Keywords: finite element method, thermal expansion, heat transfer, solar radiation, compatibility, repair mortar, masonry, stone.

During the typical degradation process in a historical stone masonry the mortar crumbles from the joints between the stones. The empty edges of joints are usually filled by new (repair) mortar. In general, the material characteristics of mortar and stone (brick) may be different in many aspects. The intention of the restorer is the application of 'compatible' mortar. The objective of this paper is to detect values of thermal stress depending on the type of masonry bond. By means of comparison with the numerical model results, the type of bond masonry that causes the highest values of thermal stress was detected. Three types of masonry bond were analyzed. This analysis of masonry is a coupled problem and was modeled by a three-dimensional finite element model. The numerical analysis was divided into two steps. The first one is the transient heat transfer analysis by which the temperature fields were detected. The computation of stress is the second step of the analysis in which the temperature fields are the loading. The masonry was modeled during the stress analysis as heterogeneous material with different values of the coefficient of thermal expansion for mortar and stone. The absolute values of thermal stress in all three models are similar. The lowest values for historically valuable stones are achieved in cyclopean masonry, the highest one in regular ashlar masonry. The shape of the stones affects the values of thermal stress in masonry.

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