Fatigue degradation of reinforced concrete under predominantly compressive loading

近几十年来，混凝土在主要压力下的疲劳变得更加重要，特别是在新的结构设计中，与传统应用相比，混凝土承受更高的应力水平和更大的载荷循环。虽然目前对混凝土疲劳的研究主要集中在均匀的未受干扰的应力场的状态，通过圆柱体试验进行评估，但工程实践中的实际观察表明，结构疲劳寿命的现实预测还必须考虑应力不均匀性的影响。这种不均匀性导致钢筋周围的局部退化，导致预期的结构寿命大幅下降。本研究建议的主要目的是促进理解一个特定的钢筋布局如何影响混凝土的压缩疲劳性能，占交叉钢筋的破坏性影响。此外，研究的目的是调查钢筋方向相对于周围的混凝土基体中的主导应力方向的影响。通过解决这些问题与混凝土的时间依赖性的行为，这项研究将包括疲劳引起的退化在关键区域的钢筋混凝土结构进行数百万次的加载循环，如钢筋和预应力桥梁和塔的混合风力发电厂。为了实现这一目标，研究将结合联合收割机量身定制的实验方法与先进的，物理上严格的，和物理上一致的数值模拟方法的发展。实验表征方法的设计目标是隔离疲劳引起的退化机制，特别强调钢筋和混凝土之间的相互作用区。建立的实验和数值框架，捕捉疲劳现象，将被用来研究三轴疲劳引起的应力重分布和时间相关的徐变和收缩发展的钢筋混凝土构件的疲劳易发区域之间的相互作用。从这项研究中获得的结果将作为改进和更现实的预测疲劳响应的区域，钢筋和混凝土相互作用，表现出不均匀的应力场的基础。基于这些改进，能够区分结构细节中特定应力配置的工程设计和评估规则将变得可行。此外，为了引入一个更现实的寿命评估与变幅循环疲劳载荷，增强评估规则的Palmante-Miner规则具有更广泛的有效性范围将被提出。这些成果将有助于通过延长疲劳寿命来提高结构混凝土设计的可持续性。

In recent decades, the fatigue of concrete under predominant compression gained a higher significance, especially in the context of new structural designs that subject concrete to higher stress levels and increased load cycles compared to traditional applications. While current research on concrete fatigue focuses primarily on states with uniform undisturbed stress fields, as evaluated through the cylinder test, real-world observations in engineering practice indicate that realistic predictions of structural fatigue life must also consider the impact of stress non-uniformity. This non-uniformity results in localized degradation around rebars, leading to a substantial decrease of expected structural lifetime. The primary objective of this research proposal is to advance the understanding of how does a particular reinforcement layout influence the compressive fatigue behavior of concrete, accounting for the disruptive effects of crossing steel rebars. Furthermore, the research aims to investigate the effect of rebar orientation relative to the dominant stress direction in the surrounding concrete matrix. By addressing these questions in conjunction with the time-dependent behavior of concrete, this study will encompass the fatigue-induced degradation in critical zones of reinforced concrete structures subjected to several millions of loading cycles, such as reinforced and prestressed bridges and towers of hybrid wind power plants. To accomplish this goal, the research will combine tailor-made experimental methods with the development of advanced, physically rigorous, and thermodynamically consistent numerical modeling approaches. The experimental characterization methods will be designed with the goal to isolate the mechanisms governing the fatigue-induced degradation, with a particular emphasis on the interaction zones between steel rebars and concrete. The established experimental and numerical framework, capturing the phenomenology of fatigue, will be utilized to investigate the interaction between tri-axial fatigue-induced stress redistribution and time-dependent creep and shrinkage development in fatigue-prone areas of reinforced concrete members. The results obtained from this research will serve as a basis for improved and more realistic predictions of fatigue response in regions where reinforcement and concrete interact, exhibiting non-uniform stress fields. Based on these improvements, engineering design and assessment rules capable of distinguishing specific stress configurations in structural details will become feasible. Additionally, in order to introduce a more realistic lifetime assessment for cyclic fatigue loading with variable amplitudes, enhanced assessment rule to Palmgren-Miner rule with a broader range of validity will be proposed. These achievements will contribute to the overarching aim to increase the sustainability of structural concrete design by extending the fatigue lifetime.