Fatigue degradation of reinforced concrete under predominantly compressive loading

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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.