Keywords: material characteristics, ledge, thermal expansion, thermal stress, finite element method, material compatibility.

The ledges of architectural as well as sculptural stone monuments often suffer from surface deterioration causing spalling of the ledges of the basic material. The repair requires using artificial stone or mortar materials for replacement of the defects. The replaced material should have the same or similar properties as the original material [1,2]. But it is nearly impossible to get two distinct materials with the same properties in the real world. It is therefore necessary to determine the maximum tolerable difference in the properties of the repair material and the original materials. The aim of this paper is to show the effect of different thermal expansion coefficients of the repair material and the original material which causes an indispensable increase of the stress in contact between the repairing and original material.

The effect of the different coefficients of thermal expansion has been simulated using a three-dimensional numerical model. Material used in the model had a linear dilatation dependency on the temperature change. The modelled element had the shape of a block. The ledge of the shape of a small block with three rectangular surfaces and two triangular surfaces was added to the one surface of the main block and created a volume of the shape of the block with the ledge (or nose).

Extreme cooling in the climatic conditions of Prague, Czech Republic has been simulated [3]. The ledge was located on the surface side exposed to the extreme changes of the temperature. The temperature array was then used to compute the stress in the structure. The stress has been computed using the finite element method. The physically linear material model was used for the computation of the stress field. The effect of the changes of the Young's modulus in the repair material has also been explored.

The effect of the different coefficients of the thermal expansion causes an indispensable increase of the stress in the contact of the repair material and original material. If the coefficients of thermal expansion differ only slightly, the material can be considered as homogenous with no considerable increase in the stress. On the other hand, if the coefficients of thermal expansion differ significantly, the appropriate increase of the stress is then inevitable. As a result the stress caused by the periodical temperature changes, cracking at the contact of the repair and original material can occur. An inappropriately chosen repair material could mainly cause the repair material to fall off and also partly the degradation of the original historic material in a thin layer close to the contact of repair and original material. It is therefore necessary to consider the coefficients of thermal expansion when designing a suitable repair material.

1

J.J. Hughes, J. Válek, "Mortars in Historic Buildings - Review of the Conservation", Technical and Scientific Literature, Historic Scotland, Edinburgh 2003.

2

H.R. Sasse, R. Snethlage, "Methods for the Evolution of Stone Conservation Treatments", Report of the Dahlem Workshop on "Saving our architectural heritage: the conservation of historic stone structures", N.S. Baer, R. Snethlage, (Editors), John Wiley & Sons Ltd., 1997.

3

P. Beran, M. Drdácký, "Thermal Expansion Compatibility of Repair Materials", 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 111, 2007. doi:10.4203/ccp.86.111

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £130 +P&P)