CHAPTER 1
INTRODUCTION
1.1 Background
In recent years, the use of layered elastic theory to evaluate and
design highway and airfield pavements has become increasingly popular
vis-a-vis existing empirical methods. The elastic layer approach, also
called mechanistic analysis, has obvious advantages over empirical
methods which are based on the correlation between the maximum deflec
tion under a load and pavement performance. It allows a rational eval
uation of the mechanical properties of the materials in the pavement
structure.
An essential part of the mechanistic process is determining real
istic elastic modulus values for the various layers in the pavement
structure. Current methods to determine the modulus of pavement
materials include various laboratory testing procedures, destructive
field tests, and in situ nondestructive tests (NDT). The problems
associated with the simulation of in situ conditions such as moisture
content, density, loading history and rate of loading of the pavement in
the laboratory are well known and recognized. Destructive field tests,
such as the California bearing ratio (CBR) and plate tests are expen
sive, time-consuming, and generally involve trenching the pavement,
which has to be subsequently repaired.
Nondestructive testing generally involves applying some type of
dynamic load or shock waves to the surface of the pavement and measuring
1