New Calibration Procedure Solves South Africa’s FWD Woes

If two devices that measure the same properties produce dramatically different results, how do you know which is right? The South African Department of Transport (DOT) faced exactly that problem when the country’s two falling weight deflectometers (FWD) began generating dramatically different readings in 1993. Extensive tests failed to explain the difference, making it impossible to know which was supplying incorrect data. The DOT called off all further deflection testing until the problem could be fixed.

Louw Kannemeyer of the South African DOT suggested that the solution might be the reference calibration procedure, which was developed under the Strategic Highway Research Program (SHRP) for use by the long-term pavement performance (LTPP) program. Kannemeyer knew that highway agencies in the United States had set up regional calibration centers to solve similar problems. The calibration procedure involves comparing readings from an FWD against independent references that meet National Institute of Standards and Technology benchmarks.

The DOT followed Kannemeyer’s suggestion and specified that FWDs must be calibrated within the 2 months preceding a network-level survey.* Rather than set up its own calibration center, the DOT required that FWDs be calibrated by an independent organization.

As a result, the two companies providing FWD testing on South Africa’s 7,000-km (4,300-mi) highway network teamed up to award the South African Bureau of Standards (SABS) with a contract to construct and operate an FWD calibration center. SABS then approached Dynatest, a U.S. vendor of FWDs, for assistance in setting up the calibration facility and obtaining the necessary manuals, software, and equipment. Construction of the center, located in Pretoria, is now under way.

According to Trenton Clark of PCS/Law Engineering, which sets up calibration centers for highway agencies and the private sector, a growing number of countries now have FWDs, and they recognize the need to keep them calibrated. Australia has already purchased the equipment needed to build a calibration center, and Denmark and several other countries have requested information on the FWD calibration procedures.

For more information on FWD calibration or on setting up an FWD calibration center, contact Max Grogg at FHWA (telephone: 518-431-4224, ext. 223; fax: 518-431-4208; email: max.grogg@fhwa.dot.gov). *

*The pavement management systems used by highway agencies differentiate between project-level testing (used for a particular site) and network-level testing (used for an entire network of highways).

Deflection testing is used to help highway agencies determine the optimal strategy for maintaining and repairing pavement. The test equipment is housed on a trailer, which is towed to the test site. The test involves dropping a heavy weight on the pavement and measuring how much the pavement deflects, or moves, in response.

A properly calibrated falling weight deflectometer is essential to a pavement management system.

Highway agencies and others in the United States can take their FWDs to any one of the four regional calibration centers:

• Minneapolis, Minnesota (phone: 612-779-5542; fax: 612-779-5535)
• Carson City, Nevada (phone: 702-888-7173; fax: 702-888-7501)
• Harrisburg, Pennsylvania (phone: 717-783-6857; fax: 717-783-6013)
• College Station, Texas (phone: 512-465-7653; fax: 512-465-3681)

The centers are operated by the highway agencies in the host States.

More information on the services provided by FWD calibration centers can be found in Guidelines for Users of the FWD Calibration Centers (FHWA-SA-95-038). The publication is available from FHWA’s Research and Technology Report Center (phone: 301-577-0818; fax: 301-577-1421).

An online discussion group provides an easy means of sharing information with, and posing questions of, fellow FWD users. The group uses email to bring together FWD users from around the world to discuss common concerns and improve practice. To join, send an email message with your name, organization name, and position or title to Egbert Beuving at E_Beuving@compuserve.com.

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