Sedimentation stability of fresh concrete
Julia Neumann, Chair of Building Materials, Ruhr University Bochum, Bochum, GermanyRolf Breitenbücher, Chair of Building Materials, Ruhr University Bochum, Bochum, Germany
In order to meet the high demands placed on modern buildings and structures, concrete has undergone a significant change in its composition. Especially the use of highly effective admixtures, like PCE-superplasticizers and air-entraining agents, enables the necessary adaptation of the fresh and hardened concrete properties (e.g. flowability, high strength, high durability and freeze thaw resistance). As a result the rheological properties of the fresh concrete also change and the tendency to be sensitive to segregation increases. Thus, stability as an extended fresh concrete property is of utmost importance in design and installation process. The workability properties (flowability, stability, etc.) of concrete can be described by rheological parameters. In this context, the paste content, paste properties and aggregate are decisive factors to describe and adjust rheology of conventionally vibrated concrete. In concretes with low paste content the supporting effect of the aggregate ensures the sedimentation stability of the mixture. With rising paste content the paste properties gain in importance as the supporting effect of the aggregate shrinks. However, up to now suitable test methods for assessing stability for design process and control on site and models for describing stability only exist for self compacting concrete. Thus, current investigations focus on the aim to develop test methods for describing and assessing stability. Therefor already available test methods for self compacting concrete, like wash-out-test and sedimentation pipe can be modified for characterizing and assessing stability under vibration. Various computer aided and manual evaluation methods for fresh and hardened concrete tests are applied or developed and tested with regard to their suitability and functionality.