Comportement des éléments en béton armé fléchis sous charge cyclique

For various types of civil structures such as bridges, skyscrapers and other slender towers, oil rigs as well as runaways, dynamic loading is predominant. This holds true especially for bridges where loads have increased considerably in the last hundred years. It is in this context that further research in the behaviour of concrete elements subjected to cyclic loading is justified and required. Within this specific field of research there exist multiple angles of attack. On the one side, one can try to better understand the fatigue life by doing long-term analysis with several million cycles to predict the failure under fatigue loading depending on stress ranges, rebar types and other parameters. On the other hand, the study of the cracking behaviour of elements under cyclic loading is just as pertinent as it allows a more fundamental understanding of the mechanical process to which cracks are subjected. This second area of research includes fracture mechanics, residual stresses as well as the loading type (tension or bending). Three beam type samples with geometry of (l = 3.00 m, b = 0.30 m and h = 0.32 m) are tested under cyclic loading, applying three different levels of load (24, 50 and 80 kN). The reinforcements are limited to longitudinal bending without any transverse ties. To observe shear as well as flexion cracks on the same sample, a 4-point bending configuration is chosen and at least 50 loading-unloading cycles are performed until stabilisation crack opening is observed. For most of the past research on structural elements which are similar to those tested for this project, strains are usually measured by gluing strain gauges onto a sample leading to very localised strain data. In the present case, a recently acquired system from LUNA FIBRE OPTICS using optical fibers which are glued onto the reinforcement bars is used to measure the complete strain profile all along the sample giving a much more detailed insight into the variations of the strain, especially around cracks. During the course of this project, all necessary procedures with regard to fiber handling and data acquisition are developed and tested. It could be shown that optical fibers present a promising addition to existing instruments, allowing for very detailed analysis of strain profiles by increasing the amount of available data considerably. Furthermore it has been shown that even at low numbers of cycles, there is a significant decrease in shear resistance due to uncontrolled shear crack development. While samples SC75 and SC76 (which represent 27% and 56% of the shear resistance of the sample) show stabilisation of crack openings and fiber strain, sample SC77 at 89% of the resistance failed prematurely during cyclic loading. In addition, extensive crack analysis from DIC imaging as well as analysis of fiber strain data has demonstrated the different behaviour of flexion and shear cracks. In addition it could be shown that the shear of reinforcement bars in pure bending zones due to the eccentric position is not at all negligible.