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

海上风能系统的扩张正在导致混凝土结构的数量增加，这些混凝土结构在不断与水相互作用时经历超过几次10^8次的加载循环。在第一个资助期(FP I)，通过声发射测量(Lohaus)，准确地将水引起的“饱和损害”(发生在减载期间)与大气疲劳损害机制(高于平均负荷)区分开来。此外，通过核磁共振研究证明，损伤效应与硬化水泥浆体(HAIST)的纳米多孔系统中的水重新定位有关。自己的数字发展也清楚地表明了干混凝土和饱和混凝土之间的降解行为的差异(Wriggers/Aldakheel)。尽管取得了这些进展，但仍然缺乏对潜在机制的详细了解。将要描述的效应发生在如此微小的尺度上，以至于在实验和数值部分的进一步研究正更强烈地面向纳米效应的测量。失效机理的建模必须进一步改进，以考虑到微观结构的复杂性质，如孔洞的产生、生长和结合。FP II的目标是在FP I期间发展的建模方法的基础上，量化、更好地理解、描述和模拟混凝土在疲劳破坏过程中的“饱和损伤”机制。为此，必须首先通过对声发射发生的分析来区分“干”和“湿”条件下不同类型的疲劳损伤。此外，还必须量化加载频率或加载前存在的损伤等影响参数。为了更好地了解作用机理，将在纳米和微观尺度上研究孔隙系统中的水重新分布过程，并检测由此导致的微观结构损伤和硬化水泥浆体和砂浆力学性能的变化，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化技术在微观结构水平上模拟。该模型还旨在借助循环跳跃技术来刻画混凝土的损伤演化，从而最终能够完整地数值描述和预测混凝土在水中的疲劳损伤。利用所开发的方法，首次在实验-虚拟实验室中描述基于微观结构参数的疲劳加载高性能混凝土在水下的退化行为。

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.