Approaching the quarter-century mark, the Nation's first comprehensive program for determining traffic and environmental impacts on pavements continues to fuel innovation.

 

 

Poor pavement performance increases traffic congestion, threatens public safety, and raises maintenance costs. Accurately predicting performance and durability is critical to improving pavement design. Yet current design procedures remain largely based on empirical data from the 1950s. In many cases, engineers design roads for much greater traffic capacity than is supported by that 50-year-old science.

Even so, the United States reaps a substantial return from investing approximately $40 billion per year in its pavements. A recent article in Roads & Bridges magazine put the annual savings from the interstate system at $737 billion when considering "safety benefits, saved time, reduced fuel, and lower consumer costs."

Since 1987, the Federal Highway Administration's (FHWA) Long-Term Pavement Performance (LTPP) program, the most comprehensive pavement research program ever undertaken, has addressed the related issues of improving pavement performance and optimizing the Nation's investment in the surface transportation system.

"The LTPP program's visible legacy is the 20-plus-year performance database for use by researchers and practitioners now and well into the future," says Ralph Haas, distinguished professor emeritus in the Department of Civil Engineering at Canada's Waterloo University. "But there is another inherent legacy, and that is the training in pavement engineering for a new generation of engineers who will become the leaders of tomorrow. They have learned about the basics of performance evaluation and the factors that affect performance, about pavement structural design, and about how to advance pavement technology through exploiting all the materials, traffic, and other data that reside in the database."

Today,FHWA researchers continue to work in partnership with State and Provincial departments of transportation (DOTs) to gather and analyze data from 2,500-plus test sections across the United States and southern Canada, of which approximately 1,000 are still active. The LTPP program collates and releases an updated database annually and distributes analysisfindings via publications and reports throughout the year to help manage existing pavements and inspire research into the pavements of tomorrow.

AASHO Road Test

Around the late 1970s, U.S. transportation agencies faced rapid road deterioration and needed better ways to understand and predict pavement performance. Most pavement management models and engineering design tools at the time were based primarily on a road test study conducted in Ottawa, IL, in the late 1950s by the American Association of State Highway Officials (AASHO), predecessor of the American Association of State Highway and Transportation Officials.

 

 

The AASHO Road Test included research on the costs of pavement damage caused by heavy truck loads. The research also advanced pavement engineering by improving the industry's understanding of the benefits of stabilized bases and became the basis for empirical design methods. But AASHO limited the test to one climate, one sub grade, and a short time span. Thus, the study did not provide an adequate knowledge base for predicting pavement performance in other locations and for longer periods.

AASHO then proposed additional research on pavement performance to overcome the road test's limitations but never implemented these further studies, partly because the engineers of the day believed that advances in pavement engineering theory and mechanistic modeling would be able to fill the gaps. That proved not to be the case, however.

 

 

Descriptions of LTPP GPS and SPS Experiments

General Pavement Studies (GPS)
GPS-1	Asphalt Concrete (AC) on Granular Base
GPS-2	AC on Bound Base
GPS-3	Jointed Plain Concrete Pavement (JPCP)
GPS-4	Jointed Reinforced Concrete Pavement (JRCP)
GPS-5	Continuously Reinforced Concrete Pavement (CRCP)
GPS-6	AC Overlay of AC
GPS-7	AC Overlay of PCC
GPS-8	Discontinued
GPS-9	Unbonded PCC Overlays on PCC

Specific Pavement Studies (SPS)
SPS-1	Strategic Study of Structural Factors for Flexible Pavements
SPS-2	Strategic Study of Structural Factors for Rigid Pavements
SPS-3	Preventive Maintenance Effectiveness of Flexible Pavements
SPS-4	Preventive Maintenance Effectiveness of Rigid Pavements
SPS-5	Rehabilitation of Asphalt Concrete Pavements
SPS-6	Rehabilitation of Jointed PCC Pavements
SPS-7	Bonded PCC Overlays on Concrete Pavements
SPS-8	Study of Environmental Effects in the Absence of Heavy Loads
SPS-9	Validation of SHRP Asphalt Specification and Mix Design (Superpave)

Source: FHWA.

 

In 1978, the Surface Transportation Act provided a first step toward a solution by directing FHWA to develop "long-term or continuous monitoring of roadway deterioration to determine the relative damage attributable to traffic and environmental factors." Prompted by the law, the research community initiated a variety of studies in the early 1980s, including pilot studies by FHWA, strategic management research by the National Academy of Sciences, and development studies by the National Cooperative Highway Research Program (NCHRP). These studies led to the launch of the LTPP program, which began operations in 1987 under the 5-year Strategic Highway Research Program administered by the National Research Council (NRC) of the National Academy of Sciences.

From Road Test to Program

Whereas the primary objective of the AASHO Road Test was to investigate cost allocation factors related to truck loading (information needed for creating the interstate highway system), the main task of the LTPP program is to understand the effects of variations in loading, environment, material properties, construction variability, maintenance, and rehabilitation on pavement performance. The end goal is to develop a knowledge base to help advance management and engineering tools to extend pavement life on the interstates and other roadways.

Unlike the AASHO Road Test, the LTPP program relies on pavement test sections constructed on public roads in all major climate zones and soil types. In one set of LTPP experiments (known as the General Pavement Studies), State and Provincial DOTs selected pavement sections that were already built based on their own design processes that met one of the nine LTPP design criteria for the study. In another set of experiments (Specific Pavement Studies), multiple test sections of specific design and thick nesses were constructed at each test site. In all, the LTPP program consists of more than 2,500 test sections, classified into 17 experiments based primarily on pavement type.

The LTPP program enlists a team of contractors who collect pavement distress ratings, deflection measurements, longitudinal profiles, transverse profiles, field materials tests, and instrumentation measurements. FHWA, State DOTs, and academia collect data on traffic loading and some materials tests. The program addresses a broad array of topics from field validation of pavement design procedures to studies of variability in traffic and materials data to the investigation of the development of pavement roughness.

 

 

Some of the LTPP test sections are holding up surprisingly well. Section 512564 in Virginia, for example, is an 8-inch (20-centimeter)-thick, continuously reinforced PCC pavement opened to traffic in 1969. Grooving performed in 1998 is the only maintenance treatment needed so far at the site. At 35 years old, the pavement has an average international roughness index rating of 108 inches per mile in both wheel paths, with low-severity punch outs beginning to form.

Data Quality

High-quality data are indispensable to any engineering research program. A primary attribute of that data is their relevance to end users. For the LTPP program, FHWA developed a plan for data collection that links user needs to data requirements and provides guidelines to help transportation agencies and researchers measure data accurately and on a regular basis. Below are some highlights of the LTPP data quality process.

Peer review. Early on, LTPP management saw the need for extensive peer review by highway agency officials and other experts. The Transportation Research Board (TRB) created the Long-Term Pavement Performance Committee in 1986to monitor LTPP studies and provide technical assistance concerning future directions for the research. Today, expert task groups focusing on specific subjects supplement the committee's work.

Experimental design. LTPP researchers used statistically based factorial experimental designs for each study. A factorial design illustrates combinations of experimental factors, which, for LTPP, includes environmental zone, soil type, surface layer thickness, base type and thickness, and pavement structural details related to the type of pavement. Internationally recognized statisticians and research engineers prepared the designs for the experimental pavements.

Data collection. In designing a plan for data collection, the LTPP program developed new procedures, protocols, and test methods.

Documentation. The LTPP program documented all phases of its activities. Program managers prepared more than 300documents detailing the planning process, experimental design, construction guidelines, agency participation requirements, data collection and processing procedures, data evaluation checks, procedures and checks for calibration of data collection equipment, data analysis results, standard data release format and data user aids, and details of construction and instrumentation installation on specific test sections.

Data quality indicators. In the early 1990s, the LTPP program managers developed indicators of data quality for inclusion in each record in the LTPP database. Refined over time, these indicators include measures for identifying missing, out-of-range, or illogical data, as well as inconsistencies in common data elements between tables. These quality indicators enable any user to determine quickly whether a particular data element might be questionable.

Data standardization. In 2000, the LTPP program adopted the data management standards of the International Organization for Standardization. As a result, all data collection contractors had to develop procedures for managing quality control.

Feedback reports. The LTPP program created a customer service and feedback process that enables data users, analysts, and others to report problems. Before releasing data to users, independent reviews are performed on the data to identify and correct problems.

Dissemination. To improve data accessibility to end users, the LTPP program developed an annual standard data release in a Microsoft^® Access^® format available on DVD.

 

 

What Is a Crack?

Defining something as "simple" as a pavement crack is a surprisingly complex task. Researchers need to classify cracks into categories to understand their different causes. To learn whether a crack is load-related requires a definition of the wheel path of heavy trucks, but developing a measurable objective definition of wheel path for all pavements is challenging, in part because asphalt and portland cement concrete (PCC) pavements have different cracking patterns.

An early task for the program was to develop a manual to provide detailed instructions on how to rate the type, extent, and severity of distresses by pavement type. Beginning in 1988, the LTPP program performed an exhaustive review of existing manuals and produced the Distress Identification Manual [DIM] for the Long-Term Pavement Performance Program (FHWA-RD-03-031). TRB's Expert Task Group on LTPP Distress and Profile Data Collection and Analysis adopted the manual for use in the accreditation workshops for raters of pavement distress, which all LTPP distress raters must pass before they can perform ratings on LTPP test sections. FHWA also conducted research on variability between distress raters.

The DIM contains color photographs and descriptions of distresses common to asphalt concrete, jointed concrete, and continuously reinforced concrete pavements. FHWA also released pocket-sized editions of the manual broken down by pavement type. Although the manual contains more distress types than most agencies will need for pavement management purposes, it serves as a comprehensive resource for training pavement raters and developing agency-specific guides for distress rating.

 

 

Measuring Pavement Smoothness

By the late 1980s, nationally accepted methods still did not exist for measuring pavement smoothness. Many DOTs were using various types of vehicle-mounted measurement devices, but the devices grew less reliable as they aged and their results could not be replicated.

Based on past research, LTPP investigators knew that measuring the relative longitudinal elevation profile in both wheel paths provided baseline data for computing a variety of pavement roughness indices. To do this at highway speeds requires the use of inertial profilers equipped with sophisticated speed and vertical distance measuring devices. At discrete intervals, subtracting the vertical motion of the vehicle from the distance to the pavement surface yields a map of the elevation change of the pavement surface. In essence, these profiles measure the bumps in the pavement that drivers experience.

Inertial profilers are complex systems whose performance is directly linked to the sophistication of the instrumentation taking the measurements. In the late 1980s, the LTPP program developed procedures for evaluating profiler equipment and comparing the performance of various models against static, ground-truth elevation measurements. LTPP devised a robust array of tools to monitor the consistency of measurements between multiple devices and wrote model procurement specifications. LTPP also developed a set of routine pre measurement equipment checks, calibration procedures, and quality-control protocols.

 

 

A fleet of inertial road profilers took to the streets as early as 1989.One of the primary publications on profiling technology is FHWA's latest Long-Term Pavement Performance Manual for Profile Measurements and Processing (FHWA-HRT-08-056) document, published in 2008.

Measuring Heavy Truck Loads

Anticipated loads from heavy trucks are the primary factor that engineers use to design a pavement's structure and decide on the thickness of the pavement's layers. At the extreme, a single pass by a heavy truck can cause a pavement to fail. Research on pavement life requires an understanding of the characteristics of heavy truck loads applied to each test section.

TRB's Expert Task Group on LTPP Traffic Data Collection and Analysis determined that weigh-in-motion (WIM) measurements are necessary to characterize the actual loads rolling over LTPP test sections. Reliance on data from enforcement of static truck loading at weigh stations does not account for overloaded trucks that might avoid the scales or times when the scales are not in service.

In the initial planning for the LTPP project, participating highway agencies were responsible for providing traffic-loading data from WIM scales for test sections in their States and Provinces. However, as data were received, the LTPP program staff noticed differences in the quality of traffic data from the various WIM equipment used by the highway agencies. So, with the goal of collecting WIM measurements of actual truck loads on each test section, the LTPP program partnered with more than half the State highway agencies to establish the pooled fund study "Long-Term Pavement Performance Specific Pavement Study Traffic Data Collection" (TPF-5(004)) to install state-of-the-science WIM scales at high-value experiment locations. Using this approach, a single scale could measure loading for multiple test sections on a Specific Pavement Study project and sometimes General Pavement Study or other Specific Pavement Study sites located in close proximity to the WIM scale.

Annual calibration of the WIM scales is a critical component of this study. The calibrations are performed to ensure that the systems are operating at peak performance. In addition, the data collected by these systems are examined daily to make sure there are no drifts or unexpected changes in the data. Many highway agencies already are using this high-quality traffic data to help with the implementation of the new pavement design guide in their respective States.

This study has raised the standard for collecting traffic data. The creation of the LTPP classification scheme, a WIM smoothness specification, and WIM workshops held around the country are just some of the byproducts that have come from this effort. The LTPP program summarized the standards for collecting high-quality data on truck loading in the FHWA draft publication LTPP Field Operations Guide for SPS WIM Sites.

How Strong Is a Pavement?

The falling weight deflectometer (FWD) has become the most common piece of equipment that engineers use to measure pavement strength. Typically mounted on a trailer towed by a truck or van, the FWD drops a weight from various heights and measures the resulting pavement deflections at a fixed distance from the load. The load pulse mimics that of a heavy truck at highway speed. Engineers then can analyze the resulting deflections formed by the dropped weight to determine the elastic modulus (stiffness) of the different pavement layers.

Colorado Implements Proven PCC Pavement Practices The Colorado Department of Transportation (CDOT) has been a consistent contributor to the LTPP program. The agency was proactive in examining performance at its LTPP test sections and then making improvements accordingly. In 2006 the agency published a report, Implementation of Proven PCCP Practices in Colorado (CDOT-DTD-R-2006-9), documenting improved PCC pavement practices based on past and current test sections. The report revealed two findings directly related to the LTPP program. First, the researchers confirmed that widening a slab from 12 feet (3.7 meters) to 14 feet (4.3 meters) was the structural equivalent of increasing the thickness by 1 inch (2.5 centimeters). Second, a single 0.125-inch (0.32-centimeter) cut was as effective as the previous standard 0.375-inch (0.95-centimeter) double cut for PCC joints. This change translated to a savings of 57 cents per linear foot of joint. Through implementing these findings, CDOT improved its specifications and overall PCC pavement performance, while saving money at the same time.

In the late 1980s only a handful of FWDs were in use in the United States. The LTPP program worked to expand use of the FWD technology by improving its application in the following ways.

Calibration procedures. To provide research-quality data from these instruments, the LTPP program developed procedures for relative and reference calibrations. A relative calibration consists of stacking all the sensors on top of each other and comparing the deflection measured by each sensor to calibrate the deflection sensors against each other. A reference calibration compares the measured deflection and load of the FWD sensors against reference devices that have been calibrated against traceable standards developed by the National Institute of Standards and Technology. In cooperation with participating highway agencies, the LTPP program created centers to perform FWD reference calibrations at locations around the United States. The calibration centers are operated by State DOTs or university research centers under contract with the State DOT in which the calibration center is located.

FHWA recently updated the reference calibration procedures under pooled fund study TPF-5(039), "Falling Weight Deflectometer (FWD) Calibration Center and Operational Improvements." For more information, visit www.pooledfund.org.

Documentation. The LTPP program developed documentation for procurement and evaluation of FWDs. Copies of these documents are available from the LTPP Customer Support Service Center at ltppinfo@dot.gov.

User manual. The LTPP program also created a maintenance and overhaul manual that provides instructions on reconditioning an FWD when it has reached the end of its practical service life. The Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual (FHWA-HRT-05-153) contains nearly 20 years of accumulated knowledge on how to troubleshoot and maintain the types of FWDs used in the LTPP program.

Pavement Forensic Investigations

Another factor of interest to engineers is why some pavements perform much better or much worse than expected. Forensic studies to determine causes of life-ending pavement distress typically require one intensive investigation. Understanding the reasons behind the condition of long-life pavements requires long-term studies consisting of a series of monitoring measurements at regular intervals.

"LTPP is on the job of determining why pavements perform the way they do," Anne-Marie H. McDonnell, P.E., principal investigator in the Connecticut Department of Transportation's (ConnDOT) Division of Research. "It is critical to capture information regarding pavement condition and response, traffic loadings, and material properties at the end of a pavement's life to reap the benefits of the LTPP experiments."

In recent years, engineers have performed forensic investigations to determine why pavements fail, measure unusual types of distress, and understand why some pavements perform well and have long service lives. At the State level, the California and Texas DOTs have developed pavement forensic guidelines. The LTPP program also has developed guidelines (available through the LTPP Customer Support Service Center) and conducted several field investigations. In addition, NCHRP's Project 1-49: Guidelines for Conducting Forensic Investigation of Highway Pavements is underway.

 

Measurements in LTPP Database Per Data Type

Data Type	Approximate Amount	Comment
Pavement smoothness (longitudinal profile)	24,000 section measurements 123,000 measurements 126 million measurement points	5 repeat measurements in typical dataset. One measurement includes 2,000-10,000 points.
Pavement strength(falling weight deflection measurement)	16,000 section measurements 603,000 measurement points 8.4 million measurements	Typical dataset has 22 measurement points and 16 measurements at each point
Pavement surface distress	26,000 section measurements
Traffic load data	2,100 site years$ 100 days/year 1,200 site years$ 200 days/year 600 site years$ 300 days/year	Traffic load data from truck WIM scales
PCC thermal coefficient of expansion	2,700 test results 2,000 test specimens	Up to 9 repeated tests on some specimens and 3 core samples from each test section
Laboratory-measured asphalt concrete resilient modulus, creep compliance, and tensile strength	1,300 test results	1 test result included resilient modulus and creep compliance at 3 temperatures, and tensile strength at 1 temperature from measurements on 3 specimens

*Data as of January 2010.

Source: FHWA.

 

Over Two Decades of Data

After 23 years in operation, the LTPP program has accumulated a significant amount of data related to pavement performance, and literally millions of measurements on pavement smoothness, strength, distress, and traffic loading. Prior to LTPP, the amount of data on modern highways available for a pavement research study rarely exceeded 100 measurements.

"The LTPP program has met a critical need by providing pavement engineering researchers with access to large amounts of research-quality data that would not have been available without the program," says Haas.

For example, certain LTPP datasets can aid in use of the Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, an NCHRP publication. Released in 2004, the design guide and its associated software package enable DOTs and other users to analyze and predict the performance of different types of pavements. The design guide includes hard-to-find data such as traffic tables containing hourly and monthly truck distribution inputs from a cross section of highway functional classifications, and for many agencies, this data only can be found on a State's LTPP test sections.

The LTPP program developed state-of-the-art methods to produce the first-ever dynamic modulus estimates and formatted them to the input requirements of the design guide's software. The program issued the estimates as part of standard data release 24, in January 2010, which also contains estimates of dynamic modulus for asphalt concrete layers on LTPP test sections from resilient modulus tests, SuperPave^® (SUperior PERforming Asphalt PAVEments) asphalt binder tests, and other mix data.

Although the LTPP program has published more than 500 research reports on the analysis of its data, the transportation industry has only begun to tap the wealth of knowledge in the database. "I am enthusiastic about the LTPP database and how agencies can use the data to investigate issues of importance to them," says Judith B. Corley-Lay, State pavement management engineer at the North Carolina Department of Transportation (NCDOT). "Last year I used the LTPP profile data to investigate the amount of annual change in profile data to see whether annual testing is justified. This year I looked at a comparison of NCDOT condition data against the very detailed LTPP condition data. This comparison is key to implementing the design guide because the guide was calibrated using LTPP data."

State DOTs and Canadian Provinces can make practical use of LTPP data to improve their current practices in several ways. For example, they can establish a schedule for monitoring profile measurements based on seasonal changes in pavement smoothness. The LTPP program initiated a seasonal monitoring study that collected seasonal and daily measurements on selected test sections, revealing unique information on the changes in pavement smoothness due to environmental influences. The large amount of pavement deflection data coupled with climate data can help agencies refine their seasonal load restriction programs.

Agencies can use the LTPP database to study wheel path rutting in PCC pavements. Conventional engineering wisdom suggests that PCC pavements do not incur ruts, but LTPP data reveal that they do. Although the rut mechanism for PCC pavements is different from that for asphalt concrete pavements, the LTPP database includes information on distortions in the transverse profile of PCC-surfaced pavements that is nearly impossible to find in other sources.

Data Analysis

FHWA encourages DOTs to use the LTPP data analysis strategic plan to develop internal studies that are of interest to them and to track findings from other research efforts. Also, by using the plan developed by the LTPP program, DOTs can optimize their limited research funds by leveraging results already achieved through other LTPP studies. The LTPP analysis structure includes seven strategic objectives: traffic characterization and predictions, materials characterization, environmental effects on design and performance, pavement condition data in pavement management, pavement response and performance prediction, maintenance and rehabilitation strategies, and performance impacts of specific design features. The LTPP program uses this plan to track completed projects and document future research needs.

"LTPP has been and continues to serve as a catalyst for innovations and improvements, influencing much more than pavement practices," says McDonnell. "The benefits and lessons learned extend to a range of cross-disciplinary topics, including how transportation professionals collect traffic data, apply climatic models, manage large datasets, and design and manage large-scale experiments."

 

 

Obtaining LTPP Resources

As the custodian of the LTPP database, library, and associated information, FHWA established the Customer Support Service Center and reference library at the Turner-Fairbank Highway Research Center in McLean, VA. Every January, the LTPP program publishes an updated version of the standard data release and reference library, available free of charge by email request to ltppinfo@dot.gov.

"A great legacy of the LTPP program is the reference library of some 600-plus documents on protocols for carrying out tests and acquisition of data, and on navigating the database, and many other technical reports and briefs," says Haas. "This is probably the most unprecedented and comprehensive repository of information ever assembled in the pavement field."

Adds Corley-Lay,"I think there will be many questions that individuals, research groups, university students, and State agencies can answer, or at least explore, using LTPP data. It is a wonderful resource."

LTPP Beyond 2009

The LTPP program holds the record as the Nation's longest research effort focused on pavement performance. In fact, the program has been a line item in the last four Federal transportation laws. FHWA management has announced publicly its commitment to continue monitoring existing test sections and to be custodian of all LTPP data and information until at least 2015.

"We see the LTPP database serving into the indefinite future as a key component of the agency's pavement research activities, and those activities will benefit substantially from the many LTPP data collection and analysis activities in [fiscal years] 2010 [through] 2015," says FHWA Administrator Victor Mendez.

Collection of research-quality data on the performance of inservice pavements is a continuing need. Researchers in 2020, 2030, and beyond will require updated, long-term data from pavement monitoring after 2015 so they can continue to advance pavement engineering and management.

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Gary Elkins is a senior engineer at MACTEC Engineering and Consulting, Inc. During his 30 years in transportation and pavement engineering research, he has worked full-time on the LTPP program in various roles since 1987. Currently he is the principal investigator on a contract to assist FHWA in operating the LTPP program. He holds bachelor's and master's degrees in civil engineering from the University of Texas at Austin.

Deborah Walker is a research civil engineer and manages traffic data collection and communication activities for the LTPP program. Before joining FHWA in 2002, she worked 11 years in the pavement field for the Texas Department of Transportation and served as the LTPP coordinator for the State. She has a bachelor's degree in civil engineering from the University of Delaware and a bachelor of arts and sciences from Lincoln University.

Kevin Senn is a principal engineer at Nichols Consulting Engineers. He has 15 years of experience in civil and pavement engineering. Currently, he is the project manager for the western regional support contractor responsible for collecting and processing LTPP data in the western United States and Canada. He holds bachelor's and master's degrees in civil engineering from Washington State University.

For more information, contact the LTPP Customer Support Service Center at ltppinfo@dot.gov or visit the LTPP Web site (https://www.fhwa.dot.gov/pavement/ltpp/) for access to research reports, database user aids, software, and other information.