Civil Engineering Reference
In-Depth Information
bond between the asphalt binder and aggregate particles due to moisture
(stripping), or other factors. Therefore, most of the tests used to characterize
asphalt concrete try to simulate actual field conditions.
Many laboratory tests have been used to evaluate asphalt concrete prop-
erties and to predict its performance in the field. These tests are performed
on either laboratory-prepared specimens or cores taken from in-service
pavements. These tests measure the response of the material to load, defor-
mation, or environmental conditions, such as temperature, moisture, or freeze
and thaw cycles. Some of these tests are based on empirical relations, while
others evaluate fundamental properties. All tests on asphalt concrete are
performed at accurately controlled test temperatures and rates of loading,
since asphalt response is largely affected by these two parameters.
The Superpave tests used for mix design, as well as Marshall or Hveem
tests discussed earlier, have been used to characterize asphalt concrete mix-
tures. Other tests are also being used, some of which are standardized by
ASTM or AASHTO, while others have been used mostly for research. The
next several sections discuss some of the common tests.
■
9.10.1
Indirect Tensile Strength
When traffic loads are applied on the pavement surface, tension is devel-
oped at the bottom of the asphalt concrete layer. Therefore, it is important to
evaluate the tensile strength of asphalt concrete for the design of the layer
thickness. In this test, a cylindrical specimen 102 mm (4 in.) in diameter and
64 mm (2.5 in.) high is typically used. A compressive vertical load is applied
along the vertical diameter, using a loading device similar to that shown in
Figure 9.36. The load is applied by using two curved loading strips moving
with a rate of deformation of 51 mm/min. (2 in./min.). Tensile stresses are de-
veloped in the horizontal direction, and when these stresses reach the tensile
strength, the specimen fails in tension along the vertical diameter. The test is
performed at a specified temperature. With 12.5 mm (0.5 in.) loading strips,
the indirect tensile strength is computed as
2P
ptD
s
t
=
(9.24)
where
s
t
=
tensile
strength, MPa (psi)
at failure, N (lb)
of specimen, mm (in.)
of specimen, mm (in.)
P
=
load
t
=
thickness
D
=
diameter
■
Diametral Tensile Resilient Modulus
To evaluate the structural response of the asphalt pavement system, the
modulus of asphalt concrete material is needed. Since asphalt concrete is
not a linear viscoelastic material, the modulus of elasticity, Young's modulus,
is not applicable. The diametral tensile resilient modulus test (ASTM
9.10.2
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