Influence of microfibers on the degradation of high performance concrete under cyclic loading

混凝土疲劳的特征是由于微裂纹导致砂浆基质退化。本项目的目的是，一方面，研究这些微裂纹的成核和演变在高强度混凝土，区分微裂纹在水泥浆和骨料的边界区。另一方面，将研究这些微裂纹的形成是否以及在多大程度上会受到细纤维（微纤维，碳纤维）的影响。在第一个资助期内，分别进行了实验和数值研究，重点是微纤维和高强度砂浆基质之间的粘结行为以及水泥浆和高强度混凝土中的损伤演化。循环裂纹张开和闭合的机制也使用计算模型进行了描述。以往的研究表明，由于循环荷载在水泥浆体中的CSH水平和高强度混凝土中的细观水平，以及它们的相互作用的结构变化的分析仍然留下的问题没有答案。在第二个供资期，将在项目线A中对这些问题进行数字和实验研究。在项目线B中，研究了不同纳米和微米纤维对硬化水泥浆体和高强混凝土疲劳性能的有效性。除了在第一个资助期已经使用的微钢纤维和碳纤维外，碳纳米管也将包括在这里。由于碳纤维和微钢纤维的尺寸不同，在高周反复荷载作用下，可以观察到不同的裂纹桥接机制。项目领域C的重点是根据项目领域A和B开发的模型，从物质（微观和中观）尺度过渡到宏观尺度，即结构层面。

Fatigue of concrete is characterized by the degradation of the mortar matrix due to microcracking. The aim of this project is, on the one hand, to investigate the nucleation and evolution of these microcracks in high-strength concrete, differentiating between microcracking in the cement paste and in the boundary zone of the aggregates. On the other hand, it will be investigated, whether and to what extent the formation of these microcracks can be affected by fine fibres (microfibres, carbon fibres). In the first funding period, experimental and numerical studies were performed with focus on the bond behavior between microfibres and the high-strength mortar matrix and on the damage evolution in cement paste and high-strength concrete, respectively. The mechanisms for cyclic crack opening and closing were also described using a computational model. Previous studies have shown, that the analysis of structural changes due to cyclic loading in cement paste at the CSH level and in high-strength concrete at the mesoscale level as well as their interactions still leave questions unanswered. These will be investigated numerically and experimentally in project line A in the second funding period. The effectiveness of different nano and micro fibres with regard to fatigue behavior is investigated in project line B both on hardened cement paste and on high-strength concretes. In addition to the micro steel fibres and carbon fibres already used in the first funding period, carbon nanotubes will also be included here as well. Due to their different sizes, it is expected that different crack-bridging mechanisms of carbon and micro steel fibers will be observed under highly cyclic loading. The focus of project line C is the transition from the material (micro- and meso-) scales to the macro-scale, i.e. the structural level, based on the models developed in project lines A and B.