Rheological study of asphalt binders with a wax-based Warm Mix Additive and its relationships to mix compaction and rutting
Date of Award
Doctor of Philosophy (PhD)
N. M. Wasiuddin
Although the energy saving Warm Mix Asphalt (WMA) technology that produces asphalt mixes at lower temperatures are around for about a decade, there still exist some concerns that are hindering its wide spread implementation. The overall objective of this study is to investigate if the viscosity reductions at higher temperatures have any impacts on asphalt mix compaction and rutting performances. This study reveals that there exists a critical temperature for each asphalt binder below which viscosity will increase with addition of Sasobit®. Therefore, compaction below the critical temperature can negatively impact density.
For PG 64-22, the critical temperature is 104 °C and for PG 76-22M, the critical temperature is about 101 °C. Another significant finding of this study is that with the addition of Sasobit®, asphalt binders become a shear-thinning liquid even at compaction temperature ranges. The shear rate dependency increases with an increase in percent of Sasobit ®. This indicates if the actual shear rate during the compaction process is higher than 6.8 s-1, then the currently recommended viscosity as well as temperature is overestimated and compaction temperature can be reduced. On the other hand, if the actual shear rate during the compaction process is lower than 6.8 s-1, then the currently recommended viscosity as well as temperature is underestimated and a higher compaction temperature should be used.
This study further reveals that an optimum amount of Sasobit® exists between 2% and 4% based on phase angle and non-recoverable creep compliance analyses of PG 64-22 and based on percent recovery of PG 76-22M. Currently used rutting factor G*/sinδ fails to indicate any optimum amount. Phase angle values suggest if rutting is performed at one grading higher temperature, an overdose of Sasobit® will increase rutting potential. Creep and recovery tests at equal-stiffness temperature indicate that addition of Sasobit®increases rutting resistance at lower stress while it increases rutting potential at higher stress. APA rut depths indicate that Sasobit®mixes prepared at WMA temperatures performed better than HMA without Sasobit ®. However, from aging evaluation of extracted asphalt binders it was revealed that reduced production temperatures reduces aging and may increase rutting susceptibility. Finally, from an exploratory study it was concluded that wax modified asphalt binders may be differentiated by its morphology using an AFM. Overall, this study provides fundamental findings with respect to rheology and performance which will help implement warm mix asphalt in the U.S.
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