Influence of stress rate and load frequency on the fatigue resistance of concrete

• Applicant: Professor Dr.-Ing. Steffen Marx
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433963337

During the last decades the fatigue behavior of concrete was increasingly investigated. The fatigue behavior was usually tested by using cylindrical concrete specimens that were dynamically loaded. These tests were performed until failure occurred or a defined number of load cycles was reached. During such fatigue tests the specimens were usually loaded with frequencies between fP = 1 Hz and fP = 10 Hz. Especially high cycle fatigue tests (N > 10^3) and very high cycle fatigue tests (N > 10^7) were conducted with a load frequency as high as possible to minimize the test durations and –costs. However, for concrete the applied load frequency has an influence on the number of cycles to failure. This phenomenon is already known and some researchers developed hypotheses that can explain this phenomenon, but according to new investigations, these hypotheses cannot be used unlimited for all stress levels. So far a description of the true damage processes inside the concrete microstructure is missing, so that the reason of the frequency influence is not clarified yet.In this project, the fatigue behavior of concrete under different load frequencies will be experimentally investigated, analyzed and described with a material model. The aim is to confirm the hypothesis that the frequency influence at high related maximum stress levels (Smax > 0.75) is mostly based on the higher value of the concrete strength according to the applied fatigue stress rate. On the other hand it is expected that at lower related maximum stress levels (Smax < 0.75) the frequency influence can be explained with the temperature influence on the concrete strength. In this proposed project the temperature of the test specimens shall be minimized by using a temperature dependend test control. Thus, the separated influence of the stress rate can be analyzed by loading specimens with different frequencies. Together with the temperature influence that is being investigated in a DFG-project the influence of the load frequency can be determined out of these two separate influences. If this hypothesis can be confirmed, this will be a fundamental step for the transferability between laboratory results and the fatigue behavior of concrete in real structural components that are usually loaded with lower load frequencies. Furthermore, the influence of the load frequency on the strain behavior will be investigated. Therefore it is planned to separate the elastic, viscous, damage induced and thermal parts of the measured fatigue strains. This will enable the analysis of the load frequency influence on each strain component. Finally a mechanically based strain model will be developed that considers the load frequency in the calculation of the strain behavior of concrete.

• DFG Programme: Research Grants