Keywords: concrete, repairs, hydration, shrinkage, creep, cracking.

Concrete structural repairs are often subject to premature cracking because the substrate prevents the free contraction of the new restoration material. This well known problem is usually analyzed with a phenomenological and experimental approach, which can typically introduce errors when changing from laboratory specimens to real applications cases. The objective of this paper is to present a powerful numerical tool to study the thermo-hygro-chemo-mechanical (THCM) behaviour of repairs taking into account the real casting and environmental conditions.

The multi-physics THCM model used is a simplified version of COMES-GEO (developed a few years ago by Schrefler, Pesavento and Gawin [1,2]) implemented in the finite elements code CAST3M. In this paper a partitioned approach is used: thermo-hygro-chemical phenomena are fully coupled and the mechanical solution depends on the thermo-hygro-chemical one but not vice versa. This is acceptable because the dependency of THC behaviour from the mechanical solution appears to be not too strong for this kind of problem. All the parameters that regulate the numerical model depend more or less directly on the hydration degree and the hydration process rate is a function of the chemical affinity of reaction, relative humidity and temperature. The mechanical behavior of concrete is modeled by an elastic damage model coupled with creep, which includes the evolution of the mechanical properties with respect to the hydration degree [3,4] and with respect to damage [5].

In the second part of the paper the numerical simulation of a repaired square column is presented. The case studied is double symmetric and is solved in two-dimensional plane strain. In order to understand what are the factors that determine the success of a repair, various mechanical simulations were made. When taking into account elastic damage, it is possible to show the cracking caused by the contraction of the new repair material prevented by the substrate, and also to estimate the opening of the cracks.

The cases analyzed confirm that the factors that mainly influence the behaviour of repairs are: the installation and environmental conditions, repair geometry and material properties.

Micro-scale and meso-scale THCM models (with respect to the hydration chemical processes) are being investigated in order to have an estimate of the input parameters for the numerical model simply from the concrete mix. In this way the numerical tool will be also easier to use, even for those without specialized knowledge of the multiphysics approach used.

1

D. Gawin, F. Pesavento, B.A. Schrefler, "Hygro-thermo-chemo-mechanical modelling of concrete at early ages and beyond. Part I: Hydration and hygro-thermal phenomena", International Journal for Numerical Method in Engineering, 67(3), 299-331, 2006. doi:10.1002/nme.1615

2

D. Gawin, F. Pesavento, B.A. Schrefler, "Hygro-thermo-chemo-mechanical modelling of concrete at early ages and beyond. Part II: Shrinkage and creep of concrete", International Journal for Numerical Method in Engineering, 67(3), 332-363, 2006. doi:10.1002/nme.1636

3

G. De Schutter, L. Taerwe, "Degree of hydration based description of mechanical properties of early-age concrete", Materials and Structures, 29(6), 335-344, 1996. doi:10.1007/BF02486341

4

G. De Schutter, "Degree of hydration based Kelvin model for the basic creep of early age concrete", Materials and Structures, 32(4), 260-265, 1999. doi:10.1007/BF02479595

5

J. Mazars, "A description of micro and macroscale damage of concrete structures", Engineering Fracture Mechanics, 25(5-6), 729-737, 1986. doi:10.1016/0013-7944(86)90036-6

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