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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-01-166
Date: November 2003

Structural Factors for Flexible Pavements—Initial Evaluation of The SPS-1 Experiment Final Report


One of the objectives of the Long-Term Pavement Performance (LTPP) studies is to develop improved design methodologies and strategies for the construction of flexible pavements. Those factors that can affect the performance of flexible pavements include, as a minimum, drainage, structural features (such as base type, base thickness, and asphalt thickness), environment, and subgrade type. The LTPP program incorporated all of these factors into a single experiment to study the structural factors for hot-mix asphalt (HMA) flexible pavements—Specific Pavement Studies (SPS) 1—entitled Strategic Study of Structural Factors for Flexible Pavements.

It is expected that the successful completion of the SPS-1 experiment will lead to improvements in design procedures and standards for construction of HMA-surfaced pavements. These improvements should contribute to achieving the overall goal of the LTPP program-increased pavement life and better utilization of resources. Investigating the effects of the specific experimental design features and site conditions (material types, layer thickness, subgrade soil, traffic, and climate), as well as their interactions on pavement performance, makes possible the evaluation of existing design methods and the performance equations. It also makes possible the development of new and improved design equations and calibration of mechanistic-empirical models.


The SPS-1 experimental plans were originally designed to incorporate project sites in all four LTPP climatic regions and on both fine- and coarse-grained subgrades. This requirement makes it possible to cover a large inference space of the continental United States. The Strategic Highway Research Program (SHRP), State highway agencies (SHAs), and the Federal Highway Administration (FHWA) made a major effort to identify appropriate SPS-1 sites and to construct all the test sections according to their original experimental design. A wide range of specific data was collected during construction and extensive field monitoring data (traffic, profile, cracking) have been collected from these test sections over time.

The original expectations for the LTPP program are summarized in the SHRP-P-395 report.(1) Originally, the following objectives were established:

The experimental designs for LTPP were developed with a clear relationship to these objectives. The following are the products identified for the LTPP program:

The following objectives of the SPS-1 (new flexible pavement) and SPS-2 (new rigid pavement) experiments are stated in the same report:

The SPS-1 experiment was also expected to identify trends associated with the various design parameters on pavement performance and life expectancy and to provide data to help improve or validate current structural design procedures. With these improved methodologies and procedures, highway agencies should be able to determine and select more appropriate and optimum strategies for the design of flexible pavements and significantly reduce the occurrence of premature failures. However, there have been many concerns expressed regarding the ability of the SPS-1 experiment to meet these expectations satisfactorily.(2,3) Some of these concerns include the following:

The full extent of the deviations and deficiencies and the potential impact of those deficiencies have not yet been quantified for the SPS-1 experiment. Issues of experimental design, construction quality, data quality, and data completeness (with respect to both current data collection guidelines and anticipated pavement engineering needs) also need to be addressed in the SPS-1 experiment.

The SPS-1 projects were constructed between 1993 and 1998, which means that they are young and may not yet directly support analysis activities to improve our knowledge in many of the above-listed areas. However, some of the SPS-1 sections have begun to exhibit distress; thus, it may now be possible to make some preliminary evaluations. To date, no in-depth assessment has been undertaken to determine to what extent this experiment will provide the necessary data to ensure the broader expectations of these experiments are attained. Therefore, this study was initiated to conduct a comprehensive review of all SPS-1 experimental sites to determine the current adequacy and potential of data from this experiment to adequately satisfy future pavement engineering needs.

This review compares the experiment sites as they exist today with both the original expectations and any new expectations for the 21st century. For example, there is a greater emphasis on mechanistic-based design now than existed a decade ago. This review will provide a sound basis for the following:

This evaluation of the SPS-1 experiment is being conducted at the same time and in cooperation with the evaluation of SPS-2 (new rigid pavement), SPS-5 (rehabilitated flexible pavement), and SPS-6 (rehabilitated rigid pavement).


As stated above, a detailed review was completed to determine to what extent this experiment will provide the necessary data to ensure that the objectives and expectations of the SPS-1 experiment are attained. Stated simply, the primary objective of the SPS-1 experiment on structural factors for flexible pavements was to determine the relative influence and long-term effectiveness of the strategic factors that influence the performance of flexible pavements. This review concentrated on the core experimental test sections and on the supplemental test sections that were built by the individual SHAs for each project. There were five specific objectives for this review, as listed below:

  1. Evaluate the set of core and supplemental test sections constructed within the SPS-1 experiment in relation to their ability to support the objectives and characterize the overall "health" and analytical potential of the SPS-1 experiment. This includes identifying areas of strength and weakness and developing a plan of recommended corrective measures as appropriate to strengthen the SPS-1 experiment to accomplish its objectives, as well as developing analysis plans for both short-term and long-term goals. This objective was further subdivided into two areas, as noted below:
    • Evaluate the quality and completeness (in relation to current data collection requirements) of the SPS-1 construction data and provide recommendations for the resolution and correction of data that are anomalous or of inadequate quality.
    • Evaluate the adequacy of existing data and current data collection requirements in relation to anticipated analytical needs. Identify areas where current requirements are excessive or deficient, and provide recommendations where adjustments (in quantity, quality, frequency, or data type) are warranted.
  2. Identify any confounding factors introduced into the SPS-1 experiment evaluated by construction deviations or other factors not accounted for in the original experimental design.
  3. Consider both short-term and long-term horizons in the evaluation and the preparation of data analysis recommendations.
  4. Evaluate the opportunities for local, regional, or national analysis of the core and supplemental test sections.
  5. Identify specific objectives and expectations that should be pursued for the SPS-1 experiment, considering the original expectations and the needs of the future. Consider expectations at the local, regional, and national levels, as appropriate.

Specifically, this report focuses on the following four areas of the SPS-1 experimental data:

  1. Review of data quality.
  2. Detailed discussions on the quantity and percentage of data that are at Level E (the highest quality data) in the Information Management System (IMS) database as of January 2000.
  3. Comparison of the designed versus as-constructed section parameters, especially those that were used for designing the experiment (e.g., experimental deviations and construction problems).
  4. Preliminary evaluation of performance and identification of future analyses that can be performed on the data.

It should be understood that the LTPP database is dynamic in nature, i.e., data are continually checked and entered. This review and detailed assessment of the experiment represents a "snapshot" of the database and the Level E data at a particular point in time, i.e., January 2000.


The report is divided into six chapters, including this introduction. Chapter 2 provides an overview of the current status of the SPS-1 experiment in comparison to the original experiment designs. Chapter 3 summarizes the project requirements for each SPS-1 project. Chapter 4 summarizes each of the SPS-1 projects that have been built, the data that are available for each project, construction difficulties, and any data deficiencies. Chapter 5 presents an analysis of the initial observations of the key distress and performance indicators completed on a project-by-project basis and across the entire experiment. Chapter 6 summarizes the effects that the data deficiencies, if any, may have on the results that can be obtained from this experiment.

More detailed information and data are provided in the appendices. Appendix A presents a summary of the construction and deviation reports, as well as other data elements that are available for each project, and appendix B presents a summary of the available construction data for each project.