Future Directions for Applications of Bio-Oils in the Asphalt Industry: A Step to Sequester Carbon in Roadway Infrastructure

The global emphasis on sustainability and net zero carbon roads combined with the increased cost of crude oil has necessitated the application of new environmentally friendly alternatives to the bitumen used in asphalt by the pavement industry. Physical aging, chemical aging, and moisture damage are three important matters encountered in the pavement industry that are harmful for asphalt; bio-oils can decelerate the rates of physical aging and chemical aging in virgin bitumen, and bio-oils can substantially improve virgin bitumen's resistance to moisture damage. Bio-oils can also restore properties in environ-mentally damaged asphalt up to a certain limit of aging. The main goal of this review is a comprehensive analysis of the literature about the potential of bio-oils produced from different biomass sources to influence the physical aging, chemical aging, and moisture damage of asphalt. Considering that bitumen's physical aging is highly impacted by the bitumen's wax content, this review also covers the effect of wax inherently present in bitumen or wax added to bitumen as a modifier. It is concluded that the chemical composition of a bio-oil has a significant impact on the bio-oil's ability to protect or recover the bitumen's original properties. Phenolic compounds found in bio-oils have antiaging effects while bio-oils' acidic compounds may lead to moisture susceptibility.According to the literature, most bio-oils have positive effects only in a specific range of temperatures: low, intermediate, or high. Complete miscibility and dispersion of a bio-oil in the asphalt matrix also have a significant influence on making a bio-oil an effective modifier. Generally, bio-oils are promising as sustainable, carbon-neutral, cost-effective alternatives to replace or modify conventional bitumen in the pavement industry. This review has identified the following critical research gaps: (1) the lack of standard methods for evaluating and reporting the performance characteristics of each bio-oil in bitumen; (2) the lack of long-term field performance data on bio-oils to support comprehensive life-cycle assessments and life-cycle analyses; (3) high variation among bio-oils made from the same feedstock through different processing methods, leading to variation in performance characteristics; (4) the lack of accurate technoeconomic analysis on industrial bio-oils to facilitate entry of bio-oils into the asphalt market.