|M.Sc Student||Michaeli Nir|
|Subject||Viscoelastic Characterization of Asphlt-Aggregate Mixes|
with the Indirect Tensile Apparatus
|Department||Department of Civil and Environmental Engineering||Supervisor||Assistant Professor Eyal Levenberg|
|Full Thesis text - in Hebrew|
This thesis deals with viscoelastic characterization of asphaltic mixes employing the indirect tensile (IDT) apparatus. The work covers the theoretical development required for characterization as well as its implementation through testing of specimens and subsequent analysis of the results. Similar to other works dealing with IDT and asphalt mixes, the characterization was limited to a linear theory and very small deformations. Unlike common practice, the test setup and accompanying modeling approach were oriented towards accounting for, and dealing with, non-recoverable deformations that are an integral part of the overall material response - but not part of the sought after characterization.
Different asphalt-aggregate mixes were prepared and tested in the study. The mixes were fabricated in the laboratory and densified with a gyratory compactor to form disks for IDT testing and cylinders for comparative uniaxial testing. All specimens were tested under 25 °C and some under 35 °C. A modern electromagnetic universal load-frame was employed for force application. The loading history included several load-rest sequences with loading periods that were (each) a few seconds in duration, followed by rest periods that were much longer - up to thousands of seconds. Transitioning between loaded and unloaded states (i.e., unloading) was achieved in milliseconds. In effect, this type of loading history allowed for viscoelastic characterization within a time range spanning six orders of magnitude under a given temperature level. Additionally, in an effort to restrain the development of non-recoverable deformations, both tensile and compressive cycles were applied interchangeably. To accomplish this unique and non-traditional IDT testing feature, specimens were glued to the load-frame. Moreover, deformations were measured on-specimen with LVDTs mounted in a non-standard arrangement, to better comply with representative volume element limitations.
The approach for viscoelastic property inference was based on solving an inverse problem with an optimization algorithm. Material properties were obtained by minimizing the error between modeling and experiment. The shape of the viscoelastic creep function was a priori assumed as a sigmoid (in a log-log scale), and the algorithm was focused on extracting the free constants. In order to account for non-recoverable deformations a viscous term was added to the analysis. Without this addition it would not have been possible to match the measurements.
Overall, the developed experimental guidelines, in combination with the applied modeling and analysis approach, enabled viscoelastic characterization with the IDT apparatus. The outcome is deemed better suited, as compared to current/accepted methods, for pavement engineering purposes.