CHAPTER 8
PAVEMENT STRESS ANALYSES
8.1 General
The mechanistic approach for evaluation and design of pavement sys
tems contains an important empirical component. It relies on empirical
relationships between pavement response and pavement performance. In
general, two relationships are used, one for predicting cracking of
bound layers (e.g., asphalt concrete) and one for predicting permanent
deformations (roughness or rutting) of the base course and subgrade
layers. These forms of deterioration are, respectively, referred to as
structural and functional by Ullidtz and Stubstad (123). The horizontal
tensile stress or strain at the bottom of the asphalt concrete and the
vertical strain or stress on top of the subgrade are both considered in
the evaluation of flexible pavements. Essentially, these relate to the
bearing capacity and riding quality, respectively, of the pavement. As
suggested by Ullidtz and Stubstad (123), it would be preferable to
interrelate the two relationships such that cracking is considered in
the model for predicting roughness and rutting. However, this is seldom
attempted in practice.
Despite the empirical component, the mechanistic design process has
obvious advantages over existing empirical methods which are based on
the correlation between the maximum deflection under a load and pavement
performance. The use of maximum deflection as an indicator of struc
tural capacity may be misleading, depending upon the stiffness of the
pavement layers relative to the subgrade moduli. For example, a stiff
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