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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-10-071
Date: August 2010

Long-Term Pavement Performance Program Highlights: Accomplishments and Benefits 1989-2009


Through both design and necessity, the LTPP program has advanced pavement measurement and monitoring practices. In addition, bias and variability—which can have large consequences on pavement treatment selection—can be thoroughly evaluated using the data collected by the LTPP program. The data collection techniques developed for the program have been fully documented and have been implemented by highway agencies across North America, providing the highway community with better and more consistent data. Improved data collection practices have resulted in twofold savings—reducing both data collection costs and pavement rehabilitation or reconstruction life cycle costs.

Pavement condition data that are of consistently high quality are extremely important in pavement evaluation, design, and management to ensure that decision makers select the best possible alternatives and timing schedules for maintaining agency networks. Distress data are also used in research studies, construction QC/QA, and pavement failure investigations—all of which demand quality data.

Based on its long history in managing LTPP activities, FHWA is uniquely qualified to assist agencies and industry with data collection issues.


Field Data Collection

The LTPP program has provided a wide variety of benefits related to field data collection equipment and procedures. Numerous LTPP data collection procedures have been adopted by AASHTO and industry, with the most widely implemented being the Distress Identification Manual for the Long-Term Pavement Performance Program (DIM). In discussions of LTPP implementation, 90 percent of State highway agencies have indicated that LTPP data collection equipment or test methods have been used by their organizations.

Distress Monitoring

The LTPP program has developed the robust and thorough DIM that highway agencies can use to improve or standardize their condition data collection. Illinois and Mississippi have used the DIM as the basis for quantifying distress on projects with pavement warranties. Additional benefit has been realized in other States, including Nevada and Oklahoma, as they update and standardize their condition data collection techniques for pavement management purposes. Many local agencies also use the DIM, which enables them to collect data on local roads without spending valuable resources to develop their own collection systems.

Distress Viewer and Analyzer (DiVA Online) is a software application that can overlay distress information for graphical analysis. Originally developed as a QA/QC review tool for LTPP distress data, DiVA is linked to the LTPP distress survey maps, photographs, and digital images. The distress data can be displayed to show a time series/time history of a pavement section under evaluation for trend analysis and variability bands for distress trends. DiVA can analyze several survey sections at the same time. It provides reports in both graphical and exportable tabular formats.

The cover of the Distress Identification Manual for the Long Term Pavement Performance Program is shown.
Distress Identification Manual, first issued in 1987, now in its 4th edition..

DiVA's ability to graphically display distress data can assist highway agencies in analyzing distress information in several ways, for example:

Other recent publications include updated manuals for FWD measurements and for profile measurements and processing. These and other LTPP documents are available on the LTPP Web site or by contacting the LTPP Customer Support Service.

Profile Monitoring

The LTPP program developed procedures for evaluating profiler equipment and comparing the performance of various models against actual elevation measurements. The program also developed tools to monitor the consistency of measurements between different devices and wrote model procurement specifications. LTPP also developed a set of routine premeasurement equipment checks, calibration procedures, and quality-control protocols. Ultimately, FHWA published the definitive work on the use of profiling technology: Long-Term Pavement Performance Manual for Profile Measurements and Processing (FHWA-HRT-08-056, 2008).

Traffic Monitoring

The LTPP program has made significant advances across the pavement engineering spectrum since its initiation in the late 1980s. It can be argued that the program's single greatest impact in pavement performance monitoring to date is in the area of collecting data on traffic volumes and loads. While the technologies for collecting automated vehicle classification (AVC) and weigh-in-motion (WIM) data were commercially available when the LTPP program began, their implementation was varied, and there were no standards for data quality, analysis, or interpretation.

In the program's early years, significant unknowns related to AVC and WIM data collection included the accuracy and reliability of the wide range of available equipment—both portable and permanent. Since then, the LTPP program has been instrumental in advancing the technology of AVC and WIM data collection. Several States, including Washington, Arizona, and Texas, have used LTPP test sections as a means to incorporate automated data collection into their standard operations.

The cover of the Long Term Pavement Performance Program Manual for Profile Measurements and Processing is shown. The cover includes photographs of profiling equipment and the logos of the Federal Highway Administration and the Long Term Pavement Performance Program.
Operational procedures for measuring longitudinal pavement profiles..

The LTPP program has developed standards and products to address variability in the data collected from the various AVC/WIM technologies:

Based on LTPP work, Smoothness of Weigh-in-Motion Systems (AASHTO provisional specification MP 14-05) was developed to assess the smoothness of pavement approaches at WIM sites. There are both long and short wavelength pavement contributions to vehicle dynamics, and this specification helps agencies to determine whether the pavement smoothness at existing locations will introduce significant errors in the resulting WIM data, as well as to identify the optimal location for new WIM installations.

Considering the importance of the SPS projects—with multiple sections at the same location to allow comparison of various design features and rehabilitation strategies—the SPS Traffic Data Collection Pooled Fund Study was established to install, calibrate, and validate continuous traffic data. The study guidelines require a site to have data that pass the LTPP quality control standards for at least 210 days a year, including field validations. Highway agencies have contributed more than $2.7 million to this effort, and the LTPP program has significantly improved the availability and quality of monitored traffic data as a result.

A stretch of highway with weigh-in-motion equipment embedded in the pavement is shown with a tractor trailer approaching. Trees are growing at the side of the highway.

Truck approaching a WIM installation in Arkansas.

Quality Control/Quality Assurance

QC/QA have been integral components of the LTPP program since its inception. Quality control plans are in place for every field data element, and while processes are in place for office processing software and detailed QC checks in the LTPP database, the LTPP program has long recognized that collecting high-quality data expedites subsequent data processing activities. Many of the LTPP products, such as the DIM and the FWD calibration centers, are the direct results of the program's emphasis on QC/QA. In 2001, when the Office of Management and Budget issued government-wide guidelines regarding data quality standards, the LTPP program already had policies in place that ensured compliance.

FWD Calibration Centers

Reliable and precise measurements of pavement strength enable pavement engineers to design and schedule appropriate repairs at cost-effective intervals. Errors in FWD data lead to errors in pavement analysis. Securing accurate data depends on the periodic calibration of these complex hydraulic-electrical-mechanical devices.

Calibrating equipment is shown beneath an falling-weight deflectometer between the tires. A silver colored round cylinder is part of the equipment. Tall computer cabinet with opened front panel revealing several “pizza box” server computers installed in stacked fashion inside.

Left: Calibrating an FWD. The reference load cell is positioned under the FWD load plate.

Right: The LTPP program periodically updates its data collection guidelines for use of FWD equipment (available at http://www.fhwa.dot.gov/pavement/ltpp/pubs/06132).

In the early days of the LTPP program, calibration procedures and regional calibration centers were identified as priority needs. The original FWD calibration protocol was finalized in 1992, and calibration centers were established in Pennsylvania, Texas, Minnesota, and Nevada (the Nevada center was eventually moved to Colorado). These centers provided calibration services not only for LTPP FWDs, but also for FWD equipment owned by highway agencies and consultants. In the first 3 years of center operations, many of the non-LTPP FWD units were found to be significantly out of calibration. The LTPP calibration centers thus provided an essential public service that resulted in significant construction savings. These savings were most notable in situations where design is driven by FWD measurements, such as in flexible pavement overlay designs and jointed rigid pavement load-transfer rehabilitation designs.

With advances in technology, FHWA recognized the need to update the FWD calibration system and initiated a pooled fund study, Falling-Weight Deflectometer Calibration Center and Operational Improvements (TPF 5(039)), for that purpose. The study resulted in a new calibration system that takes advantage of improvements in technology; an updated FWD calibration protocol: AASHTO Standard Practice R 32-09: Calibrating the Load Cell and Deflection Sensors for a Falling Weight Deflectometer; and arrangements for ongoing support of the calibration centers, ensuring that they will remain available to the pavement community.

The upgraded hardware and software in the new system allow calibration to be completed in about 2 hours, about a third of the time previously required. The system is compatible with all brands of FWDs available in the United States. Changes in the new system include the use of an accelerometer for deflection sensor calibration, the ability to calibrate all deflection sensors simultaneously by using a multiple sensor stand, use of a Windows-based programming language that can read native data formats from each brand of FWD, and use of modern data acquisition techniques to eliminate sensitivity problems.

The upgraded hardware and software in the new system allow calibration to be completed in about 2 hours, about a third of the time previously required. The system is compatible with all brands of FWDs available in the United States. Changes in the new system include the use of an accelerometer for deflection sensor calibration, the ability to calibrate all deflection sensors simultaneously by using a multiple sensor stand, use of a Windows-based programming language that can read native data formats from each brand of FWD, and use of modern data acquisition techniques to eliminate sensitivity problems.

To encourage State highway agencies and other FWD users to use the calibration facilities, FHWA has produced a video, "Calibrating the Falling Weight Deflectometer," which can be viewed at www.fhwa.dot.gov/multimedia/research/infrastructure/calibration. Also available on CD, the video illustrates the new calibration procedure, explains how to prepare for a successful calibration, describes how calibration improves the quality of backcalculated data, and explains the impact of proper calibration on overlay design.

The LTPP program also developed a maintenance manual that provides instructions on reconditioning an FWD: The Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual. Addressed to FWD owners, operators, and technicians, the manual describes the process of disassembling and reassembling the components and subcomponents of an FWD to extend its service life.

Since 1997, more than 500 FWD and heavy-weight deflectometer calibrations have taken place at the FWD calibration centers. The centers provide an important service to the highway community, particularly when FWD-derived inputs are used for rehabilitation design. With support services from the AASHTO Materials Reference Laboratory, the centers continue to serve public and private agencies.

Data Variability

An important element to consider in pavement management systems and many research applications is the variability associated with a data point. LTPP data has been analyzed to develop typical variability ranges for manual distress surveys5, 6, 7 and longitudinal profiles.8 Pennsylvania used LTPP data to verify its own acceptance limits for pavement distress data.9 Variability associated with data collection equipment has also been studied, particularly in longitudinal profile where three different makes of high-speed profilers have been used over the years. An in-depth analysis comparing profiles by equipment type, completed in 2005, determined that changes in profiler equipment did not impact the overall variability in LTPP smoothness data.10

Objective Comparisons Between Geographic Locations

Researchers must determine how much confidence to assign information obtained from different sources, particularly if those sources are from different parts of North America, where collection and testing procedures can vary. The LTPP program has established strict guidelines for data collection and processing that require specific equipment, accredited distress raters, standardized processing software, and thorough QC/QA practices that enable analysts to utilize data from sections around the country without introducing additional sources of uncertainty.

Laboratory Testing

The ability to accurately and consistently quantify material properties is an important step in pavement selection, design, and construction. The LTPP program has made significant contributions in characterizing material properties by improving test protocols, as well as in providing a database of properties that are linked to actual field performance—both of which have furthered the development and use of mechanistic approaches in pavement engineering.

Resilient Modulus Testing of Bound and Unbound Layers

When establishing characteristics in the unbound layers—including subgrade, subbase, and base materials—resilient modulus is the property most relevant to pavement design. It is no surprise that establishing granular-layer resilient modulus values has been a priority activity for the LTPP program. In the program's early days, researchers, observing that samples from the same location yielded wide variations in test results regardless of which lab was used, recognized that there was no concise test protocol for resilient modulus testing.

LTPP made a considerable investment in establishing Test Protocol P46—Resilient Modulus of Unbound Materials.11 This protocol has been widely adopted, a process that was accelerated first by a series of videos directed to administrators, engineers, and lab managers and technicians, followed by a CD-ROM containing both the videos and documentation for the LTPP Guide for Determining Design Resilient Modulus Values for Unbound Materials.12 The LTPP program also adopted a highly repeatable test protocol to determine asphalt resilient modulus: Protocol P07.13

Equipment Startup Procedures

For both bound and unbound resilient modulus testing, a key element for ensuring uniformity is the LTPP startup procedure. The LTPP resilient modulus CD-ROM contains a 15-minute video describing the startup and quality control processes. The startup procedures were developed specific to the resilient modulus protocols, but could broadly be applied to setting up closed-loop, servo-hydraulic testing equipment. This procedure has been adopted by equipment manufacturers as a quality control check during equipment production.

. The photograph shows a metal and glass testing cylinder with metal rods partly visible inside and wires attached outside.

A dynamic modulus testing apparatus (courtesy North Carolina State University).

State-Operated FWD Calibration Center Contacts


Lorina Popescu, University of California-Berkeley Pavement Research Center

510-665-3663; lpopescu@berkeley.edu


Paul J. Smith, Colorado Department of Transportation

303-398-6547; paul.j.smith@dot.state.co.us


Tim Andersen, Minnesota Department of Transportation

651-366-5455; timothy.lee.andersen@state.mn.us


John Amestoy, Montana Department of Transportation

406-444-7651; Jamestoy@mt.gov


Cal Heinl, Pennsylvania Department of Transportation

717-783-4824; cheinl@state.pa.us


John Ragsdale, Texas Transportation Institute

979-845-9921; j-ragsdale@tamu.edu

Innovations and New Products

In both the field and lab, LTPP measurement innovations and related products are benefiting the pavement community. Direct measurements at SMP projects and automated weather station locations have provided designers and researchers with an abundance of high-quality environmental data. The further step of developing virtual weather stations has already paid significant dividends (an estimated $50 million per year for LTPPBind alone), and all environmental data will support significant future savings.

Improving material characterizations for bound and unbound layers is critical for advancing mechanistic-based designs. The LTPP program has provided many advances across this spectrum—from establishing testing protocols and increasing uniformity between testing labs to providing quality data sets used in analysis activities.

Tools such as LTPP WIM Cost Online and the LTPP WIM Smoothness Index software provide agencies with the means to optimize their entire traffic data collection program—and further guidelines on calibrating and validating equipment allow loading and classification information to be utilized with confidence. Anecdotally, Arizona has reported saving $2 million in construction costs on a single project by utilizing the LTPP WIM data.

"It is important to note that the obtained data had previously been checked for accuracy and summarized into the Microsoft Access format. If this had not been the case, extensive time would have been required to manually process and edit the large amount of traffic data."

M.S. Buchanan, Ph.D.

Traffic Load Spectra Development for the 2002 AASHTO Design Guide