On measurement of dynamic modulus for bituminous mixtures

Abstract

The AASHTO:TP-79 (2010), Standard method of test for determining the dynamic modulus and flow number for hot mix asphalt using the asphalt mixture performance tester (AMPT) stipulates the test procedure for the measurement of dynamic modulus of bituminous mixtures. In this test, the stress–strain–time data for different frequencies (from 25 to 0.01 Hz) are generated and used in the calculation of dynamic modulus. In this investigation, dynamic modulus data of four types of bituminous concrete mixtures (an unmodified binder and three modified binders) are studied using this protocol. The experiments showed that the total strain decreased with time at higher temperatures and at lower frequencies while at lower temperatures and higher frequencies, the total strain was seen increasing. It was also observed that at higher temperatures and low frequencies, the phase lag decreased with a decrease in frequency. While many studies have reported such results, these are normally called as ‘anomalous’ and ascribed to the errors in measurement mechanisms. These observations seem to have not been investigated within the context of the viscoelastic behaviour of the material. This study shows that the so called ‘anomaly’ is a response shown by a viscoelastic material. In the current study, the analysis of the data showed that the increasing and decreasing trend in strain can be attributed to two phenomena: the creep due to successive loading and the recovery of the residual deformation in the material due to a reduction in load level. It is demonstrated that the observed material response for the test conducted as per this test procedure can be qualitatively predicted using many viscoelastic models, however, one has to take into account the entire load history.

Keywords:

• Dynamic modulus
• viscoelasticity
• modified binders
• Burgers model
• phase lag

Acknowledgements

The authors thank Department of Science and Technology, Govt. of India for funding this investigation. The authors acknowledge the technical assistance provided by M/s IPC Global, Australia during the conduct of the experiments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Department of Science and Technology, Govt. of India [grant number DST/TSG/STS/2011/46].