Novel method for enhancing freeze–thaw resistance of recycled coarse aggregate concrete via two-stage introduction of denitrifying bacteria

The inadequate mechanical performance and durability of recycled coarse aggregate (RCA) concrete (RCAC) have limited its widespread use, especially in frost areas where concrete can be easily damaged by freeze–thaw cycles. This research utilises denitrifying bacteria as a solution to treat RCAs. The treated RCAs are then applied to fresh RCAC to enhance concrete performance without using or creating poisonous or polluting substances. Based on thermogravimetric analysis and scanning electron microscope observations, both old and new interfacial transition zones (ITZs) and cement matrices were found to be well filled by bio-induced calcium carbonate after applying the double-stage denitrifying bacterial treatment method. Improvements of 30.3% and 19.2% were recorded for compressive strength and tensile splitting strength, respectively. The bacteria filling the open pores inside the concrete have contributed to a 33.0% decrease in water absorption, impeding the absorption of outside water and reducing the swelling caused by water freezing. Bacteria bond the ITZs, resulting in higher cohesion between aggregates and matrix as well as enhancing the resistance to pressure caused by freeze–thaw cycles. The improved resistance makes the treated RCAC withstand 75 more freeze–thaw cycles, which has enabled plain concrete to satisfy the frost resistance requirement in areas with fresh water and low-freezing areas with seawater. Overall, the treatment using denitrifying bacteria is an environmentally friendly method that can significantly improve concrete strength and durability, thus enabling the widespread usage of RCAC in frost areas.