48
correlations between the dilatometer modulus and CBR value, the use of
the CBR test makes the study empirical, to say the least.
2.4 Factors Affecting Modulus of Pavement-Subgrade Materials
2.4.1 Introduction
The response characteristics of flexible pavement materials is a
complex function of many variables, which is far-fetched from the ideal
materials assumed in classical mechanics. In general, the behavior of
these materials is dependent upon many environmental and load vari
ables. Specifically, the asphalt concrete response is primarily a
function of temperature and rate of loading. Due to its viscoelastic
nature (7,51), asphalt concrete materials become stiffer as the load
rate increases and the temperature decreases. The granular base course
and subgrade characteristics are dependent upon moisture content, dry
density, stress level, stress states, stress path, soil fabric, stress
history, and soil moisture tension (59,78,133).
Several researchers have presented relationships of resilient
modulus as a function of one or more variables, while keeping others
fixed or completely ignored. Most of these relationships were developed
from laboratory studies. A complete review of the relative effects of
the various factors on pavement-soil response, or the relationship
between modulus and other parameters measured in the laboratory can be
found in References 31, 44, 59, 78, 94, and 133. It is not the intent
of this discussion to review the various studies on this topic. The
discussion below will concentrate on two variables that are believed to
be very important in flexible pavement technology, especially when con
sidering NDT and pavement evaluation. These factors are the temperature