Mix design, development, production and policies of recycled hot mix asphalt: A review

Abstract

Worldwide, flexible pavements are the paramount type of pavements. These national assets can be recycled to serve the next design life without the need to dispose of them in landfill. The old/distressed, flexible pavement material is called, reclaimed asphalt pavement (RAP) material. In view of the sustainable development practice, 100% RAP should be recycled. However, the practical range of RAP incorporation in a drum mix plant ranges from 10% to 50% whereas in a batch mix plant, it is 10%–35%. In a batch mix plant, incorporation of the hot dried RAP (instead of cold RAP feeding) can increase the RAP incorporation level by 20%. Studies recommended that the rejuvenator should be added over the RAP material instead adding with the base binder. The addition of rejuvenator on the RAP conveyor belt (to the pug mill), provided sufficient time for rejuvenating the aged binder coating over RAP aggregates. Further, a thorough knowledge of the mix design process of recycled hot mix asphalt (RAP-HMA) is essential to produce a durable mix. For high RAP content (>20%), the RAP binder properties are important for the mix design process. However, very limited studies discussed (in detail) the recovery process of the RAP binder. The review article through light on the binder recovery process, method of RAP gradation, specific gravity determination for RAP material, minimum desirable properties of the RAP material etc. This review paper also provides a narrative review of the historical development of RAP-HMA technology, technical aspects related to the procurement of RAP material, mix design and production of the recycled mix. Moreover, the international & national policies/laws for construction and demolition waste are discussed. • Parallel heating of RAP in a pug mill increases the mixing ratio. • Performance grade, PG mixing rule was found most reliable. • The mix design process must consider the type of mix plant and RAP feeding method. • Conveyer belt found to be the most promising location for Rejuvenator addition. • The mixing temperature has an equivalent effect as that of a rejuvenator in the recycled mixture. • Stiffness modulus found a promising test to compare the performance of recycled mixtures.