Effect of uncertainty on load position in the fatigue life of steel-fiber reinforced concrete under compression

摘要

The high scatter of the compressive fatigue life of concrete and fiber reinforced concrete, which may normally reach two orders of magnitude, is caused not only by material heterogeneity, but also by the random nature of the unwanted eccentricity during the tests. It mainly arises from misalignments due to manual specimen centering, although tolerances in experimental equipment and in the geometry of the specimens may cause eccentricity too. In order to minimize it, an individualized ball-and-socket joint (i-BSJ) was designed for this research. An instrumented aluminum cube was used to characterize the probabilistic distribution of the undesired eccentricity under two testing conditions, that is, with and without the i-BSJ. The standard deviation of the loading eccentricity was one order of magnitude smaller with the new device. Secondly, two series of 15 compressive tests and two series of 15 fatigue tests were performed on 40 mm edge-length steel-fiber reinforced concrete cubes, again with and without the special device. The fatigue series range from 24 to 88% and from 36 to 82% of their respective compressive strength, f(c). The use of the i-BSJ produced an increase of 10% in the average f(c) while its standard deviation decreased by 40%. Similarly, the average fatigue life tripled for the first series and was six times longer for the second series, notwithstanding the loading limits in the low-eccentricity series referring to the enhanced f(c). Correspondingly, the standard deviation of the number of cycles until failure decreased by 50% in the two fatigue series performed with the i-BSJ.