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Publication Number: FHWA-HRT-06-132
Date: December 2006

Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements

Chapter 2. Fwd-Related Terminology

Use of proper terminology greatly eases troubleshooting and problem reporting. This manual uses specific terminology to refer to specific parts of the Dynatest® model 8002 FWD, which is the type operated by LTPP. Some of these terms also apply to other types of FWDs. This list is complete only for this manual. Parts not included here may be found in the owner’s manual.

Buffer—A rubber block attached to the underside of the weight package to control the shape of the generated load pulse. On LTPP FWDs, the buffers are roughly cylindrical, and there are four mounted on the weight package.

Control Box—Contains connectors for the geophones, load cell, temperature sensor, and other sensors mounted on the FWD. Located on the FWD trailer, the control box sends these signals to the signal processor located in the tow vehicle through the multisignal cable. The control box also has buttons for manual control of the FWD hydraulics.

Geophone—Device used to measure deflection. It is yellow, roughly cylindrical, and about 25 mm (0.98 inch) in diameter and 50 mm (1.97 inches) high. Geophones are mounted in springloaded sensor support brackets suspended along the sensor bar. Each geophone has a unique serial number that is used to identify critical calibration information in the FWD data collection software. The LTPP FWDs use 2,000-micron (80-mil, or 0.8-inch) geophones.

Load Cell—Measures the force imparted to the pavement by the FWD. The load cell is located directly above the load plate and below the swivel. The load cell has a serial number, which is visible from the rear.

Load Plate—Directly contacts the pavement surface to transmit the load. The type used by LTPP is solid, and it has a 300-mm (11.81-inch) diameter. It consists of three layers: the topmost is steel, the middle is polyvinyl chloride (PVC), and the bottommost is a ribbed rubber sheet.

Multisignal Cable—Carries electrical signals from the control box to the signal processor. These signals include the outputs from the transducers on the FWD and the command signals for the FWD hydraulics.

Signal Processor—Connects the trailer control box through a multisignal cable to the data collection computer using an RS-232 serial cable. The LTPP FWDs use a Dynatest 9000 signal processor located in the tow vehicle.

Strike Plate—A flat-surfaced column above the load plate. The weight package is dropped on the strike plate during operation of the FWD. Braces and a cylinder, housing the center geophone, and to which the swivel attaches, are welded to the underside of the strike plate.

Sensor Bar—A long bar mounted above the load plate where the geophone holders are mounted. An extension of the sensor bar continues behind the load plate toward the trailer hitch to allow a geophone to be mounted behind the load plate.

Swivel—Connects the load cell to the strike plate. It allows the load cell and load plate to rotate to provide good contact with the pavement surface.

Transducer—Any measurement device that converts a physical response into an electrical signal. Transducers on an FWD include the load cell, deflection sensors, and temperature sensors.

Weight—Removable steel weights mounted to the weight package. Their mass is 20 kilograms (kg) (44.1 pounds (lb)) each.

Weight Package—The entire assembly that is raised and then dropped to generate load.

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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