The influence of a wet environment on the fatigue behaviour of high-strength concrete has become more important in recent years with the expansion of offshore wind energy systems. According to the few investigations documented in the literature, the fatigue resistance of specimens submerged in water is significantly lower compared to that of specimens in dry conditions. However, it is still not clear how the wet environment and the moisture content in concrete influence its fatigue behaviour and which damage mechanisms are involved in the deterioration process. Here the results of a joint project are reported, in which the impact of moisture content in concrete on fatigue deterioration are investigated experimentally and numerically. Aside from the number of cycles to failure, the development of stiffness and acoustic emission (AE) hits are analysed as damage inductors and discussed along with results of microstructural investigations to provide insights into the degradation mechanisms. Subsequently, an efficient numeric modelling approach to water-induced fatigue damage is presented. The results of the fatigue tests show an accelerated degradation behaviour with increasing moisture content of the concrete. Further, it was found that the AE hits of specimens submerged in water occur exclusively close to the minimum stress level in contrast to specimens subjected to dry conditions, which means that additional damage mechanisms are acting with increasing moisture content in the concrete.

The results of compressive fatigue investigations on four high-strength concretes and their corresponding mortars are presented. The influences of coarse aggregates generally, the substitution of basalt coarse aggregate by granite, the addition of silica fume and the variation of the water to cement (w/c) ratio are investigated systematically. The numbers of cycles to failure, the developments of strain, stiffness, dissipated energy and acoustic emission hits are focused on in the analyses. The results clearly show that coarse aggregates can influence the fatigue behaviour of concretes in a negative way at higher stress levels and in a positive way at lower stress levels compared to mortars. The granite coarse aggregate decreases the adverse effect at higher stress levels due to its lower modulus of elasticity compared to that of the basalt aggregate. Silica fume improves the fatigue behaviour of concrete and mortar strongly. The increase of the w/c ratio and, thus, the increase in porosity reduces the fatigue resistance of concrete and mortar significantly, due to the weakening of the mortar matrix and of the interfacial transition zone. The results demonstrate the interaction of the coarse aggregates and the mortar matrix with their specific properties, which leads to a certain fatigue behaviour. The acoustic emission gives additional valuable strain-independent information of the damage processes occurring, possibly also on micro- and nanoscales.

The influence of the compressive strength of concrete on fatigue resistance has not been investigated thoroughly and contradictory results can be found in the literature. To date, the focus of concrete fatigue research has been on the determination of the numbers of cycles to failure. Concerning the fatigue behaviour of high-strength concrete (HPC) and, especially, ultra-high-strength concrete (UHPC), which is described by damage indicators such as strain and stiffness development, little knowledge is available, as well as with respect to the underlying damage mechanisms. This lack of knowledge has led to uncertainties concerning the treatment of high-strength and ultra-high-strength concretes in the fatigue design rules. This paper aims to decrease the lack of knowledge concerning the fatigue behaviour of concrete compositions characterised by a very high strength. Within the priority programme SPP 2020, one HPC and one UHPC subjected to monotonically increasing and cyclic loading were investigated comparatively in terms of their numbers of cycles to failure, as well as the damage indicators strain and stiffness. The results show that the UHPC reaches a higher stiffness and a higher ultimate strain and strength than the HPC. The fatigue investigations reveal that the UHPC can resist a higher number of cycles to failure than the HPC and the damage indicators show an improved fatigue behaviour of the UHPC compared to the HPC.