Analysis of digestion time and the crumb rubber percentage in dry-process crumb rubber modified hot bituminous mixes

The use of crumb rubber modifier (CRM) in bituminous mixes made by the dry process is not as widely used as the wet process. Nonetheless, this process has advantages, such as the potential to consume larger quantities of crumb rubber, thus resulting in greater savings in energy and natural resources. This research study contributes to the further development and evolution of the dry process through the analysis of the effect of the digestion time (the contact time between the crumb rubber and the bitumen) and the quantity of crumb rubber on the mix design properties. The results of the study showed that the digestion time had no influence on the selection of the optimal binder content or on the compaction of the mixture. In contrast, the digestion time was found to have an impact on the mechanical performance of the mix. In this respect, an increase in the quantity of crumb rubber contributed to a corresponding increase in the amount of bitumen needed, and also caused the mix to become less compact. This study showed that a crumb rubber percentage of less than 1% of the total weight of the mix and a digestion time of 90 min produced the best results.

Research highlights
► This study contributes to the development of the dry process in CRM hot mixes.
► Digestion time does not affect the optimal bitumen content to be added to the mix.
► Digestion time does not have any impact on the compaction of dry-process CRM mixes.
► The mechanical performance of the mix was affected by variations in the digestion time.
► The maximum crumb rubber percentage to be added is 1% of the total weight of the mix.