A composite slab is a feasible application for recycled aggregate concrete (RAC), and its long-term deflection always controls the design due to significant nonuniform shrinkage. To accurately predict nonuniform shrinkage, a universal relative humidity (RH) prediction model for RAC is required, which is currently unavailable. Therefore, this article quantifies the influence of recycled aggregate (RA) on RH evolution and moisture diffusion by a 200-day RH and weight loss test on concrete with different coarse and fine recycled aggregate replacement ratios and their combinations. The influence of RA was explained by the microcrack connectivity effect and water compensation effect. Based on these observations, the classic Bažant–Najjar diffusion model was theoretically modified using equivalent moisture flux to represent the microcrack connectivity effect and the water release content of RA to depict the water compensation effect. It was found that using 100% coarse and fine recycled aggregate can decrease the RH loss by 18%–69% and increase the moisture diffusivity by up to 73%; the water compensation effect of RA is so significant that even though the moisture diffusivity of RAC is larger, the internal RH still increases with the increase of RA content; the proposed model can provide a reliable prediction on both RH development and distribution for RAC tested in this study and the available literature.
Chen et al. (2025) studied this question.