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Publication Number: FHWA-RD-03-041

Evaluation and Analysis of LTPP Pavement Layer Thickness Data

Data Evaluation Overview

One of the project objectives was to identify and explain anomalous observations and provide recommendations for layer thickness characterization for each LTPP section. The following potential issues related to layer thickness data were identified during the preliminary data review:

Data Sources

To fulfill this task's objective, the layer thickness data in the following LTPP tables were evaluated for reasonableness and consistency (using cross-table comparison):

Table TST_AC01 and table TST_SAMPLE_LOG in the LTPP database also contain thickness related information. Table TST_AC01 contains AC core thickness measurements from the field. 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. However, records in these two tables are not keyed to the layer numbers as stored in TST_L05B and other above listed layer thickness related tables (field LAYER_NO). Therefore, the thickness measurements from these two tables can only be manually matched to the layers established in the TST_L05B table. Furthermore, some measurements span more than one layer, and thus cannot be used for any layer thickness comparison at all. As a result, tables TST_AC01 and TST_SAMPLE_LOG are not included in this evaluation. Nevertheless, these two tables can be used as raw layer thickness related data sources and be consulted for layer thickness measurements on a case-by-case basis.

The main data elements related to pavement layering structure from each of these tables are illustrated in figure 1. Double sided arrows between the table TST_L05B and tables TST_L05A, TST_AC01_LAYER, TST_ PC06, INV_LAYER, RHB_LAYER, and SPS*_LAYER schematically show that the data elements in the later tables were compared against similar data in TST_L05B table.

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Figure 1: Graph. LTPP data sources containing pavement layering data.

Essential Fields for Data Analysis

Based on the analysis of the fields in the above tables related to pavement layering structure, the following data elements were selected for detailed pavement layering data examination:

  1. Layer functional description (e.g., surface, overlay, base, subgrade).
  2. Material type description.
  3. Representative layer thickness.
  4. Layer thickness variability (discussed in the next chapter).

These four essential pavement layering characteristics (schematically identified in figure 2 as question marks and circled numbers 1 through 4) serve as key inputs for many types of pavement analyses. The selected data elements were examined and compared between different data sources (LTPP tables). The comparisons were done individually for each layer and each LTPP section. Additionally, layer thickness variability indicators were examined, as discussed in chapter 4.

Figure 2 in page 15 shows the essential pavement layering characteristics such as layer functional description (e.g., surface, base, subbase, etc.), material type, representative layer thickness, and variation in layer thickness data.
Figure 2: Graph. Four essential pavement layering characteristics.

Analysis Steps

The data review activities carried out in this task included the following:

  1. Selection of pavement layering data from different LTPP data sources.
  2. Development of a master data analysis table with the layering information from different sources included for each pavement layer.
  3. Evaluation of consistency in layer functional description.
  4. Evaluation of reasonableness and consistency in material type description.
  5. Evaluation of reasonableness of layer thickness data and layer thickness consistency between different sources.
  6. Evaluation of layer thickness variability indicators from different data sources (chapter 4).
  7. Summarize evaluation outcomes and identify reasons for data inconsistencies.
  8. Preparation of feedback reports to help ensure the data issues are resolved.

The flowchart identifying different data analysis and data evaluation activities is shown in figure 3.

Figure 3 in page 16 displays the flowchart for an eight-step approach for pavement layering data evaluation. Step 1: Obtain pavement layer data from different LTPP data tables. Step 2: Create a master table for data analysis. Step 3: Consistency in layer description. Step 4: Reasonableness and consistency in material type. Step 5: Reasonableness and consistency of layer thickness. Step 6: Variation in layer thickness data. Step 7: Identify anomalous data and investigate the reasons. Step 8: Prepare feedback reports.
Figure 3: Chart. Flowchart for pavement layering data evaluation.

In steps 1 and 2, all the data elements from different sources were prepared for the layer-by-layer review for each section. Steps 3 through 5 were used to evaluate information for major layer structure data components available in the LTPP database. Results of step 6 are presented separately in chapter 5. Under steps 7 and 8, the anomalies or suspect data in the LTPP layering information were identified, examined, and reported back to the FHWA. These activities are discussed in more detail in the following sections.

Step 2 - Analysis Data Set

Master Table for the Pavement Layering Data Evaluation

To analyze pavement layering information from different sources, a master list of all pavement layers available in the LTPP database was created. The master list contains the maximum number of unique records obtained for each LTPP section, layer number, and construction event. These records were obtained from the INV_LAYER, RHB_LAYER, TST_L05B, TST_L05A, TST_AC01_LAYER, TST_PC06, and SPS*_LAYER tables.

Reference Table Selection

The initial data review indicated that table TST_L05B contains the most recent and most complete LTPP section layering information for each layer. The main attributes of the TST_L05B table are:

A total of 96.7 percent of the unique GPS layer records (5,938 records) and 83.8 percent of all SPS layer records (9,360 records) were included in the TST_L05B table at the time of the study. As such, TST_L05B was selected as the target or reference table for the selection of analysis components and cross-table comparison of pavement layering data. Layers not included in the TST_L05B table were not used in the cross-table pavement layering data analysis. These records were examined individually for data reasonableness and identification of anomalous data.

Correspondence in Layer Numbering System between Different Sources

The review of the layer numbering scheme used in different tables indicated that layer numbering is consistent among all the tables except INV_LAYER. Thus, before the layer-related information between different tables could be compared, layers from the INV_LAYER table were aligned with the layers from the other tables.

To align the INV_LAYER records, the TST_L05B table was used as the reference. The TST_L05B table contains two fields (INV_LAYER_NO and INV_LAYER_NO2) that provide information about the corresponding inventory layers. Based on the values in these fields, several different scenarios are possible regarding layer correspondence between the INV_LAYER and TST_L05B tables. The INV_LAYER layer correspondence scenarios and consequent actions are summarized in table 5 below.

Table 5. Evaluation of layer numbering correspondence between the INV_LAYER and TST_L05B tables.
Description Number of Records (GPS and SPS) Action
Layer numbers are the same2803 (72%) Analyze
Layer numbers are different488 (12%) Align and analyze
2 INV_LAYER layers correspond to 1 TST_L05B layer90 (2%) Analyze combined thickness
Only part of INV record corresponds to TST layer69(2%) Exclude from cross-table analysis
INV_LAYER records exist but not referenced in TST_L05B468 (12%) Exclude from cross-table analysis
Total number of records in INV_LAYER3918 (100%), with 3381 (86%) analyzed

Using the scenarios outlined in table 5, 3,381 records (86 percent) with layer-related information from the INV_LAYER table were aligned with the rest of the data sources.

Data Availability for Consistency Evaluation

Based on the number of data sources available for the analysis of each pavement layer, different data availability codes were assigned to each layer:

Because the TST_L05B table was selected as a reference table, only records with analysis data availability code 1 were used in the cross-table pavement layering data analysis. Records that did not have a corresponding entry in TST_L05B were reviewed individually for data reasonableness. Table 6 summarizes the number of records used in the analysis for each LTPP experiment.

Table 6. Summary of the number of records used in the cross-table pavement layering analysis.
Experiment Number of Pavement Layers Analyzed
Type No. TST_L05B TST_L05A TST_AC01_LAYER TST_ PC06_LAYER INV_ LAYER RHB_ LAYER SPS*_ LAYER
G 1 1460 1452 526 - 961 - -
G 2 972 971 366 - 648 29 -
G 3 516 510 13 126 455 - -
G 4 247 247 1 62 223 - -
G 5 342 342 22 84 292 4 -
G 6 1763 1725 636 - 327 877 -
G 7 555 553 111 44 171 249 -
G 9 146 145 21 48 115 12 -
S 1 1214 1162 420 - - 32 1102
S 2 693 656 - 176 - - 655
S 3 3664 3648 1065 - - 313 -
S 4 496 496 - - - - -
S 5 1682 1664 553 - 165 665 1612
S 6 779 746 48 55 24 159 654
S 7 282 282 - 59 - 105 208
S 8 112 104 30 2 - - 91
S 9 416 401 56 34 - 12 409
Total 15276 15041 3856 690 3381 2391 4731
Notes: G = GPS experiment. S = SPS experiment.

Step 3 - Layer Functional Description Evaluation

The pavement layer functional description provides information about the functionality of a given pavement layer, such as overlay, surface, base, or subgrade. LTPP uses a list of codes to describe layer functional description, as shown in table 7.

Table 7. LTPP layer function description codes.
Code Description
1 Overlay
2 Seal Coat
3 Original Surface Layer
4 AC Layer Below Surface (Binder Course)
5 Base Layer
6 Subbase Layer
7 Subgrade
8 Interlayer
9 Friction Course
10 Surface Treatment
11 Embankment Layer
12 Recycled Layer

In this study, the values from the layer functional description field were compared among the following tables: TST_LO5B, TST_LO5A, INV_LAYER, RHB_LAYER, TST_AC01_ LAYER, and SPS*_LAYER. The description field in the TST_L05B table served as a reference for the functional layer description information, and the description fields from the other tables were compared against it.

The procedure for layer functional description consistency evaluation is shown schematically in figure 4.

Figure 4 in page 19 uses an example to show the procedure for layer functional description consistency evaluation by comparing the layer functional description stored in one layer thickness data table to the description in TST_L05B that serves as the reference table. In the example, by comparing Base Layer (layer functional description) stored in table INV_LAYER to Surface Layer recorded in the reference table, TST_L05B, an inconsistency in the layer functional description is identified. Similarly, the stored layer descriptions stored in tables TST_05A and RHB_LAYER are compared to TST_L05B, respectively. The number of inconsistencies is tallied for each test section.
Figure 4: Graph. Example of layer functional description consistency evaluation.

The results of the layer functional description consistency evaluation are summarized in table 8 and are shown in figure 5 separately for the GPS and SPS sections.

Records with a functional layer description field that is inconsistent between different data sources were reported to the LTPP data managers in feedback reports.

Table 8. Summary of the layer functional description consistency evaluation.
Experiment Percentage of Records with Matching Layer Functional Description
Type No. TST_L05A TST_AC01_ LAYER INV_ LAYER RHB_ LAYER SPS*_LAYER
G 1 100.0% 92.8% 91.9% - -
G 2 99.9% 95.1% 92.3% 93.1% -
G 3 100.0% 100.0% 95.8% - -
G 4 100.0% 100.0% 97.3% - -
G 5 100.0% 100.0% 97.9% 100.0% -
G 6 99.8% 93.3% 91.7% 88.7% -
G 7 100.0% 98.2% 94.7% 83.5% -
G 9 100.0% 90.5% 90.4% 100.0% -
S 1 100.0% 86.7% - - 68.8%
S 2 100.0% - - - -
S 3 100.0% 87.0% - - 79.2%
S 4 100.0% - - - -
S 5 100.0% 96.2% - 90.9% 80.3%
S 6 100.0% 89.6% - 100.0% 74.8%
S 7 100.0% - - - 68.6%
S 8 100.0% 96.7% - - -
S 9 100.0% 75.0% - - 16.7%
Notes: G = GPS experiment. S = SPS experiment.

Figure 5 in page 20 displays two pie charts that present the consistency evaluation results of comparing the layer functional descriptions from different tables with those in table TST_L05B for GPS-1, 2, 3, 4, 5, 6, 7, and 9 test sections and SPS-1 through 9 sections, respectively. The pie chart on the left shows the percentage of consistency (95 %) vs. the percentage of inconsistency (5 %) for the GPS test sections. The pie chart on the right shows the percentage of consistency (92 %) vs. the percentage of inconsistency (8 %) for the SPS sections.
Figure 5: Chart. Results of layer functional description consistency evaluation.

Note that in figure 5, the chart slice labeled "Inconsistent" represents the layers that had at least one of the evaluated tables with data (functional description) inconsistent with the data in the TST_L05B table. Similar statement applies to all other pie charts presented in Chapter 3.

Step 4 - Material Type Reasonableness and Consistency

The material type description is very important pavement layering information. Material type description data are found in tables TST_LO5B, TST_LO5A, INV_LAYER, RHB_LAYER, and SPS*_LAYER. These data were examined to determine:

Material Type Reasonableness

The purpose of the reasonableness check was to evaluate whether the material description code for the layer is consistent with the layer functional description. For example, soil material descriptions are not adequate for the paved surface layers. Table 9, based on the SPS Pavement Layering Methodology, Operational Guide [5], was used as a primary reference for evaluating material type reasonableness.

Table 9. Criteria for evaluation of material code validity.
Layer Description Code Description Valid Material Code
1 Overlay 01-08, 13, 16-20, 901
2 Seal Coat 71-73, 74-852
3 Original Surface Layer 01-08, 17-20
4 AC Layer Below Surface (Binder Course) 01, 03, 13, 20
5 Base Layer 302-310, 319-350, 21-492
6 Subbase Layer 302-310, 319-350
7 Subgrade 100-178, 200-294, 51-652
8 Interlayer 71-80, 85, 81-842
9 Friction Course 02, 20
10 Surface Treatment 11, 12, 20, 823
11 Embankment Layer 100-178, 200-294, 51-652
Notes:
1 For SPS-7 only.
2 Based on Appendix A of LTPP Data Collection Guide. [2]
3 Based on reference. [27]

While most of the records had valid material codes, some records in the evaluated tables had material codes different from those specified in table 9. Table 10 provides a summary of the records with identified erroneous material codes. Additionally, some records were missing material codes. The identified records were reported to the FHWA in the data analysis/operations feedback report.

Table 10. Summary of the records with erroneous material codes.
Table Name Number of Erroneous Records Total Number of Records Percentage of Records with Erroneous Codes
TST_L05B53 15,298 0.35%
TST_L05A49 15,189 0.32%
RHB_LAYER99 2,841 3.48%
INV_LAYER368 3,918 9.39%
SPS1_LAYER1 1,021 0.10%
SPS2_LAYER0 621 0.00%
SPS5_LAYER18 1,056 1.70%
SPS6_LAYER13 402 3.23%
SPS7_LAYER8 135 5.93%
SPS8_LAYER2 155 1.29%
SPS9_LAYER31 475 6.53%
Total 64241,1111.56%

Material Type Consistency among Different Tables

To evaluate consistency between material types reported in different tables, LTPP material code lists were reviewed first. Two sets of material codes are used in the LTPP database to describe material types in the testing tables (TST_L05A and TST_L05B tables) and in inventory-type tables (including INV_LAYER, RHB_LAYER, and SPS*_LAYER tables) in the LTPP database. As a result, for some layers, material type descriptions in tables TST_L05B and TST_L05A do not have exact corresponding material type descriptions in tables INV_LAYER, RHB_LAYER, and SPS*_LAYER. For these layers, manual reviews of individual layer descriptions and engineering judgment are necessary to identify whether the material descriptions from different tables are consistent (or similar enough).

Correlated material codes need to be formulated to evaluate the consistency in material data from all LTPP tables containing material types. For the material type codes that do not have the exact same descriptions, "similar" material groupings were developed to correlate material codes in the inventory tables and material codes in the testing tables. The reasoning for the assignment of different material categories is summarized below for different material types.

Similar Material Type Grouping for Base and Subgrade Materials

The AASHTO classification system [28] was considered the best way to group "similar" soil or granular materials. For example, clayey materials were grouped as "clayey soils," as per the AASHTO group classification A-6 and A-7. The same criteria were applied to other typical soil types, such as gravels (A-1, A-2), silty soils (A-4, A-5), sand (A-1, A-2), clayey sand (A-2), silty gravel (A-1, A-2), and silty sand (A-2). In addition, the following criteria were applied:

Similar Material Type Grouping for Asphalt Concrete Materials

The basis for grouping "similar" asphalt concrete materials included a decision-tree process. The materials were first aligned by mixture gradation (sand, open- or dense-graded) as a first filtering step. The method of production (hot- or cold-laid) was the second criterion used to distinguish asphalt groupings. Recycled asphalt concrete, maintenance seal coats, and special plant mixes (emulsions, cutbacks) were retained in individual groupings. [27]

The table of new correlated groupings of "similar" materials and corresponding material codes from inventory and testing tables is presented in appendix A.

Material Type Consistency Criteria

To test the consistency of material type data between different tables, the TST_L05B table was used as the reference for material type description information. The material type description data from other tables were compared against it using the criteria outlined in table 11 below.

Table 11. Material type consistency criteria.
Criteria Name Description Evaluation Code
ConsistentMaterial type descriptions are the same.0
SimilarMaterial types are similar based on a broad material categories developed for geological materials using the dominant material component(s).1
InconsistentMaterial type descriptions are different.2
Not evaluatedMaterial types cannot be evaluated because no material codes are available in one of the tables that make comparison pair (or if material type is available only at one location for "along the section" consistency test).3

Figure 6 shows schematically the testing procedure used for evaluation of consistency in the material type description between different tables.

Figure 6 in page 24 uses an example to illustrate the procedure for layer material type description consistency evaluation by comparing the layer material type descriptions stored in one layer thickness data table with those in TST_L05B which serves as the reference table. In the example, by comparing Silt (layer material type description) stored in table INV_LAYER to Gravelly Lean Clay recorded in the reference table, TST_L05B, an inconsistency in the layer material type description is identified. Similarly, the stored layer descriptions stored in tables TST_05A and RHB_LAYER are compared to TST_L05B, respectively. The number of inconsistencies is tallied for each test section.
Figure 6: Graph. Example of evaluation of layer material type consistency between different tables.

The results of layer material type consistency evaluation between different data sources are summarized in table 12 and figure 7, separately for GPS and SPS sections.

Table 12. Summary of the layer material type consistency evaluation.
Experiment Percentage of Layers with Layer Material Type Records Matching with Records in TST_L05B
Type No. TST_L05A INV_ LAYER RHB_LAYER SPS*_ LAYER
Exact Similar Exact Similar Exact Similar Exact Similar
G 1 98.9 0.6 32.6 39.4 - - - -
G 2 99.0 0.7 41.0 25.8 40.0 20.0 - -
G 3 98.2 1.0 41.1 24.5 - - - -
G 4 97.6 1.6 37.6 25.3 - - - -
G 5 98.8 0.6 36.6 25.3 - - - -
G 6 99.6 0.2 46.4 26.9 70.3 10.3 - -
G 7 98.1 1.1 43.8 31.4 61.4 11.4 - -
G 9 98.6 0.7 45.2 20.9 - - - -
S 1 99.9 0.0 - - - - 35.9 37.4
S 2 99.6 0.0 - - - - 24.0 56.5
S 3 95.6 1.5 - - 53.0 23.2 - -
S 4 100.0 0.0 - - - - - -
S 5 100.0 0.0 55.2 33.3 73.1 3.4 42.4 29.8
S 6 98.9 0.5 66.7 33.3 64.9 12.2 30.7 31.3
S 7 98.6 1.4 - - 63.1 33.8 24.0 47.1
S 8 100.0 0.0 - - - - 34.1 38.5
S 9 99.5 0.0 - - 25.0 0.0 33.0 31.1
Notes: G = GPS experiment. S = SPS experiment.


Figure 7 in page 25 displays two pie charts that present the consistency evaluation results of comparing the layer material type descriptions from different tables with those in table TST_L05B for GPS-1, 2, 3, 4, 5, 6, 7, and 9 test sections and SPS-1 through 9 sections, respectively. The pie chart on the left shows the percentage of consistency (81 %) vs. the percentage of inconsistency (19 %) for the GPS test sections. The pie chart on the left shows the percentage of consistency (78 %) vs. the percentage of inconsistency (22 %) for the SPS sections.
Figure 7: Chart. Results of layer material type consistency evaluation between different data sources.

Records with inconsistent material codes were identified and reported to the FHWA in the form of feedback reports.

Material Type Consistency along the Section

Table TST_LO5A contains information about layer material types evaluated at up to three locations (the beginning, the middle, and the end) along the LTPP section. In this task, the consistency of the material type along the LTPP section was evaluated using the process shown schematically in figure 8.

Figure 8 in page 26 uses an example to show the procedure for consistency evaluation of layer material type descriptions along a test section (at the beginning, the middle, and the end of the section) stored in table TST_05A. In the example, by comparing Sandy Lean Clay (layer material type description) stored in the beginning location of a test section to Sandy Lean Clay with Gravel stored in the end location of the test section, a similarity is identified. By comparing Gravelly Silt in the middle location to Sandy Lean Clay with Gravel in the end location of the section, an inconsistency is identified. The number of inconsistencies and that of similarities are tallied respectively for each test section.
Figure 8: Graph. Example of evaluation of layer material type consistency along the section.

In the TST_L05A table, 5,795 GPS records (97 percent of all GPS records) and 2,581 SPS records (28 percent of all SPS records) had layer material type information for more than one location along the section. The evaluation results of layer material type consistency along the section are summarized for GPS and SPS sections in table 13 and figure 9.

Table 13. Summary of the layer material type consistency evaluation along the LTPP section length (TST_L05A table).
Experiment Percentage of TST_L05A Layers with Material Types along the Section
Type No. Consistent Similar Inconsistent
G 1 87.0 4.8 8.2
G 2 89.4 3.0 7.6
G 3 88.2 5.1 6.7
G 4 84.7 5.6 9.6
G 5 87.5 5.2 7.3
G 6 89.9 3.7 6.5
G 7 91.4 4.0 4.6
G 9 88.2 3.5 8.3
S 1 99.3 0.0 0.7
S 2 96.3 0.5 3.2
S 3 99.9 0.0 0.1
S 4 - - -
S 5 99.9 0.1 0.0
S 6 98.1 0.9 0.9
S 7 95.6 4.4 0.0
S 8 96.2 3.8 0.0
S 9 85.5 3.4 11.2
Notes:
G = GPS experiment.
S = SPS experiment.


Figure 9 in page 27 displays two pie charts that present the consistency evaluation results of comparing the layer material type descriptions at the beginning, middle, and end locations of a test section in table TST_L05A for GPS-1, 2, 3, 4, 5, 6, 7, and 9 test sections and SPS-1 through 9 sections, respectively. The pie chart on the left shows the percentage of consistency (89 %) vs. the percentage of inconsistency (7 %) vs. percentage of similarity (4 %) for the GPS test sections. The pie chart on the left shows the percentage of consistency (98 %) vs. the percentage of inconsistency (1 %) vs. percentage of similarity (1 %) for the SPS sections.
Figure 9: Chart. Results of layer material type consistency evaluation along the section.

Step 5 - Reasonableness and Consistency of Layer Thickness Data

Evaluation of the layer thickness data was one of the most important activities under this project. Layer-specific thickness data are found in the following tables: TST_LO5B, TST_LO5A, TST_AC01_LAYER, TST_PC06, INV_LAYER, and RHB_LAYER, SPS*_LAYER, and SPS*_LAYER_THICKNESS.

The layer thicknesses in the SPS*_LAYER_THICKNESS tables are reported for different locations along the section; these data are grouped by layer type (surface, base, etc.) and material type (AC, PCC, aggregate) categories, rather than using the LTPP consecutive layer numbering scheme. The SPS*_LAYER tables contain the summary information from the SPS*_LAYER_THICKNESS tables.

The TST_LO5A table contains layer thickness measurements obtained at up to three locations (the beginning, the middle, and the end) along the section. These data serve as a source for representative layer thickness values reported in the TST_LO5B table.

The TST_PC06 table contains layer thickness measurements for PCC layers obtained using individual pavement core samples. The TST_AC01_LAYER table contains layer thickness measurements for AC layers obtained using individual pavement core samples.

The layer thickness data from the above tables were analyzed to determine:

Reasonableness of the Layer Thickness Data

To evaluate reasonableness of layer thickness data, representative layer thickness ranges were determined for different layer types. The criteria specified in SHRP-LTPP Interim Guide for Laboratory Materials Handling and Testing (PCC, Bituminous Materials, Aggregates and Soil), Operational Guide No. SHRP-LTPP-OG 004 [3] (SHRP-LTPP Lab Guide), were used to set reasonable layer thickness ranges based on the layer description codes, as shown in table 14.

Table 14. Thickness ranges used for reasonableness checks.
Layer Description Code Description Range (mm) Range (inches)
1 Overlay13 - 2290.5 - 9
2 Seal Coat3 - 380.1 - 1.5
3 Original Surface Layer13 - 3300.5 - 13
4 AC Layer Below Surface (Binder Course)13 - 2540.5 - 10
5 Base Layer25 - 6101 - 24
6 Subbase Layer76 - 12173 - 47.9
7 SubgradeN/AN/A
8 Interlayer3 - 1520.1 - 6
9 Friction Course3 - 640.1 - 2.5
10 Surface Treatment3 - 38 0.1 - 1.5
11 Embankment Layer 76 - 12173 - 47.9
12 Recycled LayerN/AN/A

The SHRP-LTPP Lab Guide [3] does not provide guidance for the representative thicknesses of the prepared subgrade and recycled layers. Also, only a few records had subgrade thickness data in the LTPP database. Thus, thickness reasonableness was not evaluated for the subgrade and recycled layers. Layer description codes from each table were used as a reference to obtain reasonable thickness ranges for different layers listed in table 14. Based on the representative layer thickness ranges, minimum and maximum thickness values were determined for each layer type.

The TST_PC06 table does not contain a field with layer functional description. To evaluate reasonableness of representative layer thicknesses reported in this table, the layer functional description from the TST_L05B table was used for the corresponding records. Thicknesses for the layers from the TST_PC06 table that did not have matching layer numbers in the TST_L05B table were not evaluated for reasonableness.

The TST_L05A table could contain thickness measurements at different locations. Reasonableness of layer thicknesses at all locations was evaluated in the study. If at least one out of the possible three layer thickness measurement values was outside of the reasonable thickness range for a given layer type, the layer was flagged as one with unreasonable layer thickness.

Table 15 provides the layer thickness reasonableness evaluation results grouped by LTPP table name and experiment type.

Table 15. Summary of the layer thickness reasonableness evaluation results1.
Experiment Percentage of Layers with Reasonable1 Layer Thickness
Type No. TST_L05B TST_L05A TST_AC01_LAYER TST_ PC06 INV_ LAYER RHB_ LAYER SPS*_LAYER
G 1 98.3 97.3 98.8 - 98.9 - -
G 2 98.2 96.2 99.7 - 99.5 100.0 -
G 3 98.9 96.8 100.0 98.4 98.6 - -
G 4 100.0 97.6 100.0 100.0 99.4 - -
G 5 99.6 98.8 100.0 100.0 99.1 - -
G 6 95.3 93.9 99.1 - 99.1 98.4 100.0
G 7 98.1 97.5 100.0 100.0 100.0 99.3 100.0
G 9 89.1 85.3 100.0 77.1 91.6 - -
S 1 99.8 99.8 100.0 - - 100.0 99.7
S 2 99.4 99.4 - 100.0 - - 99.2
S 3 98.4 98.4 99.3 - - 97.8 -
S 4 100.0 100.0 - - 100.0 - -
S 5 93.2 93.0 98.7 - 98.2 99.0 92.0
S 6 99.1 100.0 100.0 100.0 100.0 97.1 100.0
S 7 100.0 100.0 - 88.1 92.3 100.0 85.2
S 8 97.5 97.5 100.0 100.0 - - 97.4
S 9 96.7 96.3 - 100.0 100.0 55.6 96.2
Note: 1 Based on the criteria from the SHRP-LTPP Lab Guide. [3]
G = GPS experiment.
S = SPS experiment.

As a result of the layer thickness reasonableness evaluation, all thickness values outside the acceptable thickness ranges were identified and reported to the FHWA for review.

Layer Thickness Data Consistency

One of the objectives of the study was to evaluate the consistency between section-level layer thickness values available from different data sources (tables). Section-level layer thickness values could be found in the following LTPP tables: TST_LO5B, INV_LAYER, RHB_LAYER, and SPS*_LAYER.

In addition, table TST_L05A contains layer thickness values at up to three different locations along the section (beginning, middle, and end) and serves as a source of the representative layer thickness values included in the TST_L05B table. Layer thickness data from the TST_L05A table was considered consistent with the data from the TST_L05B table if at least one of the possible three thickness values in the TST_L05A table passed the consistency test. This criterion is based on the procedure for determination of the representative layer thickness, as explained in the SHRP-LTPP Lab Guide. [3]

Tables TST_AC01_LAYER and TST_PC06 contain layer thickness measurements obtained from the pavement cores taken at different locations along the section. These measurements were used to compute representative layer thicknesses for the records included in the TST_AC01_LAYER and TST_PC06 tables.

To evaluate the consistency of the layer thickness data from different sources, the criteria for allowable differences in layer thickness were developed first. The criteria were based on the layer thickness consistency values utilized in the SHRP-LTPP Lab Guide [3]. The values reported in the guide were developed for evaluating layer thickness consistency between the ends of the LTPP section (i.e., between minimum and maximum values). The comparison carried out in this study is between the representative or "average" thickness values obtained from different data tables. Based on the difference in the data statistics used in the current study compared to the analysis outlined in the operational guide ("range" versus "average" value comparison), the allowable differences used in the current study were reduced by half for the comparison of the average thickness values. The representative thickness data in table TST_L05B were used as a reference for the comparison with the representative thicknesses in the other tables.

Table 16 provides a summary of the allowable differences between representative layer thicknesses that were used in this study to evaluate layer thickness data consistency between different tables. Figure 10 schematically shows the procedure used for evaluation of consistency in layer thickness data between different tables.

Table 16. Criteria used for evaluation of layer thickness consistency between different tables.
Type of Layer Materials Layer Type Code from TST_L05B Layer Thickness from TST_L05B (h), mm Allowable Difference in Layer Thickness, mm
PCC PC < 203
> 203
38*½ = 19
50.8*½ = 25.4
Bituminous AC < 51
> 51
0.5*h*½ = 0.25*h
0.3*h*½ = 0.15*h
Bound Base or Subbase TB, TS Any 0.3*h*½ = 0.15*h
Unbound Base or Subbase GB, GS Any 0.5*h*½ = 0.25*h


Figure 10 in page 30 uses an example to illustrate the procedure for layer thickness consistency evaluation by comparing the layer thickness data stored in one table with those in TST_L05B which serves as the reference table. Using an unbound granular base in a test section as an example, if the difference in layer thickness data between one table and the reference table TST_L05B is more than the allowable percentage (e.g., 25%) of the layer thickness in the reference table, then it is concluded to be inconsistent. Different layer materials will have different allowable percentages. The number of inconsistencies is tallied for each test section.
Figure 10: Graph. Example of evaluation of layer thickness consistency between different data tables.

For thin AC layers (less than 51 mm), if the allowable difference computed using formula provided in table 16 was less than 2.5 mm (0.1 inch), the value of 2.5 mm was used as a criterion for evaluation. This decision is based on the fact that layer thickness values are recorded in the IMS database to the nearest one-tenth of an inch.

Layer thickness consistency for the subgrade or engineering fabric layers were not evaluated because no comparison criteria for these layers were established. Additionally, if layer thickness in the TST_L05B table was marked as 999.9, no comparison with the corresponding layer thicknesses from the other tables was carried out. A thickness value of "999.9" indicates that there is a considerable difference in pavement thickness values between section ends, so that no representative thickness value could be established.

Representative layer thickness values were obtained from different data tables and compared with the representative thickness data in table TST_L05B. The outcome of the thickness data consistency evaluation is summarized in table 17 and figure 11 separately for GPS and SPS sections.

Table 17. Summary of the layer thickness consistency evaluation results1.
Experiment Percentage of Layers with Consistent Layer Thickness
Type No. TST_L05A TST_AC01_LAYER TST_ PC06_LAYER INV_ LAYER RHB_ LAYER SPS*_LAYER
G 1 99.8 97.7 - 73.2 --
G 2 100.0 99.0- 72.7 87.5-
G 3 100.0 100.0 100.0 79.5--
G 4 100.0 100.0100.0 83.3--
G 5 100.0 100.0100.0 81.0--
G 6 99.9 90.6- 63.2 60.6 -
G 7 99.8 96.4 97.7 73.1 68.9 -
G 9 100.0 94.7 95.8 69.7--
S 1 99.9 80.1-- - 90.8
S 2 99.6 - 90.3-- 87.7
S 3 99.5 88.0-- 48.8-
S 4 100.0-- ---
S 5 98.7 91.7- 74.3 61.8 62.8
S 6 98.7 93.8100.0 87.5 69.8 82.0
S 7 97.6 - 84.7 - 93.9 63.1
S 8 100.0 93.3100.0-- 93.8
S 9 100.0 76.8 94.7 - 0.0 72.9
Note: 1 Based on the criteria from the table 16.
G = GPS experiment.
S = SPS experiment.


Figure 11 in page 32 displays two pie charts that show the consistency percentages of comparing the layer thickness data from different data tables with the layer thickness data in table TST_L05B for GPS-1, 2, 3, 4, 5, 6, 7, and 9 test sections and SPS-1 through 9 sections, respectively.  The pie chart on the left shows the percentage of consistency (82 %) vs. the percentage of inconsistency (18 %) for the GPS test sections.  The pie chart on the right shows the percentage of consistency (78 %) vs. the percentage of consistency (22 %) for the SPS sections.
Figure 11: Chart. Results of layer thickness consistency evaluation between different data sources.

Records with layer thickness differences between the tables exceeding the values shown in table 16 were reported to FHWA.

Step 7 - Evaluation Outcome Summary and Resolution

The anomalies, suspect data, and inconsistent information found during the pavement layering data evaluation are described below, along with a discussion of possible causes of their occurrence. Corrective or remedial measures taken to address these data issues are also discussed. Identified layer thickness data issues were reported to the FHWA for data resolution in numerous LTPP Data Analysis and Operations Feedback Reports (feedback reports).

1: Inconsistent Layer Descriptions

A total of 1,067 records had layer functional descriptions different from the description provided in the TST_L05B table-304 records from GPS experiments and 763 from SPS experiments. A feedback report was generated and sent to the FHWA for the data in these records.

2: Erroneous Material Type

Data evaluation of material and layer functional description codes indicated that, in some instances, the material description codes for the layer were inconsistent with the layer functional descriptions. For example, soil material descriptions were used for the base layers. This means that either the material code or the layer functional description code is incorrect. The summary of records with invalid material codes for specified functional layer type is provided below:

In addition, material or functional layer description codes were missing for some records. A feedback report was generated and sent to the FHWA for the data in these records.

3: Different Material Type Coding Schemes

The review of material type data used to describe different pavement layers showed inconsistencies in the material naming conventions and material codes used in the testing tables and in inventory-type tables (including INV_LAYER, RHB_LAYER, and SPS*_LAYER). As a result, for some layers, material type descriptions in tables TST_L05B and TST_L05A do not have exact corresponding material type descriptions in tables INV_LAYER, RHB_LAYER, and SPS*_LAYER. There are no established reference criteria that could be used to determine whether material types in the above tables are similar or significantly different.

As a remedial action, a materials expert was contacted to develop a methodology for evaluation of material code compatibility. As a result, a table of correlated material codes was created to enable cross-table comparison of the material codes between inventory- and testing-type tables. The results are presented in appendix A.

4: Inconsistent Material Types

A substantial number of records from the SPS*_LAYER, INV_LAYER, and RHB_LAYER tables had material types significantly different from those specified in the TST_L05B and TST_L05A tables, as summarized below.

INV_LAYER Table:

RHB_LAYER Table:

SPS*_LAYER Tables:

Some of these inconsistencies could be explained by different material coding lists used in these tables. In some instances, it was difficult to establish material "similarity." In other cases, more than one layer with different material codes in the INV_LAYER table corresponded to a single layer in the TST_L05B table. Identified problems were reported to the FHWA in the form of feedback reports

5: Unreasonable Thickness Values (Outside the Recommended Range)

The LTTP material testing guide provides typical thickness ranges for most layer types. [3] These values were compared with entries in the TST_L05B, TST_L05A, TST_AC01_ LAYER, TST_ PC06, INV_ LAYER, RHB_ LAYER, and SPS*_LAYER tables. Records that fall outside the recommended range are summarized below for each table.

TST_L05B Table:

TST_L05A Table:

Computed Representative Values based on the TST_AC01_ LAYER Table:

Computed Representative Values based on the TST_ PC06 Table:

INV_LAYER Table:

RHB_LAYER Table:

SPS*_LAYER Tables:

No remedial action was taken for the identified records. However, comment codes were assigned in the analysis summary table to the records containing such data. A feedback report was submitted to the FHWA for further data review. If the review of data sources would indicate that the reported thickness values are "true" data, we recommend adding a comment field to the relevant layer thickness tables explaining the reason for the unusual layer thickness.

In addition, in the RHB_LAYER table, thickness values of 0.0 are used to identify:

This creates some confusion because it is unclear whether the layer is removed or whether it is too thin to establish representative thickness. In the future, it is recommended to use a minimum thickness of 3 mm (0.1 in) for thin layers instead of 0.0 to differentiate between "removed" layer and existing thin layers (with thicknesses too small to determine).

6: Inconsistent Thickness Values

Based on the criteria established in table 11 in this report, layer thickness values were compared with the values in the TST_L05B table. Records that had layer thickness values significantly different from those reported in TST_L05B are summarized below.

TST_L05A Table

:

Computed Representative Values based on the TST_AC01_ LAYER Table:

Computed Representative Values based on the TST_ PC06 Table:

INV_LAYER Table:

RHB_LAYER Table:

SPS*_LAYER Tables:

No remedial action was taken for the identified records. However, comment codes were assigned in the analysis summary table to the records containing such data. A feedback report was submitted to the FHWA for further data review.

7: Multiple Records in the RHB_LAYER Table

A number of layers in the RHB_LAYER table had multiple records for the same layer and construction number. Only records with the most recent "date complete" were used in the analysis. A feedback report identifying multiple records in the RHB_LAYER table was submitted to the FHWA.

8: Missing Records in the TST_L05B Table

Analysis of the data indicated that the TST_L05B table is the most complete source of layer thickness information. However, there are still 203 (3.3 percent) GPS layers and 1,813 (16.2 percent) SPS layers available in the other tables that are not included in the TST_L05B table. Layers that are available in at least one of the following tables but not available in TST_L05B Level E release 11.5 version NT3.0 were reported to the FHWA: TST_LO5A, TST_AC01_LAYER, TST_PC06, RHB_LAYER, and SPS*_LAYER.

There are 468 (12 percent) records in the INV_LAYER table that are not referenced in the TST_L05B table. These records were reported to the FHWA for data review.

Summary of Pavement Layering Data Evaluation

The results of the pavement layering data evaluation were assessed to determine the consistency of pavement layering information between different sources. In addition, within-section layer material type consistency and material type reasonableness were evaluated using selected tables where these parameters were available.

The consistency of pavement layering data between different sources was evaluated for three data categories:

In this evaluation, data pertinent to the layer functional description, layer thickness, and layer material type were obtained from multiple LTPP data tables for each pavement layer and each LTPP section. The data were reviewed to determine consistency between multiple data sources. A layer was considered to have consistent information between different data sources if all the tables containing pertinent information had the same data for this layer. The only exception to this rule was allowed for evaluation of the layer material types. If material type records from multiple data sources had a "similar" material type, as identified in table 66 of appendix A, these records were considered "consistent." This exception was used to accommodate the comparison between the values from the tables utilizing different material classification codes (i.e., material codes for testing versus material codes for inventory tables.)

If there was inconsistency in data from one or more data sources, a layer was flagged for further review. Inconsistencies in pavement layering data were reviewed and reported to the LTPP data managers in the form of data analysis/operations feedback reports, along with recommendations for data anomaly resolution.

Table 18 contains summary results for the pavement layering data consistency evaluation for each LTPP experiment.

Additionally, reasonableness (or validity) of material type description was evaluated. The purpose of the reasonableness check was to evaluate whether the material description code for the layer is consistent with the layer functional description. While most of the records had valid material codes, 642 records out of 41,111 (1.56 percent) had erroneous material codes, and some records were missing material codes. The identified records were reported to the FHWA in the data analysis/operations feedback report.

Reasonableness of layer thickness data was evaluated using representative layer thickness ranges specified in SHRP-LTPP Lab Guide [3]. As a result of the layer thickness reasonableness evaluation, thickness values outside the representative thickness ranges were identified and reported to the FHWA for the data review.

Table 18. Summary of layering data consistency evaluation for each LTPP experiment.
Experiment Number (percentage) of Pavement Layers Analyzed
Type No. Layer Functional Description Material Type Description Representative Layer Thickness
    Consistent Inconsistent Consistent Inconsistent Consistent Inconsistent
G 1 1410 (96.4%) 53 (3.6%) 1180 (81.6%) 266 (18.4%) 933 (82.1%) 203 (17.9%)
G 2 927 (95.4%) 45 (4.6%) 748 (77.8%) 214 (22.2%) 622 (81.1%) 145 (18.9%)
G 3 496 (96.7%) 17 (3.3%) 354 (69%) 159 (31%) 306 (82.5%) 65 (17.5%)
G 4 243 (98.4%) 4 (1.6%) 165 (66.8%) 82 (33.2%) 143 (85.1%) 25 (14.9%)
G 5 336 (98.2%) 6 (1.8%) 231 (67.5%) 111 (32.5%) 209 (84.3%) 39 (15.7%)
G 6 1583 (92.8%) 122 (7.2%) 1539 (91.2%) 148 (8.8%) 1160 (82.1%) 253 (17.9%)
G 7 490 (91.4%) 46 (8.6%) 452 (84.5%) 83 (15.5%) 352 (82.1%) 77 (17.9%)
G 9 129 (92.1%) 11 (7.9%) 101 (72.1%) 39 (27.9%) 84 (75%) 28 (25%)
S 1 1138 (93.7%) 76 (6.3%) 872 (74.8%) 294 (25.2%) 794 (84.3%) 148 (15.7%)
S 2 633 (91.3%) 60 (8.7%) 559 (81.1%) 130 (18.9%) 457 (85.4%) 78 (14.6%)
S 3 3549 (96.9%) 115 (3.1%) 1353 (94.9%) 73 (5.1%) 1335 (87.3%) 194 (12.7%)
S 4 496 (100%) 0 (0%) 21 (100%) 0 (0%) 14 (100%) 0 (0%)
S 5 1393 (82.8%) 289 (17.2%) 1191 (71.8%) 467 (28.2%) 819 (59.8%) 550 (40.2%)
S 6 698 (89.6%) 81 (10.4%) 488 (66%) 251 (34%) 446 (80.9%) 105 (19.1%)
S 7 233 (82.6%) 49 (17.4%) 219 (78.5%) 60 (21.5%) 144 (67.9%) 68 (32.1%)
S 8 112 (100%) 0 (0%) 87 (77.7%) 25 (22.3%) 75 (92.6%) 6 (7.4%)
S 9 323 (77.6%) 93 (22.4%) 268 (64.4%) 148 (35.6%) 232 (69.9%) 100 (30.1%)
Total 14189 (93%) 1067 (7%) 9828 (79.4%) 2550 (15.6%) 6570 (79.1%) 1736 (20.9%)
Note:
G = GPS experiment.
S = SPS experiment.

Layer Material Type and Thickness Data Status Summary Table

Using the outcome of the data evaluation for the four major parameters related to layer structure and layer thickness (layer functional description, material type, representative thickness, and variation in thickness measurements), the quality assurance codes indicating consistency and reasonableness of pavement layering data from different data sources were assigned to each layer. A data analysis summary table containing QA codes for major layer-related parameters evaluated for each layer was submitted to the FHWA on a CD with the final report. This table includes the following information for each LTPP section on a layer-by-layer basis:


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.
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