Abstract
The magnitude of the thermal stresses that originate in an acrylic-based repair material used for the reprofiling of natural sandstone is analyzed. This kind of artificial stone was developed in the late 1970s for its peculiar property of reversibility in an organic solvent. However, it displays a high thermal expansion coefficient, which can be a matter of concern for the durability either of the repair or of the underlying original stone. To evaluate this risk we propose an analytical solution that considers the viscoelasticity of the repair layer. The temperature profile used in the numerical evaluation has been measured in a church where artificial stone has been used in a recent restoration campaign. The viscoelasticity of the artificial stone has been characterized by stress relaxation experiments. The numerical analysis shows that the relaxation time of the repair mortar, originating from a low , allows relief of most of the thermal stresses. It explains the good durability of this particular repair material, as observed by the practitioners, and provides a solid scientific basis for considering that the problem of thermal expansion mismatch is not an issue for this type of stone under any possible conditions of natural exposure.
Highlights
Many dimension stones used for the construction of historical buildings show, after a certain time, superficial degradations in the first centimeters that do not affect the stone below this depth
If we consider that the temperature representative of the repair layer is the volume-averaged temperature, namely the one measured at 0.8 cm in the middle of the patch, and the temperature of the stone the temperature at a depth just below the patch, at 2.5 cm, we find that the stone can be 6 ◦ C colder than the repair mortar during a hot summer day
We have examined the question of thermo-mechanical compatibility of an acrylic-based repair mortar and a sandstone, in case of a typical reprofiling, that is a thick stone substrate which imposes its deformation on a thin layer of repair material
Summary
Many dimension stones used for the construction of historical buildings show, after a certain time, superficial degradations in the first centimeters that do not affect the stone below this depth. For instance, during the formation of scales in a sandstone during a spalling process These alterations are formed parallel to the outer surface and are independant of the stone bedding orientation, suggesting that a combination of transport properties and environmental exposure causes the stress from a degradation process to reach critical levels only at a certain depth. Since the alteration is generally superficial, a reinstatement of natural stones would imply a removal of potentially sound original material to a depth of at least 10 cm to ensure a good placement [1], while the use of a plastic mortar that can be substituted for the lost parts would result in a minimal loss of historical material and, in addition, extend its lifetime. The latter practice is called “reprofiling” or “filling”, while the piece itself is called “plastic repair” [2], “piecing-in” [1],
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