Page 90 - MODELING OF ASPHALT CONCRETE

68 Cha pte r T h ree

Summary
With the stiffness of asphalt concrete varying with so many different factors and
conditions, there is the possibility of an endless round of confusion about how each
relates to the other and how to combine the effects of separate investigations of these
different effects on stiffness into a practical, usable predictable result. The secret to
combining all of these effects in a realistic synthesis is by the use of mechanics, as will
be discussed later in this chapter.

Importance of Asphalt Concrete Stiffness
Subsequent chapters present constitutive, mechanistically based models for describing the
behavior of asphalt concrete. An important theme in each of these models is the need for
accurate characterization of the fundamental material stiffness. Just as Young’s modulus is
paramount for predicting the deflection of steel beams in a structure, the stiffness of asphalt
concrete is critical for predicting the behavior of the material in pavement structures. The
materials that pavements are built with are very complex despite the fact that they are so
commonplace. Such complexity necessitates the use of numerical procedures, such as finite
elements, which require significant computation effort. Technology advances in recent
years have resulted in great improvements in computation speed, and in principle it is now
possible to predict the time of appearance and the rate of deterioration of asphalt concrete
distresses. That is not to say that all distresses can be predicted reliably at the present
time with existing models but it is now possible to construct a model that will provide reliable
predictions in a rapid manner. The only impediment to developing such a model is the
willingness to understand the mechanics of the selected type of distress, identify the relevant
materials properties, devise test methods that will provide these materials properties, select
the most appropriate numerical method to use, and assemble the computer model.
Having such a model available makes it possible to interpret correctly and
quantitatively the results of in-service pavement, test track, and accelerated pavement
tests and to extrapolate these results to other pavements. It also makes possible coordinated
laboratory tests that are simple, accurate, precise, and inexpensive with which to obtain
materials properties. It also makes possible the coordinated use of nondestructive testing
in the field that will provide on site measurements of the same materials properties that
are the central focus of all of these mechanics-related activities.
Computer-predicted distresses make possible and practical the use of performance-
based specifications, warranties, and construction quality assurance and quality control.
They also make possible the prediction of remaining life to allow the planning of
pavement maintenance and rehabilitation activities as well as the overall asset
management of the pavement network, including the effects of safety and cost profiles
with time. All of this is possible if stiffness, as a material property, is measured and used
properly in numerical prediction methods employing the computers that are now
capable of handling the computational tasks that pavements require.

Use of Stiffness in Computations
Asphalt concrete stiffness is used in numerical computations to calculate both the
primary responses and the mechanisms of distress. The primary responses are the
deflections, stresses, and strains in a pavement structure under the variety of