load in any pavement structure. Thus, the impulse load of a FWD is not

an exact representation of a moving wheel load. Moreover, the response

from the impact testing technique is similar to other types of dynamic

deflection testing in the sense that it represents characterization of

the entire structure. The technique does not provide information that

readily separates the effects of its various layers. Finally, the

parameters that cause plastic deformation in the structure are not

readily determinable from impact testing (79).

 2.3.2.5 Wave Propagation Technique. Wave propagation provides

methods for the determination of the elastic properties of individual

pavement layers and subgrades. Unlike the three previous methods of

NDT, these methods are not concerned with the deflection response of the

pavement. Rather, they are concerned with the measurement of the velo-

city and length of the surface waves propagating away from the load

surface (127).

 There are two basic techniques for propagating waves through pave-

ment structures: (1) steady-state vibration tests and (2) seismic

(impulse) tests. Generally, three types of waves are transmitted when a

pavement surface is subjected to vibration. These are

 1. Compression or primary (P) waves,

 2. Shear (S) waves, and

 3. Rayleigh (R) waves.

The P and S waves are body waves while the R wave is a surface wave.

Raleigh waves are the dominant waves found in the dissipation of energy

input from a vibrator on a semi-infinite half-space (75). Also, because

P and S waves attenuate rapidly with radial distance from the vibration