Optimization and utilization of air-entrained recycled brick aggregate concrete under freeze-thaw environment

Utilizing recycled brick aggregate (RBA) offers a sustainable solution to the environmental challenges posed by construction and demolition waste (CDW). However, the high water absorption and porosity of RBA accelerate freeze-thaw (FT) damage, leading to significant deterioration of both the aggregate and the concrete matrix, which compromises structural integrity. While air-entrainment has proven effective for natural and recycled concrete aggregate concrete, its application to recycled brick aggregate concrete (RBAC) remains unexplored. This study investigates the effectiveness of air-entrainment in enhancing the FT resistance of RBAC. Twelve air-entrained RBAC mixtures with varying w/c ratios (0.55, 0.45, and 0.35) and RBA substitution levels (0%, 25%, 50%, and 100%) were prepared and subjected to 300 rapid FT cycles. The results showed that RBA has a detrimental effect on the FT resistance of RBAC despite air-entrainment as evidenced by the increased mass loss, reduced RDME, and reduction in compressive strength. However, the combination of air-entrainment with low w/c ratio effectively moderated internal deterioration caused by ice expansion during FT cycling. Mercury intrusion porosimetry (MIP) analysis showed similar alteration in pore size distribution between the 0.35A-RBA-0 and 0.35A-RBA-100 specimen after FT cycling, while RBAC specimens with 0.45 and 0.55 w/c ratio showed significant expansion of mesopores and macropores (harmful pores >100 nm). X-ray computed tomography (XCT) analysis confirmed that air-entrainment preserved the mortar matrix from cracking despite localized RBA cracking at low w/c ratio. The developed FT damage model confirmed up to 46% reduction in service life of RBAC compared to NAC, depending on w/c ratio. Therefore, it is recommended to limit the RBA content to 50% while maintaining low w/c ratio at or below 0.45, particularly in cold regions where FT resistance is critical. These findings establish guidelines for optimizing RBAC mix design under cold climates, advancing sustainability and promoting circular economy practices in the construction sector.