Researcher's Guide to The LTPP Layer Thickness Data

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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

• For GPS sections, indicators such as average, minimum, maximum, and standard deviation can be found in the IMS tables INV_LAYER and RHB_LAYER. These summary statistics were provided by the highway agencies and could be either estimated or computed. No additional information is available regarding how summary statistics were derived for these tables.
• For the SPS sections, layer thickness summary statistics can be obtained from the SPS*_LAYER tables. These values were computed for the SPS sections from the elevation shots measurements. The SPS*_LAYER tables do not contain summary information on the number of data points used to derive the statistics. No information is available in the database regarding whether all of these data points were used to compute summary statistics or whether some "outlier" points were excluded.

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

• Tables TST_AC01_LAYER and TST_PC06 contain individual core thickness measurements for AC and PCC layers, respectively. These measurements are available for GPS and SPS sections. The methodology for preparing an analysis data set (including identification of outliers) and methodology for statistical analysis can be found in the Assessment of Selected LTPP Material Data Tables and Development of Representative Test Tables report. [28]
• The SPS*_LAYER_THICKNESS tables contain individual thickness measurements from elevation shots taken along the section and reported for different layer and material type combinations. These measurements are available for SPS sections only. The methodology for preparing an analysis data set (including identification of outliers) and methodology for statistical analysis can be found in chapter 4 of the Evaluation and Analysis of LTPP Layer Thickness Data report. [29]

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:

• Total Number of Measurements.
• Mean Thickness.
• Min. Thickness.
• Max. Thickness.
• Standard Deviation.
• Coefficient of Variation (COV).

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.

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

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:

• AC surface courses.
• Combined AC surface and binder courses.
• AC binder courses.
• Dense-graded aggregate bases.
• Dense-graded AC-treated bases.
• Permeable AC-treated bases.
• Lean concrete bases.
• PCC surface layers.
• PCC overlay layers.

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. 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. 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:

• Standard deviation in layer thickness generally increases with increasing mean layer thickness.
• AC layers (surface and binder layers and permeable asphalt treated bases) and dense graded aggregate bases show the highest within section relative layer thickness variability, as estimated by COV.
• PCC layers (surface layers, overlays, and lean concrete bases) show the lowest within section relative layer thickness variability, as estimated by COV.
• Individual within-section layer thickness measurements follow a normal distribution for a majority of the LTPP layers.

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