Structural Design of Pavements PART VI Structural ... - TU Delft

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3. Deflection measurement tools:
The deflection device that currently receives the highest popularity is the falling weight
deflectometer (FWD). Nevertheless other deflection measuring devices like the Benkelman
Beam (BB) and the Lacroix Deflectograph (LD) are still used at different places at the world.
Especially the Benkelman Beam deserves attention since this low cost device (the price is
approximately 1/30 th of the price of a falling weight deflectometer) is used in many
developing countries. The principles of these three devices are given elsewhere [1], here only
the main features will be described.
3.1 Falling weight deflectometer:
The principle of the FWD is schematically shown in figure 2.
Figure 2: Principle of the falling weight deflectometer.
A weight with a certain mass drops from a certain height on a set of springs (normally rubber
buffers) which are connected to a circular loading plate which transmits the load pulse to the
pavement. Load cells are used to monitor the magnitude and duration of the load pulse. The
magnitude of the load pulse can vary between the 30 and 250 kN depending on the mass of
the falling weight and the falling height. The duration of the load pulse is mainly dependent on
the stiffness of the rubber buffers. Usually pulse duration between 0.02 and 0.035 s are
measured.
The surface deflections are measured with so called geophones. These are velocity transducers
which measure the vertical displacement speed of the surface. By integration the displacements
are obtained.
Since the electronic circuits are only opened a very short moment before the weight hits the
buffers, the influence of passing traffic on the magnitude of the deflections is eliminated; only
the displacements due to the impact load are measured.
The advantage of the FWD is the short duration of the load pulse comparable to the duration
of the load pulse caused by a truck driving at approximately 50 km/h. Because of the short
pulse duration, the influence of viscous effects can be neglected.
One should however be cautious when the modulus of a saturated subgrade with a high
ground water level is determined from the deflection measurement results. In that case one
might measure the bulk modulus K of the subgrade which, in case of a fully saturated
subgrade, can be high. Because road materials are very much sensitive for shear, this high
bulk modulus value gives a wrong idea about the real stiffness of the material. This can be
illustrated with the following simple example.