Institut für Tragwerksentwurf  |  FK3 Architektur Bauingenieurwesen Umweltwissenschaften   |  Technische Universität Braunschweig
 
 
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The undirected distribution of steel fibrer in pre-fabricate concrete elements is of substantial nature. The goal of this investigation is to increase the local mechanical properties of specific concrete elements by controlling the alignment of the fibres according to the flow of forces. The investigations cover theoretical consideration and experimental research of the interaction between ferrimagnetisms and the steel fiber orientation in freshly cast fibre reinforced concrete.

UHPFRC (Ultra-High Performance Fiber Reinforced Concrete) as a composite consists of two members providing two properties: The high compressive-resistance of the matrix and the tensile strength of the fibers. Since every structural member is submitted to specific local forces, whereas the distribution of fibers is extensively undirected, conventional UHPFRC members contain more fibers than structurally needed. Novel members providing fibers aligned in zones where they are effectively needed would reduce the content of fibers. Accordingly, the entire member could be designed in a more effective, filigree and sustainable way. Furthermore the post-modeling of reinforced areas before hardening could replace the complex pre-fabricated reinforcement structures.

The ongoing research covers the investigation of the ferromagnetic realignment applied on three crucial zones of structural elements: The frontal connection zone of thin shell element, the laminar region of shell structure elements and the tensile zone of beams.
In addition three different types of steel fibers were respectively investigated: micro wire fibers, glued fibers with hooked ends and corrugated fibers.
For the first investigation form-locking frontal connections between two segments of thin shells were tested, including three different tooth-geometries.
For the second investigation perpendicular and linear grids of magnets were applied on the laminar region of molds for shell segments. The inner thickness of the mould was 20mm.
The third investigation aimed on the optimization of the tensile zone of a specimen. The dimensions were 40x40x160mm.
As a translucent substitute with similar viscosity for still liquid UHPFRC, steel fibre enriched medical ultrasonic gel was confected and used in order to achieve immediate visual control over the influence of the magnetic fields on the movement of the fibers. All moulds were made of pellucid material. The comparability of the viscosity of liquid UHPFRC and the substitute was verified with the aid of computer topographical (CT) scans of cast samplings. The chosen concrete was an UHPFRC containing 2,5Vol% of fibers. All manipulators were ultra-strong neodymium (NdFeB, N-52) magnets of different dimensions and polarity.

The results of the tests  showed repeatable interaction between the force fields and field lines of the different magnets, the geometry of the tested fibbers and the geometry of the mould. The desired micro-overlap of parallel-orientated fibrer in the tensile zones could be achieved by using sets of magnets: They were aligned due to their specific characteristics and the particular choice of the fibers. Using the effect of moving these sets on the outside of the moulds in a rotary way, the concentration of fibres affected by the magnets could be increased drastically. A following multi-stage treatment of different types and movement of the magnets led to an additional refinement of the treated zones.

The ongoing research will include deeper investigations of the described methods and the verification of the structural benefits by comparative stress tests. The development of a CNC-based automation of magnetic rearrangement of steel fibers in UHPFRC is one of the prospective goals of this research.