On the Thermal Stresses Due to Weathering in Natural Stones

Abstract

Natural weathering is known as one of the key mechanisms causing degradation in building materials. Great efforts have been made to develop new materials and new processes for protecting those that already exist. Natural stones are an example of a natural material that has been extensively used for building construction since ancient times. In addition, they fit durability, aesthetic, and mechanical requirements. Thus, they still have great importance in the construction business nowadays. Though chemical interactions in natural stones, such as oxidation or hydrolyses, have been widely studied, in the last few decades, the physical weathering due to daily temperature variations has begun to be considered as a key mechanism of degradation and has been incorporated in international standards. This process is particularly important in calcitic marble slabs, where it can cause extensive damages to facades. Consequently, there are restrictive rules for the use of marble as an external coating material in many countries. In this paper, the thermal stresses induced by daily variations in temperature are calculated using geographic and meteorological information. The concept of sol-air temperature is used to estimate the temperatures of the hidden and exposed surfaces of a slab, and Fourier’s law and the theory of elasticity are used to calculate the temperature and stress distribution, respectively. The proposed methodology allows for a detailed reconstruction of the stress induced inside marble slabs using parameters commonly acquired in meteorological stations as input data. The developed methodology was validated by comparing in-situ measurements of the temperature of a building in Pescara (Central Italy). A good correlation between the theoretical and real temperatures was found; in particular, the peak tensile stresses inside the slabs were estimated at 75 kPa.

A Numerical Model for Heat and Moisture Transfer in Porous Media of Building Envelopes

“…Solar radiation is by far the most important heat source in buildings. For this reason, there are several models available to calculate this parameter [14], which have been used [15] to study the sol-air temperature in a natural stone (marble). Following the work of [13], the authors evaluated the induced stress by daily variation of temperature in external façades using a model developed by [16], which is the model that supported the development of the simplified model by [14] for clear-sky radiation.…”

“…where t 3 is the molar mass of water, > g R7 is the partial pressure of water vapor, and , is the universal gas constant. The partial pressure of water vapor > g R7 is calculated with the relative humidity ,given by: ,-= q k Crs q kz Crs (15) where > g. R7 is the saturation pressure of water vapor. In the present study, this variable is given by Tetens equation, as shown in [23,24]:…”