Flow modelling of steel fibre reinforced self-compacting concrete

Oldrich Svec

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Concrete is one of the most widely used materials in the world. Ordinary concrete composition makes the material strong in compression yet weak and brittle in tension. Steel reinforced concrete successfully eliminates the weak tensile properties of the ordinary concrete. Steel fibres dispersed in concrete can efficiently substitute or supplement conventional steel reinforcement, such as reinforcement bars. Ordinary concrete composition further makes the material stiff and non-flowable. Self-compacting concrete is an alternative material of low yield stress and plastic viscosity that does flow and fills the formwork with a little or no effort. Steel fibre reinforced self-compacting concrete is a logical combination of the two types of concrete. The combination nevertheless creates several challenges. It has been observed by many authors that steel fibres orient and distribute according to the flow of the fibre reinforced self-compacting concrete. The orientation and distribution of the fibres results in non-homogeneous and non-isotropic mechanical properties of the structural elements.
The primary aim of this research project was to develop a numerical framework capable of predicting the fibre orientation and distribution in structural elements made of fibre reinforced self-compacting concrete. The existence of a such numerical tool is essential for the wider usage of the material. The developed numerical framework is capable of simulating free-surface flow of a suspension of explicitly represented rigid particles immersed in the non-Newtonian fluid. The non-Newtonian fluid was modelled by the novel Lattice Boltzmann fluid dynamics solver. The numerical framework, among others, allows for a two-way coupling between the fluid and the explicitly represented immersed particles. The coupling was done by means of the Immersed boundary method with direct forcing. Evolution of the immersed particles was described by Newton's differential equations of motion. The Newton's equations were solved by means of Runge-Kutta-Fehlberg iterative scheme.
Several challenges had to be overcome during the development of the numerical framework to be able to successfully simulate the flow of the fibre reinforced self-compacting concrete. Fibres are particles of high aspect ratio. To allow for efficient simulation of a large number of the immersed fibres, the fluid domain must be discretized into a coarse grid. The discrete fibre diameter then usually reduces to a sub-grid size, which significantly decreases accuracy of drag forces acting on the fibres. A function was therefore proposed to correct the drag forces. Formwork used in the structural industry is often smooth and slippery which results in an apparent slip of the fluid near formwork surface. A method to incorporate the apparent slip into the Lattice Boltzmann fluid dynamics solver was suggested.
The proposed numerical framework was observed to correctly predict flow of fibre reinforced self-compacting concrete. The proposed numerical framework can therefore serve as an efficient alternative to conventional experimental and analytical tools. Simulations performed by the numerical framework together with the experimental observations revealed several important conclusions: 1) Fibres orient under the flow of fibre reinforced self-compacting concrete. 2) Formwork surface can play an important role in the fibre orientation. 3) Fibre orientation significantly influences mechanical behaviour of the material. 4) A prevailing linear relation between the fibre orientation and tensile mechanical behaviour of the material seems to exist.
The ability to simulate the casting process of fibre reinforced self-compacting concrete, including the movement of individual immersed fibres in response to such factors as formwork geometry and surface roughness, has profound implications toward the effective use of these materials within the civil infrastructure. Together with physical experimentation, this coupled simulation of concrete casting and its load resistance in the hardened state presents opportunities for improving material performance for both ordinary and high-performance applications.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages201
ISBN (Print)978-87-78773-74-6
Publication statusPublished - 2014
SeriesByg Rapport
ISSN1601-2917

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