Falling Weight Deflectometer (FWD) Field Survey

Falling weight deflectometers (FWDs) have been in use since the 1980s. These devices are used to measure pavement deflections in response to a stationary dynamic load, similar to a passing wheel load. The data obtained are used to evaluate the structural capacity of pavements for research, design, rehabilitation, and pavement management purposes.

The basic components of a FWD are defined by reference documents such as AASHTO and ASTM standards. For example, in ASTM D4694-96, the apparatus described is composed of the following (“Standard Guide for General Pavement Deflection Measurements” 2005, pp. 487–488):

  • An impulse-generating device with a guide system. This device allows a variable weight to be dropped from a variable height.
  • Loading plate, for uniform force distribution on the test layer. When the weight affects this plate, this loading plate ensures that the resulting force is applied perpendicularly to the test layer’s surface.
  • A load cell for measuring the actual applied impulse.
  • One or more deflection sensors. (Note: Deflection basin tests require at least seven sensors.)
  • A system for collecting, processing, and storing deflection data. Recorder systems, discussed in Section 8 of ASTM D4694-96, should display and store load measurements with a 200 N (45 lbf) resolution. Such systems should display and store deflection measurements with ±1 μm (0.039 mil) or less of resolution.

Most of the FWDs used by SHAs are either towed by a vehicle or are built into a vehicle’s cargo area.

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