Page 59 - Mechanics of Asphalt Microstructure and Micromechanics

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2.4.5 Static Creep (Flow Time F ) Test
t
In the static creep test, a quick axial load (0.05 s) is applied to a test specimen until an
axial stress, usually between 69 and 207 kPa, is achieved (Asphalt Institute, 2007).
This axial stress is then maintained until the total axial strain reaches approximately
2% (20,000 microstrain) or tertiary flow (strain rate increase zone) occurs in the test
specimen. In the primary zone the strain rate decreases, while in the secondary zone
the strain rate is constant. The flow time is the time when shear deformation occurs
under constant volume. It is also the minimum point where the rate of creep compli-
ance changes as a function of time is zero. Typically, higher values of flow time (F t ) are
associated with a better rutting resistance of the HMA mixture in the pavement.

2.4.6 Dynamic Creep (Flow Number F ) Test
n
Dynamic creep testing is conducted using repeated axial stress loading cycles (usually
10,000 cycles at 63–207 kPa) and the resulting axial strain (cumulative permanent strain)
is measured (Asphalt Institute, 2007). The test lasts for approximately 3 h or until exces-
sive tertiary deformation is observed in the specimen. The axial stress is applied to cy-
lindrical specimens, 101.6 mm in diameter and 150 mm in height, for 0.1 s (loading)
followed by a rest period of 0.9 s. The starting point, or cycle number, at which tertiary
flow occurs is referred to as the flow number (F n ).

2.4.7 Flexural Fatigue Test
The flexural fatigue test is conducted to evaluate the fatigue life of a small HMA beam
specimen (380 mm long x 50 mm thick x 63 mm wide) by subjecting it to repeated flex-
ural bending until failure. The beam specimen is prepared either in the laboratory or is
sawed from field compacted HMA. The test is conducted in accordance to the proce-
dures in AASHTO T 321 as repeated haversine loads (0.1-second load and 0.4-second
unload) are applied at the third points of the beam specimen. Deflection is measured at
the center of the beam. The flexural fatigue test can be conducted in controlled stress or
controlled strain mode.
Flexural stiffness (S) can be calculated with the following formula:
(2-25)
S = s t /e t
where s t = maximum tensile stress (Pa)
e t = maximum tensile strain
2.4.8 Shear Modulus Test
The shear modulus test, provided by the AASHTO T 320 standard, is used to describe
the viscoelastic behavior of an asphalt mix. In this test, a sinusoidal shear strain of
0.01% (peak-to-peak) is applied to a Superpave-gyratory-compacted specimen at 10
frequencies (0.031 to 10 Hz). Specimens are usually 150 mm in diameter and 50 mm in
height. A diameter-to-height ratio of 3:1 or greater should be used for all specimens.
The obtained shear stress and strain are then used to calculate the complex shear mod-
ulus (G*) of the asphalt mixture. This test can be used to determine the effect of loading
frequency (traffic speed) and temperature on an asphalt mixture. The results of this test
are used as an input into the mechanistic-empirical design guide for pavement thick-
ness design.