In this study, the mechanism of segregation during the filling of fresh concrete into formwork is numerically investigated. A mathematical model, which considers fresh concrete as a non-Newtonian fluid, is developed. The aggregates are considered as Lagrangian particles whose trajectories determine segregation. Aggregate segregation is partially affected due to concrete viscosity. The study presented here will show the relationship between viscosity and segregation during fill of fresh concrete in formwork. The relationship in between aggregate segregation and viscosity during the fill of fresh concrete for both MC concrete mixture and MCS concrete mixture is investigated. MCS concrete mixture includes superplasticizers for the same composition of MC. Cylindrical formwork (30 cm by 150 cm) is employed. It is found that MC concrete mixture has an aggregate segregation level of 50%, while it reduces to a level of 20% for MCS concrete mixture. The use of superplasticizer reduces aggregate segregation. (C) 2004 Elsevier Inc. All rights reserved.