ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 336

During testing, one of the parallel plates is oscillated with respect to the
other at preselected frequencies and rotational deformation amplitudes
(or torque amplitudes). The required amplitude depends upon the value of
the complex shear modulus of the asphalt binder being tested. Specification
testing is performed at an angular frequency of 10 rads/s. The complex
shear modulus and phase angle are calculated automatically by the
rheometer's computer software. The complex shear modulus and the phase
angle define the asphalt binder's resistance to shear deformation in the linear
viscoelastic region.

G * 1 2 1 d 2

Bending Beam Rheometer Test The bending beam rheometer measures the
midpoint deflection of a simply supported prismatic beam of asphalt
binder subjected to a constant load applied to its midpoint (ASTM D6648).
The bending beam rheometer test system consists of a loading frame, a con-
trolled temperature bath, and a computer-controlled automated data acqui-
sition unit, as shown in Figure 9.17. The test temperature is 10°C higher
than the lower temperature rating of the binder (e.g., a PG 64-22 is tested
at ). The sample is conditioned in both the rolling thin-film oven and
pressure-aging vessel prior to testing. An asphalt binder beam is placed in
the bath and loaded with a constant force of for 240 seconds.
As the beam creeps, the midpoint deflection is monitored after 8, 15, 30, 60,
120, and 240 seconds. The constant maximum stress in the beam is calcu-
lated from the load magnitude and the dimensions of the beam. The maxi-
mum strain is calculated from the deflection and the dimensions of the
beam. The flexural creep stiffness of the beam is then calculated by divid-
ing the maximum stress by the maximum strain for each of the specified
loading times.
The low-temperature thermal-cracking performance of paving mixtures is
related to the creep stiffness, defined as the slope of the logarithm of the creep
stiffness versus the logarithm of the time curve of the asphalt binder con-
tained in the mix.

- 12°C

980 ; 50 mN

Deflection transducer

Asphalt beam
original position

Asphalt beam
deflected position

FIGURE 9.17 Schematic of
the bending beam rheometer.