# Optimization of Inspection Period in Natural Stone Claddings
## 1 Introduction  
As the first line of defense against the environmental agents, the claddings are the most susceptible element to degradation [1]. However, for many years, little or no attention has been paid to their maintenance [2] since building owners/managers assumed that this element plays a more decorative than structural role [3]. As a result, severe degradation of the building facades has become a serious problem, both economic and cultural, in several countries [4; 5]. However, over the last years, this belief has been changed, and stakeholders start to understand that appropriate and continuous maintenance is the most economical way to ensure the long-term durability of assets and improve the quality and aesthetic perception of the cities without compromising the assets' reliability or safety [6; 7]. Consequently, nowadays, decision-makers on service life management of the built environment are confronted by high demands of deteriorated assets and lower financial and human resources [8]. In the scope of the service life management of the built environment, significant work still needs to be done since consolidated guidelines are still not available [9]. For example, regular inspections toassess the condition and to define maintenance activities, in general, are not mandatory or regulated [10], depending mainly on the country [11; 12] and the type of element to be assessed [13], and the definition of the end of the service life is subjective, depending on the owners'/users' demands, aesthetic appearance, and budgets [14]. Nevertheless, a significant amount of effort has been undertaken to understand the natural aging phenomena and predict the degradation processes of buildings' facades and claddings [5; 15], to define more adequate repair techniques for anomalies' correction and elimination of their causes [16; 17; 18; 19; 20], and to develop approaches for establishing optimum management of maintenance strategies [21; 22; 23; 24]. For decision-makers, establishing an approach for the definition of optimum maintenance plans is the ultimate goal. However, the definition of an optimum maintenance plan implies a trade-off between distinct parameters. For example, usually, stakeholders pursue maintenance plans that minimize service life costs and maintain the assets in a safe and minimal degradation condition [25]. In this sense, multi-objective optimization procedures can be successfully used to achieve an optimal relationship between the multiple goals associated with service life management [8]. Specifically, these types of procedures can provide multiple trade-off solutions for decision-makers to select a rational alternative by using higher-level information [8]. Since the definition of the best maintenance plan results in a conflict of objectives, in this study, a multi-objective optimization procedure using genetic algorithms (GA), available in software MatLab(r)[26], is implemented to schedule the optimal time interval between inspections of building claddings. For that purpose, a condition-based maintenance model based on Petri nets is used to analyze the influence that different maintenance plans have on the overall degradation of natural stone claddings (NSC). In this model, the maintenance activities can only be performed after an inspection has been carried out. In condition-based maintenance plans, the maintenance activities are performed according to prescribed condition criteria (i.e., condition, cost, availability, risks, among other factors). Therefore, inspections should precede the decision-making process and are essential to evaluate the cladding's condition and plan their correction. In other words, the information collected during inspection helps to select the best interventions in order to reduce the risk of failure of the cladding.