Journal Published Online: 01 November 1989
Volume 17, Issue 6

Nondestructive Testing: Interpretation of Deflection Bowl for Falling Weight Deflectometer Tests on Flexible Pavements



Nondestructive testing methods such as Dynaflect or Falling Weight Deflectometer (FWD) provide dynamic or impulse deflection data which represent a deflection basin of the flexible pavement under the test load. The magnitude of the deflections and the shape of the deflection basin in relation to the magnitude and configuration of the test load provide excellent data for a diagnostic “probe” of structural pavement strength at the location and time of testing. Meaningful comparison between tests at different times or locations requires adjustment of the measured and processed data for a standard temperature and a standard test load.

A program (PROBE) has been developed which implements this fundamental consideration for the FWD test. Different versions of the PROBE calculate the following primary response parameters under the test load: curvature and horizontal strain in the asphalt concrete (AC) layer; elastic stiffness or modulus of the subgrade; and vertical deflection, stress, and strain on top of the subgrade. In its backcalculation feature, based on a calibrated method of equivalent layer thickness (ELT), the program determines the combined effect of AC and base layer stiffnesses on the subgrade. The horizontal AC strain is computed kinematically (i.e., directly via the curvature of the deflection basin).

With regard to the accuracy of actual response parameters, the program has been tested against numerous forwardly calculated cases, using linear-elastic layer theory as a benchmark. It has also been tested on FWD field test data from a test site in Ontario.

All response parameters influenced by changes in the AC temperature are adjusted for a chosen standard AC temperature, and new concepts of modelling such adjustments are introduced. Differences are discussed between results from ideally elastic and actual soil materials as used at a test site in Ontario.

Author Information

Jung, FW
Ministry of Transportation Ontario, Downsview, Ontario, Canada
Pages: 11
Price: $25.00
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Stock #: JTE11140J
ISSN: 0090-3973
DOI: 10.1520/JTE11140J