Page 91 - MODELING OF ASPHALT CONCRETE

Overview of the Stif fness Characterization of Asphalt Concr ete 69

loading, thermal, and moisture conditions that are applied to pavements. The
principal mechanisms of distresses are fracture and flow. Fracture is predicted by
the use of fracture mechanics while flow is predicted by the use of the various forms
of plasticity theory. Both processes use energy in causing the asphalt to deteriorate
and this fact is used in damage theory, which describes in mathematical form how
the various components of energy are used in predicting the rate and magnitude
of the damage that is done by the loads and environmental stress that are applied to
the asphalt concrete. Because asphalt concrete is a viscoelastic material, the
components of energy that affect its response to loading are that which is stored and
can be recovered and that which is dissipated in a loading and unloading process.
Some of that energy is used in overcoming the viscous resistance of the material
while the rest of the dissipated energy is available to damage the material. The
correct representation, measurement, and use of asphalt concrete stiffness as a
material property are essential to being able to predict correctly this partitioning of
the energy.

Tests to Determine Stiffness
In order to be useful in mechanics- and computer-based numerical predictions of both
primary responses and distress of pavements, asphalt concrete stiffness must be a
material property and not an index property. In the laboratory, the material property
must be measured accurately, precisely, and repeatably by testing machines and
transducers that are operating within the response range of which they are capable
including response time or frequency, stroke, and magnitude. The test measurements
must be made in that part of the test specimen where there is a uniform stress field, a
uniform strain field, and the sample is held in a condition of uniform temperature and
moisture. Examples of this are measurements in the middle third of triaxial compression
and tensile tests and measurements in the center of the indirect tension test. It is only in
this way that the measured material property can be assured to be independent of test
apparatus, sample size, and geometry. Failure to do this produces large variances of the
test results, low repeatability, large variances in the predicted results, and higher levels
of risk to those who must rely upon them.
Material properties such as asphalt concrete stiffness can be back-calculated from
tests where there is no uniformity of stress, strain, temperature, or moisture but this
requires the use of a computer program that is based upon the mechanics of the sample
loading and geometry and may need to include the buildup of damage in all three
dimensions. The material properties that are determined by this indirect method cannot
be determined directly from the test measurements and this is a major source of their
higher variability.
The best tests to use for determining the asphalt concrete stiffness and, in fact, all
of the material properties of asphalt concrete are those tests in which both the load
and the displacement of the test sample are both known precisely by actual
measurement on the sample and by the use of feedback control of the test with such
rapid response that it is much faster than the loading or frequency rate being used in
the test.
These are general principles. It is understood that nondestructive tests in the field
will always require the use of back calculation in order to arrive at material properties.
This is inherently the reason for the larger variances of the results.