The heating up of high-performance concretes under fatigue load in cyclic pressure threshold test raises the question of the mechanisms and influences of this heat development. Of particular interest is how the heating affects the fatigue strength and what the causes of this are. In order to be able to answer this basically, two test series-one with an HPC and one with a UHPC-were carried out in which the temperature influence on the static compressive strength in a range of À25 C to 90 C was determined. In addition, different storage conditions are being considered in order to question previous findings. In this article, the state of the art is briefly presented and the test program based on it. Then, the production and storage of the specimen, their preparation and the used measurement technique are explained. The results obtained are presented with regard to concrete strength, type of storage, moisture content and, in the main part, the influence of temperature. A discussion of the results in relation to the state of knowledge follows. General insights are acquired, which will be very helpful for further work in the field of fatigue research on high performance concretes.

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.

High-performance concrete (HPC) is a topic of current research and construction projects, due to its outstanding compressive strength and durability. In particular, its behaviour under high-cycle fatigue loading is the focus of current investigations, to further pave the way to highly challenging long-lasting constructions; e.g., bridges or offshore buildings. In order to investigate the behaviour of HPC with different moisture contents in more detail, a mixture of silica sand and basalt aggregate with a maximum grain size of 8 mm was investigated with three different moisture contents. For this purpose, cyclic compressive fatigue tests at a loading frequency of 10 Hz and different maximum stress levels were performed. The main focus was the moisture influence on the number of cycles to failure and the development of concrete temperature and strain. In a further step, only the mortar matrix was investigated. For this purpose, the mixture was produced without basalt, and the moisture influence was investigated on smaller-sized test specimens using dynamic mechanical analysis (DMA) and X-ray computed tomography (XRCT). It was shown that the moisture content of HPC had a significant influence on the fatigue damage behaviour due to the number of cycles to failure decreasing significantly with increased moisture. In addition, there was also an influence on the temperature development, as well as on the strain development. It was shown that increasing moisture content was associated with an increase in strain development. XRCT scans, in the course of the damage phases, showed an increase in internal cracks, and made their size visible. With the help of DMA as a new research method in the field of concrete research, we were also able to measure damage development related to a decrease in sample stiffness. Both methods, XRCT and DMA, can be listed as nondestructive methods, and thus can complement the known destructive test methods, such as light microscopy.

Due to research activities on the topic of concrete mixtures during the last decades concrete compressive strength could be increased significantly. At the same time the lifespan of concrete structures has been continuously extended. Therefore, the fatigue behavior of modern High Performance Concrete (HPC) becomes more important with respect to service life. Previous studies on normal strength concrete have shown that the number of cycles to failure under pure compressive cyclic loading mainly depends on frequency and amplitude. An increase of the loading frequency leads to a higher number of load cycles to failure. Recent research work shows that the humidity of the specimen is also influencing the fatigue behavior of concrete. Conditioning of the test specimen to high humidity led to significant lower fatigue life. Other research activities on high strength concrete point out the effect of the loading frequency on load induced heat effects of the concrete. The experimental study presented here aims on the influence of the humidity on the temperature development under fatigue compressive loading and on the number of cycles to failure. It comprises three fatigue test series at a test frequency of 10 Hz with sealed cylindrical specimens of the same concrete with different degrees of humidity. In addition, destructive and non-destructive tests were carried out on damaged and undamaged concrete.

No abstract