The response of the pavement to the impulse loading is normally

measured with a set of geophones placed at varying radial distances from

the center of the plate. These deflection measurements can, in princi-

ple, be used to characterize the structural properties of the pavement

layers.

 Three manufacturers currently market impulse testing equipment in

the United States. These are the Dynatest, KUAB and Phoenix falling-

weight deflectometers. The Dynatest falling-weight deflectometer (FWD)

is the most widely used impulse loading device in North America and

Europe (109). Its newest version--the Dynatest 8000 FWD testing

system--was used in this study and will be described later in this

report. Other experimental impulse testing devices have been evaluated

by Washington State University and Cornell Aeronautical Laboratory (79).

 The impulse testing machines have several advantages over other NDT

instruments. The magnitude of the force can be quickly and easily

changed to evaluate the stress sensitivity of the pavement materials

being tested. Perhaps the greatest advantage is the ability to simulate

vehicular loading conditions. Several investigators (11,35,46) have

compared pavement response in terms of stresses, strains, and deflec-

tions from an FWD-imposed load to the response of a moving wheel load.

All these comparisons have shown that the response to an FWD test is

quite close to the response of a moving wheel load with the same load

magnitude. Figure 2.6 shows such an example of pavement response com-

parison.

 However, the deflection basin produced by an impulse loading device

is symmetrical about the load and Lytton et al. (63) have argued that

the deflection basin under a moving wheel is not symmetrical about the