Effect of aggregate gradation, aging, and compaction energy on swelling characteristics of rubber modified asphalts mixtures

Use of ground tire rubber (GTR) in asphalt has been experienced for decades. There has been extensive research on rubber modified asphalts and many pavements have been built using this material. In general, use of rubber in asphalt has been shown to be beneficial both from environmental point of view (landfill saving) and field performance (reduction in rutting and increase in fatigue resistance). A challenge faced with the use of rubber modified asphalts for certain types of aggregate gradation is the swelling of rubber particles, and the rebound of the material after compaction. This phenomenon is more severe in the dry process (when rubber is first blended with the aggregate); however, it does also pose a challenge in the wet process (when rubber is first blended with asphalt). Such behavior affects mechanical properties of the mix, and results in reduction of density in the field. A laboratory experiment was undertaken to quantify the level of this swelling and to determine the magnitude of specimen swelling as affected by the aging level and compaction energy when the wet process is utilized. Specimens were prepared under short-term and long-term aging processes and compaction was conducted using a gyratory compactor at different energy levels. The experiment was also conducted in a way to determine the gradation effect, and to separate the effect of rubber swelling on specimen rebound from the effect of compaction energy on this rebound. It was found that the rebound and swelling are highly affected by the aggregate gradation, with denser gradations resulting in higher swell. The study also indicated that extended aging reduces the specimen swelling significantly. Similarly, higher rebound was observed under higher compaction energy. Finally, the mix swelling and rebound were correlated with the rubber modified binder characteristics through multiple stress creep and recovery (MSCR). The findings of this study are significant in assisting the mix designer with a tool for better control of the mix behavior and reduction of swelling both in the laboratory and In the field.