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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-03-041

Evaluation and Analysis of LTPP Pavement Layer Thickness Data

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The mission of the Long-Term Pavement Performance (LTPP) program is to foster increased pavement life through: [1]

Layer structure and thickness information is one of the most important data elements for any type of pavement performance study. Among the studies where layer structure and thickness information is critical are backcalculation of pavement moduli, mechanistic analysis of pavement structures, and performance modeling. In fact, the accuracy of layer thickness data has a strong impact on the outcome of practically all analyses of performance.

Layer Structure and Thickness Information Collected by the LTPP Program

A large amount of data related to layer structure and thickness has been collected as part of the LTPP program. The data have been collected from several sources, including the following:

The pavement layer thickness data from these sources exist in many different LTPP tables. For example, tables TST_AC01, TST_AC01_LAYER, and TST_PC06 contain core measurement data. The inventory or planned layer thickness data are stored in various other tables (e.g., INV_LAYER and RHB_LAYER). Tables SPS*_LAYER and SPS*_LAYER_THICKNESS contain field elevation data. The design layer thickness data are found in the experimental designs for newly constructed SPS sections.


Additionally, material types and depths to strata top and strata bottom are identified or measured in the field from holes, test pits, and probes. Table TST_SAMPLE_LOG stores information about the samples taken from holes, pits, and probes, and is a good raw data source for unbound layers.

Using the above information, the LTPP Regional Support Contractors (RSC's) complete tables TST_L05, TST_L05A, and TST_L05B. Table TST_L05 stores project-level material type information for SPS experiments with multiple sections constructed at the same SPS site. Table TST_L05A summarizes measured layer material type and thickness data at the beginning, within, and at the end of a section, based on the core measurements and field test pit information. The TST_L05B table provides the representative thickness for the section. These representative thicknesses are the recommended analysis level layer thicknesses in the LTPP database.

Following is a list of relevant LTPP tables that contain layer material type or thickness data:

Additional information about the LTPP program, field sampling, materials testing, data collection guidelines, and LTPP database can be found in the following documents:

Need for Review of LTPP Pavement Layer Thickness Data

The LTPP database contains a wealth of layer material type and thickness data. However, some discrepancies have been observed in these data, raising some concerns about data quality. For some sections, design thickness or highway agency inventory thickness was reported in the TST_L05B table because of the lack of materials testing data. This is especially true for many rehabilitated sections. In addition, some sections are missing layer thickness information, which severely limits the use of these sections in data analysis studies.

Study Objectives

The goal of this study is to assess and improve the LTPP layer material type and thickness data quality for data that are currently available in the LTPP database. The main objectives for this study are as follows:

One important product from this study is a Researcher's Guide to the LTPP Layer Thickness Data. The Guide is presented in a separate report.

Report Organization

The report contains seven chapters. Chapter 1 (this chapter) provides an introduction to the issues related to the LTPP layer material type and thickness data, study objectives, and report organization. Chapter 2 summarizes layer structure and thickness data availability and completeness. Chapter 3 discusses the results from evaluation of the LTPP layer material type and thickness data reasonableness and consistency. Chapter 4 provides a summary of layer thickness variability data evaluation. Chapter 5 summarizes characteristics of the within-section thickness data variation for SPS layers with extensive elevation measurements. Chapter 6 discusses evaluation results on comparing designed versus as-constructed or measured thicknesses. Finally, chapter 7 presents a summary, conclusions, and recommendations from this study.

Additional material is included in three appendixes. Appendix A contains a table of material codes used to correlate material type data from inventory and testing tables. This table was developed to enable cross-table comparison of material types specified in several LTPP database tables using different material coding schemes. Statistical formulations used in the skewness-and-kurtosis test are provided in Appendix B. Appendix C contains description of a statistical procedure that was considered for evaluation of within-section layer thickness variability characteristics.

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