Mix design considerations for foamed bitumen stabilized materials: A review

Both economic and environmental drivers are strong in support of low-energy technologies such as bitumen stabilisation with foamed bitumen. The resulting material of such bitumen stabilisation (BSM) is distinctly different from conventional hot-mix asphalt (HMA) in terms of its composition, structure and properties. These differences imply that the mix design approach of these mixes does not fit into relatively well-established frameworks of traditional mix design methods such as that of HMA. This has created a need for new mix design methods for BSMs. Several road agencies developed guidelines for the design of BSM which are the outcome of few decades of research efforts to standardise the mix design. The present paper aims to outline this research development in terms of various mix design considerations associated with the BSMs with emphasis on the evolution of different mix design specifications. While doing so, the paper identifies the differences between BSM and HMA (and unbound materials) and link these differences to different aspects of BSM mix design. The gaps in the current understanding of various mix design aspects of BSMs are also highlighted. In the later part of the paper, mix design methods recommended by various road agencies are compared. It is found that these design methods have many common elements as well as differences. Finally, the knowledge gap in the existing literature is highlighted with discussion on the way forward to address the same that may eventually lead to the widespread use of the material.