Due to the growing demand for renewable energies, the field of offshore wind turbines is becoming increasingly important. In the process of "grouting", ultra-high-performance concrete (UHPC) is used to fill out the annular clearance between the tower structure and the foundation pile (monopile) of wind turbines. UHPC is also used in bridge constructions, leading to a reduction of the dead weight and therefore making it possible to build more delicate structures.

As a common feature of both examples, winds, heavy swell or traffic are acting upon the used concrete. Frequent observations of structural defects suggest that these high-performance concretes are more susceptible to fatigue than plain concrete leading to an increase of research in this field.

Even though UHPC has a highly optimized and extremely dense structure, crack initiation from capillary pores can not necessarily be assumed as it is the case in plain concrete. Hence the aim of the project is to localize the crack initiation and to track the crack growth with its main factors for binder and concrete. On the basis of these results, a multi-scale modeling approach will be developed, which maps the elasto-plastic fatigue behavior in its three characteristic phases of failure (Figure 1).

Figure 1: Cylindrical UHPC or binder specimen under compression load with its typical 3-phases of fatigue failure

For this purpose, UHPC samples are suspended to cyclic compressive loads of up to 50 000 cycles and examined at various points of damage using SEM, TEM and CT. By this means molecules shall be identified (Figure 2), which might initiate and promote the crack propagation in UHPC during fatigue. Moreover, additional information about the distribution of the various components of the UHPC structure is to be gathered.

Furthermore, various load tests on pure cement and aggregate samples are to be carried out in order to obtain material characteristics for the simulation parameters, e.g. Youngs modulus, strength, friction.

Figure 2: TEM with element analyses of UHPC concrete.

Simulations are carried out with the simulation software MUSEN, which was specially developed by the SPE institute. The used bonded-particle model (BPM) is an extension of the discrete element method (DEM), in which the particles are connected by bonds. Since concrete is a structure in which aggregate grains and other additives are compounded by the hydrated cement, this model provides a realistic capability, whereby a major focus is on defining a rheological model of the bonds.

Since cyclic simulations require a high computational effort, different types of simulation have to be performed on different scales (Figure 3): On the microscale, the distribution of the various UHPC components is approximated, whereas on the meso and macro scale, the actual mechanical modeling and simulation takes place. The behavior of the concrete from the meso level (3-phase model with cement, aggregate and contact zone) is then upscaled to create a homogenized 1-phase model at the macro level.

Due to the fact, that it is not possible to run simulations with up to 50,000 load/unload cycles despite upscaling the behaviour, hybrid approaches are to be applied in which certain cycles can be extrapolated in the simulation and thus skipped to minimize the computational effort.

Figure 3: Modelling at micro-/ meso-/ and macro scale