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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRDI-13
Date: April 2004

Framework for LTPP Forensic Investigations - Final

PDF Version (277Kb)


Prepared by:
FHWA–LTPP Technical Support Services Contractor
MACTEC Engineering and Consulting of Georgia, Inc.
12104 Indian Creek Court, Suite A
Beltsville, MD 20705–1242

Prepared for:
Office of Infrastructure R&D
LTPP Team, HRDI-13
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101

United States Department of Transportation L T P P, Long-Term Pavement Performance
U. S. Department of Transportation
Federal Highway Administration
Long-Term Pavement Performance
Serving your need for durable pavement


The idea of forensic investigations has been discussed within the Long-Term Pavement Performance (LTPP) program for many years. The concept is that after a test section has gone out of service or is scheduled for rehabilitation, a forensic-type investigation should be performed to examine the details surrounding the causes and mechanisms of distress. In cases where the test section is going out of study or is scheduled for rehabilitation for reasons other than pavement failure, forensic investigations can also be performed to examine what worked and why.

Accordingly, the purpose of this document is to provide a framework for forensic investigations at LTPP test sections. The framework is intended to promote consistency and uniformity within the program and to ensure that maximum benefit is derived from the investigations. The word "framework" was purposely chosen over "plan" because no two sections are alike and therefore a unique, detailed plan cannot be developed that applies to all LTPP test sections. A key element of the framework detailed in this document is the development of a forensic plan tailored to each test section. Another key element of the framework is that, as appropriate, it goes beyond the distress mechanism investigations and addresses the collection of missing or desirable test section data.

Because each test section is unique and no two investigations will be the same, the development of tables in the LTPP pavement performance database to store forensic data is not envisioned at this time. Thus, the primary mechanism for dissemination of the forensic investigation results will be technical reports. Production of a "Forensic Investigations CD–ROM" may be considered later, depending on the actual number of investigations performed. Likewise, future consideration will be given to including a new table in the database listing test sections where a forensic investigation has been performed and whether an associated report is available.

Besides the report, additional missing or desirable data will be collected as part of the investigations and, when possible, entered into the existing database tables.

Investigation Framework

The proposed LTPP forensic investigation framework consists of the following three elements:

  1. Recommendation for forensic investigation.
  2. Development of forensic investigation plan.
  3. Implementation of forensic investigation plan.

This document provides details on the activities to be performed in each element and the assignment of responsibilities.

1. Recommendation for Forensic Investigation

The LTPP program is nearing its 16-year mark. The number of active test sections in the program, which once totaled almost 2,500, is rapidly decreasing and the number of sections being rehabilitated or going out of study is expected to accelerate in the coming years. Ideally, forensic investigations would be performed on all test sections going of out of study. However, this will not be possible using only LTPP program funding because of current and anticipated constraints. Thus, a decision on whether to proceed with forensic investigations will need to be made on a section-by-section basis.

Assessment of Data Availability

The first step in the decisionmaking process is to assess both the quantity and quality of data available in the database for the test section in question. The purpose of this assessment is twofold:

  1. To compile pertinent information such as pavement structure (layer thicknesses, material types, material characteristics), traffic loads/volumes, existing pavement condition and performance history (distress, roughness, structural capacity), and climatic conditions for use, if needed, in development of a detailed forensic plan.
  2. To compile a list of missing or questionable data that may be addressed as part of the forensic investigation (e.g., missing materials data or layer thickness information).

The product of this step is a test section report detailing the results of the assessment in terms of the two issues addressed above. The LTPP regional support contractors (RSCs) are responsible for completing the test section report.

Assemble Forensic Team

The next step in the process is to assemble a forensic team to help with the decision and, depending on the decision, to help develop and implement a forensic plan. The forensic team shall consist, at a minimum, of the following:

The RSC offices, in cooperation with the FHWA LTPP Team, are responsible for assembling the forensic team. The RSC offices will nominate a team leader, subject to approval by the FHWA.

Decision Process

Once the team is finalized, each member of the forensic team will receive a copy of the test section report. The team will then be required to decide whether a forensic investigation will be conducted on the test section in question. Critical issues to consider when making this decision include the following:

The relative importance of the sections to the overall program should be based on the following priority listing in order from highest to lowest. This is provided to help in cases where the final decision rests in selecting one or a few test sections from a group of many.

  1. Seasonal Monitoring Program (SMP) test sections.
  2. Category S1 and S2 test sections: Specific Pavement Studies (SPS) –1, –2, –5, –6, and –8.
  3. Category G test sections: SPS–9 and General Pavement Studies (GPS) –1, –2, –3, –4, –5, –6B/C/D/S, –7B/C/D/F/R/S, and –9.
  4. Category C test sections: SPS–3, –4, and –7 and GPS–6A and –7A.

The RSCs will work with highway agency and FHWA LTPP personnel to address the above (and other) issues to arrive at "go" or "no go" decision for the test section. A face-to-face meeting with the members of the forensic team may be required but is not mandatory. The background and results of the decision process should be recorded on the "LTPP Forensic Investigation Go–No Go Record" in Appendix A and retained by the RSC and copies provided to the FHWA LTPP team and the highway agency.

If a decision is made to proceed with a forensic investigation, the steps detailed in the remainder of this framework are to be followed.

2. Development of a Forensic Investigation Plan

Once a decision to proceed with an investigation is made, the next step is to develop a detailed forensic investigation plan. Toward this end, the forensic team will first conduct a preliminary site inspection to obtain information on site-specific conditions, distresses, likely causative mechanisms, and other factors needed to formulate a final plan. It is expected that the preliminary site inspection can be completed within a working day or less.

The RSCs are responsible for coordinating activities with the highway agency and forensic team, including scheduling of site inspection (date and time), logistic arrangements (hotel, meeting place, etc.), and setup of traffic control. In addition, the RSCs are responsible for assembling and compiling observations and/or notes made during the site inspection by forensic team members. During the preliminary site inspections, meetings with appropriate State and Federal highway agency officials should be considered to discuss logistics, site access, traffic control, material handling, material tests, and other practical considerations for developing the final plan.

A detailed forensic investigation plan, tailored to each test section, will then be developed based on the results of the data assessment (contained in the test section report) and the preliminary site inspection. Responsibility for developing the plan rests on the RSCs, but it should be closely coordinated with the FHWA LTPP Team, forensic team, and highway agency. Final review of the plan by the FHWA LTPP Team is required before its implementation.

The plan should consist of two parts:

  1. Investigations aimed at examining the details surrounding the causes and mechanisms of distresses.
  2. Activities aimed at addressing missing, questionable, or desirable data.
Distress Mechanism Investigations

Some types of investigations and field tests that should be considered for this type of work include the following:

Crack cores: By coring through a crack it may be possible to determine the location of the crack starting point and if it extends completely through the bound surface layer.

Rut trenches: If rutting is the major distress, transverse trenches at selected locations can be used to determine which layer in the pavement structure contributed to the rutting mechanism.

Drainage evaluation: Depending on the specific features of the site, drainage evaluation can provide valuable supplemental information on likely distress mechanisms. On sites with in-pavement drainage features, it could also lead to the need for further functional drainage evaluation tests, such as water injection, video inspection of edge drains, and excavations of edge drains, lateral, and other features to examine them for signs of functionality and intrusion of fines.

Joint seal evaluation: Depending on the nature of the pavement structure, if a portland cement concrete (PCC) pavement has exhibited suspected moisture-related distresses at the joints, such as pumping, theIowa vacuum joint seal tester can be used to evaluate condition of the seals.

Voids under PCC pavements: Slab removal and the Strategic Highway Research Program (SHRP) epoxy core test can be used to examine the extent and sources of possible voids. In addition, nondestructive techniques may be used, such as GPR or slab impulse response techniques.

Joint/crack faulting and joint/crack width: Excavation and removal of concrete around joints and cracks can be performed to allow inspection of the condition of steel reinforcement, dowel bars, and tie bars.

Table 1, "Distress Mechanism Investigation Matrix," outlines appropriate actions for the various types of distress that may be found in test sections. These are broken down by the type of structure.

Collection of Missing or Desirable Data
Missing Data

Data requirements identified in the initial data availability assessment that can be satisfied through field sampling, testing, and laboratory testing must be reflected in the forensic plan. The data elements will require particular test protocols, which in turn will require samples to be retrieved from the test section. This sampling and testing must rely on existing materials sampling and testing protocols and shall be indicated based on the scope of the missing data. Test results from these efforts are expected to be fully compatible with database protocols.

End-State Physical Properties

While forensic activities tend to be based on what failed, equally important and more difficult to address is the investigation of what worked and why. Knowing what worked requires understanding what did not work. For example, in the case of superior performing pavements, it may be deemed prudent to conduct additional exploratory tests to discover and confirm the absence of factors thought to contribute to distress formation.

Table 1. Distress Mechanism Investigation Matrix.
Structure Type Distress Actions Comments
AC, AC/AC Fatigue cracking Core Within and outside area
Longitudinal WP Core  
Longitudinal NWP Core  
Rutting Trench Layer profile
Transverse cracking Core  
AC/JCP, AC/CRCP Longitudinal WP Core  
Longitudinal NWP Core  
Rutting Core Within and outside to note differences
Transverse Mill off AC Observe joint/crack condition
JCP Transverse Core  
Corner Break Core  
Longitudinal Cracking Core  
Pumping Test pit Void detection using NDT
CRCP Punchout Core  
Pumping Test pit Void detection using NDT
Site specific conditions not listed above should be investigated at the discretion of the forensic team.

Note: NDT = nondestructive testing, NWP = nonwheelpath, WP = wheelpath.

All sections identified for forensic investigation must be evaluated to capture the physical properties of the system and its components. This will require sampling and testing of materials beyond examining the distress mechanisms. LTPP policy requires closeout monitoring of sections going out of study: falling weight deflectometer (FWD), manual distress survey (MDS), and profile measurement. For forensic sections these activities should be supplemented with the following elements:

While extensive excavation activity may not be desirable for sections undergoing major rehabilitation short of reconstruction, the agency will decide the type of sampling (e.g., trenches and test pits) to be conducted. It must be emphasized that this is a national program and that the greater the detail and extent of data collected, the greater the eventual benefit to the stakeholders.

The following sections provide details of investigative activities according to the type of pavement structure needed to collect end-state physical properties to supplement LTPP data. These lists are not all inclusive and additional testing may be identified during development of the plan. Table 2 summarizes the field activities anticipated and table 3 presents the laboratory testing envisioned for this effort. Appendix C presents guidelines for trench excavation in AC and AC/AC pavement structures. Appendix D provides suggested locations for cores and nondestructive testing.

AC and AC/AC
  1. Excavate trenches at three locations to confirm layering: stations 0, 76 meters (m), and 152 m.
    • Yields layer dimensions.
    • Identifies location of permanent deformation.
    • Allows in situ moisture and density testing.
  2. Conduct bulk sampling for possible laboratory testing: AC, unbound base, subgrade materials.
  1. Test pits at two locations (stations 30 m and 121 m). This would allow in situ density and moisture content determination and provide bulk samples of bound and unbound pavement layer materials for missing materials tests.
  2. Remove AC surface at several locations to visually assess transverse joint condition, measure width periodically during the sampling day, and observe amount of spalling, if any.
  3. Take temperature gradient measurements throughout the sampling day.
  1. Test pits at two locations (stations 30 m and 121 m). This would allow in situ density and moisture content determination and provide bulk samples of bound and unbound pavement layer materials for missing materials tests.
  2. Remove AC surface at several locations to visually assess transverse crack condition, measure width periodically during the sampling day, and observe amount of spalling, if any.
  3. Take temperature gradient measurements throughout the sampling day.
  1. Test pits at two locations for in situ density and moisture.
  2. Take bulk sample at test pits for missing materials tests.
  3. Use Dipstick device for warp and curl measurement periodically during sampling day.
  4. Take temperature gradient measurements throughout the sampling day.
  5. Measure transverse joint faulting.
  6. Conduct pachometer studies for sample of slabs to determine depth of steel and number of bars.
  7. Conduct pachometer studies at joints to evaluate dowel orientation and depth.
Table 2. Field Testing Activities for Collecting Supplemental Data.
Field Activity Pavement Type Purpose
Core at crack All Visual determination of crack origin
Core at saw and seal locations AC/JCP Locate joint relative to saw cut
Core at reflection cracks AC/JCP, AC/CRCP Determine PCC joint versus crack
Trench AC, AC/AC Detailed layering study; determine location of any permanent deformation
Test pits JCP, CRCP Sampling and in situ density and moisture testing
GPR All Layer thickness, material condition
SASW All Layer thickness, voids under PCC
Pachometer JCP, CRCP Location and depth of steel, dowel, and reinforcement
Drainage evaluation All Excavate and assess condition of drainage elements (laterals, collectors, filter materials)
Dipstick All Cross slope


Table 3. Tests for End-State Physical Properties of Pavement Materials.
Sites Laboratory Test
All forensic  
From bulk of unbound base  
  Moisture-density relationship
  Resilient modulus
  Engineering properties (Atterberg limits, gradation)
  Specific gravity
From bulk of subgrade  
  Moisture-density relationship
  Resilient modulus
  Engineering properties (Atterberg limits, gradation)
AC and AC/AC  
From cores/bulk samples of AC  
  Mix properties, density, voids, AC content
  Resilient modulus
  SHRP properties from SST
  PG of extracted/recovered AC
  Core inspection for stripping
JCP and CRCP  
From cores  
  Compressive strength
  Splitting tensile strength
  Elastic modulus
  Inspect core for corrosion
  Inspect core for ASR

Note: ASR = alkali-silica reaction, PG = performance grade, SST = Superpave® Shear Tester.

  1. Test pits at two locations for in situ density and moisture.
  2. Take bulk sample at test pits for missing materials tests.
  3. Conduct pachometer studies for a sample of pavement length (three locations each 3 m by full lane width) to determine depth of steel and number of reinforcing bars.
  4. Conduct in situ corrosion and chlorides testing at same locations as steel survey.
  5. Measure point location of transverse cracks (at intersection of crack and midlane).
  6. Measure crack width at same spot as point location measurement periodically during the sampling day.
  7. Take temperature gradient measurements throughout the sampling day.

Sites in the Seasonal Monitoring Program require effort beyond that outlined above. End-state conditions are highly desired to confirm material properties used for calibrating the instrument data. Therefore, detailed excavation, sampling, and testing at locations where instruments were installed are critically important. Appendix B is a memorandum that details a plan used in the successful forensic investigation of an SMP site inConnecticut. This plan, coupled with the activities presented above, should be the basis for forensic plans for SMP sites.

Additional Requirements

Each candidate forensic site will have a different set of data needs. As such, the activities presented are not considered all inclusive. The RSC and other members of the forensic team must examine the available information and this document and then develop a comprehensive plan to collect the needed data. The RSC is responsible for compiling the forensic plan and presenting it in a written recommendation to the FHWA LTPP Team. It is the responsibility of the FHWA LTPP Team to review the plan and make suggestions on funding sources and potential sources of assistance so the plan can be implemented successfully.

3. Implement Forensic Investigation Plan

The FHWA LTPP Team will review and work with the RSC and forensic team to finalize the forensic investigation plan before implementation. The purpose is to refine the plan details and objectives in the context of cost sensitivity and program benefits. The RSC will provide cost estimates for the plan to the FHWA LTPP Team for review and consideration. Once the plan is fully approved, implementation by the RSC in coordination with the forensic team and FHWA LTPP Team will begin.

Because each forensic plan is section specific, the scope of data collection will vary. A project manual for each plan must be developed to clearly define the data to be collected, methods and protocols to be used, and data sheets to be completed. Based on the range of possible field activities, a number of additional protocols are required to support consistent data collection. Many protocols exist from the implementation of the LTPP program, but Appendix C addresses specific procedures for trench excavations and Appendix D addresses cores taken at crack locations and nondestructive testing of PCC pavement (SASW, GPR, and pachometer).

Additional data collection protocols and supporting sheets may need to be developed based on unanticipated site-specific conditions. In these cases the RSC is responsible for identifying the needed procedure and coordinating its development with the LTPP technical support services contractor. The extent of new forms and protocols must be balanced with the need for consistency in data types collected. Reliance on existing LTPP forms and procedures is critically important to render the forensic data readily accessible to users.

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