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

混凝土的疲劳是由于微裂缝引起的砂浆基质的降解。本项目的目的一方面是为了研究高强混凝土中这些微裂纹的形核和演化，区分水泥浆体中的微裂纹和骨料边界区的微裂纹。另一方面，将研究细纤维(超细纤维、碳纤维)是否以及在多大程度上影响这些微裂纹的形成。在第一个资助期，分别对超细纤维与高强砂浆基质之间的粘结行为以及水泥净浆和高强混凝土中的损伤演化进行了试验和数值研究。用计算模型描述了循环裂纹张开和闭合的机理。以往的研究表明，在CSH水平的水泥浆体和在细观水平的高强混凝土中循环加载引起的结构变化以及它们之间的相互作用的分析仍然没有答案。这些将在第二个资助期的项目A中进行数字和实验研究。在B项目中，研究了不同纳米和微纤维对硬化水泥浆体和高强混凝土疲劳性能的影响。除了第一个资助期已经使用的微型钢纤维和碳纤维外，这里还将包括碳纳米管。由于碳纤维和微型钢纤维的尺寸不同，在高周反复荷载作用下，碳纤维和微型钢纤维的裂纹桥联机制可能不同。项目项目C的重点是根据项目项目A和B中开发的模型，从材料(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.