# Rheological Performance of Bio-Char Modified Asphal with Different Particle Sizes
## 6 Summaries and Conclusions  
The bio-char was investigated as a favorable modifier to improve the rheological performance of asphalt binders. Bio-char with two particle size ranges of 75 \(\upmu\)m-150 \(\upmu\)m and less than 75 \(\upmu\)m were used as the modifiers. The content of bio-char was 2%, 4%, and 8% respectively. The flake graphite with a size range of less than 75 \(\upmu\)m and a content of 4% was utilized as a modifier of asphalt, for comparison. The Scanning Electron Microscopy characterization, Rotational Viscosity test, Dynamic Shear Rheometer test, and the Bending Beam Rheometer test were conducted to evaluate the modified binder performance. The conclusions obtained are as follows:  
1. Compared with dense and smooth flake graphite, the bio-char particle had a more porous microstructure and rough surface texture.
2. Both bio-char and graphite could increase the rotational viscosity of the asphalt, with the added contents. The bio-char modified asphalt binders with smaller-sized particles had higher viscosities than those with larger-sized particles. They all satisfied the requirement for the specifications of rotational viscosity--less than or equal to 3000 mPa\(\cdot\)s at 135 \({}^{\circ}\)C.
3. The porous structure and rough surface of bio-char could lead to a larger adhesion interaction in asphalt binder than those of the smooth flake graphite. As a result, the bio-char modified asphalts had a better high-temperature rutting resistance and better anti-aging properties than that of the graphite modified asphalt. A more noticeable improvement was found for the bio-char modified asphalt with the smaller-sized and higher number of particles. Moreover, the bio-char modified asphalts with the smaller-sized particles had the highest critical temperatures among all other binder types.
4. Though the crack resistance, at low temperatures, of the bio-char modified asphalts decreased with the added bio-char, as seen in the BBR test results, the low critical temperatures were still lower than \(-18\)\({}^{\circ}\)C, with the smaller-sized bio-char particles. The bio-char modified asphalts had similar low-temperature properties as that of the graphite modified asphalt binder.  
In summary, it is possible to use bio-char as a modifier for the petroleum asphalt binder. This study showed that the bio-char with a size range of less than 75 \(\upmu\)m and a content of less than 4% could increase the anti-aging property and rutting resistance of the asphalt binder, as well as keep a good low-temperature crack resistance in the asphalt binder. Thus, this type of bio-char particles could be used as an environment-friendly and favorable modifier for the asphalt binder. In future studies, the porous structure of bio-char and the interface bonding between the bio-char and the asphalt will be investigated to facilitate this application.  
Figure 6: Low critical temperatures of control, graphite modified and bio-char modified asphalts.  
**Author Contributions:** Conceptualization and methodology, R.Z., Q.D., Z.Y., H.W; Writing-Original Draft Preparation, R.Z.; Writing-Review & Editing, Q.D., Z.Y., H.W. and C.P.; Funding Acquisition, R.Z. and Z.Y.  
**Funding:** This research was funded by the Fundamental Research Foundation of the Central Universities of China under Grant No. 300102218701 and the U.S. National Science Foundation (NSF) under Grant CMMI 1300286.  
**Acknowledgments:** This work was completed with the assistance of [PERSON] from the Department of Materials Science and Engineering at Michigan Technological University. [PERSON] is supported by China Scholarship Council under Grant No. 201606560020. Any opinion, finding, and conclusions expressed in this paper are those of the authors and do not necessarily represent the view of any organization.  
**Conflicts of Interest:** The authors declare no conflict of interest.