The Office of Engineering Research and Development of the Federal Highway Administration (FHWA) is constantly looking for new materials and methods to improve the quality of our nation's highway system. At the Turner-Fairbank Highway Research Center (TFHRC), research on chemically modified asphalts is an ongoing project that has already resulted in furfural-modified asphalt, compatibilized crumb rubber asphalt (CCRA), and chemically modified crumb rubber asphalt (CMCRA), all of which were developed by the authors of this article. But at FHWA, the search for the optimal asphalt continues, and this time, it has simply taken a different route.

Chemically modified crumb rubber (CMCR) has been used successfully to improve both high- and low- temperature performance grade specifications in many types of asphalts, including those with low asphaltene levels. Data shows that the performance grade of an asphalt with an asphaltene content of only 0.3 percent can be improved significantly. Test results show the expansion of its performance grading from 46-16 to 76-32, which expands its useful temperature range from 62°C to 108°C. CMCRA is also more homogenous and has a lower separation range after heated storage than conventional crumb rubber- modified asphalts (CRMA). Such a low separation range is an exciting result because it is expected to deter pavement raveling, leading to longer life for these pavements.

So far, CMCRA has only been tested in small laboratory quantities. However, the results indicate that it may possibly be one of the most cost-efficient modified asphalts in the industry. As the National Asphalt Pavement Association's vice president Dale Decker stated, "This new crumb rubber technology may produce high-performance pavements, but the costs and mix performance must be tested in full-scale production before it can be considered a viable product for replacing other modified asphalts." Consequently, scaled-up production is soon to be held, where different quantities of CMCRA will be tested to determine whether the binder's reactivity, separation, and rheological properties are consistent at each stage of production with those recorded at TFHRC's laboratories .

After seeing that a chemical modification to crumb rubber significantly upgraded the performance of neat or virgin asphalts, researchers at TFHRC pursued a similar procedure with polymer-modified asphalts. Unlike conventional polymer-modified asphalt, CMCRA enhanced the interaction of polymer functional groups within polymer-modified asphalt system, containing both normal and low asphaltene contents. This new development led to an improvement in the rheological properties of the chemically modified binder by one performance grade for both high and low temperatures, as well as in its homogeneity. For example, Stylink^TM, a trade name for a particular polymer in asphalt, has exhibited separation features very similar to those of virgin asphalt after being combined with chemically modified crumb rubber. That is, the difference in stiffness between the top and bottom third of a chemically modified Stylink binder after 24 hours of heated storage was found to be only between 2 percent and 4 percent. Figure 1 compares the propensity of four types of asphalt to separate or settle during heated storage.

Figure 1. Speed profile with flags for required speed reduction.

American Association of State Highway and Transportation Officials (AASHTO) binder performance data also showed that the predicted fatigue performance of three Stylink binders is improved by one performance grade (brought down from 19°C to 16°C) when combined with CMCR. Although further research is needed on combining CMCR and polymer-modified asphalts, the procedure has been disclosed for patenting purposes, and interest is gaining momentum across the asphalt industry.

Dr. Brian Chollar is a research chemist in the Office of Engineering Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. Dr. Chollar has worked for FHWA for 23 years. His areas of expertise are asphalt chemistry and snow and ice chemicals for winter maintenance. He received his bachelor's degree in chemistry from Purdue University and his doctorate in organic chemistry from the University of Minnesota.

Dr. Mohammed Memon is also a research chemist working in the Office of Engineering Research and Development at TFHRC. He has worked for FHWA as a contractor for six years. His areas of expertise are asphalt and polymer chemistry. He received his doctorate from the University of Sindh in Pakistan.