Shear behaviour and pore characteristics of biochar amended recycled concrete aggregates subjected to freeze-thaw cycles

Using recycled concrete aggregates (RCAs) as road base materials provides environmental and economic benefits in preserving natural resources and reducing solid waste. Biochar derived from the pyrolysis of agricultural waste has been proven to serve as a sustainable and eco-friendly amendment, used to mitigate the environmental risks associated with construction waste. This study conducts a series of drained triaxial tests to assess the effects of biochar on the shear behaviour of RCAs under freeze-thaw (F-T) cycles. Three biochar contents are considered in this study (0 %, 5 % and 10 % (g/g)). According to the results, biochar helps to increase the peak shear stress of RCA by up to 60 % without F-T cycles. However, due to the pore structure evolution, the shear stress for biochar-amended cases is lower than the bare RCA after 5 F-T cycles. Adding biochar increases the critical state friction angle by 20 %. Nevertheless, biochar has an insignificant influence on the critical state shear strength under low confining stress regardless of F-T cycles. Moreover, the differences in the shear strength behaviour between specimens with 5 % and 10 % biochar are insignificant. The amendment of 5 % biochar is thus suggested as it can substantially enhance the mechanical behaviours of RCA during shear.