The main concern in working with fresh concrete is the workability during filling of formwork. In this study, the mechanism of segregation during the filling of fresh concrete into formwork is numerically investigated considering the rheological properties of fresh concrete. A mathematical model, which considers fresh concrete as a non-Newtonian fluid that, is used for the investigation of relationship between slump values and rheological properties of fresh concrete. The aggregates are considered as Lagrangian particles whose trajectories determine segregation. This study will provide an insight to the relationships between workability and various rheological parameters of fresh concrete that is, yield stress, segregation, viscosity during filling of fresh concrete into formwork. The relationships between workability and rheological properties during the fill of fresh concrete for both concrete mixtures with no admixtures (MC) and concrete mixtures with admixtures (MCS) are investigated. MCS concrete mixture includes super-plasticizers for the same composition of MC. Cylindrical formworks for three different heights of 50, 100 and 150 cm are employed. It is observed that MCS mixtures have higher slump values and less aggregate segregation (%) compared to MC mixtures for the same compositions of fresh concrete. It is found that, slump values decrease as yield stress increases for both MC and MCS fresh concrete mixtures. When MCS mixtures are used, there is a significant reduction in yield stress compared to MC mixtures.