Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-03-040
Date: July 2002

Researcher's Guide to The LTPP Layer Thickness Data

Previous | Table of Contents | Next

  1. Data Sources
  2. Typical Values for Variability from LTPP
  3. Layer Thickness Distribution Type
  4. Summary of Layer Thickness Variability Assessment

This chapter contains a discussion about LTPP database sources for layer thickness variability data. In addition, typical layer thickness summary statistics used to characterize layer thickness variability are provided. These summary statistics were derived from the LTPP layer thickness data. The chapter also contains information on the extent that layer thickness variation within a section follows typical statistical distributions. Guidelines are provided for the selection of the appropriate sources for the LTPP layer thickness variability data based on layer, material, and experiment type.

Data Sources

Layer thickness summary statistics such as average, minimum, maximum, standard deviation, and coefficient of variation are used to determine layer thickness variability along the LTPP section. Most of these values are found in the following LTPP tables

Alternatively, layer thickness summary statistics could be computed using LTPP layer thickness data obtained from individual core measurements or from elevation measurements. The following data sources are available in the LTPP database

Typical Values for Variability Indicators from LTPP

To estimate typical values for layer thickness variability indicators, layer thickness data for SPS experimental sections with newly constructed layers were obtained from TST_AC01_LAYER and TST_PC06 tables (core thickness), and from SPS*_LAYER_THICKNESS tables (elevation thickness). These newly constructed SPS layers were selected for estimation of the typical variability indicators because they have documented target thickness values. The target thickness values were found to affect some of the layer variability indicators significantly. Data obtained using different measurement methods were analyzed separately. The analysis was done for the sets of data grouped by target design thickness, material, and layer type. The following statistical indicators were computed:

The analyses were done separately for the thickness data obtained from core measurements and for the thickness data from elevation measurements. Data from the GPS experiments were analyzed separately from the SPS experimental data because the construction quality control used during the construction of new SPS sections was different from the quality control for the GPS sections constructed prior to inception of the LTPP program.

Table 13 summarizes layer thickness COV and standard deviations by different layer and material types. These summaries are based on the analysis of core thickness data for PCC and AC layers. Table 14 summarizes layer thickness COV and standard deviations by different layer and material types obtained for the SPS sections based on the analysis of elevation measurements.

The COV values from tables 13 and 14 could be used as approximate estimates of the expected layer thickness variability along the project for a given design layer thickness, material, and layer type.

Table 13. Summary of layer thickness COV and standard deviations based on core measurements.

Experiment type Description Number of analysis layers Mean COV, % Min COV, % Max COV, % Mean St. dev., mm Min St. dev., mm Max St. dev., mm
GPS
AC Binder Course
396 10.1 0.78 83.19 7.46 0.87 110.28
ATB Layer
88 6.83 1.02 46.92 8.34 1.3 61.38
AC Surface Layer
506 9.76 0.7 93.24 5.44 0.52 107.46
AC Overlay
259 10.68 1.48 59.92 5.44 0.87 44.9
SPS
AC Binder Course
382 10.41 0.62 71.38 7.89 1.27 95.19
ATB Layer
139 12.66 0.85 184.88 14.79 1.47 135.97
AC Surface Layer
488 10.21 0.69 64.28 5.34 1.14 45.58
AC Overlay
160 10.7 0.72 70.71 4.9 1.14 25.85
GPS
PCC Surface Layer
336 2.36 0.4 10.92 5.44 1.04 31.14
PCC Overlay
24 2.92 0.55 13.1 6.22 1.04 20.74
SPS
Lean Concrete Base
34 4.62 1.12 23.38 7.37 1.8 38.8
PCC Surface Layer
233 2.66 0.51 27.97 6.31 1.14 65.21
PCC Overlay
29 5.19 1.61 12.59 7.22 2.19 14.63

Table 14. Summary of layer thickness COV and standard deviations based on SPS elevation measurements.

Material Type Number of Analysis Layers Mean COV,% MinCOV,% MaxCOV,% MeanSt. Dev., mm MinSt. Dev., mm MaxSt. Dev., mm
DGAB 219 8.78 1.9 37.44 13 3.2 55.76
DGATB 97 5.31 1.79 15.1 9.5 3.87 24.48
LC 48 5.69 2.55 20.33 8.96 3.81 32.38
PATB 129 8.74 3.45 21.21 8.91 3.59 20.41
PCC 177 4.18 0.98 17.98 8.61 2.88 22.96
SB 319 8.32 2.01 35.8 8.41 2.47 21.1

The methodology used to create an analysis data set, identify outliers, and compute summary statistics is documented in the report titled, Evaluation and Analysis of LTPP Pavement Layer Thickness Data. [29]

Layer Thickness Distribution Type

Layer thickness data from the SPS elevation measurements were analyzed to determine the extent to which the variation of layer thickness within a section follows typical statistical distributions. The layers used in the analysis include different material types and functional classifications, such as:

To assess layer thickness distribution characteristics, descriptive statistics such as mean, standard deviation, skewness, and kurtosis were computed for each section. Using descriptive statistics, a likely shape of layer thickness distribution was analyzed. The results of exploratory analysis indicated that, for most of the sections, the distribution is likely to be normal. For a more rigorous analysis, a combined test for skewness and kurtosis was selected to test normality of layer thickness distribution. The summary of the testing procedure is documented in reference. [29]

The analysis results for 1,034 SPS layers indicated that thickness variation within a section follows a normal distribution for 84 percent of all layers. These results could serve as an input to pavement engineering applications involving design reliability, and also for construction quality control and quality assurance applications. Figures 1 to 2 provide examples of layer thickness frequency distributions obtained from the elevation measurements data for AC and PCC surface layers, respectively. Theoretical normal distributions are superimposed over field frequency data to provide means for comparison between field data and theoretical distributions.

Figure 1: Chart showing example distribution of layer thickness measurements along the section for AC surface. Figure 1 the frequency (number of observations) distribution of the 55 surface and binder layer thickness data points over the layer thickness ranging from 46 to 80 mm or more with 4-mm increment for the SPS-1 Section 55-0118. The mean of the distribution is 65 mm and the standard deviation is 7 mm. The distribution appears to be normal and the data were determined to be reasonably normal based on skewness and kurtosis tests at selected level of significance. Click here for more details.
Figure 1: Chart. Example of distribution of layer thickness measurements along the section for AC surface and binder layer for the SPS-1 Section 55-0118.

Figure 2: Chart showing example distribution of layer thickness measurements along the section for PCC surface layer. Figure 2 shows the frequency (number of observations) distribution of the 60 PCC surface layer thickness data points over the layer thickness ranging from 177 to 216 mm or more with 4-mm increment for the SPS-8 Section 39-0809. The mean of the distribution is 200 mm and the standard deviation is 8 mm. The distribution appears to be normal and the data were determined to be reasonably normal based on skewness and kurtosis tests at selected level of significance. Click here for more details.
Figure 2: Chart. Example of distribution of layer thickness measurements along the section for PCC surface layer for the SPS-8 Section 39-0809.

Summary of Layer Thickness Variability Assessment

Based on the assessment of the LTPP layer thickness values and analysis of layer thickness variability indicators, the following findings have been established:

Previous | Table of Contents | Next


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration