Laboratory and field compaction of warm rubberized mixes

Binder in asphalt rubber mixes is generally stiffer than the binder in conventional mixes and requires higher compaction temperatures. These temperatures should be determined carefully because it can change the viscosity of the mix and affect its compactibility. Crumb rubber in the asphalt has the potential to swell which causes the mixture to swell after it is placed and compacted. While adjusting the gradation to allow for “gaps” in the gradation for the crumb rubber helps, there are still instances where sample compaction in the laboratory and field pose a challenge. If not compacted appropriately, samples in the lab tend to swell and the mat in the field also tends to rebound, leading to increasing air voids even after compaction. With increasing use of warm rubberized mixes, there is a need to identify rolling temperatures and patterns that can effectively compact warm rubberized mixes without significant rebound. This paper presents findings from laboratory and field compaction of warm rubberized mixes to minimize swelling of the mixes. The findings from a lab study indicate that rubberized mixes with finer crumb rubber and compacted at lower temperatures tend to swell less compared to mixes containing coarser crumb rubber and compacted at higher temperatures. Findings from the field portion indicate that compacting mixes using low amplitude vibration and with fewer vibratory passes kneads the mix instead of pounding it, leading to lower rebound and improved densities.