Microcrack and fatigue behavior of high-performance fiber-reinforced concrete under cyclic compressive loading

The ongoing trend to create slender constructions makes the use of high-performance building materials indispensable. One problem with lightweight construction is an increasing vulnerability to fatigue loads caused by time-variant loadings, which may ultimately lead to earlier fatigue failure. Examples include bridge girders, onshore and offshore installations, which are affected by time-variant loadings such as traffic, wind, and waves simultaneously. The design of filigree and slender constructions implies an increased sensitivity to vibrations due to various external loads. This, however, may cause higher fatigue loads under cyclic stresses, which may further lead to damage or even failure of the concrete components before the maximum static strength is achieved. High-performance concretes have a denser matrix in comparison to normal-strength concrete and associated with this is a very brittle material behavior in the event of failure. Steel fibers can positively influence this brittle failure behavior and improve the ductility of the high-strength matrix. The extent to which this can have a beneficial effect on fatigue behavior has not yet been sufficiently researched experimentally. In this context, systematic investigations were carried out to gain knowledge about the influence of two different types of microfibers (steel and carbon). The fibers were selected in such a way that they can bridge microcracks in the range of <50 micrometers. To be able to investigate and compare the influence of the fibers on the fatigue behavior and the damage progress, the strain in the axial direction and an extensive microscopic evaluation of the microcrack behavior were carried out. For this purpose, sections were taken from the specimens according to defined numbers of load cycles, and thick sections were prepared. These thick sections were examined microscopically and evaluated concerning the amount, size, position, and orientation of the microcracks. The number of load cycles to failure tended to be slightly lower for the fiber-reinforced specimens compared to the fiber-free specimens. The strains in the axial direction of the fiber-free specimens are lower until fracture. The material degradation was investigated by means of a microscopic analysis of the microcrack development. This analysis shows that in both high-performance concrete, and fiber-reinforced high-performance concrete microcracking primarily takes place in the interfacial transition zone (ITZ), and in the hardened cement paste, the crack width changed only marginally during the applied load changes, predominantly newly formed microcracks appeared with increasing degradation. This was found in both the fiber-free and fiber-reinforced samples. The fiber-reinforced specimens show a reduced number of microcracks compared to the fiber-free specimens. Additionally, it can be concluded that the increasing degradation is essentially caused by the formation of new microcracks. 1 11th International Conference on Fracture Mechanics of Concrete and Concrete Structures FraMCoS-11 J. M. Chandra Kishen, A. Ramaswamy, S. Ray and R.