Water-induced damage mechanisms of cyclic loaded high-performance concretes

海上风能系统的扩张导致混凝土结构的数量增加，这些结构在不断与水相互作用的同时，经历了超过10^8的加载周期。在第一个资助期（FP I），通过声发射测量（Lohaus），水引起的“饱和损伤”（发生在载荷减少期间）与大气疲劳损伤机制（高于平均载荷）明确区分。此外，通过核磁共振研究表明，损伤效应与硬化水泥浆纳米孔系统中的水重定位有关（Haist）。同时，自己的数值发展也清楚地证明了干混凝土和饱和混凝土在退化行为上的差异（Wriggers/Aldakheel）。尽管取得了这些进展，但对其潜在机制的详细了解仍然缺乏。要描述的效应发生在如此微小的尺度上，因此在实验和数值部分的进一步研究更强烈地倾向于纳米级效应的测量。破坏机制的建模必须进一步完善，以解释微观结构的复杂特性，如孔隙的产生、生长和聚并。FP II的目标是在FP i期间开发的建模方法的基础上，量化、更好地理解、描述和建模混凝土疲劳破坏期间的“饱和损伤”机制。为此，必须首先使用声发射频率分析来区分“干燥”和“潮湿”条件下不同类型的疲劳损伤。此外，必须对加载频率或加载前存在的损伤等影响参数进行量化。为了更好地理解其工作机制，将在纳米和微观尺度上研究孔隙系统内的水重分配过程，并检测硬化水泥浆和砂浆的微观结构损伤和力学性能变化，然后将其转化为简化的分析工程模型。最后，在纳米尺度上发生的损伤过程将通过适当的均质化技术在微观结构水平上进行模拟。该模型还拟借助循环跳变技术来描述混凝土水致疲劳损伤的演化过程，从而实现对混凝土水致疲劳损伤的完整数值描述和预测。利用所开发的方法，将首次在实验-虚拟实验室中实现基于微观结构参数的高性能混凝土水下疲劳退化行为描述。

The expansion of offshore wind energy systems is causing the number of concrete structures which experience loading cycles in excess of several 10^8 while constantly interacting with water to increase. During the first funding period (FP I) a water-induced “saturation damage” (occurring during load decrease) was unmistakeably distinguished from an atmospheric fatigue damage mechanism (above average load) by acoustic emission measurements (Lohaus). Additionally, it was demonstrated by way of NMR-investigations that damage effects correlate with water relocations within the nano-porous system of the hardened cement paste (Haist). Own numerical developments also clearly demonstrated the difference in the degradation behaviour between dry and saturated concrete in the meantime (Wriggers/Aldakheel).In spite of these advancements a detailed understanding of the underlying mechanisms is still lacking. The effects to be described occur at such minute scales that further investigations in the experimental and numerical portions are being more strongly oriented towards measurement of nanoscopic effects. The modelling of failure mechanisms must be further refined in order to account for the complex properties of the microstructure, such as the creation, the growth and the coalescence of pores.The objective of FP II is to quantify, better comprehend, describe and model the mechanism of “saturation damaging” during fatigue failure of concrete, building on the modelling approaches developed during FP I. For this purpose, the different types of fatigue damage in “dry” and “moist” conditions must first be differentiated from one another using the analysis of acoustic emission occurrences. Further the influencing parameters such as loading frequency or damage existing prior to loading must be quantified. In order to better understand the mechanisms at work, water redistribution processes within the pore system on the nanoscopic and microscopic scales will be investigated and resulting microstructural damages and changes in the mechanical properties of the hardened cement paste and mortar will be detected, which may then be translated into a simplified analytical engineering model. Finally, damage processes occurring at nanoscopic scale are to be modelled at the microstructural level by way of adequate homogenisation techniques. This model is intended also to portray the damage evolution with help of cycle-jump-techniques, so that finally the water-induced fatigue damage of concrete may be completely numerically depicted and predicted.With the developed methods the description of the degradation behaviour of fatigue loaded high-performance concrete under water on the basis of microstructural parameters will become possible for the first time in the Experimental-Virtual-Lab.