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The limited understanding of the fatigue behavior of reinforced and prestressed concrete members is one of the reasons why many structures do not reach their expected service life. Without a deeper theoretical understanding of the fatigue phenomena in the various fatigue process zones, reliable fatigue life predictions are not possible. This paper provides a classification of the major fatigue process zones and mechanisms in reinforced and prestressed concrete members, accompanied by a brief review of recent developments in design rules, experimental characterization, and modeling approaches specific to each fatigue process zone and mechanism. The limitations of current approaches to fatigue characterization and modeling are also discussed, highlighting the need for further research in this area. [ABSTRACT FROM AUTHOR]

The establishment of a simple engineering rule for predicting the fatigue failure of concrete has been pursued over the past decades. An energetic approach to the matter seems to be an attractive option that many researchers have embraced. In the present work, the authors attempt to contribute to the establishment of such a rule. In particular, the energy dissipation of confined concrete subjected to shear cyclic loading is studied and quantified. For this purpose, a microplane fatigue model recently introduced by the authors, referred to as MS1, is used. It aims to capture the fundamental inelastic mechanisms driving the tri‐axial stress redistribution within a material zone during the fatigue damage process in concrete. To this end, the fatigue damage evolution is linked to a measure of cumulative inelastic shear strain at the microplane level, reflecting the accumulation of fatigue damage due to internal shear/sliding between aggregates at subcritical pulsating load levels. To isolate the dissipative mechanism mentioned above, test configurations with dominant shear stress seem to be more appropriate. In the present work, a punch‐through shear test (PTST) FE model is used to induce shear‐dominated stresses and strains along the ligament of a specimen. Numerical studies are first presented to evaluate the behavior and energy dissipation at the elemental interface level. The interface is introduced in the MS1 microplane material model, which is capable of reproducing the concrete behavior under monotonic, cyclic, and fatigue loading with consistent set of material parameters. Quantification of the energy dissipation for each introduced dissipative mechanism is performed at each microplane and integrated via a well‐established homogenization scheme to evaluate the macroscopic energy dissipation. Later, an analysis of the energy dissipation of the PTST process zone is performed for cyclic loading under two different subcritical cyclic load amplitudes. [ABSTRACT FROM AUTHOR]

The fatigue of concrete has been the subject of research for many years, and yet, there are still open questions. In particular, the fatigue‐induced damage evolution accompanied by a stress redistribution process propagating through the concrete structure is still not fully understood. So far, there are only few experimental studies addressing the fatigue propagation through a material zone occurring, for example, in beams with a pulsating, nonuniform stress profile. To investigate the influence of such stress configurations on the material and structural degradation process, a measuring concept has been developed combining digital image correlation, fiber optic sensors, and conventional strain gauges. The presented experiments visualize the propagating fatigue degradation in the compression zone of prestressed concrete beams subjected to fatigue loading. Based on the developed measurement concept, the quantitative foundation for a comprehensive validation of design rules and material models accounting for concrete fatigue can be significantly extended. Highlights: Design of a test setup for the analysis of compressive fatigue propagation in concreteEvaluation of a coordinated monitoring concept combining various measurement techniquesVisualization of the propagating fatigue degradation of prestressed concrete beams [ABSTRACT FROM AUTHOR]

The load scenario with a compressive force applied to an anchored bar (push-in mode) has not been sufficiently addressed so far with reference to fatigue, since most of the studies available in the literature are focused on bond behavior under tensile forces (pull-out mode). However, special structures like the towers of wind turbines subjected to alternating moments (and tensile-compressive forces) due to the variable wind direction, are fostering the interest for the fatigue behavior of concrete-bar bond under compressive forces, all the more because several millions of load cycles may be applied. An extensive experimental investigation has been carried out in this research project using a modified beam-end test in the push-in mode. Fifty beam-end specimens were tested under monotonic loads, as well as under low-cycle and high-cycle fatigue loads, with different bar diameters and bonded lengths. The failure modes and bond behavior with the end sections of the anchored bars either unloaded (free ends) or pushing against the concrete (compression ends exhibiting the well-known tip-bearing effect) were investigated as well. The results of the cyclic tests in the push-in mode are compared with those available in the literature in the pull-out mode, through the usual Wöhler curves and fatigue-induced creep curves. Based on this comparative analysis, the failure mechanisms of bond under fatigue loading are identified, with specific attention to the interaction between bond behavior and splitting cracks. [ABSTRACT FROM AUTHOR]

A realistic prediction of the concrete fatigue life exposed to high-cycle loading scenarios with variable amplitudes is of utmost importance for a reliable and economically efficient design of civil engineering infrastructure for transport and energy supply. Current design codes estimate the fatigue life under variable amplitudes using the Palmgren–Miner rule, which assumes a linear scaling between lifetimes measured for uniform cyclic loading scenarios. Several experimental series conducted in the past, however, indicate that this assumption is not valid and that it may lead to unsafe design. In this paper, an experimental and theoretical investigations of the fatigue loading sequence effect in normal- and high-strength concrete behavior are presented, which confirm this observation. In particular, a test campaign with 135 cylinder specimens, including three concrete grades and six different loading scenarios has been conducted. Several response characteristics of the fatigue behavior including Wöhler curves, fatigue creep curves and evolving shapes of hysteretic loops have been evaluated. To substantiate the experimental results, a theoretical explanation of the observed sequence effect is formulated based on the assumption, that energy is dissipated uniformly within the volume of a test specimen during subcritical, compressive cyclic loading. Then, superposition of energy dissipation profiles along the lifetime measured for constant amplitudes becomes possible and a theoretical justification of the experimentally observed sequence effect can be provided. Moreover, a reverse sequence effect reported in the literature for bending fatigue of concrete can then be explained by an unevenly distributed energy dissipation over a cracked specimen. Supported by the theoretical consideration, the processed experimental data is used to validate existing fatigue life assessment rules by testing their ability to reflect the load sequence effect. [ABSTRACT FROM AUTHOR]