Evaluation and Analysis of LTPP Pavement Layer Thickness Data

6. CHARACTERIZATION OF VARIATION BETWEEN AS-DESIGNED AND AS-CONSTRUCTED LAYER THICKNESSES

The main purpose of this chapter is to characterize the extent of differences in the layer thickness data between as-designed and as-constructed (measured) thicknesses for the newly constructed SPS layers. Only these new SPS layers have design thicknesses accurately documented.

Data sources for the analysis are discussed first, followed by an overview of as-designed thicknesses for the newly constructed SPS layers. After that, typical thickness deviations from the target thicknesses are summarized, as well as their distribution types. Finally, the results of the statistical analysis are presented.

Data Sources

Two thickness data sources with multiple measurements on a given layer exist in the LTPP database:

• Elevation measurements in SPS*_LAYER_THICKNESS tables for experiments SPS-1, SPS-2, SPS-5, SPS6, SPS-7, and SPS-8
• Pavement core measurements in testing tables TST_AC01_LAYER and TST_PC06.

According to the SPS construction guidelines [35-40], rod and level survey measurements are to be taken at a minimum of five offset locations (edge, outer wheel path, midlane, inner wheel path, and inside edge of lane) at longitudinal intervals no greater than 15 m (50 ft). Typically, 55 elevation measurements are available for each regular SPS test section.

The number of cores taken at each section depends on experiment and layer type and is defined in the corresponding Sampling and Testing Guide [6-11]. The number of cores per section ranges between 1 and 9.

All sections with available thickness data in either one of these tables are studied to quantify as-designed versus as-constructed variations in layer thickness.

For the section/layer combination, an analysis cell is defined to represent a specific layer in a test section, for which the target thickness was documented. The following fields from TST_L05B or EXPERIMENT_SECTION table in LTPP database along with the design target layer thickness define a unique analysis cell:

• EXPERIMENT_NO (Experiment number).
• LAYER_TYPE (Layer type).
• MATL_CODE (Material type description)
• Target layer thickness.

Design Thicknesses

For newly constructed SPS layers, the design thicknesses are defined in the corresponding SPS Experimental Designs [12-17]. The design thicknesses are available for the following layer types:

• SB - AC surface and binder thickness (SPS-1, SPS-5, SPS-6, SPS-8).
• DGATB - Dense-graded asphalt-treated base (SPS-1).
• PATB - Permeable asphalt-treated base (SPS1, SPS-2).
• PCC - Portland cement concrete (SPS-2, SPS-7, SPS-8).
• LC - Lean concrete (SPS-2).
• DGAB - Dense-graded aggregate base (SPS-1, SPS-2, SPS-8).

The design thicknesses for all these SPS experiments and layer types are presented in tables 38 through 43.

Study Methodology

For both the elevation and core as-constructed thickness measurements, typical mean layer thickness deviations are established by the following:

• Descriptive summary statistics of the average thicknesses deviations between as-designed and as-constructed values for the layers with the same layer material type and same design thickness.
• Kurtosis and skewness tests of the distribution of the mean thicknesses for the layers with the same layer material type and the same design thickness.
• Two types of comparisons are made in relation to their as-designed thicknesses or target values:
• Evaluation of the percent of the individual measurements that are either within or outside specific values from the target thickness.
• Statistical analysis of the measured mean thickness values versus the designed values.

Descriptive Summary Statistics of the Thickness Deviations

The mean thickness difference between as-designed and as-constructed thicknesses was computed for each layer using both core and elevation thickness measurements.

The following statistical indicators were computed:

• Total number of sections or layers.
• Mean thickness deviation.
• Minimum thickness deviation.
• Maximum thickness deviation.
• Standard deviation of thickness deviation.
• COV of thickness deviation.

The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements.

Layer Thickness Deviation Distribution Type

Mean thickness deviations from layers or sections were analyzed to determine whether they follow typical statistical distributions. Skewness and kurtosis analyses were conducted for this purpose, using the methodology outlined in chapter 5.

Percentage Distribution of the Individual Measurements

To evaluate the variation between as-designed and as-constructed thicknesses, deviations of the individual measurements in relation to the target values are computed for each analysis cell. These deviations are then summarized into three deviation levels: 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in), for different material types and target thickness values.

This evaluation provides information regarding variations between as-constructed and as-designed thicknesses at individual measurement level.

Statistical Analysis of Sample Measurement Means

Statistical analysis is performed to evaluate variations for each analysis cell. The goal of statistical analysis is to assess deviation of the measurement population means from the target thicknesses. Two types of the thickness comparison are performed for both data sources:

Two-sided t-tests with 95 percent confidence level for each section and layer, to determine whether the differences between as-designed and as-constructed thicknesses are significant.

Figure 45: Equation. The null and alternative hypotheses for two-sided t-test.

The null hypothesis for this test is that average of core or elevation thickness data is equal to the target thickness, i.e.: If the null hypothesis is rejected (i.e., the result of the two-sided t-test is significant), then the measured mean thickness is different from the design thickness at the 95 percent confidence level. On the other hand, if the null hypothesis is not rejected or the test result is not significant, then there is no evidence that the measured mean thickness is different from the design value.

One-sided t-tests with 95 percent confidence level for the difference between as-designed thickness and the mean as-constructed thickness and for tolerance level of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in). The null hypothesis is that the absolute value of the difference between the mean and target thickness is less than or equal to the tolerance level with the alternative hypothesis being that the absolute value of the difference is greater than the tolerance level. For example, for elevation data, for allowance of 6.35 mm (0.25 in), the null and alternative hypotheses are:

Figure 46: Equation. The null and alternative hypothesis for one-sided t-test.

If the null hypothesis is rejected (i.e., the result the one-sided t-test is significant), then the measured mean thickness deviates from the design thickness by more than the specified allowance (in this example 6.35 mm) at a 95 percent confidence level. On the other hand, if the null hypothesis is not rejected or the test result is not significant, then there is no evidence that the measured mean thickness deviates from the designed value by more than the specified allowance value, in other words, that the mean thickness is within the allowance value (in this case 6.35 mm) from the designed thickness.

Typical Deviations between Mean Measured and the Design Thicknesses

Descriptive Summary Statistics

Mean layer thickness data for SPS experimental sections with newly constructed layers were obtained from the TST_AC01_LAYER and TST_PC06 tables (core thickness), and from the SPS*_LAYER_THICKNESS tables (elevation thickness), to compute measured thickness deviation from the design value. 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 sections or layers
• Mean thickness deviation
• Minimum thickness deviation
• Maximum thickness deviation
• Standard deviation of thickness deviation
• COV of thickness deviation

The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements. Table 44 summarizes layer thickness deviations by different layer and material types based on analysis of elevation measurements. Table 45 summarizes mean core examination layer thickness deviations from their designed values by different layer and material types.

Figures 47 through 61 present the frequency distributions of the thickness deviations for different layer types and target thicknesses for both core and elevation thickness measurements.

The following observations are made based on these summary statistics:

• The computed description statistics using elevation measurement data are different from those using core examination data. However, based on statistical analyses, the differences in the mean layer thicknesses and standard deviations at the section or layer level are not significant for a majority of the layers.
• The mean constructed layer thicknesses for PCC layers and lean concrete base layers are generally above the designed values.
• For the same layer and material type, the mean constructed layer thicknesses tend to be above the designed value for the thinner layers, and below the design value for the thicker layers.

These summary statistics for the differences between as-designed and mean as-constructed layer thicknesses can be used as benchmarks for use in pavement design reliability and other research studies.

Figure 47: Chart. The Frequency distribution of mean thickness deviations for all four target thicknesses of the DGAB layer.

Figure 48: Chart. Frequency distribution of elevation and core thickness measurements deviations for DGATB with 102-mm (4-in) target thickness.

Figure 49: Chart. Frequency distribution of elevation and core thickness measurements deviations for DGATB with 203-mm (8-in) target thickness.

Figure 50: Chart. Frequency distribution of elevation and core thickness measurements deviations for DGATB with 305-mm (12-in) target thickness.

Figure 51: Chart. Frequency distribution of elevation and core thickness measurements deviations for LC with 152-mm (6-in) target thickness.

Figure 52: Chart. Frequency distribution of elevation and core thickness measurements deviations for PATB with 102-mm (4-in) target thickness.

Figure 53: Chart. Frequency distribution of elevation and core thickness measurements deviations for PCC with 76-mm (3-in) target thickness

Figure 54: Chart. Frequency distribution of elevation and core thickness measurements deviations for PCC with 127-mm (5-in) target thickness.

Figure 55: Chart. Frequency distribution of elevation and core thickness measurements deviations for PCC with 203-mm (8-in) target thickness.

Figure 56: Chart. Frequency distribution of elevation and core thickness measurements deviations for PCC with 279-mm (11-in) target thickness.

Figure 57: Chart. Frequency distribution of elevation and core thickness measurements deviations for SB with 51-mm (2-in) target thickness.

Figure 58: Chart. Frequency distribution of elevation and core thickness measurements deviations for SB with 102-mm (4-in) target thickness.

Figure 59: Chart. Frequency distribution of elevation and core thickness measurements deviations for SB with 127-mm (5-in) target thickness.

Figure 60: Chart. Frequency distribution of elevation and core thickness measurements deviations for SB with 178-mm (7-in) target thickness.

Figure 61: Chart. Frequency distribution of elevation and core thickness measurements deviations for S with 203-mm (8-in) target thickness.

Layer Thickness Deviation Distribution Type

Mean thickness deviations from layers or sections were analyzed to determine whether they follow typical statistical distributions. Skewness and kurtosis analyses were conducted for this purpose. The statistical test results are presented in table 46 for both the elevation and core mean layer thicknesses. Examples of the thickness deviation distributions are shown in figures 62 and 63.

As shown in table 46, there are some discrepancies between the distribution types drawn from elevation data and core data. For the layers with both elevation and core data, the distribution of the thickness deviation derived from the core data is normal for more layer type and design thicknesses than from the elevation data.

The conclusions drawn from both the descriptive statistics and the kurtosis and skewness tests of their distribution types will be useful for pavement designers and researchers. They will be especially useful in reliability based mechanistic-empirical pavement performance analysis and design.

Figure 62: Chart. Example of normally distributed thickness deviations (elevation data, LC, target thickness 152 mm [6 in]).

Figure 63: Chart. Example of a skewed distribution for layer thickness deviation (core data, PCC, target thickness 279 mm [11 in]).

Statistical Analysis of Elevation Measurements

Analysis of the Percentage Distribution

The overall percentage distribution of elevation measurements as a function of the three tolerance levels is presented in table 47.

The distribution of measurements by layer type for tolerance levels of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in) are presented in tables 48, 49, and 50, respectively.