Investigation of rutting performance of dense and gap graded recycled asphalt mixtures

Aggregate gradation used in the production of bituminous mixes has a significant influence on the rut resistance of bituminous mixes. However, despite the increase in the use of RAP in the production of bituminous mixes, a limited attempt has been made to investigate the effect of aggregate gradation on the rut resistance of RAP-modified bituminous mixes. Hence in a present study, an attempt has been made to compare the rut resistance of RAP-modified bituminous mixes made with dense and gap gradations. Two dense and two gap gradations were used to prepare the bituminous mixes at varying RAP content. Rutting characteristics of different mixes considered in the present study were evaluated in terms of dynamic modulus rutting factor (E*/sinφ), dynamic creep, and flow number (FN). The results of these tests indicate that rutting performance improved significantly with an increase in the proportion of RAP material in the mix. Also, the statistical analysis results indicate that dense-graded mixes have better resistance to rutting than gap-graded mixes. However, the average increase of 136% in flow number of gap-graded mixes compared to 34% observed in dense graded-mixes with a change in RAP content from 25 to 35% indicates that rate of increase of rut resistance with RAP content was higher in gap-graded mixes. The higher rate of increase of rut resistance without significant reduction in fatigue lives makes the gap-graded mixes more favorable for utilization of higher RAP content than dense-graded mixes.