Researcher's Guide to The LTPP Layer Thickness Data

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1. Inventory Layer Thickness Data
2. Rehabilitation Layer Thickness Data
3. Laboratory Core Examination Data
4. Field Elevation Measurements
5. Field Layer Material Type and Thickness Measurements
6. Layer Material Type and Thickness Summary Data
   
   

Several types of layer thickness data were collected under the LTPP program, and these data reside in different LTPP database modules and tables. This chapter provides detailed discussions about each of these data sources and data tables.

The following lists LTPP tables that contain layer material or thickness information:

• TST_AC01-Asphalt concrete (AC) core examination and thickness. Contains measured AC core thicknesses.
• TST_AC01_LAYER-AC core exam and thickness information. Contains field layer and actual layer number.
• TST_PC06-Portland cement concrete (PCC) core examination and thickness.
• SPS*_LAYER-Summarized layer descriptions and thicknesses for newly constructed SPS layers. (Sheet 4).
• SPS*_LAYER_THICKNESS-Field elevation layer thickness measurements (Sheet 12).
• TST_SAMPLE_LOG-Information about the samples taken from holes, pits, and probes.
• INV_LAYER-Layer descriptions and thickness data collected from highway agencies (Data Sheet: Inventory 3).
• RHB_LAYER-Layer descriptions and thickness data collected from highway agencies on rehabilitated layers (Data Sheet: Rehab 2).
• TST_L05-Table containing laboratory material testing data, project level for SPS experiments only.
• TST_L05A-Table containing layer descriptions for all constructions, section level - measured data.
• TST_L05B-Table containing layer descriptions for all constructions, section level - analysis section.

Additional information about the LTPP program, field sampling, materials testing, layer structure and layering information, data collection guidelines, and the LTPP database can be found in the following documents:

• Data Collection Guide for Long-Term Pavement Performance Studies, Operational Guide No. SHRP-LTPP-OG-001, Strategic Highway Research Program, Washington, DC, 1993. [1]
• SHRP-LTPP Interim Guide for Laboratory Material Handling and Testing, Operational Guide No. SHRP-LTPP-OG 004, SHRP, Washington, DC, 1991. [2]
• Field Materials Sampling, Testing, and Handling Guide No. SHRP-LTPP-OG 006, Version 2.0, Strategic Highway Research Program, Washington, DC, 1992. [3]
• LTPP SPS Pavement Layering Methodology, Federal Highway Administration, McLean, Virginia, January 1994. [4]
• Specific Pavement Studies, LTPP Material Sampling and Testing Requirements for SPS Experiments. [5-10]
• Specific Pavement Studies, LTPP Experiment Design and Research Plan for SPS Experiments. [11-16]
• SHRP-LTPP Protocol P01 for SHRP test designation AC01: Visual Examination and Thickness of Asphaltic Concrete Cores, SHRP, Washington DC, February 1991. [17]
• SHRP-LTPP Protocol P66 for SHRP test designation PC06: Visual Examination and Length Measurement of Portland Cement Concrete Cores, SHRP, Washington DC, February 1991. [18]
• LTPP Information Management System: IMS Quality Control Checks, Federal Highway Administration, McLean, Virginia, 2000. [19]
• Specific Pavement Studies, Data Collection Guidelines for SPS Experiments. [20-25]

Note that information presented in this Guide about LTPP table structures, field names and descriptions, and code lists reflect the current LTPP database structure and may be changed in the future. For the updated information, the users should refer to the most recent IMS Quality Control Checks document [19].

Inventory Layer Thickness Data

Inventory or design layer structure data for the existing LTPP pavement sections are collected by LTPP and stored in table INV_LAYER. The data are generally based on highway agency records. Inventory data are historical in nature and exist for all GPS sections. For SPS experiments, only the rehabilitation projects (SPS-5, SPS-6, SPS-7, and SPS-9) and the sections that are linked to a GPS section have records in the INV_LAYER table. [1, 20-25]

The essential fields other than the section and layer identification fields in table INV_LAYER are the following:

A list of the codes and their descriptions for the field DESCRIPTION (code name "DESCRIPTION") is given in table 1. The code name for field "MATERIAL_TYPE" is "MAT_TYPE." Table 2 provides the code list and descriptions for the MATERIAL_TYPE field. Up to four numbers were provided to indicate the minimum, maximum, mean, and standard deviation of thickness for each specific layer. If only a single specified design value for thickness was available, it was entered as the mean value. When a number of boreholes were made for sampling materials, thickness measurements may also have been taken. In such cases, the mean thicknesses were then verified or revised and variability information (MIN, MAX, and STD_DEV) added as a result of these field measurements. [2,3]

Table 1. Code list and code descriptions for "DESCRIPTION"

Table 2. Code list and code descriptions for code "MAT_TYPE", continued.

Rehabilation Layer Thickness Data

Each highway agency is expected to notify the LTPP regional office prior to doing rehabilitation on a highway segment containing an LTPP section [1]. Rehabilitation activities include resurfacing, reconstruction, and the addition of lanes. Rehabilitation sometimes alters the pavement structure. In these cases, layer data are recorded in table RHB_LAYER from information provided by the highway agencies. Table RHB_LAYER contains layer thickness and material type information for sections that were rehabilitated during the LTPP program.

The essential fields other than the section and layer identification fields in table RHB_LAYER are the following:

The code list and descriptions for the following fields were provided previously: DESCRIPTION (table 1) and MATERIAL_TYPE (code MAT_TYPE, table 2). Up to four numbers are provided to indicate the minimum, maximum, mean, and standard deviation of thickness for each rehabilitated layer. If only a single specified design value for thickness was available from project historic records, it was entered as the mean value. For SHRP/LTPP, when a number of boreholes were made for sampling materials, careful thickness measurements may also be made. In such cases, the mean thicknesses were then verified or revised and variability information (MIN, MAX, and STD_DEV) added as a result of these field measurements and measurements of cores in the laboratory. [2,3]

Laboratory Core Examination Data

Field and laboratory tests are conducted to establish material properties and layer characteristics. Characterization of material properties and the variations in these properties between and within the test sections is required to evaluate causes of performance differences between test sections.

Laboratory core examination data are stored in three LTPP tables: TST_AC01 and TST_AC01_LAYER for asphalt bound layers and TST_PC06 for PCC layers

For the asphalt bound layers, SHRP protocol P01 [17], Visual Examination and Thickness of Asphaltic Concrete Cores, provides detailed procedures for identifying and determining the thickness of individual layers within the AC core. A single asphalt concrete core taken from the field may contain more than one layer. For example, a single AC core identified in the field as AC material may contain hot-mix AC (HMAC) wearing, binder, and base layers.

The test results for an entire AC core are stored in table TST_AC01, and the test results for the individual layers within the AC core are stored in table TST_AC01_LAYER

The essential fields other than the section and layer identification fields in table TST_AC01 are:

Since the thickness measurements stored in table TST_AC01 is not keyed to LAYER_NO field and may not be for a single layer, it is difficult to identify which layer the measurement belongs to. The code lists for codes VISUAL_ACPC and COMMENTS are provided in tables 3 and 4 respectively.

Table 3. Code list and code descriptions for code "VISUAL_ACPC"

Table 4. Code list and code descriptions for code "COMMENT," continued.

The essential fields other than the section and layer identification fields in table TST_AC01_LAYER are as following:

The code list and descriptions for field LAYER_DESCRIPTION (code DESCRIPTION, table 1) were provided previously.

Material Sampling Plan for AC Cores

Material sampling for AC cores was performed according to guidelines provided in several LTPP documents and reports, including the SHRP-LTPP Interim Guide for Laboratory Material Handling and Testing and the SPS Guidelines [2,3,5-16]. For GPS experiments, test samples were collected at specific locations outside the monitoring sections of the LTPP test sections. For SPS projects, cores were extracted from designated locations adjacent to the pavement test sections. Core thickness examination and thickness measurements were performed on all cores retrieved. Basically, two types of cores were extracted:

• Type A-152-mm diameter cores obtained from the approach and leave ends of a monitoring section.
• Type C-102-mm diameter cores obtained from the approach and leave ends of a monitoring section.

Sampling and testing requirements for AC core examination and thickness testing are presented in tables 5 and 6. The tables show the minimum number of core specimens required for testing for the various LTPP experiments, along with the designated sampling locations.

Table 5. Sampling requirements for visual examination and thickness of AC cores.

Notes:  
¹Asphalt-treated base.
² Outside Diameter.

Table 6. Detailed SPS sampling requirements for visual examination and thickness of AC cores.

PCC Core Examination and Thickness

Visual examination and length measurement are performed on PCC core specimens as part of the material characterization program for the LTPP study. These basic tests are conducted on all PCC cores before they are subjected to compressive strength, split tensile strength, and static modulus of elasticity testing.

Usually, cores are taken at specified periods after construction (e.g., 14, 28, and 365 days) for newly constructed pavements and several years after construction for in-service pavements adopted into the LTPP program. The number of cores taken from a given location depends on both the pavement test section properties and LTPP experiment type.

Procedures used for measuring core length (layer thickness) and examining the cores are presented in the following SHRP test protocol and AASHTO/ASTM test procedures:

• SHRP P66-Visual examination and length measurement of PCC cores. [18]
• AASHTO T148-Measuring length of drilled concrete cores. [26]
• ASTM C856-Petrographic examination of hardened concrete. [27]

The PCC core specimens are examined visually to determine their general condition, presence of distresses, presence of defects such as cracks, voids, D-cracking, alkali-silica reactivity, and problems with layer separation (for overlaid pavements). The general type and shape of aggregates (e.g., rounded gravel or angular crushed stone) were also documented. The cores were also examined to determine their suitability for length measurements and other testing. Cores with serious defects, such as uneven surfaces and segregated aggregates, were noted and not used for further testing.

The length measurement and visual examination test results are stored in the LTPP database after undergoing quality checks that ensure anomalies (e.g., negative PCC core thickness values) are identified and corrected. Data that are of acceptable quality after the quality control (QC) checks are classified as Level E and are stored in table TST_PC06. The following essential data other than the section and layer identification fields are maintained in table TST_PC06:

The code list and descriptions for the following fields were provided previously: VISUAL_EXAM_1 to VISUAL_EXAM_6 (VISUAL_ACPC, table 3) and COMMENTS_1 to COMMENTS_6 (COMMENT, table 4).

Material Sampling Plan for PCC Cores

PCC core specimens are collected as directed by the SHRP P66 protocol from designated locations within the GPS and SPS experiments (GPS experiments are cored by SHRP contractors; SPS experiments are cored by highway agencies or contractors). Cores are then prepared according to procedures outlined in SHRP P66 [18], AASHTO T148 [26], and ASTM C856 [27] before being shipped to certified laboratories for testing.

For GPS pavements, cores are taken from both the approach and leave ends of the pavement test section. Both core locations are sited some distance from the monitored test section to avoid damaging the actual test section. For SPS experiments, cores are taken at designated locations from various test sections within a given site.

For SPS experiments, sampling can be complicated because these experiments consist of a diverse matrix of test sections, some of which are newly constructed with different PCC target thicknesses and strengths (e.g., SPS-2), and the others are in-service pavements that have been overlaid and, therefore, consist of both in-service and newly constructed layers (e.g., SPS-7).

Table 7 presents a summary of the minimum core specimens required by the LTPP materials testing and sampling program for core length measurements and visual examination for each LTPP experiment [2,5-10]. Although the minimum number of tests per layer indicated in the table directly translates to the required minimum tests per analysis cell for GPS projects, the same is not true for SPS projects. Therefore, a more detailed description of the testing requirements for various SPS experiments is presented in table 8.

Table 7. Summary of the sampling and testing plan for thickness measurement and visual examination of PCC cores.

Table 8. Detailed SPS sampling requirements for PCC core visual examination and length measurement.

Field Elevation Measurements

For SPS newly constructed layers, elevation measurements of the final finished surface are conducted throughout the test section. The measurements normally are made at five offset points at every 500 feet along the section. Subsequently, layer thicknesses of these layers were then determined from before and after elevation measurements at each point where elevation measurements were made.

The measured layer thicknesses were recorded for each material layer type instead of for each relevant layer in the section layer structure. These layer thickness results at each measurement point are stored in the following tables:

• SPS1_LAYER_THICKNESS
• SPS2_LAYER_THICKNESS
• SPS5_LAYER_THICKNESS
• SPS6_LAYER_THICKNESS
• SPS7_LAYER_THICKNESS
• SPS8_LAYER_THICKNESS

The layer thickness measurements in the above tables are not keyed to individual layer numbers but are provided for each identified layer types. Aside from the section identification fields and coordination fields (STATION_NO and OFFSET), the following essential fields are contained in each of the above tables:

For SPS1_LAYER_THICKNESS:

For SPS2_LAYER_THICKNESS:

For SPS5_LAYER_THICKNESS:

For SPS6_LAYER_THICKNESS:

For SPS7_LAYER_THICKNESS

For SPS8_LAYER_THICKNESS

In addition to being provided at individual elevation measurement points, the summary layer thickness values (mean, minimum, maximum, and standard deviation) derived from elevation measurements are also stored in the LTPP database for individual layers for each section. These summaries are maintained in the following tables:

• SPS1_LAYER
• SPS2_LAYER
• SPS5_LAYER
• SPS6_LAYER
• SPS7_LAYER
• SPS8_LAYER
• SPS9_LAYER

The following essential fields other than the section and layer identification fields are present in all the above tables:

The code list and descriptions for the "DESCRIPTION" field were previously given in table 1, while the code list and descriptions for field "MATERIAL_TYPE" (code name "MAT_TYPE") were previously provided in table 2.

Field Layer Material Type and Thickness Measurements

Material types and depths to strata top and strata bottom are identified or measured in the field from holes, test pits, and probes. 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 this table are not keyed to the layer numbers as stored in TST_L05B and TST_L05A tables (field LAYER_NO). The thickness measurement from this table can only be manually matched to the layers established in the TST_L05B table. Nevertheless, this table can be used as raw layer thickness related data source and be consulted for unbound layer material type and thickness on a case-by-case basis.

The following essential fields other than the section and layer identification fields are present in table TST_SAMPLE_LOG:

The code list and descriptions for the field "MATERIAL_CODE" (code name "MATERIAL") are provided in table 9.

Table 9. Code list and code descriptions for code "MATERIAL".

Once layer structure information was collected from different sources, the FHWA RSC's summarize these layer structure data into the following three LTPP tables:

Layer Material Type and Thickness Summary Data

• TST_L05-Table containing laboratory material testing data (project-level) for SPS projects only.
• TST_L05A-Table containing layer descriptions for all constructions, measured data at the section level.
• TST_L05B-Table containing layer descriptions for all constructions, analysis data at the section level.

Prior to completion of the L05 tables, the FHWA RSC evaluation personnel are required to review the following information:

• Inventory data, if available.
• Field material sampling and testing data, including photographs of cores, test pits, etc. that were taken in the field.
• Materials testing data packet, including any photographs taken during core examinations, etc.
• Project-specific material sampling and testing plan
• Construction plans or typical cross-sections, when available.
• Appropriate State supplemental documents.
• Construction data sheets for the experiment, including rod and level survey data.
• For SPS sections, appropriate Specific Pavement Studies Experimental Designs [11-16]. (Used to establish the expected layer structure for new construction.)
• Any other information that the evaluation personnel deem relevant, including Falling Weight Deflectometer (FWD) information and profilometer (i.e., roughness) data.

Table TST_L05

The TST_L05 table contains layer material information for SPS projects only. Unlike the GPS studies, SPS projects consist of multiple test sections. To keep track of pavement layering and test results from various test sections, a "Project-Level Layering Structure" was developed and stored in table TST_L05. The ultimate purpose of the Project-Level Layering is to set up an accounting system to be used to link material tests for a given pavement layer in a particular section to other similar materials throughout the project. This project layering information in table TST_L05 is important to researchers interested in the material properties at the project level. The following is a list of the essential fields other than the SPS project identification fields in table TST_L05:

The code name for field "MATL_CODE" is "MATERIAL," and its code list and descriptions were previously given in table 9.

Table TST_L05A

TST_L05A includes field or laboratory determined material classification and measured thicknesses for a given pavement layer on a test section basis. TST_L05A is a final laboratory summary of pavement layering material and thickness information at each end of the pavement section, as well as within a section. The following essential information other than the section and layer identification fields is contained in this table:

The code list and descriptions for the following fields were provided previously: DESCRIPTION (table 1), MATL_CODE_STATION0, MATL_CODE_ WITHIN, and MATL_CODE_STATION5 (code MATERIAL, table 9). The code list and descriptions for field LAYER_TYPE are provided in table 10. The code name for the following fields is "MEASURE_TYPE":

• MEASURE_TYPE_1_STATION0, MEASURE_TYPE_2_STATION0, MEASURE_TYPE_3_STATION 0
• MEASURE_TYPE_1_WITHIN, MEASURE_TYPE_2_WITHIN, MEASURE_TYPE_3_WITHIN
• MEASURE_TYPE_1_STATION5, MEASURE_TYPE_2_STATION5, MEASURE_TYPE_3_STATION 5

Table 10. Code list and code descriptions for code "LAYER_TYPE".

The code list and descriptions for code "MEASURE_TYPE" are provided in table 11.

Table 11. Code list and code descriptions for code "MEASURE_TYPE"

Table TST_L05B

TST_L05B establishes the final pavement layer structure for each test section. All information available for the test section is used to derive this layer structure. Therefore, this table can potentially use data actually derived from the section itself, or if information is not available, data from other test sections in close proximity to the section. Table TST_L05B contains the following essential fields other than the section and layer identification fields:

The code list and descriptions for the following fields were provided previously: DESCRIPTION (table 1), LAYER_TYPE (table 10) and MATL_CODE (code MATERIAL, table 9). The code name for field LAYER_COMENT 1, LAYER_COMENT 2, and LAYER_COMENT 3 is "L05B_COMMENT_CODES," and its code list and descriptions are provided in table 12.

Table 12. Code list and code descriptions for code "L05B_COMMENT_CODES".

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