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Publication Number: FHWA-SA-96-019
Date: August 1996
The falling weight deflectometer (FWD), originally used for pavement research, has become an important tool for State highway agencies' pavement management systems. In Arizona, FWD data are crucial to the majority of pavement design and rehabilitation work. . According to Larry Scofield of the Arizona Department of Transportation (DOT), such accuracy would not be possible without the Federal Highway Administration's (FHWA) regional calibration centers, which use the reference calibration method developed by SHRP researchers.
Arizona DOT has one FWD for measuring the structural strength of the roughly 26,000 lane-kilometers (16,000 lane-miles) of pavement the agency oversees. Data collected by FWD tests are used extensively in Arizona's pavement management system, both at the network level and at the individual project design level.
Accurate FWD data are particularly important at the project level, according to Scofield. "About 75 percent of the pavement work Arizona DOT does is overlays," he says. "The thickness of overlays is directly predicated by FWD tests. Therefore, it is critical that FWD data be accurate." Without accurate data, Arizona DOT pavement designers might recommend an overlay that was too thin for a site, making it insufficiently durable, or too thick, making it unnecessarily expensive.
Because FWD testing is so important to Arizona DOT's pavement management, the agency has made FWD calibration a routine part of its operations. The DOT staff regularly perform the relative calibration procedure, a technique developed by SHRP that checks whether the seismic sensors in an individual FWD produce consistent results and are functioning properly. But the relative calibration procedure can't show how readings from Arizona's FWD compare to data from other FWDs throughout the country. To find that out, once a year DOT staff make the 12-hour drive to FHWA's calibration center in Reno, Nevada, for reference calibration.
Reference calibration checks an FWD against independent references that meet National Institute of Standards and Technology (NIST) benchmarks. In addition to the Reno center, there are regional calibration centers in Harrisburg, Pennsylvania; Minneapolis, Minnesota; and College Station, Texas.
Without reference calibration, Arizona DOT could never be certain that its FWD results were valid. "Before SHRP developed reference calibration, there was really no way to determine whether data were accurate," Scofield explains. "The equipment manufacturers' ability to produce quality equipment had outstripped everyone's ability to test how accurate the equipment was. Imagine if you had the most accurate piece of equipment of its kind in the world. If it's off, how do you know?"
That's what makes the calibration centers "a real plum for SHRP," Scofield says. "Now that there is a standard calibration method that every State highway agency has access to, it guarantees a higher level of quality for our pavement tests."
Now that States are familiar with the advantages of having a perfectly calibrated FWD, many want to be able to run their own reference calibration tests. Arizona DOT is closely following research underway in Texas into an affordable means of performing the reference calibration procedure. If the process became easier and required less expensive equipment, Arizona and other States could check their FWDs against national benchmarks whenever engineers saw evidence of a problem.
"SHRP has raised the bar," Scofield says, explaining why the program has generated this desire for superior FWD accuracy. Now that the calibration centers and the reference calibration procedure have made it possible for State highway agencies to get highly accurate data, and therefore make better pavement management decisions, agencies want to recalibrate their FWDs more often. Eventually, Scofield predicts, all States will be able to perform some form of high-level calibration on their own equipment: "It's the natural progression toward quality that's been fostered by the SHRP calibration centers."
The falling weight deflectometer (FWD) has been an important tool for research into pavement design for more than a decade. More recently, the device has earned a major role in pavement management. The Strategic Highway Research Program (SHRP) adopted the FWD as a key piece of equipment for assessing the structural strength of long-term pavement performance (LTPP) test sections.
The FWD measures pavement structural strength and other factors by dropping a large weight to apply loads ranging from 7 kN to 120 kN (1,500 lb to 27,000 lb force) to a section of pavement. The degree to which a pavement deflects is measured with seven seismic sensors in the FWD. The test is often performed several times along the same section of road to ensure that the data accurately reflect the pavement's true condition.
The resulting data serve several functions. They provide the primary means of estimating the pavement's structural capacity, or strength. They also provide a basis for estimating material properties of a pavement section, as well as the degree of load transfer across joints or cracks in rigid pavements. And FWD tests give researchers some indication of how a test section as a whole compares with core samples of the test section assessed in the laboratory. Repeated FWD tests over time provide data that can be used to determine how a section of pavement is affected by changes in temperature and moisture.
These data are central to State's pavement management systems. Using data from FWD testing and other tools, State highway agencies can determine where pavement maintenance and rehabilitation work will have the greatest benefit. At individual projects, FWD data are used to design pavements of the needed strength and durability.
Under the LTPP program, FWD testing is used at all General Pavement Studies (GPS) and Specific Pavement Studies (SPS) test sites. Most sections are tested about once every 5 years. At selected sections, FWD tests are performed 12 to 14 times a year, every second year, to check for seasonal changes in the pavement's strength.
For more information, contact Debbie Walker, FHWA, 703-285-2745 (fax: 703-285-2767).
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