Page 116 - MODELING OF ASPHALT CONCRETE

94 Cha pte r F o u r

FIGURE 4-5 Loading pattern for compressive dynamic modulus testing.

of the asphalt mix. They used test temperatures ranging from 15 to 55°C and frequencies
from 25 to 0.1 Hz.
In the original ASTM protocol, capped cylindrical specimens, having a height to
diameter ratio of ≥2, are loaded with a uniaxial compressive haversine stress pattern
shown in Fig. 4-5. Testing is performed in load control keeping the load constant
throughout the frequency sweep. However, load levels are decreased as a function of test
temperature to keep measured strain levels small and to achieve steady-state straining
pattern after the initial creep, that is, recoverable and nonrecoverable (permanent) strain
accumulation, of the specimen.
This same loading approach was used by Witczak et al. in their testing program at
UMd, where they created a large database that was used to develop the predictive dynamic
modulus equation (latest form by Andrei et al. 1999), which will be used in the new
pavement design guide as an initial approximation of the stiffness of asphalt mixtures.
In recent years, the simple performance test and the new pavement design guide
work has led to a new research and protocol development for the dynamic modulus
test. The next sections discuss protocol issues that should be addressed when conducting
cyclic sinusoidal testing.

Specimen Fabrication and Instrumentation Different instrumentation techniques have
evolved in the testing over the years. In the early development of the test, strain gauges
were used to measure displacements, whereas today, the deformations are measured
using spring-loaded LVDTs. In the early approach used by Witczak et al. at UMd, these
LVDTs were clamped vertically on diametrically opposite specimen sides.
A new method of specimen instrumentation has been developed by the Superpave
Models Management research team (Witczak et al. 2000). Axial deformations are
measured with LVDTs mounted between gauge points glued to the specimen, as shown
schematically in Fig. 4-6. LVDTs are secured in place using brackets screwed to the
studs. In addition, guiding rods, which prevent the LVDTs from bulging out at high
temperatures, were added to the instrumentation. This instrumentation allows the use
of membrane around the specimen for confined testing. It also prevents any constraining
that the clamp around the specimen may have at higher temperatures. The deformations
are measured at a minimum of two locations 180° apart; however, three locations located
120° apart are recommended to minimize the number of replicate specimens required
for testing. This new approach has been found to be mandatory for asphalt specimens
that exhibit large nonrecoverable deformation during testing.
Compaction methods for fabricating test specimens in the laboratory have varied
over the years. The early compaction methods included kneading compactor and rolling