# On the Thermal Stresses Due to Weathering in Natural Stones
## 1 Introduction  
Durability against natural weathering has been considered one of the greatest challenges that building materials must face when they are used in external environments. Artificial materials, such as polymers and glasses, have been developed to overcome this problem [1, 2, 3, 4]. Many studies have been conducted on rock-like materials to develop new processes and technologies for increasing their performance (in terms of durability and mechanical properties), as well as their competitiveness with other construction technologies. Natural stones just partially fit these requirements, but due to their unique aesthetic features and the low CO2 emissions in their exploitation chain (which is very important nowadays), they still have great importance as building materials.  
Nonetheless, the durability of natural stones may be reduced by natural weathering. Staining, detachment, oxidation, and efflorescence are the pathologies most commonly found in building-covering systems, but, in the last two decades, a phenomenon known as bowing, already studied in the last century, has become the object of numerous studies [5; 6; 7; 8; 9; 10; 11; 12; 13].  
Bowing affects carbonate rocks, mainly calcitic marbles, and it is characterized by plastic deformation that might be easily visually noticed. In some cases, the warping is imperceptible, but the internal damage can be even greater. This key feature might help to distinguish this pathology from the others. The causes of this process are still debated: Some researchers consider the mineralogical influence as basically responsible for leading to this deformation [12; 13; 14; 15]. Others also take into account the humidity and the anchorage system [16; 17]. However, one of the most accepted hypotheses is the anisotropic thermal expansion of grains. One of the most known cases occurred at the Finland Hall's facade, in which Carrara marble was used as an external coating; after some years, the deformation was so evident that renewal of the whole facade was demanded. However, after a few months, the same problem affected the building again. The testing and assessment of marble and limestone [18] have verified that the occurrence of this pathology is spread throughout many countries.  
Thermal action has been demonstrated to be an important factor that leads to accelerated degradation in many carbonate rocks, such as marbles [19; 20; 21; 22; 23]. For this reason, it is considered in European standards that are focused on the applicability of natural stones in external environments. In marble, the relationship between thermal action and weathering depends on its mineralogical composition. Calcite belongs to the trigonal-hexagonal mineralogical system, which means that the \(\ell\) and \(\hat{a}\) optical axes are perpendicular to each other. This orthotropic behavior has also been verified in the thermal expansion coefficients of this mineral. During heating-cooling cycles, the grain tends to expand in one direction, but it contracts in the direction perpendicular to that, which leads to internal stresses.  
The main goal of this paper is to propose a methodology for the evaluation of thermal stresses induced by insolation on natural stones used as an external coating material. Meteorological and geographical data were used for forecasting the thermal strain generated within marble slabs. The proposed methodology has the advantage of using parameters that are commonly measured in meteorological stations; consequently, it is promising as a cost- and time-saving procedure. Data coming from the monitoring of the facade of the Pescara Justice Court were used for the validation of the proposed methodology. Therefore, Carrara marble's parameters were used in the calculation. It is worthwhile to note that this methodology's application is not limited to the evaluation of the stress state in facades, but it could be applied to any structures used in external environments that are subject to natural thermal weathering.