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Publication Number: FHWA-RD-03-040
Date: July 2002

Researcher's Guide to The LTPP Layer Thickness Data

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  1. Data Sources
  2. Typical Deviations Between the Mean Measured and the Design Thicknesses
  3. Analysis Results From Designed Versus Constructed Thickness Comparisons

This chapter summarizes analysis results concerning characterization of the differences in the as-designed and as-constructed (measured) thickness data for the newly constructed SPS layers. Only these new SPS layers have design thickness values accurately documented in the LTPP program. Typical summary statistics for as-designed versus as-constructed mean thicknesses are presented by layer types and target thickness values. Statistical test results are provided regarding whether the mean layer thickness deviations by layer type and design thickness levels are normally distributed. Finally, analysis results are presented from analyzing the percentage of the thickness measurements, as well as for the t-tests results comparing the mean layer thickness and the designed thickness values.

Data Sources

Measured Thickness Data

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

According to SPS construction guidelines [30-35], rod and level survey measurements are to be taken at a minimum of five offset locations (edge, outer wheel path, mid-lane, inner wheel path, and inside edge of lane) at longitudinal intervals no greater than 15.2 m (50 ft). Typically, 55 elevation measurements are available for a section.

The number of cores taken for each section depends on experiment and layer type and is defined in the corresponding sampling and testing guides [5-10]. The number of cores per section ranges between 1 and 9.

All sections and layers with available thickness data in either one of these tables were studied to quantify design versus constructed variations in thickness.

For these section/layer combinations, an analysis cell is defined to represent a specific layer for which the target thickness was documented. The following fields in LTPP tables define a unique analysis cell:

Design Thickness

For newly constructed SPS layers, the design thicknesses are defined in the corresponding SPS Experimental Designs [11-16]. The designed thicknesses are available for the following layer types:

The design thicknesses for all these SPS experiments and layer types are presented in tables 15 through 20.

Table 15. Design Layer thickness for the SPS-1 experiment.

SHRP_ID Design Layer Thickness, mm (in)
DGAB1 PATB2 DGATB3 SB4
101203 (8)  178 (7)
102305 (12)  102 (4)
103  203 (8)102 (4)
104  305 (12)178 (7)
105102 (4) 102 (4)102 (4)
106102 (4) 203 (8)178 (7)
107102 (4)102 (4) 102 (4)
108203 (8)102 (4) 178 (7)
109305 (12)102 (4) 178 (7)
110 102 (4)102 (4)178 (7)
111 102 (4)203 (8)102 (4)
112 102 (4)305 (12)102 (4)
113203 (8)  102 (4)
114305 (12)  178 (7)
115  203 (8)178 (7)
116  305 (12)102 (4)
117102 (4) 102 (4)178 (7)
118102 (4) 203 (8)102 (4)
119102 (4)102 (4) 178 (7)
120203 (8)102 (4) 102 (4)
121305 (12)102 (4) 102 (4)
122 102 (4)102 (4)102 (4)
123 102 (4)203 (8)178 (7)
124 102 (4)305 (12)178 (7)

Table 16. Design layer thicknesses for the SPS-2 experiment.

SHRP_ID Design Layer Thickness, mm (in)
DGAB1 PATB2 LC3 PCC4
201152 (6)  203 (8)
202152 (6)  203 (8)
203152 (6)  279 (11)
204152 (6)  279 (11)
205  152 (6)203 (8)
206  152 (6)203 (8)
207  152 (6)279 (11)
208  152 (6)279 (11)
209102 (4)102 (4) 203 (8)
210102 (4)102 (4) 203 (8)
211102 (4)102 (4) 279 (11)
212102 (4)102 (4) 279 (11)
213152 (6)  203 (8)
214152 (6)  203 (8)
215152 (6)  279 (11)
216152 (6)  279 (11)
217  152 (6)203 (8)
218  152 (6)203 (8)
219  152 (6)279 (11)
220  152 (6)279 (11)
221102 (4)102 (4) 203 (8)
222102 (4)102 (4) 203 (8)
223102 (4)102 (4) 279 (11)
224102 (4)102 (4) 279 (11)

Table 17. Design layer thickness for the SPS-5 experiment.

SHRP_ID Design Layer Thickness, mm (in)
SB1
5010
50251 (2)
503127 (5)
504127 (5)
50551 (2)
50651 (2)
507127 (5)
508127 (5)
50951 (2)

Table 18. Design Layer thickness for the SPS-6 experiment.

SHRP_ID Design Layer Thickness, mm (in)
SB1
6010
6020
603102 (4)
604102 (4)
6050
606102 (4)
607102 (4)
608203 (8)

Table 19. Design Layer thickness for the SPS-7 experiment.

SHRP_ID Design Layer Thickness, mm (in)
PCC1
7010
70276 (3)
70376 (3)
70476 (3)
70576 (3)
706127 (5)
707127 (5)
708127 (5)
709127 (5)

Table 20. Design layer thickness for the SPS-8 experiment.

SHRP_ID Design Layer Thickness, mm (in)
DGAB1 PCC2 SB3
801203 (8) 102 (4)
802305 (12) 178 (7)
803203 (8) 102 (4)
804305 (12) 178 (7)
805203 (8) 102 (4)
806305 (12) 179 (7)
807152 (6)203 (8) 
808152 (6)279 (11) 
809152 (6)203 (8) 
810152 (6)279 (11) 
811152 (6)203 (8) 
812152 (6)279 (11) 

Typical Deviations Between the Mean Measured and the Design Thicknesses

Typical mean layer thickness deviations are established by the following:

Descriptive Summary Statistics

Mean 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), 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:

The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements. Table 21 summarizes layer thickness deviations by different layer and material types based on an analysis of elevation measurements. Table 22 summarizes mean core examination layer thickness deviations from their designed values by different layer and material types. The following observations are made based on these summary statistics:

These summary statistics characterizing 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

Table 21. Summary of differences between mean elevation thickness measurements and target thicknesses.

Mat. Type Target Thickness Total Number of Sections Mean Difference Standard Deviation Min. Max. Difference
Difference
mm in mm in mm in mm in mm in
DGAB1024840.40.0110.30.4-28.6-1.1333.41.32
152655-1.2-0.0514.40.57-51.5-2.0338.21.51
2038400.90.0412.70.5-26.8-1.0545.21.78
3051240-6-0.24301.18-173.3-6.8234.91.37
DGATB1024271.80.0780.31-12-0.4721.10.83
2038420.50.0216.30.64-62.5-2.4628.91.14
3051228-2.1-0.0815.90.63-35.1-1.3838.11.5
LC1526485.50.2210.60.42-25.8-1.0236.91.45
PATB10241291.20.0510.50.41-17.1-0.6741.91.65
PCC7631218.20.7211.50.453.40.1342.61.68
12751216.50.6511.60.465.10.2391.53
2038765.40.2112.20.48-32.6-1.2853.32.1
27911774.70.18110.43-24.8-0.98391.54
SB512464.80.1919.90.78-27.8-1.167.92.67
1024125-2.2-0.0918.50.73-58.9-2.3231.71.25
127546-4.4-0.1720.10.79-70.6-2.7838.31.51
178795-8.2-0.3223.90.94-73.3-2.8959.42.34
20387-2.7-0.1122.90.9-36.9-1.4536.31.43

Table 22. Summary of differences between mean core thickness measurements and target thickness.

Mat. Type Target Thickness Total Number of Sections Mean Difference Standard Deviation Min. Max. Difference
Difference
mm in mm in mm in mm in mm in
DGATB102422-0.9-0.0410.90.43-22.9-0.920.30.8
2038341.10.0421.50.85-64.3-2.5338.11.5
3051222-5.4-0.2125.10.99-88.9-3.5210.83
LC1526368.20.3212.60.5-19.1-0.7538.91.53
PATB102432-19.7-0.7839.41.55-87.2-3.43113.54.47
PCC7631020.30.810.70.425.90.2335.91.41
12751213.40.5313.50.53-9.9-0.3937.11.46
2038719.80.39140.55-22.5-0.8952.32.06
2791171-0.7-0.0328.31.12-94.7-3.7331.81.25
SB5124516.20.6421.40.84-17.1-0.6859.72.35
10241145.20.2170.67-63.5-2.5471.85
1275479.10.3623.60.93-39.4-1.5573.22.88
178794-4.3-0.1721.80.86-96.5-3.865.42.58
20386-18.4-0.7351.62.03-118.1-4.6516.50.65

Distribution Type of the Deviations between Designed and Constructed Layer Thicknesses

Table 23. Distribution of the mean thickness deviations from the design thickness based on kurtosis and skewness tests.

Mat. Type Target Thickness Elevation Measurement Data Core Examination Data
mm in No. Layers Distribution Type No. Layers Distribution Type
DGAB 102 4 84 Normal No Data
152 6 55 Wide spread and skewed left
203 8 40 Wide spread and skewed right
305 12 40 Wide spread and skewed left
DGATB 102 4 27 Normal 22 Normal
203 8 42 Wide spread and skewed left 34 Normal
305 12 28 Normal 22 Wide spread and skewed left
LC 152 6 48 Normal 36 Normal
PATB 102 4 129 Skewed right 32 Normal
PCC 76 3 12 Normal 10 Normal
127 5 12 Normal 12 Normal
203 8 76 Wide Spread 71 Normal
279 11 77 Normal 71 Wide spread and skewed left
SB 51 2 46 Skewed right 45 Normal
102 4 125 Skewed left 114 Wide spread and skewed left
127 5 46 Normal 47 Normal
178 7 95 Skewed left 94 Wide spread and skewed left
203 8 7 Not enough data 6 Not enough data

As shown in table 23, for the elevation data, eight of the distributions appear to be normal while three are skewed to one side, five to the other side, and one sample tested wide spread but not skewed. The only reasonable distribution (of the differences between mean thicknesses from elevation measurements and target thicknesses) to assume for all material types and thickness is then the normal distribution. Again since the core data are of the same type as the elevation data, it is reasonable to assume normality for these data as well. Supporting the assumption for core data samples is that for the actual tests of skewness and kurtosis nine out of thirteen material types and thickness groups were determined to be normal, while the remaining five groups were skewed and wide spread.

For the cases where distributions were skewed, the reasons were further investigated, and in some cases explanations were discovered by simply examining the data points. For example, figure 4 shows an example of unreasonably large differences from the target thickness for seven PCC layers. Five layers with the largest differences belong to the same SPS project (10-0200). It appears that all core values for these layers are about 8 inch, while elevation data for the same layers show that they are 11 in or thicker. One explanation might be that the incomplete layer thicknesses were obtained during coring.

The following conclusions could be obtained from the analysis of the distribution of the differences between target and measured mean thickness values:

As figure 4 indicates the skewness could be due to unreasonable outlier sections where it is possible that (1) incomplete layer thicknesses were obtained from the core, (2) contractor consistently deviated from the design thickness (under design or over design), or (3) erroneous data exist in the database.

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 3 Deviation from Target Thickness, mm. Figure 3 shows an example normally distributed chart for the deviation of the mean elevation-measured lean concrete base layer thickness from the target thickness of 152 mm. The horizontal axis of the chart is the deviation between the mean elevation-measured thickness of a section and the corresponding target thickness, ranging from -44.5 mm to 50.8 mm with 6.3-mm increment.
Figure 3: Chart. Example of normally distributed thickness deviations (elevation data, LC, target thickness 152 mm [6 in]).

Figure 4: Chart - Deviation from Target Thickness, mm. Figure 4 in page 47 shows an example skew-distributed chart for the deviation of the mean core-measured PCC surface layer thickness from the target thickness of 279 mm. The horizontal axis of the chart is the deviation between the mean elevation-measured thickness of a section and the corresponding target thickness, ranging from -101.6 mm to 50.8 mm with 6.3-mm increment. The distribution of the deviations appears to skew to the right.
Figure 4: Chart. Example of a skewed distribution for layer thickness deviation (core data, PCC, target thickness 279 mm [11 in]).

Analysis Results From Designed Versus Constructed Thicknesses Comparisons

This section presents a summary and conclusions derived from two types of statistical analyses that compared as-constructed versus as-designed thicknesses or target values. Detailed discussion of these comparisons can be found in a report titled, Evaluation and Analysis of LTPP Pavement Layer Thickness Data. [29]

First, both elevation data and core examination data were evaluated to establish the percentage of the individual measurements that is either within or outside the specified values from the target thickness. Three tolerance levels of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in) were used for this comparison.

Second, a statistical analysis of the measured mean thickness values versus the as-designed values was performed. Two types of thickness comparisons were performed for both data sources. The two-sided t-test with 95 percent reliability level was used for each section and layer to estimate whether the difference between as-designed and as-constructed thicknesses was significant. The one-sided t-test with 95 percent reliability level was used for each layer for the difference between as-designed thickness and the mean as-constructed thickness and for tolerance levels of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in). Analysis results from these comparisons and statistical analyses are presented in the following sections.

Analysis of the Percentage Distribution Results

Based on percentage distributions of the elevation measurements:

Based on percentage distributions of the individual core thickness measurements:

Statistical Analysis Results

Analysis Results from Two-sided t-tests

Based on the elevation measurements:

Based on the core thickness measurements:

A comparison between analysis results from the elevation and core thickness measurements shows that the percentage of measurements within tolerance limits for all three tolerance levels is approximately the same. However, the percentage of measurements lower than target value is consistently higher for core measurements than for elevation measurements.

Analysis Results from One-sided t-tests

Based on the elevation measurements:

Based on the core thickness measurements:

Different conclusions were drawn between the statistical tests performed on the elevation measurements and the statistical tests conducted on the core examination measurements. For example, more core measurement data than elevation measurement data suggest that the measured mean layer thicknesses are not significantly different from their designed thicknesses.

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