Critical review on use of biochar as a modifier in asphaltic binders for pavement construction: Critical review on use of biochar as a modifier in asphaltic binders…: I. Wani, A. Garg

Biochar provides a sustainable carbon sequestration technology, an effective fertilizer in agriculture, a step forward for the profitable and safe disposal of bio-wastes, reduced carbon dioxide emissions and global warming, and a renewable energy source. Using biochar as a bitumen modifier in asphalt pavement construction is under active research. It can prove a sustainable and environmentally friendly alternative, provided it meets the efficiency, strength, and economy challenge. This review focused on the available literature on utilizing biochar as a bitumen modifier for the construction of asphaltic roads. The studies show that biochar's physical and chemical nature has helped project it as a promising bitumen modifier. The biochar, being porous and fibrous, provides a strong, stiff frame in the asphaltic mast and results in the enhancement of both stiffening point and viscosity. This, in turn, leads to a reduction in penetration or increased deformation resistance. This is perhaps the reason for the high performance of biochar-modified asphalt at high temperatures. The increase in viscosity of asphaltic masts was also observed due to biochar amendment, making asphalt more sensitive to temperature. The two important factors, the complex modulus and the rutting factor of the asphalt, were noticed to increase with the addition of 10% biochar. The biochar amendments of up to 20% increased fatigue resistance temperature by 4.6 °C. The improvement in the resistance to deformation at high temperatures, probably due to a reduction of phase angle due to adding biochar, is also seen as a significant function of biochar. However, biochar applicability in the field is mainly related to its cost efficiency and performance as a bitumen modifier for asphaltic pavements. So far as the cost economy is concerned, the mean price for biochar (as per available literature) was very high, from $2.65 to $0.09/kg for blended biochar. The price was as high as $3.29/kg in the Philippines to $0.08/kg in India and in the US to $13.48/kg, implying that the market price of biochar is variable worldwide and dependent mainly on the biochar feedstock, cost of labor/living of the area and land costs. On the other hand, its efficiency has not yet been satisfactory at low temperatures. The other noticeable limitations that need to be explored in further research are long-term effects on strength, rutting resistance, and ageing. Also, field studies to support the research and, more importantly, cost economy viz-a-viz other available modifiers need exploration.