Page 89 - MODELING OF ASPHALT CONCRETE

Overview of the Stif fness Characterization of Asphalt Concr ete 67

of the test data. Test specimen geometry in situ is usually dictated by the pavement
structure. In the laboratory, there are many more options such as uniaxial tension and
compression, triaxial tests in compression and extension (Chaps. 4, 7, and 11), various
forms of shear tests (Chap. 10), bending and torsion tests, and indirect tension tests
(Chap. 5) of a test sample. Some test specimens are constructed and tested in the
laboratory with load, layer, and test conditions that are intended to be severe simulations
of field conditions such as moisture or temperature exposure. These are commonly
referred to as torture tests. It is uncommon to be able to extract material properties, and
asphalt concrete stiffness specifically, from torture tests. Torture tests are commonly
used as screening tests for the suitability of the composition of the asphalt concrete.
Other test specimens are constructed with such simple geometries that the direct
measurement of both stresses and strains make the direct determination of asphalt
stiffness possible. Examples of these are the uniaxial and triaxial tension and compression
tests and torsional tests on cylindrical samples. These tests are characterized by a high
degree of accuracy, precision, and repeatability, as will be shown in subsequent
chapters.

Loading Pattern
A variety of loading patterns are used to measure asphalt concrete stiffness under a
variety of loading conditions. These include monotonic loading of both stress-controlled
or strain-controlled tests, frequency sweep tests, impulse and wave propagation tests,
repeated load tests, creep, relaxation, and creep and recovery tests. A creep test is one in
which the applied stress is held constant and the strain is measured as it grows with time.
The ratio of the strain divided by the constant stress is termed the creep compliance. A
relaxation test is one in which the applied strain is held constant and the diminishing
stress is measured with time. The ratio of the stress divided by the constant strain is the
relaxation modulus.

Rate, Temperature, and Age
The stiffness of asphalt concrete rises as the rate of loading increases and it decreases as
the temperature increases. The age of the asphalt concrete is determined less by its
chronological age than by its exposure to air, heat, and solar radiation conditions which
will increase its rate of reaction with oxygen and make its stiffness increase along with
its susceptibility to brittle fracture.

Moisture
The stiffness of asphalt concrete is affected by the amount of moisture that is held within
the asphalt binder; the solubility of various components of the asphalt; the amount of
asphalt that becomes emulsified; the strength of the adhesive bond between the asphalt
and aggregates, with and without water present on the interface; and the strength of the
cohesive bond within the asphalt, with and without water present on the surface of
microcracks within the asphalt. The fact that asphalt can absorb water within the thin
films of mastic that are present within a mixture, together with the fact that different
asphalts can absorb widely differing amounts of water at a given level of water vapor
pressure, makes the effect of water on the stiffness of asphalt concrete very highly
dependent on the composition of the bitumen. It is also a fact that the rate at which
water can diffuse through asphalt films differs greatly with the composition of the
bitumen.