Synergistic modification of recycled aggregate concrete with waterborne epoxy resin and steel fibers: impermeability and microstructural analysis

Abstract Recycled aggregate concrete (RAC) is commonly limited by high porosity, weak interfacial transition zones, and poor impermeability. In this study, waterborne epoxy resin (WER) and steel fibers (SFs) were jointly used to improve the performance of RAC. Concrete with a target compressive strength class of 40 MPa and a 50% recycled coarse aggregate replacement ratio was selected as the main system. Mixtures with different WER dosages and SF contents were prepared. Mechanical properties, porosity, impermeability, microstructure, and direct material cost per cubic meter were investigated. The results showed that WER and SF produced a clear synergistic effect. WER mainly improved matrix compactness and interfacial bonding, while SF mainly restrained crack initiation and propagation. The optimum performance was obtained at 4 wt% WER and 1.0 vol% SF. Compared with the unmodified RAC control, the optimized mixture showed higher strength, lower porosity, and better impermeability. Scanning electron microscopy observations further confirmed the formation of a denser internal structure and improved interface quality. Although the direct material cost increased after modification, the optimized RAC showed better overall engineering applicability, particularly for water-resistant and durability-critical structures.