Migration behavior of reclaimed asphalt pavement mastic during hot mixing

The production of hot central plant recycling asphalt mixture (RHMA) is more complex and uncertain than conventional hot mix asphalt. An adequate blend of virgin and aged asphalt is essential to recovering the per-formance of aged asphalt in reclaimed asphalt pavement (RAP). Therefore, the amount of aged asphalt migrating from the RAP into the recycled material system determines the level of blending and the durability of RHMA. To reveal the influencing factors of migration rate, we used fine aggregates in RAP as tracers to measure the RAP mastic concentration in the blended mastic. The considered factors included mixing time, asphalt-aggregate ratio, rejuvenator content, mixing sequence, RAP pseudo ratio, and RAP agglomeration rate. Then, the migra-tion behavior of aged mastic was investigated from the viewpoints of the temperature field and microscopic features using the infrared thermogram and CT scan techniques, respectively. In addition, FTIR, GPC, and AFM were used to study the influence of migration of RAP mastic on the composition of blended asphalt. Finally, the effect of RAP mastic migration on moisture stability of RHMA was investigated using moisture induced sensi-tivity test, freeze-thaw indirect tensile test, and water immersion rutting test. The results show that the migration rate of RAP mastic during hot mix can be improved by reducing RAP content, raising the preheating temperature of RAP, extending the mixing time, increasing the asphalt content, and using dry mixing method, whereas the amount of regenerator has no significant effect. The temperature is the essential factor affecting the migration level. The RAP mastic migration is highly synchronized with RAP aggregate movement under forced mixing. Furthermore, increasing the migration level of RAP mastic can significantly increase the proportion of aged asphalt in blended asphalt. At last, the higher RAP mastic migration can prevent void development in RHMA under freezing-thawing and hydrothermal washing, thereby improving the moisture stability of RHMA and alleviate peeling aggregates and mastic. This study contributes to current knowledge by explaining the essence of RAP mastic migration during hot mixing, the results should be interested by road contractors, producer of RHMA, and academic society.