Damage development of steel fibre reinforced high performance concrete in high cycle fatigue tests

In this contribution, damage development in high cycle fatigue of steel fibre reinforced high and ultra-high performance concretes is described and analyzed. For this purpose, high cyclic flexural tests were conducted on one HPC mixture containing macro steel fibres with single end anchorages and one UHPC mixture with smooth micro steel fibres. The concrete mixtures investigated range from fibre-free reference samples through specimens with 0.3 vol.-% fibre content, showing typical softening behavior after initial cracking in static tests, up concretes with fibre contents of 0.75 and 1.5 vol.-%, displaying strain-hardening behavior. The development of damage itself is evaluated by three indicators: the plastic crack mouth opening at lower load level for each load cycle, the residual stiffness in each loading cycle, and the accumulated damaging energy, i.e. the work performed on the crack opening until the respective cycle. When testing specimens without pre-cracks, a succession of two fatigue tests occurs, firstly fatigue of pure concrete matrix before cracking and secondly fatigue of the fibre concrete composite after crack initiation. However, this behavior can be smeared depending on the fibre content and fibre position at the crack opening. The damage indicators presented here show distinct changes in damage development depending on the fibre content. Furthermore, the development of the selected damage parameters is primarily related to the amount of fiber used, while the respective top load level on the specimen has a negligible influence. All damage parameters behave qualitatively the same for both mixtures considered. Overall, the residual stiffness and the damage energy seem to provide a greater selectivity than the plastic portion of the crack opening.