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Evaluation of Laboratory Methods for Estimation of Mixing and Compaction Temperatures of Rubber Modified Asphalt Mixtures

Ebrahimzadeh, Mohammad | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52493 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Tabatabaee, Nader
  7. Abstract:
  8. One of the major challenges in construction of rubber modified asphalt mixtures is determination of mixing and compaction temperature (MCT). In order to estimate MCT according to Superpave volumetric mix design procedure, a temperature-viscosity relationship for the binder should be developed using ASTM D2493. This approach provides reasonable temperatures for unmodified binders which are Newtonian fluids at high temperatures and their viscosity is independent of shear rate. However, this method often yields excessively high temperatures for modified binders which exhibit non-Newtonian pseudoplastic behavior. The reason is that viscosity of modified binders depend on not only temperature but also on shear rate. Several methods have been proposed for determining MCT of modified binders. The goal of this research was to evaluate feasibility of some of those methods which might be applied to crumb rubber modified (CRM) binders in hot-mix asphalt (HMA). In this regard, three candidate methods were selected for further laboratory evaluation. Two of them were based on binder rheology and the other one was based on mixture volumetrics. Indirect tensile strength and cracking tolerance index tests were also conducted to assess the effect of different compaction temperatures on cracking resistance of mixtures. Traditional equiviscous method resulted in unreasonably high temperatures for CRM binders. High Shear Rate Viscosity method yielded lower MCTs than those from the traditional equiviscous method for CRM binders. However, linear temperature-viscosity relationship assumed for unmodified binders is not valid for CRM binders. Assuming linear relationship led to erroneous compaction temperatures, such that compaction temperature for 18% CRM binder was lower than that of 15% CRM binder. Phase Angle method also resulted in lower MCTs than equiviscous method for CRM binders. However, for different crumb rubber contents, no remarkable difference in compaction temperatures was observed, which could be due to the lower impact of CRM dosage on phase angle in comparison to viscosity. Thus, it seems that phase angle method cannot effectively capture workability of the mixtures, observed in the field. Prediction of compaction temperature using the compaction plot (compaction temperature against air void content) yielded in lower MCTs campared to equiviscous method. This approach considers the effects of CRM binder’s viscosity, aggregate type, and gradation on workability of rubber modified HMA mixtures by which consequently a remarkable difference in compaction temperatures are made as CRM dosage changes. The results of mechanical tests did not show a trend for mechanical properties with neither temperature nor CRM dosage. These results also exhibited high coefficient of variation (COV) values, so they could not be used as an assessment tool to determine effect of different compaction temperatures on performance of rubberized asphalt mixtures. Finally among the evaluated methods, using compation plot (compaction temperature against air void content) was proposed as an appropriate method for determining compaction temperature of rubber modified HMA mixtures
  9. Keywords:
  10. Modified Bitumen ; Crumb Rubber Modifier ; Workability ; Phase Angle ; Mixing and Compaction Temperature ; Compaction Plot ; High Shear Viscosity

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