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Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 1 > LTPP: The Next Decade

July/August 1998
Vol. 62· No. 1

LTPP: The Next Decade

by Charles J. Churilla

SHRP Road Test segment sign. The Federal Highway Administration's (FHWA) Long-Term Pavement Performance Program (LTPP) is the largest and most comprehensive pavement study in the world. The program has collected a wealth of information that can be used to benefit the global highway community.

Indeed, this is already happening. LTPP data were used to develop a module for the World Bank's Highway Design and Maintenance Model software. It is also being used for the development of the AASHTO 2002 Pavement Design Guide. And perhaps, most significantly, LTPP information is now readily available to the entire highway community via DataPave, an easy-to-use CD-ROM.

These efforts illustrate the enormous value of LTPP; nevertheless, today, as it enters its next decade, LTPP faces a significant challenge - a challenge that requires the support and active participation of the U.S. states and Canadian provinces.

LTPP's First Decade

LTPP was established as part of the Strategic Highway Research Program (SHRP), and it is now managed by the FHWA.

In 1987, LTPP began a series of rigorous long-term field experiments monitoring more than 2,400 asphalt and portland cement concrete pavement test sections across the United States and Canada. The program was designed as a partnership with the states and provinces.

To ensure that its results were technically sound, valid, and defensible, LTPP used a scientific approach in its study of pavement performance. Significant variables were identified. These included: pavement structure; materials properties of the pavement layers; traffic loads applied to the pavement structures; climatic conditions; pavement conditions; and related influencing factors, such as quality of construction, the influence of maintenance, and other special or unique design features. Experimental designs were then developed under the two broad categories of General Pavement Studies and Specific Pavement Studies. Lastly, procedures for analyzing the information and data acquired from LTPP studies were established.

LTPP's goal is to help the states and provinces make decisions that will lead to better performing and more cost-effective pavements. To this end, early in the program, LTPP set standards for pavement monitoring and materials testing and developed equipment calibration procedures. More recently, LTPP developed a new, refined set of tools, such as the improved procedures for the design of jointed concrete pavements that were adopted by the American Association of State Highway and Transportation Officials (AASHTO) in 1997. In fact, over the past several years, LTPP has proved to be a powerful resource for many members of the highway community.

"LTPP is a wealth of information," said David Holt, executive director of the Minnesota Asphalt Association. "When we looked around for data for a study we were doing regarding Superpave implementation, we found that, not only did LTPP have the best available data, it had the only data available." As a result of the study, contour maps that clearly illustrate the distribution of performance grades (PG) with reliabilities throughout Minnesota were developed. The contour maps provide practicing engineers with a simple, yet effective, tool for selecting binder grades that are more cost-effective for regional or local conditions and that meet Superpave PG concepts.

LTPP's Next Decade Challenge

Today, as it enters its second decade, LTPP must focus and refine the way it does business if it is to provide the data that will be the foundation and verification for new pavement design procedures for years to come. Understanding pavement performance is a complex task that requires not only gaining a knowledge base on "how" pavements perform but also understanding "why" pavements perform as they do.

What's the difference between understanding "how" pavements perform and "why" pavements perform in a certain way, and why is that important? Knowing how pavements perform can help us to duplicate past successes and avoid recreating past failures. It doesn't, however, provide enough information to allow us to correct the underlying causes of failure or build upon our successes. Understanding why pavements perform as they do will provide the tools that engineers and managers need to deal with an ever-changing world and to more effectively manage the risks inherent in highway design, construction, maintenance, and rehabilitation.

Over the past decade, LTPP has developed a solid knowledge base for understanding how pavements perform. LTPP's challenge throughout its second decade is to build on this foundation by furthering the understanding of why pavements perform as they do.

Addressing LTPP's Next Decade Challenge

To address the challenge of understanding why pavements perform as they do, FHWA has:

  • Developed a Strategic Analysis Plan that establishes a clear focus for FHWA's LTPP analytical activities and that drives the data collection activities.
  • Launched a data resolution effort to address the gaps and questions regarding data collected over the past several years.
  • Initiated a monitoring adjustments effort to identify appropriate adjustments to improve the quantity and completeness of the data.

All of these efforts are interrelated. The outcome of the data resolution effort will influence the monitoring adjustments effort. Both, in turn, will influence the ability to proceed with certain analytical activities.

Strategic Analysis Plan

"The purpose of the Strategic Analysis Plan is to provide a structured approach for addressing LTPP's challenge," explained Cheryl Richter, LTPP highway research engineer. "The plan establishes a target by which results can be measured and appropriate data collection and analysis efforts can be established." Within the plan, analytical efforts are organized into four major stages: (1) preliminary analyses, (2) investigation of site and design factors, (3) development of detailed performance-prediction models, and (4) development of performance-prediction and strategy-selection tools.

"Each analysis stage builds upon the foundation provided by the LTPP data and outcomes of previous analysis stages. Throughout all stages of the plan, LTPP will be coordinating with other national research initiatives to maximize the effectiveness of LTPP's contribution to pavement research," said Richter. Figure 1 illustrates the time line for each stage of the plan.

Data Resolution

"Analyses of current LTPP data," said Monte Symons, LTPP program manager, "have indicated that there are gaps in the data, along with questions related to the accuracy inherent in the data collection procedures. To resolve uncertainty about some of the available data and to fill in some of the missing data, FHWA launched a data resolution effort in 1997."

The first stage of this effort involved modernizing LTPP's computer hardware and software along with reorganizing its data-processing procedures to ensure that data backlogs will be kept to a minimum and that missing and questionable data are identified in a timely manner.

The second stage of this effort involved processing all of the LTPP data collected to date and compiling results into data status reports for each of LTPP's 2,400 test sections. The data status reports identify where there are gaps and questions regarding the current data for each state and province.

In the final stage of the effort, FHWA will conduct a series of data resolution meetings with the states and provinces during the summer of 1998. The purpose of these meetings is to review the data status reports, discuss both data completeness and quality issues related to specific test sections, identify which data issues can be resolved, and develop a data-resolution action plan for data issues that can be resolved.

Acknowledging the importance of this effort to the highway community, AASHTO's Standing Committee on Highways passed a resolution in support of it on April 18, 1998.

"AASHTO member departments recognize the value of LTPP data as it applies to the performance analysis of pavements," stated Haleem Tahir, AASHTO's SHRP Product Implementation Coordinator. "With better quality data and more complete information, a higher level of confidence can be placed in these analyses. Therefore, it is in the interest of the states to help LTPP resolve the data deficiencies."

"Resolving issues relating to missing and questionable data will enable the states, provinces, and FHWA to focus their resources on those test sections with the maximum potential for contributing to an improved understanding of pavement performance. In those cases where data issues cannot be resolved, the states, provinces, and FHWA will be able to reduce or eliminate the need for future monitoring, enabling all to allocate resources more effectively," Symons said.

The data resolution effort applies equally to state- and province-supplied data, as well as data collected by FHWA. As such, by mid-1998, FHWA will resolve two key issues: the asphalt resilient modulus test procedures routine and photographic distress surveys.

Monitoring Adjustments

The data resolution effort will be completed by the autumn of 1998. Once this is done, adjustments in the data collection program and monitoring frequency will be made. This effort will include altering the frequency of collection for data already being collected and also the collection of additional data at some of the test sites.

Key considerations that will drive monitoring adjustments include:

  • LTPP has a decade's worth of monitoring data - factual information it can use to adjust monitoring programs for maximum effectiveness.
  • Test sections that have the most complete data sets have greater potential to contribute to the development of meaningful conclusions.
  • Some of the data elements needed to explain why pavements perform as they do, do not have to be collected at every site.
  • The states, provinces, and FHWA have limited resources. An increase in data collection in one area must be offset by a decrease in another area.

Working with the states and provinces, FHWA has targeted adjustments to the data collection program to be completed by mid-1999.

Critical Issues

As LTPP enters its second decade, it is crucial that the states and provinces maintain their full and active participation in LTPP. State and provincial support and participation in LTPP's current data resolution effort - in addition to continuing to provide materials testing, traffic data, and traffic control for data collection and to participating in additional data collection that may be needed - are essential. The success of the data resolution effort is vital to LTPP. Since these data are the foundation of and means for verification for new pavement design procedures for years to come, the consequences of missing or incomplete data are quite significant. While much can be done with the available data, LTPP cannot fully deliver its promised benefits without resolving the current data issues.

Conclusion

Understanding pavement performance is vital to building and maintaining highway systems. Throughout its first decade, LTPP has furthered the understanding of how pavements perform. As it moves into its second decade, LTPP seeks to build on these efforts to further knowledge of why pavements perform as they do.

Charles J. Churilla is the chief of FHWA's Pavement Performance Division, Office of Engineering Research and Development. The division is responsible for several of FHWA's key pavement research initiatives: the Long Term Pavement Performance Program, performance-related specifications for asphalt and portland cement concrete pavements, and truck-pavement interaction. He began his highway career with the Pennsylvania Department of Transportation in 1965 and joined FHWA in 1981 as a geotechnical engineer. Prior to becoming division chief, he served as the Strategic Highway Research Program Implementation Coordinator in FHWA's Office of Technology Applications.

The Partnership

Since its inception, LTPP has depended on the cooperative efforts of the highway engineering community. As LTPP moves into its second decade, each of these partners will continue to play a key role in helping LTPP achieve its full potential.

States and Provinces

As the owner agencies of the LTPP test sections, the state and provincial highway agencies have been vital to LTPP's achievements to date, and they play a pivotal role in LTPP's future. In addition to providing the test sections, the states and provinces collect traffic volume and weight data, perform materials testing, report maintenance and rehabilitation activities, and provide traffic control for pavement data collection activities.

As builders of today's highways, the states and provinces also play a key role as the primary users of the results garnered from the LTPP program. Now, with the availability of the DataPave CD-ROM, the states and provinces can expand this role to include using the actual LTPP data to address local and regional pavement technology needs.

Federal Highway Administration

FHWA's Office of Engineering Research and Development manages the day-to-day operation of LTPP. This includes the collection, processing, and dissemination of data, along with national analysis activities.

In 1996, FHWA established an LTPP Implementation Team, composed of engineers from FHWA regional offices. The team's mission is to take LTPP research findings and turn them into practical engineering tools. Its most notable accomplishment has been the introduction of DataPave in 1998. For the longer term, FHWA is working to integrate LTPP product development and delivery activities into its broader Pavement Technology Program. (Contact the Pavement Division, Office of Engineering, at paul.teng@fhwa.dot.gov for more information on the Pavement Technology Program.)

American Association of State Highway and Transportation Officials

While the individual states and provinces play the key role in getting the LTPP data collected, the collective leadership role of AASHTO has been vital to past successes in test section recruitment and the adoption of LTPP-developed methods, procedures, and guidelines as standards for pavement engineering. AASHTO's continued leadership and support are critical to the success of the current data resolution initiative and to the adoption of future LTPP outputs.

Transportation Research Board

The Transportation Research Board (TRB) provides an independent forum for the states, industry, and academia in which they can provide input and advice on the conduct of LTPP research and implementation activities. The TRB-LTPP Committee provides management-level input on the conduct of LTPP. A subcommittee that provides advice and counsel on matters relating to strategic planning, data resolution, and monitoring adjustments supports the TRB-LTPP Committee. In addition, several topic-specific expert task groups provide input on technical details.

Highlights of LTPP's First Decade of Accomplishments

These highlights represent just a few of the many outputs LTPP has produced:

  • Introduced in 1998, DataPave presents LTPP data on an easy-to-use CD-ROM. DataPave unlocks the potential of LTPP data for the development of products by the states, provinces, and FHWA.
  • Engineers have long believed that the long-term performance of a pavement is related to the initial, as-constructed smoothness. In fact, a number of agencies pay a bonus for smooth pavements. Through analysis of the LTPP data, the long-term performance benefits of building smooth pavements have been quantified.
  • Analysis of the LTPP traffic data has yielded previously unavailable information on how much traffic data is needed to accurately estimate total design traffic loads at a particular site.
  • Testing, monitoring, and calibration procedures developed to support LTPP data collection have provided a foundation for AASHTO standards that helps agencies collect high-quality data to support the decisions they make every day. For example, LTPP-developed procedures ensure consistency in resilient modulus testing of soils, and LTPP-developed falling-weight deflectometer calibration procedures are now widely used by state and provincial highway agencies.

Four Stages of LTPP's Strategic Analysis Plan

Preliminary Analyses

Preliminary analyses focus on determining the quality, quantity, and completeness of LTPP's available data. The results will provide information on variability in pavements, traffic, and the measurements used to monitor and assess their performance. The results derived from these analyses will enable managers and engineers to make better decisions as to where and how to invest monitoring and evaluation resources, as well as maintenance, rehabilitation, and construction funds.

Site and Design Factors

The second stage of the analysis process focuses on the cause-and-effect relationships inherent in pavement performance. Studies will address both site (the traffic, subgrade, and climatic conditions in which pavements exist) and design factors (the pavement type, structure, layer thicknesses, and details that can be varied or adjusted to suit the conditions and constraints associated with a given project). These studies will yield extensive knowledge of what works and what doesn't and begin to explain the "why" of pavement performance.

Performance Prediction Models

In the third stage of the analysis process, component models will be assembled to form complete performance-prediction models. Initially, work will focus on evaluating, testing, and calibrating currently available models to see how they fit real-world pavements. New models will be pursued only when necessary. The models resulting from this analysis phase will improve engineers' ability to accurately predict and understand pavement performance and the factors that affect it.

Performance-Prediction and Strategy-Selection Tools

In the fourth stage of the analysis process, the models and information that have been validated or developed in prior stages will be developed into practical tools for use in routine pavement engineering practice. The outputs will be in the form of procedures and guidelines for pavement design and management to facilitate wise strategy selection and reliable performance prediction.

Excerpt of the AASHTO Standing Committee On Highways (SCOH) Resolution on "Addressing the Data Deficiencies in the Long Term Pavement Performance Program (LTPP) Studies"

NOW, THEREFORE, BE IT RESOLVED that SCOH encourages the FHWA, as steward of the LTPP database, to inform each member agency by August 1998 of the status of the data set, identifying apparent deficiencies for each of its pavement sections, and emphasizing the benefits of continued monitoring of some sections and discontinuation of monitoring of other sections; and

BE IT RESOLVED that the AASHTO member agencies, as LTPP Partners, act to eliminate the data deficiencies on their pavement sections by October 1998; and

BE IT RESOLVED FURTHER that the FHWA is encouraged to provide SCOH a progress report in October 1998 and a final report in April 1999 on the status of data collection on the pavement sections on which monitoring will be continued.

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