Stability of concrete subjected to vibration – Analysis of the nano- and microscopic structural build-up and structural breakdown behavior of cementitious suspensions

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Trends in modern concrete construction are moving towards slimmer, lighter, more complex shaped and more closely reinforced components. For such components comparatively soft and flowable concretes are required, which have to subject under vibration additionally for full filling the whole formwork. But also in conventional concrete construction more flowable concretes are used to achieve more favorable processability properties of the concrete, a faster construction progress or even to an advanced mechanization and automatization of the concrete installation progress. Modern construction chemistry has contributed significantly to the further development of fresh concrete properties, in particular by providing highly effective superplasticizers. At the same time, however, this increases the complexity of the building material system. The development of self-compacting concrete (SCC) is the culmination of these trends so far. Nevertheless, SCC, which must not subjected to vibration because of the risk of segregation, has rarely been used in Germany until now. This is due to the increased sensitivity of SCC in the application. For very narrow reinforcement layouts and complex concreting situations, even the high flowability of SCC is not always sufficient, since not all cavities can be safely filled without additional vibration energy. Also in the field of normal concrete increases with increasingly softer or flowable consistency the risk of segregation. In any case, current case of damage show that problems with the stability of concrete can occur with increasing flowability and complexity of the building material system as well as under difficult and narrow concreting conditions. Although there are well-founded approaches in the field of SCC ensuring a sufficient stability of concrete, comparable approaches for soft to flowable concretes under vibration are lacking. Therefore the segregation stability of flowable concretes subject to vibration is to be described on the basis of engineering model approaches. The model approaches are based on the description of structural build-up and structural breakdown processes in the cementitious suspension. The influence of early hydration products on the shear- and time-dependent behavior of the suspension should be recorded. By taking account of the composition of the concrete, optimization strategies for increasing the segregation stability of flowable concrete under vibration can be derived simultaneously.