State departments of transportation (DOTs) in Minnesota, Nevada, Pennsylvania, and Texas.
2002 Design Guide
The American Association of State Highway and Transportation Officials' (AASHTO) Guide for the Design of Pavement Structures is widely used in the design of new and rehabilitated highway pavements. However, the current Design Guide, published in 1993, is broadly recognized as being inadequate for the design challenges currently faced by highway agencies. To address the inadequacies, the National Cooperative Highway Research Program undertook a project called "Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures," otherwise known as the 2002 Design Guide.
The importance of LTPP to the development of the 2002 Design Guide was recognized from the very start. LTPP data document the performance of an extensive array of pavement designs, in a wide range of service conditions, making possible the calibration and validation of broadly applicable models to predict pavement performance. The models are key elements in pavement design procedures. The LTPP database also provides a strong basis for the development of guidelines for the selection of default or typical values for design inputs.
In addition, LTPP is providing one of the most important elements in technology development through field calibration and validation. The use of LTPP data helps demonstrate to the user community that the models and technologies being developed match real-world pavement performance. This applicability, in turn, helps encourage the States to adopt new technologies and approaches. Given the funds spent on highways—in 1995 highway agencies spent approximately $17 billion on pavement-related projects, according to FHWA's 1999 Highway Statistics—it is extremely important that the 2002 Design Guide be accurate and up-to-date, and reach mainstream audiences for implementation as soon as possible. Developers of the 2002 Design Guide estimated that the new procedures could result in savings of nearly $1 billion annually on pavement rehabilitation.
Calibration centers help ensure that Pennsylvania's pavement designs and rehabilitation strategies are based on the most accurate data possible. With the dollars that Pennsylvania spends each year on rehabilitating our roads, we need the most accurate data we can get.
One of the Long Term Pavement Performance Falling Weight Deflectometers is collecting deflection data from an LTPP test section.
In recent years, use of the calibration centers has been on the rise. In 2000, the four FWD calibration centers were used to calibrate a total of 79 FWDs, including 57 owned by State highway agencies. With the anticipated delivery of AASHTO's 2002 Guide for the Design of Pavement Structures (see "2002 Design Guide" on page 47) and increased focus on mechanistic-empirical design principles, use of the FWD calibration centers is expected to increase.
FWD calibration centers help States like Pennsylvania obtain accurate data, resulting in better decisions about when and where to conduct rehabilitation work, how to use budget dollars more cost effectively, and how to make better decisions when designing new pavements and overlays.
A New View on Skewed Joints
LTPP studies on the efficacy of skewed joints provided the information needed to support a cost-saving change in Pennsylvania's policy on joints.
The studies analyzed LTPP data to identify what worked and what did not control the development of joint faulting. Based on PennDOT's review of the studies, the Commonwealth decided to change its practice of using skewed joints.
Pennsylvania will be able to reduce costs initially because perpendicular joints are less expensive than skewed joints. In addition, we believe the Commonwealth will save money by eliminating construction problems and ensuring that future maintenance is easier. Not only will we save money initially, but also throughout the entire life cycle of a pavement project.
LTPP's analysis program addresses a broad array of topics, from field validation of pavement design procedures and studies of variability in traffic and materials data to investigating the development of pavement roughness. In turn, findings from these analyses provide key information that helps highway engineers in their day-to-day operations.
2002 Guide Implementation
Pennsylvania is also relying on LTPP products and the LTPP database as the Commonwealth begins implementing the principles outlined in the 2002 Guide for the Design of Pavement Structures. Specifically, Pennsylvania has started to take advantage of improvements in resilient modulus testing that came about as a result of LTPP.
The Long Term Pavement Performance Program is transporting a Dynatest Falling Weight Deflectometer.
The resilient modulus—or stiffness of the soil and rock in the subgrade at a paving project—is a critical factor in determining the thickness and composition of pavement layers. Resilient modulus testing is a standardized laboratory procedure for measuring the resilient modulus of subgrade materials, along with a related laboratory startup and calibration verification procedure.
For years, measuring the resilient modulus of a particular site was at best a complex and difficult task, but using LTPP data, it is expected that the process will become a great deal easier. AASHTO has adopted the LTPP resilient modulus procedures as a provisional standard.
LTPP data will be used to validate the 2002 Guide for Pennsylvania conditions. Highway engineers may find it useful to employ the LTPP data from their own State to test the applicability of general models to the unique conditions on their particular roads.
This aerial photo shows the John J. Shumaker Memorial Highway, a bypass around Dauphin Borough in Dauphin County, Pennsylvania.
Asset Management
LTPP provides the data necessary to support pavement performance models that help engineers and planners develop reliable predictions regarding the resources required for efficient highway maintenance, rehabilitation, and construction strategies.
The LTPP Story
The need for the LTPP was identified in the "infrastructure crisis" of the early to mid-1980s. Pavements were simply not living up to expectations. LTPP was initiated in 1987 as part of the Strategic Highway Research Program.
LTPP has six program objectives. The first five can be summarized as providing factual and accurate answers as to how and why pavements perform as they do. The
sixth goal is the creation of a national LTPP database to support objectives 1-5.LTPP monitors approximately 2,500 sections of pavement sections across the United States and Canada. Of these, 780 were selected from existing highway pavements and are the subjects of LTPP General Pavement Studies (GPS). The remaining sections were constructed by State highway agencies for LTPP Specific Pavement Studies (SPS).
Data collection on the GPS sections began in 1989, while the SPS data collection started in the early 1990s. The collected data characterize climatic conditions, material properties, traffic volumes and loads, pavement layer thicknesses, and pavement performance. LTPP uses extensive quality control and quality assurance measures to achieve complete, accurate, and consistent results.
With the conclusion of the Strategic Highway Research Program in 1992, FHWA agreed to take over operation of LTPP. To maintain State, industry, and academia ownership, involvement, and input, FHWA asked the Transportation Research Board (TRB) to create a TRB-LTPP committee. The 15-member committee is chaired by a State chief administrative officer, with seven of the members drawn from the State DOTs.
For asset management, the wealth of data available in the LTPP database provides a strong foundation for developing empirical performance prediction models. Similarly, LTPP data also offers a strong foundation for the evaluation, validation, and verification of empirical or mechanical models.
Benefits in 2001
Throughout 2001 the opportunities to benefit from LTPP continued to grow. Specifically:
- The release of DataPave 3.0 software, which provides quick access to most of the LTPP data, makes it easier for everyone to benefit from this valuable resource.
- LTPP findings regarding the adequacy of 3- and 5-point rut bar data helped Pennsylvania's neighbor New York make informed decisions in its specification of a data collection vehicle.
- The release of LTPP findings on the topic of pavement smoothness—arguably the most important measure of performance to highway users—provided the basis for new smoothness index relationships developed for use in construction quality control.
- Both the AASHTO Subcommittee on Materials and the American Society for Testing and Materials (ASTM) balloted LTPP-based standards for temperature correction of FWD test results.
LTPP Benefits All of Us
Although the transportation community cannot and should not expect LTPP to be the solution to all of its problems, the industry can rely on it as a preeminent source of pavement performance data, answering all kinds of questions yet to be conceived.
The number of ways in which States can benefit from LTPP continues to grow, but the extent to which the transportation community benefits will depend on the investment made in learning about, testing, and applying the findings and products of LTPP.
The exact relationship between the investments made in LTPP and the benefits accrued is difficult to calculate. However, what we do know is that if we do not seize the opportunity to invest in learning about, adopting, adapting, and applying the findings and products of LTPP, we will not achieve all the benefits that are possible.
One of the Long Term Pavement Performance Falling Weight Deflectometers is collecting deflection data from an LTPP test section.
State Route 79 in Allegheny County, Pennsylvania.
Gary L. Hoffman, P.E., was appointed to the position of chief engineer of highway administration for the Pennsylvania Department of Transportation (PennDOT) on June 26, 1995. As chief engineer, he directs a team of more than 650 employees involved with the maintenance, operations, restoration, and expansion of Pennsylvania's transportation infrastructure. The focus of his organization is setting policy and standards, performing quality assurance, identifying and implementing technological change, and providing for development of the technical workforce. Chief Engineer Hoffman is a licensed professional engineer and surveyor with more than 33 years of experience, 29 of which have been with PennDOT. He has steadily moved through the ranks at PennDOT serving as director of maintenance and operations prior to his appointment as chief engineer. From 1983 to 1992, Hoffman was the director of the Bureau of Bridge and Roadway Technology where he was contract manager for developing the department's RMS, BMS, and NBIS Safety Inspection Systems. Hoffman received both a B.S. and M.S. in civil engineering from Drexel University in Philadelphia. He has authored or co-authored more than 40 published technical papers and articles, primarily in the areas of transportation materials, pavement designs, construction practices, and management systems. Many of these papers and articles have received national exposure. Hoffman is active in strategic planning and innovation implementation at the national level and serves as chairman or as an active member of several National Academy of Science- and AASHTO-sponsored boards and committees. He chairs the new AASHTO Committee on Technology Implementation, is a vice-chair of the Subcommittee on Maintenance, and chairs the Task Force on Retroreflectance of Signs and Markings. He is a member of AASHTO's Asset Management Committee of AASHTO's Land Transportation Security Task Force and the National Partnership for Highway Quality and is an active participant in the Standing Committee on Highways. He is also a member of the TRB Steering Committee on Long Term Pavement Performance and the TRB Committee for Research on Improved Concrete Pavements.
Other Articles in this issue:
A Hallmark of Context-Sensitive Design
Do Better Roads Mean More Jobs?
Exciting Opportunity for ITS Work
Benefitting from LTPP—A State's Perspective
