Open channel flow behaviour of concrete in the presence of obstacles and mechanisms of flow blockage (OCF-Blockage)

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The prediction, whether a fresh concrete can fill a given formwork without leaving substantial voids and without segregating is of great importance for the construction industry. Numerous studies have been carried out, in which the form filling behaviour was studied. Nearly all of these works have in common, that they were focussed on self-compacting concretes (SCC), primarily considered concrete as a homogeneous (non-segregated) substance having Bingham properties and investigated non-stationary conditions. Contrary, in the proposed study flowable ordinary concretes shall be studied under stationary conditions. The key goal of the proposed project is to model the open channel flow and blockage behaviour of concrete. Herefore, the flow behaviour will be examined in an open channel under obstacle-obstructed flow conditions and is monitored by a stereo-optical camera system, which provides the essential information of the flowing concrete for evaluation and represents a new method. The results to be obtained in this project will allow us to get a much more detailed understanding of when and how blockage of concrete occurs and how such a blockage can be avoided. Combined with the research results from funding period FP I, a complete picture of practically relevant segregation mechanisms (FP I: bleeding and sedimentation; FP II: blocking) will be obtained. Secondly, by incorporating obstacles with different geometrical configurations in the open channel and studying the engulfing of these obstacles, information on the rheological properties of the concrete shall be gained. This information can be used as a quality test method for acceptance testing of fresh concrete. The first step of the working programme consists in developing the open channel for the experiments. Here, the submitters can build up on extensive preliminary work, which will be outlined. In the following work package, the flow behaviour without and with model obstacles – such as square or spherical bodies – inserted in the flow channel is studied. Our pre-tests clearly show, that by studying the engulfing behaviour of these obstacles, conclusions on the rheological properties of the concrete can be drawn. In the subsequent step, the effect of obstacles onto the blockage behaviour is studied. The emphasis is hereby laid onto investigating which concrete compositions are more likely to exhibit blocking and how this risk can be predicted based on the rheological properties of the concrete. Finally, it is the goal to develop a rheological model with which on the one side the open channel flow behaviour of the concrete can be quantified and used for reverse analysis to determine rheological parameters such as yield stress and plastic viscosity from open channel flow observations. The second part of the model consists of a hydrodynamics-based engineering approach, which is intended to quantify the possible risk for blockage.