Flowable concretes tend to segregate. The risk of segregation is particularly high when the concrete is vibrated during the compaction process. A well-known segregation phenomenon is the so-called "bleeding". This is a rise of water to the surface of the freshly poured concrete due to the difference in density between the mixing water and the concrete's denser solid components (aggregates, cement and additives). This type of segregation occurs particularly within the paste. The focus of this paper is, therefore, on the sedimentation behavior at the microscale of concrete and especially on the influence of this process on rheological properties of the cement paste. In addition to common bleeding tests of cement suspensions using standing cylinders, rheometric measurements were performed on the suspensions during the bleeding process. A measuring procedure was developed for the rheometric measurements of the sedimenting cement suspensions. The rheological properties of the investigated cement suspensions were determined at four specific measuring times and at four specific measuring heights (i.e., positions) each. With this method it could be shown that the cement suspensions are not homogeneous over their height and that bleeding has a great influence on the rheological properties of cement suspension.
Keywords: bleeding; cement suspension; rheology.