The modeling of fresh concrete flow can significantly contribute to the durability and strength of a structure and it is necessary for design optimization of casting procedure. The purpose of this paper is to give an overview of fresh concrete rheology and to present a numerical model based on a homogeneous approach for the simulation of fresh concrete casting.

The introduction to the fresh concrete rheology is given. In the subsequent sections, commonly used constitutive models are introduced and corresponding testing methods, that can be used to determine material parameters, are discussed. It is widely recognized that concentrated suspensions, such as concrete, typically behave as non-Newtonian fluids. The flow of concrete must be defined by at least two parameters, one being the yield stress, because it shows an initial resistance to the flow which should not be neglected, and the other one being plastic viscosity that governs the flow after it was initiated. The yield stress is a consequence of the inter-particle forces, that are irreversibly broken by shearing the material. A summary of state of the art computational approaches used in the field of fresh concrete modeling is presented, discussing three main directions: homogeneous fluid simulations, discrete particle simulations, and simulations based on particles suspended in the fluid.

In the rest of the paper, recent results from numerical modeling of fresh concrete flow that is treated as a homogeneous non-Newtonian fluid will be presented. The Bingham model is considered with the yield stress and plastic viscosity as parameters. As the characteristic flow velocity is very small compared to the speed of sound in fresh concrete, the fluid is treated as incompressible. The solution algorithm is based on a stabilized Eulerian FEM formulation to prevent potential numerical instabilities. The stabilization techniques include streamline-upwind / Petrov-Galerkin (SUPG) and pressure-stabilizing / Petrov-Galerkin (PSPG) formulations [1,2]}. To track the position of free surface, two interface-tracking techniques based on Volume-of-Fluid (VOF) [3] and level set method [4] are introduced. Both approaches are based on the volume tracking concept which allows it to naturally handle topological changes of free surface and straightforward generalization into multiple dimensions (level set method).

The numerical simulations of small scale casting problems are presented as well as their comparison with available experimental data. The numerical results show a good agreement with measured data, validating the initial assumptions and mathematical model and illustrate high potential towards simulation of complex casting problems.

[1]

A.N. Brooks, T.R.J. Hughes, "Streamline Upwind/Petrov-Galerkin Formulations for Convection Dominated Flows with Particular Emphasis on the Incompressible Navier-Stokes Equations", Computer Methods in App. Mechanics and Engng., 32, 199-259, 1982.

[2]

T.E. Tezduyar, "Stabilized Finite Element Formulations for Incompressible Flow Computations", Advances in Applied Mechanics, 28, 1-44, 1992.

[3]

C.W. Hirt, B.D. Nichols, "Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries", Journal of Comput Physics, 39, 201-225, 1981.

[4]

S. Osher, J.A. Sethian, "Fronts propagating with curvature-dependent speed: Algorithms based on Hamilton-Jacobi formulations", J. Comput. Phys., 79, 12-49, 1988.

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