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

现代混凝土结构的趋势是向更薄、更轻、更复杂的形状和更紧密的钢筋构件发展。对于这样的部件，需要相对柔软和可流动的混凝土，这些混凝土必须在额外的振动下才能完全填充整个模板。而且在传统的混凝土施工中，更多的流态混凝土被用来实现更好的混凝土的加工性能，更快的施工进度，甚至是混凝土安装过程的先进机械化和自动化。现代建筑化学对新拌混凝土性能的进一步发展做出了重大贡献，特别是通过提供高效的高效减水剂。然而，与此同时，这增加了建筑材料系统的复杂性。到目前为止，自密实混凝土(SCC)的发展是这些趋势的顶峰。然而，到目前为止，SCC在德国很少使用，因为它不能因为隔离的风险而受到振动。这是由于SCC在应用中的敏感性增加。对于非常狭窄的钢筋布置和复杂的混凝土浇筑情况，即使SCC的高流动性也不总是足够的，因为不是所有的空洞都可以在没有额外振动能量的情况下安全填充。此外，在普通混凝土领域，随着更软或更易流动的稠度，离析的风险也会增加。无论如何，目前的破坏情况表明，随着建筑材料系统流动性和复杂性的增加，以及在困难和狭窄的混凝土条件下，混凝土的稳定性问题可能会发生。虽然在自密实混凝土领域已经有了确保混凝土足够稳定性的有充分依据的方法，但对于振动下的软混凝土到可流动混凝土，还缺乏类似的方法。因此，在工程模型方法的基础上描述振动作用下的流态混凝土的离析稳定性。模型方法是基于对胶凝悬浮体中结构的堆积和结构破坏过程的描述。应记录早期水化产物对悬浮液剪切和时间相关行为的影响。通过考虑混凝土的组成，可以同时得出提高流动混凝土在振动下的离析稳定性的优化策略。

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.