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The fatigue behavior of plain concrete has been studied for decades, usually under compressive or tensile loading. Shear loading (mode II) has been almost completely neglected in the past. In contrast to cylindrical compression tests, this type of loading offers the advantage of precise load determination and a small, well-defined fracture surface. This paper presents a comprehensive experimental campaign of 66 shear tests, which was conducted to systematically investigate the monotonic, cyclic, and fatigue response of high-strength concrete under mode II loading. Since the material behavior under shear stress is strongly dependent on the concurrent lateral compressive stress, a new test setup was developed which allows simultaneous control of compressive and shear loading. One potential utilization for these shear fatigue tests is the validation of a promising hypothesis that suggests that the development of fatigue damage in concrete at subcritical load levels is governed by a cumulative measure of shear sliding. The qualitative influence of the lateral compressive loading on the displacement and damage development, fracture behavior, and fatigue life is analyzed and discussed. The test results indicate that there is no influence of the lateral compressive load level on the shear fatigue life, as long as the increase in shear strength is considered. Furthermore, concrete under mode II loading seems to have a longer fatigue life than concrete in standard cylindrical specimens under compressive loading.

<p>A cost-efficient yet safe design of reinforced and prestressed concrete structures under fatigue loading is a highly complex and elaborate task. One of the main reasons for this is the still insufficient understanding of the fatigue damage phenomenology of concrete. A promising hypothesis states that the evolution of fatigue damage in concrete at subcritical load levels is governed by cumulative internal sliding between aggregates. With the objective of collecting experimental results supporting the fundamental postulated hypothesis, a systematic experimental campaign was developed to investigate the fatigue behavior of high-strength concrete under shear-compression loading using newly adapted Punch-Through-Shear-Tests (PTSTs). The test setup is capable of applying a controlled fatigue shear loading with simultaneous constant fatigue compressive loading, without causing secondary cracks. Various phenomena of confined concrete shear fatigue behavior, such as the evolution of confinement during fatigue life and the loading and unloading behavior, are discussed.</p><p>Finally, numerical studies reproducing experimental results using a pressure-sensitive fatigue interface model are presented. This material model is able to capture the material degradation due to internal sliding between aggregates, as the fatigue damage evolution is linked to a measure of the cumulative shear strain. Simulations at the single material point level showed that the model can reproduce the evolution of the shear and confining stresses under monotonic loading, as well as its ability to simulate pre- and post-peak cyclic behavior. The material model was used as well in a FEM simulation for modeling the behavior of the PTST. The calculated results show good agreement with experimental tests and allow a more profound investigation of the dissipative mechanisms occurring in the process zone.</p>

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.