Fall 1996· Vol. 60· No. 2

Test Roads:

Designing the Pavements of the Future

by Terry Mitchell

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Innovations don't happen overnight. What begins as a theory or idea requires a well-planned and documented program of testing and evaluation to determine if the idea will pan out. Pavements are no exception. New theories and models developed by researchers to improve roadway design and construction must be validated through real-world experience. The Federal Highway Administration (FHWA) and a number of partners are gaining this experience by conducting pavement studies around the country designed to push the boundaries of road technology. From full-scale test tracks to machines that simulate traffic loads, today's pavement testing will lead to better roads tomorrow.

A New Approach to Pavement Testing

The new age of pavement testing is illustrated by FHWA's recently opened WesTrack. (See WesTrack article) The two-year study is designed to collect data that will be useful in developing performance-related specifications for hot-mix asphalt (HMA) pavements by analyzing how differences in materials and construction impact performance. The project will also provide verification of the performance prediction models used in the SuperpaveTM mixture design and analysis procedures that were developed under the Strategic Highway Research Program (SHRP).

In-Service Pavement Studies

Another approach to evaluating pavement designs and construction is the use of in-service test roads. Placing test sections in pavements that carry conventional traffic rather than test vehicles minimizes the cost of loading the pavements although it does generate additional costs for traffic monitoring and vehicle weighing. The SHRP/FHWA Long-Term Pavement Performance (LTPP) Program is a 20-year study of 2,200 in-service test sections in the United States and Canada. The program zeroes in on the long-term performance of different pavement structures under various loads, climatic factors, subgrade soils, and maintenance programs.

The LTPP program is split into two separate components: General Pavement Studies (GPS) and Specific Pavement Studies (SPS). GPS experiments, which are under way at 780 sites across North America, focus on the most commonly used pavement structural designs. In contrast, the 1,400 sections in the SPS experiments concentrate on the effects of selected maintenance, rehabilitation, and design factors on pavement performance.

Several states are currently operating large-scale in-service test road projects to collect important pavement information. The Minnesota Road Research Project (Mn/Road), for example, was started in 1994 to examine the effects of climate and traffic on road materials, including different soil, base, subbase, and pavement types. Sponsored by the Minnesota Department of Transportation (MnDOT) and the University of Minnesota, Mn/Road features a state-of-the-art pavement research facility located along I-94 in Otsego. The facility, which contains a 5.6-kilometer (km) mainline test road and a 4-km low-volume paved loop road, will provide Minnesota and other northern states with important new data on how freezing, thawing, and moisture affect pavement performance. Mn/Road will also help to both verify the empirical pavement design models used today by Minnesota and the American Association of State Highway and Transportation Officials and to develop new mechanistic design models for the future.

Forty test sections have been built on the two roads, and a total of 4,572 sensors are located in the pavement layers for monitoring purposes. These sensors collect more than 30 million bytes of information daily, which are being used to compare the performance of different pavement designs. More than 75 individual research projects are planned for the Mn/Road facility over the next several years

. Ohio has also implemented a long-term pavement study, known as the Ohio SHRP Test Pavement. Started in 1994 in partnership with FHWA, the project is located approximately 65 km north of Columbus on U.S. Route 23. It features a 4.8-km stretch of four-lane roadway containing 38 HMA and portland cement concrete test sections. The two primary objectives of this project are to provide FHWA with basic performance data for the SPS experiments and to measure structural response to dynamic loads.

Response information will be collected with environmental and dynamic sensors installed in 33 of the test sections. This instrumentation will continuously monitor temperature and moisture in the pavement structure, as well as the pavement's structural response to controlled loads applied with a falling weight deflectometer, a truck from the National Research Council of Canada, and Ohio DOT dump trucks. The Ohio DOT trucks will be equipped with assisted-guidance systems that help drivers maintain their lateral position on the roadway as they travel over buried pavement sensors.

Table 1 -- Pavement Testing Machines

Program (Machine)	Pavement Type	Load (kN)	Passes	Date Opened	Funding Source
FHWA (Accelerated Loading Facility)	HMA	40-100	3802	1986	F1
Louisiana (Accelerated Loading Facility)	Composite	40-100	380	1994	S/F
Indiana (Accelerated Loading Facility)	HMA	40-90	1333	1991	S/F/I
California (Heavy Vehicle Simulator)	HMA	20-200	850	1995	S/F

¹F=FHWA, S=State; I=Industry/Private
²Applying an 80-kN (18,000-lb) load at 380 passes to a test section is equivalent to applying ESALs at an annual rate of 2,100,000.

Pavement Testing Machines

While test tracks and in-service test roads are often the most visible pavement research sites, the push for better roads doesn't stop there. Currently, four facilities in the United States -- and more are in the planning stages -- use mechanical testing devices to conduct accelerated pavement tests. These centers use accelerated loading facility (ALF) machines, heavy vehicle simulators, and other devices to simulate the effect of traffic on a pavement, allowing critical data to be collected in a relatively short period of time.

FHWA's pavement testing facility, located at its Turner-Fairbank Highway Research Center in McLean, Va., uses two ALF machines to simulate traffic on 12 full-scale pavement test sections. The simulations have helped researchers to focus on factors, such as high tire pressure and wide-base single tires, that affect the performance of HMA pavements. The facility is also collecting data to validate the Superpave^TM performance-based asphalt binder and mixture specifications developed by SHRP. Complementing FHWA's pavement testing facility are state testing centers in Indiana, Louisiana, and California. Indiana's testing center in West Lafayette is currently evaluating its recently completed study of crushed-aggregate HMA mixtures. "This research is already providing valuable information," says Indiana DOT's Brian Coree.

"We are seeing more realistic performances of the mixtures in our facility as compared to previous testing in the laboratory," he says. "It's a more complete picture." The center is also about to partner with FHWA on a pooled-fund Superpave^TM mixture validation study.

Louisiana's pavement research facility in Baton Rouge uses an ALF machine to simulate traffic loads on several full-scale pavement test sections. The facility's research is currently focused on improving pavement base course design.

In California, the pavement testing program uses two heavy-vehicle simulators to conduct accelerated pavement tests. Current research objectives are focused on improving pavement design, construction, and rehabilitation. The specific objectives include testing: (1) drained versus undrained pavement sections, (2) construction variables during night paving, and (3) rubberized asphalt versus conventional asphalt in HMA overlays.

The state's program represents a collaboration between the California Department of Transportation (Caltrans) State and Local Project Development Program, the University of California at Berkeley, the Council of Scientific and Industrial Research in South Africa, and Dynatest.

Table 2 -- Track and In-Service Pavement Tests

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Program	Loading Type	Pavement Type	Number of Test Sections	ESAL Loading Rate (Annual) ( x 102)	Total ESALs per Section (x 103)	Project Duration	Funding Source
AASHO Road Test	Track	PCC HMA	836	500	1,114	1957-1961	P/F/FA/I
Penn State Track	Track	PCC HMA	28	600	1,200	1971- present	SP&R/S
WesTrack	Track	HMA	26	5,000	10,000	1994-1998	F
LTTP-GPS	In-Service	PCC HMA	780	85-300 minimum	1,700-6,000 minimum	1989-2009	S/F
LTTP-SPS	In-Service	PCC HMA	1,400	85-200 minimum	1,700-4,000 minimum	1991-2011	S/F
Mn/Road Mainline	In-Service	PCC HMA	23	1,000	20,000	1994-2014	S/FA/P/FO
Mn/Road Loop	Track	PCC HMA Aggregate	17	20	50-100	1994-2014	S
Ohio SHRP Test Pavement	In-Service	PCC HMA	38 18 HMA	980 HMA	19,600 HMA	1995-2016	S/FA/F

¹ESAL=Equivalent 80-kN (18,000-lb) Single Axle Load
²F=FHWA; FA=Federal Aid; FO=Other Federal Agencies; S=States; I=Industry/Private; SP&R=State Planning & Research (Federal Funds, State-directed); P=Pooled SP&R (Multi-State)

FHWA's Pavement Technology Program

The data collected from test tracks, in-service test roads, and pavement testing machines complement and, in many cases, contribute directly to the work of FHWA's current pavement technology program. FHWA's integrated, three-pronged effort includes the long-term pavement performance (LTPP), Superpave^TM implementation, and performance-related specifications (PRS) programs.

The LTPP program is a 20-year study of in-service pavements across North America. The program's primary objective is to increase pavement service life by investigating both new and rehabilitated pavement structures using different materials and operating under different loads, environments, subgrade soils, and maintenance programs.

FHWA is also working to build better pavements through its Superpave^TM implementation program. Superpave^TM is an asphalt pavement mixture design and analysis system developed under SHRP. The system allows an asphalt mix to be designed to accommodate a location's specific traffic loads and climatic conditions, resulting in pavements that better resist rutting and cracking. FHWA is currently spearheading a national program to encourage the adoption and use of the Superpave^TM system. Finally, the PRS program's objective is to develop improved construction specifications, based on test results that have been shown to be quantitatively related to pavement performance. The goal is to develop a system for identifying and specifying the construction of a target or "as-designed" pavement and then to equitably reward or penalize the contractor for the quality of the "as-constructed" pavement.

Facilities Under Development

The FHWA and state centers will soon be joined by four new pavement testing facilities. The U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory is installing an accelerated pavement testing machine in its Frost Effects Research Facility (FERF) in Hanover, N.H. FERF is a large, specially designed chamber that allows researchers to investigate the combined effects of load and environment.

Kansas recently dedicated their accelerated loading facility at Kansas State University in Manhattan. The project is sponsored by a coalition that includes industry, academia, and state agencies.

Meanwhile, Texas DOT is developing a mobile load simulator. The simulator will use full truck axles and will achieve high loading rates by increasing the number of actual test wheel passes on pavement sections at normal loads rather than overloading axles as many simulators do.

Finally, the Federal Aviation Administration is developing a large accelerated testing facility at its Technical Center in Atlantic City, N.J. Researchers will use the facility to study the effects of aircraft loading on pavement structures and materials.

Teamwork Key to Success

As these new facilities join the ones already operating, pavement testing will play an increasingly critical role in road research. Achieving superior new roads depends more than ever on teamwork. The innovations of tomorrow cannot be designed in isolation, and so, FHWA recently created an expert task group to support implementation of the SHRP SuperpaveTM specifications through a coordinated national program of accelerated pavement testing. Success can only come from FHWA, the states, industry, and academia working together.

Terry M. Mitchell is a research materials engineer in the Pavement Performance Division at the Turner-Fairbank Highway Research Center in McLean, Va. He joined FHWA in 1971. He received bachelor's degrees in aeronautical engineering and mathematics and a doctorate in nuclear engineering from the University of Michigan.

For More Information About:

WesTrack
University of Nevada at Reno
Jon Epps, (702) 784-6873 (fax: (702) 784-1429; E-mail: epps@unr.edu)
Turner-Fairbank Highway Research Center, FHWA
Terry Mitchell, (703) 285-2434 (fax: (703) 285-2767; E-mail: terry.mitchell@fhwa.dot.gov)

FHWA's Pavement Testing Facility
Turner-Fairbank Highway Research Center
Ray Bonaquist, (703) 285-2629 (fax: (703) 285-3105; E-mail: ramon.bonaquist@fhwa.dot.gov)

Louisiana Pavement Research Facility
Louisiana Transportation Research Center
Bill King, (504) 767-9137 (fax: (504) 767-9108)

Indiana Accelerated Pavement Testing Facility
Indiana Department of Transportation
Brian Coree, (317) 463-1521 (fax: (317) 497-1665)

California Accelerated Testing Program
New Technology Research Program
Wes Lum, (916) 654-9776 (fax: (916) 654-9977; E-mail: weslum@trmx3.dot.ca.gov)

Long-Term Pavement Performance Program
Turner-Fairbank Highway Research Center
Charlie Churilla, (703) 285-2355 (fax: (703) 285-2767; E-mail: charles.churilla@fhwa.dot.gov)

Minnesota Road Research Project (Mn/Road)
Minnesota Department of Transportation
Richard Sullivan, (612) 779-5509 (fax: (612) 779-5616)

Ohio SHRP Test Pavement
Ohio Department of Transportation
Bill Edwards, (614) 752-5272 (fax: (614) 752-4835)

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