Compared with natural aggregates, the water absorption behavior and particle shape of recycled concrete aggregates (RCA) are the main factors affecting fresh behavior. Based on the excess paste theory, the discrete element method (DEM) was employed to simulate the role of RCA in fresh concrete, explicitly accounting for its particle morphology and water absorption behavior. The DEM model was calibrated and validated by using slump flow tests under varying superplasticizer dosages (0.5–1.0%), water to binder ratios (0.409–0.479) and mortar paste volume. The DEM model achieved prediction errors within 10% under high flowability condition. Based on the calibrated model and the assumed time-dependent water absorption of RCA, the theoretical rheological properties of mortar were derived to predict the fresh behavior of concrete at different absorption times. Compared with 90 s of water contact, the yield stress of the mortar increased from 33.78 to 43.94 Pa after 1200 s, while the plastic viscosity rose from 6.48 to 8.36 Pa·s. The slump flow of concrete decreased from 570 to 501 mm. The results indicated that the water absorption behavior of RCA influenced the fresh properties of concrete with time by increasing the mortar’s yield stress and plastic viscosity while reducing the paste film thickness. The proposed DEM model provides an approach for analyzing and predicting the time-dependent workability of recycled aggregate concrete.
Xiao et al. (Mon,) studied this question.