Page 88 - MODELING OF ASPHALT CONCRETE

66 Cha pte r T h ree

stiffness in the laboratory and in the field, the composition and condition factors that
stiffness depends upon, and finally the methods of mathematical characterization of
asphalt concrete stiffness in both the damaged and undamaged states.

Asphalt Concrete
Asphalt concrete is a composite material made up of aggregate particles, bitumen, air,
and other components such as additives, modifiers, fines, and water in either liquid or
vapor forms. The emphasis in this chapter is that the material is a composite and its
composition is important in determining its useful properties for engineering and
construction applications.

Asphalt Concrete Stiffness
Asphalt concrete stiffness is a material property. Precisely, it is a slope of a stress-strain
curve of the asphalt concrete. What is unique about a material property is that it is
independent of the test apparatus, or the sample size or geometry that is used to
measure it. The converse of this is that if a measured result of a test does depend upon
the test apparatus or the sample size or geometry, then that measured result is not and
cannot be a material property. There are several types of stress-strain curves from which
asphalt concrete stiffness can be measured as a material property.

Methods of Measuring Stiffness
Asphalt concrete stiffness can be measured either in the laboratory, as will be discussed
in Chaps. 4 and 5, or in the field. The geometry of the test specimen is important because
it determines whether the material property can be measured directly with instruments
mounted on the sample or must be inferred by analyzing the response of the test sample
to the imposed load. The loading pattern that is applied to the test sample also
determines what the measured stiffness will be. The rate of loading and the temperature
and moisture at which the loading is applied will also determine the measured stiffness.
Finally, the age of the test sample will also affect the measured stiffness.

Location of Test
In the field, asphalt concrete stiffness may be measured either destructively or
nondestructively. In the laboratory, tests can be conducted on cores taken from the field
or from samples that were compacted in the laboratory. Destructive tests in the field
include “small aperture” testing instruments such as cone penetration or pressure meter
devices or “large aperture” methods such as test pits, or accelerated loading tests using
full-scale or scale-model vehicle simulation devices. Nondestructive tests include static,
cyclic, impulse, and surface wave instruments. In general, the test data from all of these
devices must be analyzed by some form of inverse analysis in order to produce a
material property of the asphalt concrete, and specifically, its stiffness.

Geometry of the Test Specimen
The geometry of a test specimen, either in the field or in the laboratory, is an important
consideration because if some geometries are selected, material properties can be
inferred directly from the test measurements without the need for the inverse analysis