Page 115 - MODELING OF ASPHALT CONCRETE

Complex Modulus Characterization of Asphalt Concr ete 93

FIGURE 4-4 Schematic of shear frequency sweep test.

SST-Shear Frequency Sweep Test
The shear frequency sweep test conducted with the SST was developed in the SHRP
research program. The test protocol was first introduced as SHRP Designation M-003:
“Standard Method of Test for Determining the Shear Stiffness Behavior of Modified and
Unmodified Hot Mix Asphalt with Superpave Shear Test Device” (Harrigan et al. 1994).
Later the test protocol was adopted by the American Association of State Highway and
Transportation Officials (AASHTO) as a Provisional Standard: AASHTO Designation:
TP7-94 (AASHTO 1994).
The shear frequency sweep at constant height is a strain-controlled test; the
maximum shear strain is limited to 100 microstrains. During the test, a horizontal
shear strain is applied at a frequency of 10 to 0.01 Hz using a sinusoidal straining
pattern. At the same time the specimen height is kept constant by compressing or
pulling the specimen axially based on the closed loop feedback given by the vertical
LVDTs (linear variable differential transformer) attached to the sides of the specimen.
The specimen is sheared from the bottom as Fig. 4-4 schematically presents. The
cylindrical test specimen diameter is 150 mm, height 50 mm and it is glued between
two aluminum platens. Based on the TP7 protocol, testing is conducted at 4, 20, and
40°C, but higher temperatures have been used. The strain control mode in both
actuators makes the test difficult to run and very soft mixtures can cause severe control
problems at high temperatures. Also, test temperatures are limited above 4°C because
at colder temperatures mix stiffness may exceed the stiffness of the glue and specimens
may shear off the platens.

Compressive Dynamic Modulus Test

Background The complex dynamic modulus test was originally adopted in 1979 by the
American Society of Testing and Materials (ASTM) as a standard method: “Test Method
for Dynamic Modulus of Asphalt Concrete Mixtures” ASTM D 3497-79 (ASTM 1979).
The test was conducted applying a haversine load between 0 and 241 kPa using
temperatures of 5, 25, and 40°C and frequencies of 1, 4, and 16 Hz.
Additional temperatures and frequencies have been added to the protocol by
Witczak et al. at University of Maryland (UMd) (Witczak et al. 1996; Witczak and
Kaloush 1998; Pellinen and Witczak 1998) to enable construction of a full mastercurve