Page 111 - MODELING OF ASPHALT CONCRETE

CHAPTER 4

Complex Modulus

Characterization of

Asphalt Concrete

Terhi K. Pellinen

Abstract
There are two complex modulus tests that have been used for characterization of asphalt
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mixtures in the United States: The dynamic modulus |E | test and the dynamic shear
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modulus |G | test, better known as the simple shear tester (SST) shear frequency sweep
test. The dynamic modulus test has been selected to be used in the new AASHTO pavement
design guide and it will replace the resilient modulus test currently used in pavement
design. A new test protocol development for the dynamic modulus test has involved
improving specimen instrumentation techniques and analysis methods for the test data.
Research has shown that the modulus is less sensitive for test data imperfections than the
phase angle when obtained using different signal analysis methods. However, a limit for
feedback waveform for deviations of a perfect sine wave would improve data quality for
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both the modulus and phase angle. Comparison between uniaxial |E | and SST shear
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modulus |G | has shown that measured shear modulus was 2 to 30 times lower than
theoretically would be expected, and deviations increased as the test temperature increased.
Also, the phase angle values obtained from the SST testing were substantially higher than
that of the uniaxial testing. It is assumed that instrumentation and specimen size problems
in the SST testing are contributing to the differences. Also, differences in the specimen
loading modes may have affected obtained parameter values. A new method of constructing
mastercurves for cyclic modulus data has been developed that uses a sigmoidal fitting
function and experimental shifting. The sigmoidal function approaches asymptotically to
the limiting mix stiffness at cold and warm temperatures. At cold temperatures, the limiting
stiffness is dependent of the glassy modulus of the binder and at warm temperatures it is
dependent of the modulus of aggregate skeleton. Research has also shown that the stiffness
correlates well with rutting and fatigue cracking and could be used as a simple performance
test to complement the Superpave volumetric mix design method.

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