Page 50 - Mechanics of Asphalt Microstructure and Micromechanics

Mechanical Proper ties of Constituents 43

resulting forces are measured and converted to stresses. G I is calculated from the area
under the load displacement curve to failure using the following formula:
∂ W
G (2-20)
I ∂
Ba
where ∂W = area under the load-displacement curve
B∂a = specimen area in the plane
The test procedure is covered by ASTM D 2094.

2.1.9.9 Monotonic Test
Monotonic, recovery, and cyclic behaviors of pure asphalt bitumens were studied by
Ossa et al. (2005a) to develop a simple phenomenological model. These tests are de-
scribed below. Mean stress, axial strain, and accumulated strain are determined in rela-
tion to permanent deformation.
The monotonic stress versus strain behavior of AC was determined from the con-
stant strain-rate and constant stress creep tests. In the constant strain-rate test, a certain
·
uniaxial tensile strain rate, e is applied to the specimen and the resulting nominal ten-
sile stress s and strain e are recorded. In the constant stress creep test, a constant nomi-
nal tensile stress is applied instantly to the specimen and the nominal tensile strain e is
recorded as a function of time, t.
For this study, a hydraulic testing machine was employed for the tensile tests on
dumbbell-shaped specimens. The nominal stress in the specimen was defined by the
load measured with a 2-kilonewton load cell, while the load line displacement defined
the nominal strain. The specimen grips were diametrically split cups whose inner sur-
faces were shaped to match the heads of the dumbbell specimens. Tests have been per-
formed at –5, 0, 10, and 20°C, which are representative operating temperatures in the
United Kingdom. Samples were conditioned for 2 hours in an environmental chamber
prior to testing.

2.1.9.10 Creep Recovery Test
In the creep recovery tests, a series of single load/unload tests are applied to the pure
bitumen or mastic to investigate their creep recovery behavior. A stress, s, is applied
very quickly to the specimen and then held constant, and the material is allowed to
creep to a specified total nominal tensile strain, e . At this strain, the loading stress is
T
·
released with the tensile strain monitored until the strain rate is zero, e 0. The strain
r
T
r
pl
at this point, e = e − e , is the irrecoverable strain and e is the recoverable strain. Such
tests are repeated for a series of strains, e , and creep stresses, s, using the same speci-
T
mens as for monotonic tests.
2.1.9.11 Continuous Cyclic Test
In the continuous cyclic tests, the nominal tensile stress s is varied between s min and
s max . The load levels are calculated with the formulae provided below:
σ σ + σ
R min ; σ min max (2-21)
σ m 2
max
In this test, the loading rate is defined by the frequency, f, of the triangular wave-
form. The nominal tensile strains are measured as a function of time and the tests are