A systematic review of bio-asphalt for flexible pavement applications: Coherent taxonomy, motivations, challenges and future directions

Abstract From the perspective of highway engineering, bio-asphalt binders have economic, social and environmental benefits and, therefore, can be partially substituted for petroleum-based asphalt binders. A typical raw material for producing bio-asphalt binder used as a renewable energy source is commonly called a biomaterial. Examples of these biomaterials are swine manure and waste cooking oil, as well as castor, sunflower, cotton, linseed and soybean oils. The aim of this review was to survey research efforts on bio-asphalt binder technology for flexible road applications, map the research view from the literature into a coherent and systematic taxonomy and determine the motivations behind using biomaterials in road applications. Meanwhile, an extensive taxonomy was developed based on the literature reviewed and analysed in terms of pure bio-asphalt binder, biomaterials with warm mix asphalt, biomaterials with reclaimed asphalt pavement and biomaterials with capsulation technology. Moreover, critical rheological characteristics and their performance characteristics in bio-asphalt mixture applications were reviewed. The current review concluded that most of the biomaterials can improve the low-temperature rheological properties, but at the same time affect the high-temperature rheological properties. The ageing of most bio-asphalt binders was also found to have a negative effect on the performance of bio-asphalt based on feedback sources and the resulting by-products, which cause unstable physiochemical and morphological behaviours. The fatigue performance of most of the bio-asphalt mixtures was positively enhanced, whereas rutting and moisture resistance were negatively affected. Overall, the different types of bio-asphalt binders possess properties that are either positive or negative. Future research is recommended to mitigate the ageing properties of bio-asphalt binders. The environmental impact and life-cycle assessment of bio-asphalt and traditional petroleum-based asphalt binders must be compared. Determining the optimum biomaterials to be used in pavement applications without compromising performance is an interesting task. This systematic review is expected to contribute to understanding of available gaps and options for other interested researchers to participate in this line of research.