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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 86
PROCEEDINGS OF THE ELEVENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Paper 117

Modelling the Sequential Casting Procedure of Foundation Slabs: Continuous Model for Concrete Creep

T. Krejcí1, T. Koudelka1 and J. Šejnoha2

1Department of Structural Mechanics, Faculty of Civil Engineering, Czech Technical University in Prague, Czech Republic
2Centre for Integrated Design of Advanced Structures, Prague, Czech Republic

Full Bibliographic Reference for this paper
T. Krejcí, T. Koudelka, J. Šejnoha, "Modelling the Sequential Casting Procedure of Foundation Slabs: Continuous Model for Concrete Creep", in B.H.V. Topping, (Editor), "Proceedings of the Eleventh International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 117, 2007. doi:10.4203/ccp.86.117
Keywords: concrete creep, continuous model, drying and shrinkage of concrete, damage, heat and moisture transport.

Summary
The foundations of buildings in deep ditches, under the ground water level which is highly elevated during floods, is a serious problem. Cracking arising due to the combination of hydration, creep and drying processes in concrete and contact forces between the foundation slab and the subsoil impairs the impermeability of the slab. Repairs of such damaged structures are inadmissibly expensive. The computer simulation of a foundation slab in an early stage, when the damage or initials cracks can influence the final permeability of the slab, leads to the coupled thermo-hydro-mechanical problem. Discretization of such a problem using the finite element method (FEM), we obtain a system of non-symmetric and non-linear algebraic equations. It is convenient to undertake numerical solution by Newton-Raphson method for non-linear system of equations, mainly if material properties have strong non-linear dependence. This fact dramatically increases solution time and the requirements for the computer hardware (CPU speed and RAM size). The type of matrix storage and the type of solution method is therefore changing.

Constitutive equations for concrete creep can be formulated either in the integral form or in the differential form. The integral form is less convenient for numerical treatment as it requires storing the data for all preceding time steps. The incremental form of constitutive equations, which is free of this deficiency, requires a suitable representation of the kernel J(t,tau) of the compliance function. From the numerical point of view, the most convenient choice is the degenerate kernel. The overall strain is calculated as a sum of the individual strains corresponding to the links of the Kelvin-Voigt chain, which is a representation of the degenerate kernel. An effective approach is to introduce the continuous Kelvin chain model with infinitely many Kelvin units and retardation times spaced infinitely closely [1]. This algorithm has a positive effect on speed up and computational time.

For realistic modelling of the building process, the computer code was extended by the time controlled switching on/off of particular structural parts. This was realized by the time controlled change of nodal code numbers.

The results of the two-dimensional computer simulation of the behaviour of a concrete slab in the early stages are presented. Its behaviour was solved as a coupled problem, where the mechanical analysis was solved together with heat and moisture transport.

References
1
Z.P. Bazant, W. Xi, "Continuous retardation spectrum for solidification theory of concrete creep", J. Engrg. Mech., 121(2), 281-288, 1995. doi:10.1061/(ASCE)0733-9399(1995)121:2(281)

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