The main issue in working with fresh concrete is the workability during filling of formwork. One of the problems found in workability is occurence of segregation depending on w/c ratio. Segregation is strongly related to w/c ratio. The fresh concrete is usually considered as a non-Newtonian fluid since it is a mixture of aggregate, cement and water. The flow behaviour of the fresh concrete, a characteristic that is strongly related to w/c ratio, plays a crucial role in the quality of high performance concretes. The aggregates in fresh concrete cause segregation in the final product depending on the flow conditions. In this study, the mechanism of segregation in such a system was theoretically investigated. The mould filling of fresh concrete was numerically investigated and aggregates were considered as Lagrange particles. Segregation was identified from trajectories of such particles. Within this framework, fresh concretes with no admixtures (MC) and those with high range water reducer admixtures (HRWRA) (MCS) were investigated. Minimum agregate segregation in fresh concrete mixtures without HRWRA admixtures was observed to have been higher than in mixtures containing HRWRA admixture.