# Effect of Chemical Composition of Bio- and Petroleum-Based Modifiers on Asphalt Binder Rheology
## 4 Discussion
### Summary of Modified Asphalt Binder Rheology  
Modifiers' dosages were selected to meet PG 58-28 with true grades close to each other for all MABs to ensure reasonable comparison of rheological properties, except for binder modified with K (ReOB). It was observed that increasing the dosage of K increased high PG, decreased intermediate PG, and decreased low PG stiffness. However, there was no improvement in the relaxation properties of the modified binder with increasing amount of modifier K in the blend. Therefore, dosage for modifier K resulting in a continuous PG closest to PG 58-28 was selected for further investigation. The selected dosage varied from one modifier to the other. This might be one of the reasons for the observed differences in rheology. For instance, increasing the dosage of modifier C can result in a similar performance as that of AB modified with modifier G; but would result in a different Superpave continuous-PG. The focus of the study was to compare binders with similar Superpave characteristics. Dosage variation/optimization is not within the scope of this study, but appears to be a promising research path.  
\(\Delta\)T\({}_{\rm c}\) was determined to evaluate the low temperature cracking susceptibility of modified binders for PAV, 2 PAV, and 3 PAV aging conditions. Relatively high \(\Delta\)T\({}_{\rm c}\) indicates better relaxation properties at low temperatures, which results in better resistance to cracking. Modification of S1 improved the \(\Delta\)T\({}_{\rm c}\) for all MABs except K. Significant improvements were observed when modifiers G and C were used, which even showed better relaxation properties than unmodified binder (S5). G-modified binder has the highest \(\Delta\)T\({}_{\rm c}\) for 2 PAV and 3 PAV conditions, followed by binder modified with C. Differently, K-modified binder has the lowest \(\Delta\)T\({}_{\rm c}\) values in all aging conditions. MABs containing A, D, and E have \(\Delta\)T\({}_{\rm c}\) values close to S5 only after 2 PAV and 3 PAV aging conditions.  
Complex shear modulus master curves show that modulus consistently increased for all aged binders. K-modified binder showed distinctly stiff behavior at PAV that was also observed in 2 PAV and 3 PAV conditions. Other MABs stiffness trends shifted with aging conditions. At UA condition, G-modified binder was the stiffest and after 2 PAV and 3 PAV, it was the softest binder; which is desired. However, mechanisms of change in modulus for G-modified binder after aging need to be investigated. Modifier G, as discussed before, has distinctive characteristics and needs to be explored with additional testing. Aging after 2 PAV and 3 PAV, other modifiers have master curves closer to unmodified binder (S5).  
The black space diagram was used to evaluate the impact of aging on phase angle (\(\delta\)). The \(\delta\) for selected range of G\({}^{\bullet}\) shows similar trends after PAV, 2 PAV and 3 PAV aging. The differences in \(\delta\) of MABs become more distinct with aging but are always noticeable, which makes \(\delta\) at a certain G* a useful parameter to distinguish MABs. Note that this might not be the case when polymers are in the blend [44]. As mentioned earlier, a higher \(\delta\) at a certain G* indicates that the material is less prone to cracking in a brittle way at service conditions. Again, MAB containing G has the highest phase angles, followed by C, S5, E, D, and A, while K has the lowest. In addition, the evolution of GR parameter also suggests that G-modified binder is the most resistant to aging while K is the least.  
Based on rheological testing, G-modified binder is least susceptible to cracking followed by C-modified binder whereas K-modified binder is the most susceptible. MABs modified with A, D and E show similar rheological characteristics to S5.