In this sense, in this study, a methodology for the service life prediction of indoor wood flooring is proposed, analysing in-service performance data, in order to acquire some knowledge related to the impact of the floors' characteristics in their expected service life. This research follows the methodology proposed by Gaspar and de Brito [14,15] for the service life prediction of buildings' envelope elements, which has been applied to several external façade claddings [16,17]. As a research hypothesis, it is considered that this general methodology for service life prediction of the elements of the building envelope can also be applied to predict the service life of wood flooring systems. Therefore, in this study, this methodology is applied, for the first time in the literature, to predict the service life of an element in the interior of the building. The methodology adopted is based on data collected by visual inspection, during a fieldwork survey, concerning the degradation condition of wood flooring systems. In this study, 34 Portuguese in-use houses have been inspected, corresponding to a total of 96 wood flooring systems. These data are analysed and converted into a numerical indicator, which portrays the overall degradation condition of the wood floorings analysed, which allows a graphical description of the loss of performance of these components over time and according to their characteristics. These analyses allow identifying an estimated service life of the wood flooring systems analysed, identifying the most relevant parameters for the degradation of these elements over their service life.

#### **2. Materials and Methods**

A fieldwork survey is used to collect all the relevant information for the classification of the anomalies present in the cases studies analysed, to define a model for predicting the service life of wood floorings. This survey is crucial for the definition of degradation curves, which allow evaluating the loss of performance of the wood flooring systems over time. For that purpose, 96 wood floorings, with different ages (i.e., at different stages of their service life) and various characteristics, were analysed, to obtain a degradation pattern of

these floorings, through the comparison of the degradation condition of different examples. Some floors were analysed several years after installation, which makes it impossible to know in detail some constructive characteristics of the wood floors analysed, such as the thickness of the wood elements, the type of fastening to the support and the species of the wood, which can be impossible to obtain without a significant margin of error in several case studies.

The application of additional diagnosis techniques (e.g., application of laboratory tests to evaluate more specific characteristics of the wood species) may lead to more accurate results, but they are of little use in practice, both due to time constraints and additional costs for the analysis of the floors' degradation conditions. The existence of simple tools makes it possible to provide an estimation of the service life of wood flooring systems, which is certainly more relevant than making informed assumptions, due to the lack of technical or material capacity to apply more advanced or laboratory techniques for analysing wood floors. This study thus intends to provide an empirical tool, based on visual inspections, which is, in most situations, the technique used to assess the condition of the floors, assisting the decision to carry out maintenance actions, to provide some information regarding the degradation of the floors, their service life, and the factors that condition it.

The sample analysed is in Lisbon, Portugal, with construction periods between 1930 and 2018. The indoor wood floors analysed are subjected to regular maintenance, for example, either dry cleaning or wet cleaning, and, sometimes, the finishing layer is replaced. However, in the sample analysed, six case studies have been subjected to a generalised maintenance intervention (with the replacement of wood elements), and, in this scenario, it is assumed that this intervention restores the initial condition of the floor, which implies that the "age" of these floors correspond to the period between the last intervention and the inspection time.

The degradation condition of the indoor wood flooring systems is evaluated though in situ inspections, aided using a moisture-meter in singular cases of humidity problems, as anomalies caused by the presence of humidity do not necessarily imply the detection of high levels of humidity, at the time of inspection.

The anomalies observed in wood floorings can be considered a symbol of these degradation factors, which may be caused by natural or human actions, inadequate use, lack of maintenance, or unpredictable events (accidents or vandalism). Figure 1 shows some examples of the anomalies observed in the wood floorings analysed during the fieldwork survey.

In this study, the main anomalies that can occur in wood flooring systems are divided into three main groups [18,19]:

Different levels of degradation have been defined for each group of anomalies and, in some cases, for each specific anomaly. The degradation levels of each anomaly consider the type of anomaly and the number/size of its defects. Consequently, to distinguish the severity of the anomalies, percentages of their extent have been defined on the surfaces to be analysed. These percentages and the proposed degradation levels for each of the groups of anomalies are presented in Tables 1–3.

Staining or colour change Cigarette marks Scratches or wrinkles Wearing or detachment of the finishing layer

Stains Cracking of elements and/or joints

Detachment or loss of wood elements

Broken or splintered elements

Xylophage attack

Wear of the wood material

Rot Disaggregation Moisture stains Pulverulence

Loss of the filling material of the joints Change of the joint size

**Figure 1.** Illustrative examples of the anomalies observed in the wood floorings analysed.

**Table 1.** Degradation levels of aesthetic anomalies.

#### **Table 2.** Degradation level of joint anomalies.

**Table 3.** Degradation levels of functional anomalies.

In this study, only the anomalies arising from use and due to the natural degradation process are considered, thus not contemplating major design or constructive errors (e.g., the presence of sapwood is considered a design characteristic and not an anomaly, in other words, it may be considered as a defect, but it is certainly a constructive defect), nor discrete events (such as vandalism actions). These phenomena are not considered, as they do not represent the natural evolution of the degradation of wood floors over time and are therefore not liable to be mathematically modelled.

These percentages were defined based on experts' opinions, considering the pathological context of the wood floors analysed, also taking into account and calibrating the

degradation scale initially proposed by Prieto and Silva [18] for wooden façades. Each anomaly is rated on a discrete scale from 0 to 4, according to the following definitions: level 0 corresponds to a floor with no visible degradation; level 1 to the presence of visible anomalies, even though the floor remains in a good overall condition; level 2 to a floor that shows slight degradation signs; level 3 to a floor that shows moderate degradation; and level 4 to a floor that presents generalized degradation.

In this study, the service life prediction method initially proposed by Gaspar and de Brito [14,15] is adopted. This methodology is a deterministic empirical method, which intends to evaluate the loss of performance (or the evolution of degradation over time) of wood floorings in real service conditions and in different phases of their service life. The inclusion of the value of the areas affected by each type of anomaly in the proposed model allows assessing the extent of degradation and in parallel proceeding to the respective weighting in relation to the level of severity of each one. In this empirical method, the qualitative levels of degradation presented in Tables 1–3 are converted into quantitative information, in other words, a numerical index that establishes the overall degradation condition of the flooring, called severity of degradation (*Sw*). This numerical index is given by the ratio of the weighted degraded area to a reference area, equivalent to the whole flooring with the highest possible degradation level—Equation (1).

$$S\_{w,wf} = \left(\sum \left(A\_i \cdot k\_n \cdot k\_{a,n}\right) + \sum \left(A\_f \cdot k\_n \cdot k\_{a,n}\right) + \sum \left(A\_j \cdot k\_n \cdot k\_{a,n}\right)\right) / \left(A \cdot \sum \left(k\_{\max}\right)\right) \tag{1}$$

The parameters that are taken into account in the equation that is associated with the model used are: *Sw,wf*—severity of degradation of the wood floorings, in %; *A*—flooring area (m2); *Ae*—area affected by aesthetic anomalies (m2); *Af*—area affected by functional anomalies (m2); *Aj*—area affected by joint anomalies (m2); *kn*—multiplication factor for anomaly *n*, as a function of its degradation level (*k* varies between 0 and 4); *ka,n*—weighting coefficient corresponding to the relative weight of the detected anomaly; ∑(*kmax*)—sum of the weighing constants, corresponding to the highest possible level of degradation (4 + 4 + 4).

Therefore, after collecting fieldwork information on the degradation condition of the various wood floorings analysed, their service life is predicted using a graphical and statistical analysis of the evolution of their severity of degradation index over time.

To estimate the severity of degradation index, the weighting coefficients presented in Tables 4–6 are used. These coefficients allow obtaining values closer to reality, in other words, values that reflect, in a more adequate way, the reality observed during the inspections carried out on the floorings. The weighting coefficients are defined considering the repair costs of each anomaly, as well as the propensity of the anomaly to cause new anomalies or increase the propagation rate of the existing ones, and the effects of the anomaly in decreasing the capacity of the flooring to fulfil the minimum performance requirements.

**Table 4.** Weighting factors associated to functional anomalies.

**Table 5.** Weighting factors associated to aesthetics anomalies.

**Table 6.** Weighting factors associated to joint anomalies.

#### **3. Results and Discussion**

#### *3.1. Definition of a Degradation Curve for Wood Flooring Systems*

In this study, and through the adoption of the proposed model, the evolution of the degradation condition of wood floorings over time can be characterised by a degradation curve. This curve (Figure 2) is a graphical representation of the evolution of the degradation condition of wood floorings over time, and is obtained through a regression analysis between the numerical index that expresses the degradation of wood floorings (*Sw,wf*) and their age, considering the sample analysed in the fieldwork survey. The regression model is used to evaluate the variability of *y* (i.e., the severity of degradation index) that varies with the variability of *x* (i.e., the age of the wood floors) [16], or in other words, the regression analysis allows identifying the percentage of the variability of the severity of degradation of wood floors that are explained by their age.

**Figure 2.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age.

In this study, a linear degradation pattern is adopted to describe the degradation of the sample analysed (97 wood floorings). According to various authors [20–23], a linear degradation pattern is adequate and valid to describe the degradation of interior components, in standard service conditions. This pattern describes a time-dependent linear degradation of wood floorings, which are subjected to consistent and continuous degradation agents over their service life, especially, when the floorings' users are the same throughout their lifetime, adopting consistent conditions of use and maintenance. Moreover, the climatic degradation agents (e.g., UV radiation, humidity, and temperature) acting on the interior floors are less aggressive and less variable (e.g., interior floors are not subjected to rainfall and direct solar radiation, whose intensity varies over the year, as well as during the day) than the agents acting on the external claddings [24].

The estimated service life of the wood flooring systems can thus be determined based on this overall degradation curve, through the intersection between the degradation curve and the theoretical limit adopted to establish the end of service life of this component. As mentioned in various studies [16,25], the end of service life is a conventional limit that it is not easy to specify, which relies on subjective criteria as the users demands and expectations, the funds available for maintenance actions, among other parameters, are difficult to model.

Figure 3 shows different examples of three levels of severity of degradation of wood floorings, revealing that a flooring with a severity of degradation of 20% still presents an adequate condition level, and therefore, adopting a maximum severity of degradation of 20% seems excessively conservative to establish the end of service life of wood flooring systems. Conversely, a floor with a severity of degradation of 40% already shows clear signs of generalised deterioration, and the presence of some anomalies that compromise the floors' functionality. In this sense, a limit of 40% for the severity of degradation, to establish the end of service life, seems too high and inadequate to fulfil the users' demands. Therefore, in this study, it is assumed that a severity of degradation of 30% corresponds to the end of service life of a wood flooring.

**Figure 3.** Illustrative examples of the overall condition of the wood floorings for different levels of the severity of degradation index.

Having established this limit, the estimated service life of the wood flooring systems can be obtained graphically; for the sample analysed and based on the overall degradation curve defined in Figure 2, an estimated service life of 44 years is obtained, which is in accordance with the literature and empirical knowledge about the durability of these elements. According to the existing literature on the durability of wood flooring systems, the service life of these floors can vary significantly. Nebel et al. [26] obtained an estimated service life of 10 years for multilayer parquet, while Seiders et al. [27] state that a floor can last up to 100 years, with a service life similar to the building's. Anderson et al. [28] obtained an estimated service life of 20 years for multilayer parquet glued to the substrate. Nebel et al. [26], for 8 mm parquet and 10 mm parquet, obtained an estimated service life of 25 years. Jönsson et al. [29] and Jönsson [30] estimated a service life of 40 years for pine flooring. Scharai-Rad and Welling [31] and Petersen and Solberg [32] suggested a

service life of 45 years for oak flooring, while Eaton and Hale [33] and Asdrubali et al. [34] state that oak woods present an estimated service life ranging between 35 and 50 years. Adalberth [35] and Mithraratne and Vale [36] obtained an estimated service life of 50 years, for 22 mm parquet. Gunther and Langowski [37] obtained an estimated service life of more than 50 years for parquet. Aktas and Bilec [38] concluded, with an 80% confidence interval, that the service life of a wooden housing floor is, on average, 40 years. This confidence interval was defined in accordance with ISO 15686-1 [39], to define minimum and maximum limits for the variance of the results of the estimated service life, with a minimum of 15 years and a maximum of 73 years.

#### *3.2. Influence of the Characteristics of Wood Flooring Systems on Their Service Life*

The dispersion of values proposed in the literature for the expected service life of wood flooring systems reveals that the wide range of characteristics of wood floorings strongly influences their behaviour over time, affecting their durability. Reinprecht [40] refers that the service life of wood elements depends essentially on the natural durability of the wood, but also varies considerably according to the design characteristics, the protection applied, and the exposure and maintenance conditions. In this sense, the evolution of the degradation condition of wood floorings should also be analysed as a function of the different characteristics of these floors.

In this study, different curves are proposed according to the relevant characteristics of the wood floorings analysed. The type of wood is the first characteristic analysed, and five categories are considered to typify the sample analysed: (i) oak; (ii) eucalyptus; (iii) mahogany; (iv) pine; and (v) tropical woods (e.g., *Couratari oblongifolia*, *Dipteryx odorata* and *Diplotropis* sp.). In the sample analysed (Figure 4), oak and eucalyptus wood floorings present the lower estimated service lives, around 33 years, followed by mahogany floors, with an estimated service life of 35 years, and by tropical woods, with an estimated service life of 43 years, and, finally, by pine wood floorings, with an estimated service life of 50 years.

**Figure 4.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age, according to the type of wood.

Various studies [41,42] emphasise the crucial role of the natural durability of the wood species on the service life of a wood flooring. The results obtained may seem incongruous with the empirical perception regarding the durability of wood species. Usually, pine is

considered as a less durable type of wood and non-resistant to decay due to xylophage attack [43,44]. However, as a natural material, the physical properties of a wood element vary significantly, even within the same wood species [45]. Cruz et al. [46] refer that, usually, the wood species are divided into softwood (e.g., pine) and hardwood (e.g., oak, eucalyptus, and some tropical species) and among hardwood species, the properties of wood range between not very resistant nor durable to very resistant and durable woods (as some Brazilian species, as the ones included in the category "tropical" analysed in the present study).

Therefore, the results reveal three major conclusions (the sample analysed is relatively small, so the results should be analysed with some caution): (i) oak, eucalyptus and mahogany species present similar degradation curves, revealing lower estimated service lives than tropical or pine wood floorings; (ii) tropical floors are usually more durable, since more naturally durable wood species have been applied while greater care was taken at the time of execution given the specificity of the material, and greater attention is taken in the floors' maintenance; and, (iii) the main reason why pine floors are more durable, within the sample analysed, is because 75% of the pine floors inspected are traditional tiles, which have a significant thickness, and whose maintenance implies polishing and applying wax, as a protective layer, weekly or monthly, thus mitigating anomalies that may occur over the service life of the floor.

In this sense, while the quality of the wood used is undoubtedly relevant to the service life of wood flooring systems, correct design also plays an important role [34]. In the sample analysed, three types of floor solutions are analysed [47–49]: (i) parquet, consisting of a set of wooden slats, with a total thickness around 8 mm; (ii) traditional floorboard, entailing several wooden strips that can be solid or laminated, with a thickness between 18 and 20 mm; and (iii) traditional tiles of solid wood, with a thickness between 17 and 22 mm. The degradation curve obtained according to the type of floor (Figure 5) reveal that parquet floors are the less durable of the sample analysed, with an estimated service life of 33 years, followed by traditional floorboard, with an estimated service life of 37 years, and by traditional tiles, with an estimated service life of 49 years. The thickness of the wood elements seems to play an important role in the durability of the wood floorings; naturally, thicker floorings, have longer estimated service lives, in addition to allowing more intrusive maintenance actions, which allow extending the service life of the elements in more adequate conditions of use.

**Figure 5.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age, according to the type of flooring system.

Other intrinsic characteristics of wood may also be analysed, as its hardness, which is a relevant property for the selection of a given wood used in flooring indoors [50,51]. The hardness of the wood elements is intrinsically related with their density, which depends on several factors, such as the tree species and growing conditions, which are variable in the same species and in the same tree [3]. In this sense, the sample analysed was divided into two categories: (i) softwood species, which usually present lower densities [3]; and (ii) hardwoods, usually more resistant to impacts, scratches, and wear [52,53]. Due to the multiplicity of characteristics that influence the hardness and density of a wood species, it is not possible to draw unequivocal conclusions regarding the impact of this characteristic in the service life of wood flooring, but Figure 6 presents some statistical analysis regarding the influence of the wood's hardness in the presence of related anomalies, revealing that hardwood floorings are less prone to suffer scratches, wrinkles, and wear of the wood material.

**Figure 6.** Statistical analysis of the presence of scratches or wrinkles and wear of the wood material, according to the hardness of the wood.

The type of surface coating of the wood floorings is also a relevant parameter to protect the floors from liquid water and direct UV radiation, avoiding anomalies that compromise the natural and aesthetic characteristics of the wood floorings, while increasing the service life of these floors [54]. Concerning the type of coating of the wood floorings, three types of protection treatments are analysed: (i) oil-based matte finishes; (ii) wax; and (iii) varnish. Figure 7 shows the degradation curves obtained for the sample under analysis, according to the surface coating. The sample corresponding to the oil-based matte finishes is small (7 case studies) and the older case study has 26 years, and therefore, no unequivocal conclusions can be drawn regarding the influence of this coating on the service life of wood floorings. The varnish coatings present a lower estimated service life (around 38 years), when compared with wood floorings with a wax coating (with an estimated service life around 52 years).

**Figure 7.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age, according to the surface coating.

Richardson [55] states that varnish can be an attractive solution to protect wood applied in interiors, but almost all varnishes suffer from a preferential wetting failure due to the fact that varnish is hydrophobic, and wood is hydrophilic, and in the presence of water, the interaction between the two materials may privilege the deterioration of the wood elements. Moreover, when exposed to the direct incidence of UV radiation, varnishes tend to oxidize and become opaque. Specifically, varnish degrades over time, and its efficiency is limited in time, losing elasticity, and becoming brittle, thus making the wood susceptible to various agents of deterioration. Unlike varnish, which is usually applied as a protective film, wax is usually impregnated in the wood [56], with generally longer periods of action, increasing the dimensional stability of the wood floorings exposed to moisture [57], and promoting a more homogeneous and regular degradation of the wood floorings over time. Furthermore, in Portugal, wood floorings with wax coatings tend to be maintained more regularly, with the removal of old wax and application of a new layer of wax.

The installation of wood floorings in contact with moisture or near moisture sources is seen as a risk factor that promotes the deterioration of these flooring systems [19,58]. Various studies [18,59,60] identify the presence of moisture as one of the main causes of anomalies in wood used as coating material. Figure 8 presents the degradation curve of wood floorings according to the exposure to moisture sources (e.g., sanitary facilities, kitchens). In the sample analysed, a wood flooring near to moisture sources has a 70% probability of presenting some anomaly related to the presence of water (e.g., moisture stains, rot, cracking). The results obtained confirm the that a wood flooring near or exposed to moisture sources reaches the end of its service life sooner (after 38 years) than a flooring protected from moisture sources, which only reaches the end of its service life after 45 years.

**Figure 8.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age, according to the exposure to moisture sources.

The degradation of wood floorings also occurs due to the action of mechanical or some chemical actions [46]. In the sample analysed, the case studies are not subjected to chemical degradation agents and, therefore, this degradation mechanism is not considered. On the other hand, the use conditions influence the degradation of the wood floorings analysed. Figure 9 shows two degradation curves according to the type of use and to the in-use conditions. First, the analysis of the type of use reveals that wood floorings in services buildings present a higher estimated service life (around 51 years) than floors in housing (with an estimated service life around 40 years). This result reveals that, even though the floorings in services buildings are more exposed to an intense circulation and use, a more frequent and careful maintenance increases their estimated service life.

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**Figure 9.** Regression analysis between the numerical index that expresses the degradation of the 96 wood flooring systems (*Sw,wf*) and their age, according to the type of use (**a**) and in-use conditions (**b**).

Moreover, in housing, some case studies are exposed to the presence of pets, children and some are used to have meals, which increases the risk of impacts and presence of liquids. When this risk is analysed, as in Figure 9b, an increased risk of impacts and presence of liquids leads to a lower estimated service life (around 39 years) than floors in normal in-use conditions (with an estimated service life of 45 years). A more detailed analysis is performed, evaluating the differences in the expected service life of wood floorings according to the house room, and although these values should be analysed with caution, given the small sample size, the following conclusions can be drawn: (i) dining rooms present the lowest estimated service life, around 28 years; (ii) followed by entrance halls, in which the circulation is more intense, with an estimated service life of 38 years; (iii) in third place come bedrooms, with an estimated service life around 42 years; (iv) classrooms and living rooms present estimated service lives of 46 and 47 years, respectively; and (v) finally, offices present an estimated service life of 49 years.

These results reveal two conflicting phenomena: on the one hand, floorings with more intense circulation should have lower estimated lives and this occurs in the case of halls. However, in the case of public buildings, offices and even classrooms, although circulation is more intense, maintenance is also more thorough and frequent, which counteracts with the higher rate of degradation of spaces. In other words, maintenance helps to preserve floors in adequate conditions for a longer time, while in residential buildings maintenance actions tend to be postponed, sometimes due to economic reasons.

Various studies [61,62] reveal the relevance of maintenance to mitigate the degradation of wood flooring systems and the impact of these actions to extend the floor's service life. In the sample analysed, the frequency of maintenance actions is divided into three categories, according to the information collected during the fieldwork survey, leading to the following results: (i) dry and wet cleaning, performed weekly, with an estimated service life of 44 years; (ii) dry cleaning, performed weekly, and wet cleaning performed monthly, with an estimated service life of 43 years; and (iii) dry and wet cleaning, performed monthly, with an estimated service life of 43 years. The differences obtained are insignificant, since all the case studies are frequently subjected to maintenance actions and, therefore, the impact of having regular maintenance cannot be fully evaluated. The type of maintenance varies between floorings, being more or less strict depending on the users; however, the actions carried out are not evaluated in detail (e.g., the cleaning products applied), thus not allowing assessing of the differences between the types of maintenance carried out in the different floorings analysed. The impact of the maintenance it is therefore more perceptible when other characteristics of the floorings are analysed, such as the surface protection or conditions of use, where the type of maintenance has an indirect impact on the floorings' service life according to these characteristics.

#### **4. Conclusions**

This study proposes an empirical degradation model to evaluate the on-site performance of wood flooring systems, considering a sample of 96 case studies. The evolution of the degradation condition of the sample analysed is discussed as a function of the different characteristics of these floors, and an estimated service was obtained for each one of the characteristics analysed. Table 7 presents a summary of the results obtained for the various characteristics of the wood floorings analysed. The sample analysed is small, from a statistical point of view; nevertheless, some statistically significant values were obtained. In this sense, a deeper statistical analysis (e.g., a multiparametric statistical analysis) would lead to debatable results, as the sample size could lead to the bias of the conclusions obtained. In future studies, with the improvement of the sample, it will be possible to carry out multiple regression analyses, which made it possible to consider the simultaneous effect of several variables on the degradation of wood floors.

The results obtained revealed, as expected, that the wide range of characteristics of wood floorings strongly influence their behaviour over time, affecting their durability. In the sample analysed, serious errors of construction or use are not considered. Additionally, when severe situations of degradation occur, the users usually perform maintenance and repair actions, in order to mitigate the progression of the degradation of the wood floorings, and, in some cases, restore their initial performance. In the sample analysed, all floorings are regularly maintained, which explains why this parameter, when grouped into the three categories considered, which already include a high maintenance periodicity, does not lead to significant differences in the overall service life of the wood floorings.

Moreover, the type of maintenance performed, as well as the cleaning products applied, are difficult to assess and can affect the state of degradation of the wood floorings. These conditions of maintenance are indirectly reflected on the use conditions. The type of use shows a variation in the estimated service life of 11 years, between floorings in public spaces and floorings in housing. The in-use conditions seem to be less relevant, with a lower impact on the overall estimated service life, with a small variation in the estimated service life (*ESL*) obtained for floorings exposed to high risks of impact and presence of liquids and floorings in normal conditions of use. The level of the users' demands associated with the sample analysed can also explain this result, since users that are more careful tend to lighten the relevance of this parameter.

The presence of moisture leads to a variation in the estimated service life of wood floorings of seven years, revealing that the proximity of a moisture source increases the incidence of anomalies related with this degradation agent, thus reducing the service life of these floorings.

**Table 7.** Summary of the estimated service life obtained according to the different wood floorings' characteristics.

Regarding the type of protection or finishing, the type of wood, and the type of flooring, these characteristics show a very significant impact on the estimated service life, in other words, a variation of 18, 17, and 16 years, respectively, is obtained for these characteristics. Therefore, these characteristics significantly influence the durability and service life of wood floorings, considering the high variation obtained for the estimated service life as a function of these characteristics, taking into account that the estimated service life of the overall sample is 44 years. This result confirms the influence of the susceptibility of wood to degradation mechanisms and the impact of design in the estimated service life of wood flooring systems.

The results obtained in this study allow quantifying of the variability introduced by the different characteristics of the wood floorings on their estimated service life, leading to an estimation in years, which is the first step to adopt more sustainable solutions during the design stage, adopting preventive measures and selecting adequate solutions for an intended use. By understanding the degradation mechanisms of the wood floorings and based on the results obtained in this study, the durability of wood floorings can be increased through the adoption of regular inspections and maintenance actions, thus promoting the sustainability of the solution and reducing its environmental impacts.

**Author Contributions:** Conceptualization, P.C., A.S., and J.d.B.; methodology, P.C. and A.S.; formal analysis, P.C., A.S., and J.d.B.; investigation, P.C., A.S., and J.d.B.; data curation, P.C. and A.S.; writing—original draft preparation, P.C. and A.S.; writing—review and editing, J.d.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Portuguese Foundation for Science and Technology (FCT) through project BestMaintenance-LowerRisks (PTDC/ECI-CON/29286/2017).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due since belongs to the research group and is part of an ongoing research.

**Acknowledgments:** The authors gratefully acknowledge the support of the CERIS Research Centre (Instituto Superior Técnico—University of Lisbon).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Degradation Assessment of Natural Stone Claddings over Their Service Life: Comparison between Tehran (Iran) and Lisbon (Portugal)**

**S. H. Mousavi 1, Ana Silva 2,\*, Jorge de Brito 2, A. Ekhlassi <sup>1</sup> and S. B. Hosseini <sup>1</sup>**

**Abstract:** Now more than ever, the construction sector is aiming to adopt more sustainable solutions. To achieve this purpose, more durable solutions must be adopted, making rational decisions at the design and maintenance stages regarding the conditions of environmental exposure and use. In this sense, knowledge regarding the service life of building components is crucial. This knowledge should not be a general concept, or a standard value, and adapting practices from one country to another is extremely challenging. In this sense, this study analyses the service life of natural stone claddings. We adopt a methodology initially proposed for Lisbon (Portugal), intending to evaluate its applicability to other geographical contexts, in order to perform a more reliable service life prediction of stone claddings located in Tehran (Iran). An estimated service life of 65 years was obtained for a sample of 162 stone claddings directly adhered to the substrate, located in Tehran, which were analysed by in situ inspections. The impact of different conditions (e.g., type of stone and environmental exposure conditions) on the service life of stone claddings in Tehran was quantified, which revealed that the exposure to environmental agents, such as wind, rain and pollutants, is the main cause of degradation of the natural stone claddings.

**Keywords:** durability; in situ inspections; service life prediction; natural stone claddings

#### **1. Introduction**

The durability of buildings is one of the fundamental elements in assessing the quality of life of contemporary societies and is indispensable to their social and economic stability [1]. The durability of buildings can be jeopardised by increasing the degradation of their components, resulting from the natural ageing process of materials [2], leading to the loss of a building's performance [3]. Due to this degradation process, the buildings tend to become obsolete and unable to meet the performance demands established during the design stage [4]. Therefore, the service life prediction of building elements is of utmost value for a more sustainable environment [5]. The knowledge about the durability of building elements allows a more rational management of resources, which can reduce the economic and environmental costs of buildings during their lifetime [6].

Building components are categorised in various studies [7–10] according to the time they take to reach the end of their service life, i.e., as durability layers. The façade is considered the "skin" of the building, acting as a protective layer against environmental degradation agents [2]. Therefore, the cladding is more prone to anomalies [11], which is why the building envelope failures can be held accountable for more than 50% of all building deficiencies [12]. The adoption of natural stone as a cladding solution is growing due to its durability and mechanical strength [13].

**Citation:** Mousavi, S.H.; Silva, A.; de Brito, J.; Ekhlassi, A.; Hosseini, S.B. Degradation Assessment of Natural Stone Claddings over Their Service Life: Comparison between Tehran (Iran) and Lisbon (Portugal). *Buildings* **2021**, *11*, 438. https:// doi.org/10.3390/buildings11100438

Academic Editor: Wahidul Biswas

Received: 27 July 2021 Accepted: 23 September 2021 Published: 27 September 2021

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A review of the literature shows that a variety of approaches can be used in the description of models to replicate the degradation of façade claddings during their lifetime. Gaspar and de Brito [14] classified these under two categories, namely, (i) laboratory testing and (ii) fieldwork surveys, to evaluate the degradation condition of the elements in real exposure situations. In spite of using laboratory testing methods because of the quickness with which they yield results [15], some authors indicate that the simulation of the complete effect of environmental agents that simultaneously contribute to the degradation of the building elements is not possible in such an artificial environment, since the degradation effects are analysed in a much reduced way [16]. In addition, real life situations and the synergy between different degradation agents are difficult to model in a laboratory setting [17]. The studies conducted by Searls and Tomasen [18] and Henriksen [19] reveal that laboratory tests are more appropriate to model the effects of a given degradation agent [20].

To overcome this criticism, an evaluation of the influence of the combined actions of different agents on claddings' service life by observing the existing condition was used. This research method, called ex post facto, which literally means 'after the fact' or 'retrospectively', refers to those studies that investigate possible cause-and-effect relationships by examining the elements under analysis in real situations and searching back in time for plausible causal factors [21]. This course of action to assess the durability of building components was conducted in several studies [22–28].

However, most of the research in the field of the degradation processes of exterior claddings in real conditions in retrospect have been carried out in a specific region with a specific climate type, and the identification of the influence of some properties on the evolution of the degradation can come to the light by comparing the same building components in different climate types. In this sense, Haagenrud [3] suggests that the relation between the degradation agents and their effects on the degradation process can be explained when the different geographical coordinates and climates are taken into consideration in the characterisation and mapping of the most significant degradation agents. Therefore, the impact of environmental agents and façade specifications determined by comparing the natural stone cladding behaviour in Tehran (Iran) and Lisbon (Portugal), performed within a PhD study [29] and unique in the literature, is described in this study.

Regardless of the innovations in cladding materials used in exterior walls, natural stone is still one of the most efficient and durable options [13]. Therefore, this cladding solution was selected as a case study in the present study. For decades, many methods to determine the adherence of stone plates to a building's external walls have been developed, which can be classified under the headings of (i) a 'direct fastening system', which corresponds to the complete bond between the substrate and the stone elements, and (ii) an 'indirect fastening system', adopting mechanical anchorage methods [30,31]. Considering the fact that the indirect fastening system is a relatively new technology and that it is therefore difficult to find a significant number of case studies, natural stonewall claddings with a direct fastening to the substrate were chosen in the current study. The sample analysed comprises 203 stone claddings located in Lisbon (Portugal) and 162 stone claddings in Tehran (Iran), which were inspected in situ.

The aim of this study is essentially to propose an empirical method to establish a degradation path for stone claddings over time, through a simple tool to predict the service life of these claddings, considering characteristics that can be easily assessed by visual inspections in situ. This study intends to analyse and quantify the impact of different factors, such as the characteristics of the stone elements and environmental exposure conditions on the service life of natural stone claddings in Tehran. This knowledge is extremely useful for the adoption of more sustainable and rational solutions at the design stage, as well as for the definition and optimisation of maintenance strategies to promote the durability of these claddings.

#### **2. Materials and Methods**

#### *2.1. Climatic and Environmental Exposure Conditions*

In this study, according to Haagenrud [3], different geographical coordinates and climates are taken into consideration at various scales. The macro scale usually describes the overall meteorological conditions based on meteorological indicators (e.g., air temperature, precipitation, among others). Tehran is categorised as having a cold semi-arid climate (BSk) based on the Köppen–Geiger classification [32], which denotes that at least one month's average temperature is below 0 ◦C. By contrast, the Lisbon area is situated in a hot summer Mediterranean climate (Csa), where the average temperature is above 0 ◦C even in the coldest months. With respect to the *meso* climate, the effects of the ground and of the built environment are considered. In this context, Tehran is spread from 35◦34 to 35◦51 north latitude and from longitude 51◦06 to 51◦38 east with an average elevation of 1190 m, comprising 22 regions. The current study focused on the sixth district, which is geometrically located in the centre of Tehran and was founded in the 1950s (Figure 1).

**Figure 1.** Map of the 6th district, located in the centre of Tehran.

The other research area is in the Lisbon area at 42◦38 north latitude and 09◦08 west longitude, located in western Portugal on the estuary of the Tagus River along the Atlantic coast.

The impact of environmental conditions on cladding degradation varies according to the building's exposure circumstances, which mainly depends on the local climate [33,34]. Accordingly, the local specifications, i.e., the local conditions in the building's proximity, were collected at the data acquisition stage.

#### *2.2. Data Acquisition and Limitations of the Method Proposed*

As Silva et al. [2] mentioned, in order to appraise the service life of building elements considering their current condition, various methods can be used, including destructive and non-destructive tests carried out in situ. The former procedures, however accurate in regarding the information provided, usually demand costly equipment, in addition to requiring repair actions afterwards. By contrast, the latter techniques, notably visual inspections, are generally fast, less expensive and adequate to establish the degradation condition of the elements under analysis [35]. Moreover, in most real situations in the geographical contexts analysed, the maintenance actions are carried out based on subjective criteria that depend on the users' perception of the state of degradation of the façades acquired through simple visual inspections.

Consequently, in the current study, an in situ survey of buildings, through visual inspections, was used to collect data regarding the degradation condition of natural stone claddings. The fieldwork survey encompasses the following steps (Figure 2): (i) each case study is selected and characterised based on complementary information, such as the location of the case study, existing drawings and other relevant documents from the municipality; (ii) an inspection file is used to collect all the relevant information to characterise the case study (e.g., façade's dimensions, exposure conditions and the description of the anomalies observed, as well as the area of the façade affected by these anomalies); (iii) during fieldwork, the case study is photographed, and, aided by diagrammatic sketches, the anomalies observed are identified and their extent is quantified using design and calculus software.

**Figure 2.** Illustrative example of one case study analysed in Tehran: (**a**) photograph of the façade; (**b**) diagrammatic sketch to represent the anomalies observed; (**c**) direct measurements and identification of different anomalies in the natural stone cladding.

The method proposed, in which the fieldwork is based on visual inspections only, presents some limitations regarding the collection of relevant information to characterise some properties of the claddings, namely (i) the factors related to the characteristics of the materials applied in the execution stage, such as the bonding material applied or the material used to fill the joints, cannot be determined during the visual inspections; (ii) the execution conditions and the level of control over the construction processes (e.g., the quality of the workforce work, material storage environments, among others) are also difficult or even impossible to characterise. The use of inadequate materials or execution errors irreparably affect the durability of stone claddings, but there is a lack of information in municipalities about the execution processes, which do not allow consideration of these factors when using visual inspections only. However, in this study, the case studies that presented anomalies due to clear execution errors or choice of inappropriate materials were excluded from the analysis, as they do not characterise the physical degradation phenomenon of natural stone claddings.

Information about the maintenance and rehabilitation actions carried out on the façades analysed was collected in the municipalities and through contact with the owners. The type of actions carried out is not within the scope of this study, as the models

consider the natural evolution of degradation of stone claddings without maintenance. In other words, the "age" is considered as the period since construction/last intervention/maintenance action until the inspection time.

Despite these limitations, the method used has numerous practical advantages. The main advantage relies on the time required to obtain relevant results regarding the influence of exposure conditions on the degradation of the claddings. The combined action of the degradation agents is not easily reproduced. In this sense, the field data on the degradation condition of buildings should be used when possible, since they provide relevant information on the real degradation of components when exposed to a particular set of degradation agents, which act simultaneously and synergistically, whose action it is not possible to simulate with precision in an artificial way.

#### **3. Description of the Samples Analysed**

Accordingly, with respect to the samples located in the Lisbon area, fieldwork data from previous research [22,24] were analysed in this study, related to the inspection of 203 stone claddings. The newest building was built in 2008, and the oldest building was built in 1891 (being subjected to a maintenance intervention in 1948). As for the case studies in Tehran, based on the research of Silva et al. [22] for the classification system of anomalies in stone claddings with direct fastening to the substrate, a visual inspection plan was developed. This fieldwork generated data from 52 buildings in the sixth district of the Tehran (Iran) area, whose claddings presented various anomalies, totalling 162 case studies. In the Tehran dataset, the oldest building is from 1969 and the newest one from 2010. The resulting data can generally be arranged into the headings of "materials' characteristics" and "defects". The former category comprises the type of stone, size and colour; the type of finishing; and the position of the cladding in the façade; in addition, complementary information (e.g., orientation, exposure to wind-driven rain, among other parameters) was collected in advance to put each case study into a better perspective (Table 1).

**Table 1.** Description of the samples collected in Tehran and in Lisbon.

According to Silva et al. [2,22], four groups of anomalies are considered:

From the preliminary analyses of the data collected, as shown in Figure 3, the most and least frequent defects detected during the fieldwork in both research areas are aesthetic anomalies and those related to the fastening to the substrate, respectively. In the Tehran dataset, the aesthetic anomalies are followed by defects related to the loss of integrity (32%) and, finally, joint defects (15%). On the contrary, the analysis of the Lisbon cases shows joint defects in 29% of the inspected façades, while the defects associated with the loss of integrity occur in 28% of the case studies analysed.

**Figure 3.** Statistical analyses of the defects observed in the case studies analysed.

The frequency of the different defects in the aesthetic anomalies group observed in the cases under analysis can be compared in Figure 4. Among the aesthetic degradation defects in both research areas, superficial dirt, caused by the accumulation of pollution debris on the façade, is the most common one. In Tehran, this is followed by colour change and stains (occurring in 46% and 44% of the sample analysed, respectively), while an inverse tendency in these defects is observed in the inspected façades in Lisbon. The fourth ranked frequency of the aesthetic degradation defects observed in both fieldwork surveys relates to flatness deficiencies, which is followed by efflorescence and biological colonisation, respectively, in Tehran, whilst the Lisbon samples exhibit inverse behaviour. The incidence of vegetation growth only occurs in the Lisbon sample.

Concerning the defects related to the joints, as shown in Figure 5, degradation of the materials used to fill the joints is more frequent than the loss of material (which corresponds to a more serious defect with severe consequences, such as penetration of water in the substrate). Concomitantly, given the higher age of the samples under analysis in Lisbon (less than 89 years) than in Tehran (less than 48 years), the first sample was naturally subjected for a longer period to the degradation mechanisms, thus leading to a higher incidence of joint defects.

**Figure 4.** Frequency of the defects in the aesthetic anomalies group observed in the samples under analysis.

colonisation

**Figure 5.** Frequency of the joints' defects in the samples under analysis.

Concerning the defects related to the fastening to the substrate (Figure 6), the scaling of the stone near the edges is the major defect in the sample analysed. This is followed by a partial loss of stone material and detachment, respectively, while a partial gap in the stone elements does not occur in any inspected cases in Tehran. Conversely, the analysis of the Lisbon data reveals that the presence of partial gaps in the stone elements is the second most common defect, followed by a partial loss of stone material.

Notwithstanding the lower age of the studied samples in Tehran (with an average age of 26 years and a maximum age of 48 years) in comparison with those in Lisbon (with an average age of 38 years and a maximum age of 89 years), the defects related to the loss of integrity are more frequent in Tehran than in Lisbon, as presented in Figure 7. This result can be justified by the presence of high levels of pollutants in Tehran, which occasionally reach dangerous levels, jeopardising the health of its residents and their quality of life [36,37]. Higher air pollution increases the deterioration and weathering of stone materials [38], causing different types of anomalies related to the loss of integrity of stone

plates (e.g., erosion). Moreover, the action of pollutants also promotes discoloration of the stone elements [39], as observed in 46% of the Tehran sample, and a higher deposition of sediments (Figure 4).

**Figure 6.** Frequency of the fastening to substrate defects in the samples under analysis.

**Figure 7.** Frequency of the different loss of integrity defects in the samples under analysis.

In this study, the defects inspected visually during the fieldwork survey are discreetly ranked based on the proposal defined by Silva et al. [2,22] regarding the degradation condition of natural stone claddings. In this classification system, which was used in several studies [24,25,27,40], the defects are ranked from 0 (no visible degradation) to 4 (generalised degradation), considering the area of the façade affected by each anomaly and its relative importance within the degradation scale adopted (Table 2).

(

**Table 2.** Classification of the degradation condition of natural stone claddings (data sourced from Silva et al. [2]).

**\***) Material degradation is meant to be every anomaly that involves loss of volume of the stone material. ( \*\*) Data from Silva et al. [2,22].

> A preliminary analysis of the Tehran data shows that every cladding presents visible defects. Most of the anomalies observed are ranked level 1 (52% of the sample), followed by level 2 (45%) and, finally, by level 3 (3%); none of the inspected claddings belong to level 4. In the Lisbon data, the defects ranked in level 2 are the most common with a frequency of

55%, followed by those in levels 1 and 3, which occur in 27% and 16% of the case studies, respectively, and, finally, anomalies related to level 4 occurred in only 2% of the façades in question [2,22].

#### **4. Service Life Prediction Model**

According to the literature, several methods can be used to predict the service life of claddings over time, which can be categorised as analytical, statistical, empirical and experimental [41]. Regarding the probabilistic nature of degradation phenomena in addition to the capability of statistical models to establish probabilistic and statistical means to predict the performance and failure probability [42], the statistical model proposed by Gaspar and de Brito [14] was adopted in the current study. This model was effectively used for the quantification of the degradation condition of various types of claddings and in other geographical contexts, namely in South America [43,44], and this is the first application of this model in Asia. This approach relies on the calculation of a numerical index (severity of degradation), which is given by the ratio among the area of the façade affected by the different anomalies (weighted based on the degradation level and the severity of the anomalies observed), and a reference area corresponding to the maximum hypothetical extent of the degradation for the façade under analysis—Equation (1).

$$S\_w = \frac{\Sigma(A\_{\rm tr} \times k\_{\rm tr} \times k\_{\rm tr,\rm tr})}{A \times \Sigma(k\_{\rm max.})} \tag{1}$$

where *Sw*—normalised severity of degradation of the façade as a percentage; *An*—area of the cladding affected by a defect *n*, in m2; *kn*—defects' "*n*" multiplying factor as a function of its condition (between 1 and 4); *ka,n*—weighting coefficient to encompass the relative consequences of each defect, which considers the cost of repair of defect *n* (*ka,n* ЄR+) (in absence of more information, *ka,n* = 1); *kmax*—weighting factor equal to the worst condition level; and *A*—total area of the cladding, in m2.

In this study, the groups of defects are weighted according to their repair costs [22] (i.e., the ratio between the total cost of the intervention needed to repair the anomaly and the cost of replacing the cladding). A higher weighting coefficient is thus assigned to the more complex and costly anomalies as shown in Table 3. The values of the weighting coefficient (*ka,n*) are different for the two samples, since these values are adjusted to the costs applied in each geographical context.

**Table 3.** Weighting coefficients corresponding to the relative importance of each group of defects.

( \*) Data from Silva et al. [22].

To evaluate the degradation of stone claddings over time, four typical deterioration patterns were proposed by Shohet et al. [20], defined based on specific degradation agents and mechanisms, comprising linear, convex-shaped, concave-shaped and S-shaped patterns. In this study, an S-shaped pattern was used, which allows taking into account different rates of the degradation of stone claddings; i.e., it is capable of describing the process of the cladding's deterioration, which starts slowly and apparently stabilises over time but accelerates again near the end of the cladding's service life. Accordingly, "S"-shaped

degradation curves are obtained through a nonlinear regression, where a third-degree polynomial line is fitted to the case studies analysed in the fieldwork (Figure 8). The degradation curves of the Tehran and Lisbon data sets lead to a determination coefficient (R2) of 0.80 and 0.77, respectively. This reveals that a high percentage of the variance of the variable "severity" can be described by the proposed curves, i.e., there is a good correlation between the observed values and those predicted by the model.

**Figure 8.** Degradation curves of the Tehran and Lisbon samples.

To estimate the service life of the natural stone claddings, a threshold degradation level that establishes the end of its service life must be defined. The relevant literature on this subject suggests that establishing the conventional limit for the end of service life is not simple, depending on acceptance criteria, not certainly determined by "pure" scientific methods. Therefore, and according to previous research on natural stone claddings [2,22,27], a degradation severity level of 20% was chosen in this study to establish the end of service life. Accordingly, an estimated service life of 65 years was obtained, using the degradation curves in Figure 8, for the claddings located in Tehran, and an estimated service life of 68 years was calculated for the claddings in Lisbon.

#### **5. Analysis of the Degradation Evolution According to Claddings' Characteristics**

Various studies [2,10] discuss the relevance of several contributory factors for the presence of defects in natural stone claddings, which are responsible for the claddings' deterioration. In fact, the stone claddings show a distinct behaviour in terms of deterioration because of the great variability of their characteristics. In order to compare the degradation evolution in Tehran and Lisbon, the claddings' features chosen in the research conducted by Silva et al. [2] for the Lisbon samples were also analysed in this work for the Tehran cases. They comprise the type of stone and related properties (colour, size and type of finishing), location of the cladding in the facade, the facade's orientation and their exposure to environmental conditions (wind/rain action).

Natural stone is generally classified into three groups—igneous, sedimentary and metamorphic [30,31]—according to its mineral and chemical composition, the texture of the constituent particles and the genesis processes. Therefore, the samples under analysis here were grouped into three categories: (i) granite and similar stones, (ii) travertine and limestone and (iii) marble and crystal. Figure 9 provides the degradation evolution over time according to the type of stone plates used in the claddings.

**Figure 9.** Degradation curves of the Tehran and Lisbon samples, according to the type of stone.

In both research areas, granite stones present a longer estimated service life, even though the Lisbon data show a curve that is statistically unreliable due to a high scatter of results. In second and third place, the degradation curves reveal that travertine and limestone are more durable than marble and crystal in exterior wall claddings. An estimated service life of 64 and 63 years was obtained for limestone and marble, respectively, in Tehran, while 78 and 66 years were the estimated service lives obtained for limestone and marble, respectively, in Lisbon. These results are consistent with those of Schouenborg et al. [45], who found that granite is the most durable cladding stone, followed by limestone and then marble, by analysing the strength capacity of stone plates of 200 buildings.

Regarding the colour of the natural stone plates (Figure 10), both light and dark colours exhibit a good correlation between the field results and the degradation curves. Figure 9 shows that the degradation progress of stone claddings with light colours is faster than those with dark colour stones at the preliminary 50 years of the façade's service life in both Tehran and Lisbon. On the contrary, after that time interval, there are no case studies in Tehran, and more data regarding claddings with dark colour stones must be collected in order to obtain unequivocal conclusions. The literature does not provide any indication regarding the relevance of colour for the deterioration of stone claddings [2]. In the Tehran sample, light colour stone claddings tend to present a higher incidence of defects than the dark colour ones, mainly because some defects, such as staining or soot deposition, are easily identified in light colour claddings [14], and, moreover, dark colours are usually associated with more durable stones, such as granites and other eruptive rocks [22].

**Figure 10.** Degradation curves of the Tehran and Lisbon samples, according to the colour of stone.

In terms of type of finishing (Figure 11), subtle variations in the surface finishing produce some differences in decay patterns and the degradation evolution of stone claddings. Various studies suggest that the polishing of stone surfaces would partially inhibit the weathering effect because of the reduction in the open porosity of the stone surface, which exerts an influence on salt decay by fog sea-salt deposition in coastal areas, such as Lisbon, or soot deposition in the polluted air of Tehran [46–48]. In other words, claddings with a rough finishing present higher degradation indexes mainly due to having a higher area of stone exposed to the climatic degradation agents [49]. The results from this study (Figure 11) are coherent with previous findings, and an estimated service life of 67 and 61 years was obtained for claddings with smooth and rough finishings, respectively, in Tehran, while an estimated service life of 69 and 67 years was calculated for Lisbon for the same categories.

Regarding the evolution of the degradation condition of stone claddings according to the size of the stone elements (Figure 12), the results reveal that claddings with medium size plates (area < 0.4 m2) present longer estimated service lives than those with larger stone plates (area ≥ 0.4 m2). As mentioned by Silva et al. [2], this can probably be explained by the fact that the larger the size of the stone elements, the greater the cladding's susceptibility to weather effects, in addition to the lower relative area of the joints in the larger plates and a resulting higher concentration of stresses, which can promote a higher incidence of defects. Based on the degradation curves (Figure 11), an estimated service life of 67 and 61 years was obtained for the Tehran claddings with medium and large stone plates, respectively, and 71 and 66 years for the Lisbon sample.

**Figure 11.** Degradation curves of the Tehran and Lisbon samples, according to the type of finishing.

In terms of the location of the cladding (Figure 13), the samples under analysis were grouped depending on whether the stone cladding was located in the taller areas of the

façade (integral or partial elevated cladding) or only in the lower floors of the building (partial bottom cladding).

**Figure 13.** Degradation curves of the Tehran and Lisbon samples, according to the location of the cladding.

Westberg et al. [50] suggested that the higher levels of façades are more exposed to the environmental agents, which can lead to a rapid deterioration of the stone claddings. Furthermore, bottom wall claddings are more accessible to carry out maintenance actions, so they are expected to have longer estimated service lives [2,22]. This study also leads to the same results, as shown in Figure 13, since an estimated service life of 63 years was obtained for partial or elevated claddings and 87 years for bottom wall claddings in the Tehran sample, while an estimated service life of 69 and 65 years was obtained for the bottom wall and elevated stone claddings in Lisbon, respectively.

The literature review on the degradation of building façades reveals that moisture is one of the main factors for materials' deterioration [51]. Wind and solar radiation are responsible factors for moisture settling on the façade and are variable in accordance with the geographical orientation [52]; hence, an analysis of the degradation curves according to the claddings' orientation seems relevant for the analysis of the degradation over time of stone claddings, and they are illustrated in Figure 14.

According to Gaspar and de Brito [14], regarding the rate of the erosion process according to the façades' orientation, the authors concluded that north and west are the most critical orientations in Portugal. This has been confirmed by Emídio et al. [24], who suggested that the stone claddings facing east and south are more durable, while claddings facing west and north tend to present lower service lives. The outcomes of the current study for the Lisbon sample reveal that the most favourable cladding orientations are, in decreasing order, south, east, north and west, with estimated service lives of 71, 70, 67 and 64 years, respectively, which corroborate the findings of the previous works.

**Figure 14.** Degradation curves of the Tehran and Lisbon samples, according to the façades' orientation.

In accordance with the meteorological statistics [53], the prevailing winds in Tehran come from the west. Therefore, the probability of the incidence of wind-driven rain is high in that orientation, thus being more prone to suffer from anomalies due to the presence of moisture. Accordingly, in the sample analysed (Figure 14), stone claddings facing north/northeast and west/northwest show lower estimated service lives (63 and 64 years, respectively) in comparison with those facing south/southwest and east/southeast (with estimated service lives of 74 and 78 years, respectively).

Concerning the exposure to the combined action of wind and rain (Figure 15), two categories are considered: (i) stone claddings exposed to a severe action of wind and rain, in façades that are not protected by surrounding buildings or vegetation, in elevated areas; and (ii) stone claddings with a normal or moderate exposure to this action. As mentioned before, in the analysis of the impact of the façades' orientation on the deterioration of stone claddings, the action of driven rain promotes the deterioration of stone claddings, being one of the main causes of erosion and other loss of integrity deficiencies [51,54]. Therefore, stone claddings with a severe exposure to wind–rain action reach the end of their service life sooner (64 and 67 years for Tehran and Lisbon samples, respectively) than claddings with a moderate exposure (85 and 68 years for Tehran and Lisbon samples, respectively).

**Figure 15.** Degradation curves of the Tehran and Lisbon samples, according to the exposure to wind and rain action.

#### **6. Results and Discussion**

Table 4 shows a summary of the results obtained for the two samples. In the previous section, as the degradation curves were presented, the results were discussed, and the conclusions of this study can be summarised as follows:

mechanical properties. The results in Table 4 reveal that the type of stone is the most influential parameter in the service life of natural stone claddings in Lisbon. On the other hand, exposure to prevailing winds and application of the cladding in bottom walls, which are more protected from the environmental agents, seem to be the most relevant factors for the deterioration of stone claddings in Tehran (these characteristics have the highest impact on the loss or gain of the estimated service life of stone claddings in Tehran);

**Table 4.** Summary of the results obtained for the estimated service life of natural stone claddings in Tehran and in Lisbon.

\* The estimated service life is determined using the equations of the degradation curve shown in Figures 7–14. \*\* These values are obtained through the ratio between the estimated service life for each characteristic and the average estimated service life, which is 65 and 68 years for Tehran and Lisbon samples, respectively.

#### **7. Concluding Remarks**

This study analyses the service life and durability of stone claddings in two environmental contexts, namely in Lisbon, Portugal, and in Tehran, Iran. The methodology applied in this study was initially developed for rendered façades in Lisbon [14], with the aim of being a general framework for predicting the service life of building elements. In this sense, this empirical method was adapted and applied to other claddings, in particular to natural stone claddings [2,22]. In the previous approaches, the authors mentioned that this method could be applied to other elements and to other geographical contexts. In fact, even though there are significant differences between the two locations, the proposed

methodology proves to be able to accurately predict the service life of stone claddings in Tehran according to their intrinsic properties, such as the type of stone, their design conditions and their exposure to environmental agents.

Similar degradation patterns were identified in both samples in accordance with the physical deterioration process of natural stone claddings over time. The results obtained revealed that the characteristics of stone claddings strongly influence their service life. The climatic or atmospheric agents present a significant impact on the deterioration of stone claddings. However, the most conditioning climatic agents for the deterioration of stone claddings seem to change in the two geographical contexts. In Lisbon, the exposure to damp seems to be the most conditioning factor, while in Tehran, the exposure to driven rain appears to be a very significant degradation agent, mainly because there are also high levels of atmospheric pollution associated with this action.

This study proposes an empirical tool to evaluate the service life of natural stone claddings, considering their on-site performance. The application of this method to two different locations proves the validity and applicability of the tool to different contexts. This study provides some knowledge about the degradation and service life of stone claddings, quantifying the variability of the estimated service life introduced by the different characteristics analysed. This information can be extremely useful both in design and maintenance stages for the adoption of more rational and durable solutions.

**Author Contributions:** Conceptualisation, S.H.M., A.S. and J.d.B.; methodology, S.H.M. and A.S.; formal analysis, S.H.M., A.S. and J.d.B.; investigation, S.H.M. and A.S.; data curation, S.H.M. and A.S.; writing—original draft preparation, S.H.M. and A.S.; writing—review and editing, J.d.B., A.E. and S.B.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Portuguese Foundation for Science and Technology (FCT) through project BestMaintenance-LowerRisks (PTDC/ECI-CON/29286/2017).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors gratefully acknowledge the support of the CERIS Research Centre (Instituto Superior Técnico-University of Lisbon).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Most Frequent Problems of Building Structures of Urban Apartment Buildings from 2nd Half of 19th Century and the Start of 20th Century**

**Klara Kroftova**

Faculty of Civil Engineering, Czech Technical University in Prague, 166 36 Prague 6, Czech Republic<EMAIL_ADDRESS>
**Abstract:** An urban residential building from the second half of the 19th century and the start of the 20th century, the so-called tenement house, is a significant representative of the architecture of the developing urban fabric in Central Europe. The vertical and horizontal load-bearing structures of these houses currently tend to show characteristic, repeated defects and failures. Their knowledge may, in many cases, facilitate and speed up the design of the historic building's restoration without compromising its heritage value in this process. The article presents the summary of the most frequently occurring defects and failures of these buildings. The summary, however, is not an absolute one, and, in the case of major damage to the building, it still applies that, first of all, a detailed analysis of the causes and consequences of defects and failures must be made as a basic prerequisite for the reliability and long-term durability of the building's restoration and rehabilitation. An integral part of the rehabilitation of buildings must be the elimination of the causes of the appearance of their failures and remediation of all defects impairing their structural safety, health safety and energy efficiency.

**Keywords:** apartment building; 19th century; horizontal structure; vertical structure; defects; failures

#### **1. Introduction**

The apartment building became a new phenomenon in Central Europe in the 19th century, although rental flats had been quite common in historic cities, practically from the Middle Ages. However, the situation in individual cities differed. Architect Pavel Janak summed up the situation in his article "One Hundred Years of a Residential Rental House in Prague" (1933): "The situation in Prague and our towns in the 19th century in general is characterized by the non-existence of a family house. Towns are made up of large houses with many flats, built by individuals and rented out. Our culture, or its lack, is based on collective housing in a large building. A family house and family living is not a component of our culture. In the second half of the 19th century, the type of a large house became the subject of unprecedented business. The primary reason why it was built was not to provide flats, but to make money fast. The size of the house and its massive scale were only paralyzed at the end of the 19th century by increased demands for the value of an apartment—leading to the shrinkage of the house to only two apartments on one floor and to the appearance of a family house, a hitherto unknown type of housing" [1].

The courtyard tracts of historic houses represent an initial early stage in the development of apartment buildings in Czech cities—while the solidly built foundation of a house with a vaulted ground floor was often left even during a radical reconstruction, the newly built courtyard tracts connected with the house by galleries emerged as completely new structures [2]. In Prague (the capital city of the Czech Kingdom of the Austro-Hungarian Empire of the time) three or, less often, even four-storey houses were built in the busiest streets on the threshold of the 19th century (Figure 1). On side streets, the houses mostly had two to three storeys, and single-storey houses were an exception. The houses had

**Citation:** Kroftova, K. Most Frequent Problems of Building Structures of Urban Apartment Buildings from 2nd Half of 19th Century and the Start of 20th Century. *Buildings* **2021**, *11*, 27. https://doi.org/10.3390/buildings 11010027

Received: 8 December 2020 Accepted: 1 January 2021 Published: 12 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

courtyard tracts and the space was often built-up to the maximum extent. The situation differed in other large Czech cities (e.g., Pilsen or Ostrava), where three-storey houses were a rarity; moreover, they were only built in the main squares. Two-storey houses, and single-storey houses in side streets, were much more common there. The former historic royal towns, which usually had around 220 to 250 houses in the "walls", used to have mostly single-storey houses, and a two-storey house was an exception. At the same time, the smaller the town, the fewer the number of rental flats.

**Figure 1.** Section (**a**) and elevation (**b**) of a typical four-storey tenement house from the end of the 19th century on the student work of A. Censky, 1890 [3].

> Until the beginning of the 18th century, there were no binding rules in force for the design of buildings. The fundamental construction principles were based on the builders' efforts to create the required interior space volume defined by vertically laid-out loadbearing structures, divided horizontally by floor structures and enclosed by the roof.

> From about the mid-18th to the mid-19th century, the most important areas related to building law were refined, mainly in connection to fire safety. The buildings were designed on the basis of builders' experience where the thickness of vertical load-bearing structures was determined by empirical formulae depending on the number of storeys, the height of the storeys and the depth of the tract. During the 19th century, three waves of building codes were issued, which partly reflected the development of urban construction practice. The first wave includes the Czech Building Regulations of 1833, the Moravian and Silesian Building Regulations of 1835 and the amended version of the Czech Building Regulations of 1845. The second wave may be represented by the Czech Building Regulations of 1864, the Moravian Building Regulations of 1869 and the Silesian Building Regulations of 1883. The third wave covers the Prague Building Regulations of 1886, which were adopted by Pilsen and Cesk ˇ é Budˇejovice in 1887, the Czech Building Regulations of 1889 applying to the rest of the country and two Moravian Building Regulations of 1894 [4].

#### **2. Materials and Methods**

Vertical and horizontal load-bearing structures of apartment buildings from the second half of the 19th century currently show, in many cases, characteristic, recurring defects and faults (Figure 2). The main goal of this paper is to identify the most common defects and failures of masonry structures based on field and archival research of building structures and details of masonry buildings from the period. Knowledge of endangered structural details of vertical and horizontal structures, including, for example, the issue of basic construction conditions, is a suitable step to mitigate the consequences of construction and technical problems of historic buildings. At the same time, their knowledge can facilitate and speed up the design of the restoration of a historic building so that no monumental values are lost during this process [5].

**Figure 2.** Images clearly showing the most common failures of structures from the second half of the 19th and the early 20th centuries caused by degradation processes initiated in these cases by the combination of moisture, an incorrect design solution and neglected maintenance. The following are clearly visible on images: (**a**) fallen plaster, damaged ledge by massive leakage, mold and mosses; (**b**) degradation and disruption of plaster, significant surface efflorescence of salts, mold and mosses.

> A summary of the most common defects and failures of these buildings, presented in the following parts of the article, is based on field research, especially of Prague apartment buildings. The archival research is based on historical building literature of the 19th century and a search of drawing documentation of students of construction schools, which is available at the National Technical Museum in Prague [6].

#### *2.1. Vertical Structures*

The primary structural function of vertical masonry structures is to transfer the effects of vertical and horizontal loads from individual floor structures through the foundation structure to the foundation subsoil. Tenement houses from the 19th century, but also shops, schools, hospitals, public buildings, etc., were characterized by the longitudinal layout of load-bearing walls, parallel to the street tract. Inside the building, the load-bearing walls formed the so-called middle walls, where the depth of the tracts most often ranged between 4 and 5 m, exceptionally up to 8 m (palaces, public buildings), while internal tracts were usually less than 4 m deep (Figure 3a). The vertical load-bearing structures of urban apartment buildings were most often executed in mixed masonry and, above all, masonry of solid burnt bricks of the so-called classic format of 290 mm × 140 mm × 65 mm [7,8]. In exceptional cases, the walls of overground storeys in buildings from this period could already be built of hollow burnt-clay bricks or hollow brick-like blocks of lightweight concrete (slag concrete or slag pumice), or blocks of another "modern" material. The end of the 19th century was marked by an increase in the use of rolled iron elements (beams, columns, etc.), to be later followed by a global development of concrete and reinforced concrete structures.

Pursuant to the above building codes, urban residential masonry buildings were characterized by massive walls 300–900 mm thick, where, starting from the second wave of building regulations (i.e., from the second half of the 19th century), a minimum wall thickness of 450 mm was required to prevent frost penetration and was further increased in a downward direction (Figure 3b). Lime or cement-lime mortar was used as the most common bonding material, while the use of cement mortar increased at the end of the century, and, subsequently, in the 20th century.

**Figure 3.** The floor plan and section of urban apartment buildings: (**a**) floor plan of house with four and two-room apartment, both are equipped with bathrooms (arch. V. Reznicek, early 20th century) [9]; (**b**) part of a four-storey house with a clearly legible arrangement of the vertical supporting structure with increasing thickness downwards (V. Douda, 1902) [10].

The stability of the load-bearing system of masonry buildings with several storeys was ensured by longitudinal—front and middle—walls, on which floor structures and transverse walls properly bonded to the longitudinal walls were mounted. The transversely laid-out walls—gable walls, staircase walls, brick partitions 150 mm and 300 mm in thickness—coupled the longitudinal, primarily load-bearing walls, ensuring their stability and contributing to the spatial performance of the load-bearing system, and played an important role in enhancing stability.

The so-called wall and beam ties fulfil an important function in terms of ensuring spatial stiffness, stability and resistance to the effects of forced deformations (Figure 4). From the beginning of the 19th century, wrought iron wall ties were used in walls and vaults, where they served to stiffen and clamp the building and absorb oblique compressive loads, particularly during masonry settling and the additional compression of foundation soil.

#### *2.2. Horizontal Structures*

In the majority of urban apartment buildings from the second half of the 19th and the start of the 20th century, various types of floor structures can be found. From a fireproof point of view, ceiling structures are one of the most important parts of the building. At the same time, the fire safety properties of horizontal structures not only reflect their historical development, but also represent an important criterion in categorization. "Numerous construction ceilings can be divided into more or less completely combustion—if it had used exclusively wood because besides still other substances—and incombustible in which wood is unfit on at all." [7] (p. 72).

With regard to the ability of horizontal structures to resist the effects of fire, ceilings of the monitored period can be divided into:

According to current knowledge, the above-mentioned division of ceiling structures, from the point of view of fire resistance, is overly simplified and inaccurate [11,12]. Nevertheless, at the time of formulating the above-cited building regulations, it was a significant step to increase the safety of buildings, i.e., also city apartment buildings.

**Figure 4.** The wall and beam ties: (**a**) the beam wrought iron ties in the drawing on the school work of V. Vasatko from 1910 [13]; (**b**) the detail of the wall ties and their location (red lines) in the floor plan of the house [14]; (**c**) wall ties made of wrought iron, including adjustment with a lock, on a period photograph (1932) when performing a masonry structure [15].

The dominating types of horizontal structures used were combustible and semicombustible floor structures. Specific types of floors include, in particular, wooden beam floors with subflooring and, later, also with reed plaster, or an upright plank floor with diagonal braces with reed plaster, subflooring and an embankment, etc.

In rooms with strict fire safety requirements, i.e., mainly in public buildings, but also in some areas of apartment buildings (e.g., the ground floor), non-combustible floor structures were mostly designed. In the apartment buildings of the 19th century, vaults were considered to be a safe, non-combustible ceiling structure (or were required by law and building regulations for the Czech Kingdom). The vaults were used (or required by the building codes) in gateways, entrances to houses, cellars and potentially, if necessary, in staircases, corridors and on the ground floor of apartment buildings were considered as safe, non-combustible floor structures. They were usually of a wagon vault type with arch rings, and, later, flat mirrored vaults were used. In this period, the non-combustible ceiling structures used also included segmental vaults made of solid fired bricks vaulted between the beams made from cast iron, wrought iron or later steel (Figure 5). The structures used at the turn of the 19th and early 20th centuries also included various types of masonry floor structures derived from a barrel vault, so-called straight or flat vaults made up of ceramic masonry elements, and, later, of plain or reinforced concrete.

**Figure 5.** Detail of a vaulted ceiling in a school project, unknown author, 1920s. Section (**a**) and top view (**b**) of a brick vault into traverses with a clearly visible composition of bricks [16].

#### **3. Results**

The main causes of degradation processes, which generally lead to the deterioration of the required properties of materials and structures, are, on the one hand, specific properties of building materials (their composition, structure, etc.) and, on the other hand, time-variable parameters of the exterior environment in which buildings and their parts perform [17,18]. Different degradation processes caused by climatic factors differ from each other in their duration and type of effect. Some last for fractions of a second, while others for days, months or even years. These parameters, together with material parameters, create the conditions that will initiate or accelerate mechanical, mineralogical, physical, chemical and biological degradation processes [19]. It may be summarized that the defects and failures of structures from the second half of the 19th and the start of the 20th century are most frequently caused by design defects, material defects, by degradation processes triggered mainly by moisture, by incorrect design solutions, faulty workmanship, usage and by neglected maintenance [20–22].

#### *3.1. Vertical Structures*

A frequent cause of failures of vertical masonry structures of buildings from the second half of the 19th and the start of the 20th century, which are manifested by cracks, crushing and spalling of surface layers, etc., is poor quality and strength of the masonry in compression and tension plus some other effects, such as different properties of the masonry elements made of mixed masonry or multi-leaf masonry, respectively, low compressive

and tensile strength of masonry elements and mortar, etc. [23]. The quality of the masonry and its load-bearing capacity result from the quality of the masonry elements, the bonding mortar and masonry workmanship, but, in particular, from compliance with the principles of proper masonry bonding [24]. The quality of masonry is also significantly affected by increased moisture levels or its aggressiveness and associated degradation processes.

Defects in masonry are caused by imperfect project documentation or imperfect or incorrect execution. The most common execution defect is non-compliance with the principles of proper bonding. The defects in masonry subsequently manifested by the occurrence of failures, insufficient load-bearing capacity, low resistance to degradation processes and others, include:

In the case of buildings from the second half of the 19th and the start of the 20th century, the cause of these failures resulting from a change in the footing bottom shape is an error in their project preparation, which lacked a geological survey of foundation conditions. At the same time, the empirical design of building structures and insufficient knowledge of the above-mentioned issues led to the construction of foundation structures with insufficient stiffness, without expansion joints and to insufficient reinforcement of the load-bearing masonry at the level of floor structures (by wall and beam ties, bond beams, tie rods, etc.).

The failures of vertical load-bearing masonry structures are crucial in terms of ensuring the mechanical resistance and stability of the load-bearing system. Structural (mechanical) failures of masonry structures and buildings are caused by the response of the building structure to static loads and the deformation effect of some loads and non-force effects, or to static loads with a dynamic component. The most common manifestations of these failures are excessive deformations and strains, cracks (failure in tension and shear), crushing (failure in compression) and local mechanical damage arising due to insufficient loadbearing capacity, in places of stress concentrations, as a result of a change in loading, longterm deformations, degradation and disintegration of masonry, or due to unprofessional interventions. (Figure 6) [28–30]. These failures are directly related to the structural behaviour of the load-bearing structure and affected by their occurrence in the structural performance of the load-bearing system, either locally or as a whole—they are critical in terms of ensuring the mechanical resistance and stability of the supporting system [31,32].

Non-structural failures of masonry structures and buildings are caused by the interaction of materials used for individual structures with the external environment, manifested, above all, by increased moisture levels, chemical, mineralogical and biological processes which weaken the required properties of structures and cause their gradual deterioration and disintegration—degradation processes [33].

The failures of masonry are closely related to the failures of the adjoining masonry structures—vaults, springers and lintels, relief vaults, cavettos and soffits of wooden ceilings. Attention must also be paid to potential defects without visually observable manifestations (cavities and plastered grooves in masonry, different quality of individual masonry components, etc.).

In summary, the most frequently occurring defects and failures of vertical structures of buildings from the second half of the 19th and the start of the 20th centuries include:

• tensile and shear cracks due to the effect of spatial stress states arising in the place of the concentration of compressive stresses in masonry exerted by the mounting of floor beams and girders (or masonry crushing due to the effect of concentrated compression);

**Figure 6.** Examples of structural failures of structures from the second half of 19th and the start of the 20th century: (**a**) prominent tensile cracks in masonry weakened by openings, with the representation of pressure trajectories, 1—area of normal stresses and tensile cracks; (**b**) crack between the perimeter structure and the ceiling caused by forced deformation due to the effect of a change in the footing bottom.

A change in the shape of the footing bottom and subsequent masonry failures might also have occurred as a result of building activity in the surroundings, e.g., extension of the building, vehicle traffic (dynamic effects causing compaction of the subsoil, shocks of the building), or a change in hydrogeological conditions, waterlogging or frost penetration into the footing bottom of the existing foundation during the erection of a neighbouring building, additional loading of the foundation, dynamic effects and shocks during the construction of a neighbouring building, etc. (Figure 7) [37].

A serious problem of urban masonry apartment buildings is the degradation and disintegration of damp masonry and surface coatings due to the action of moisture contaminated by soluble salts and weak acid solutions. The chemical processes disintegrate the binder component of masonry elements, and the binder and the salts which are the product of these reactions create pressures in the masonry pore system during a change in moisture and degrade its integrity, causing the spalling and disintegration of coatings and layers.

The prevailing parts of historic buildings from the second half of the 19th and the start of the 20th century suffer from increased moisture levels in the foundation masonry and masonry above the foundation line as a consequence of non-functional or damaged waterproofing of the masonry above the foundation line and the masonry of underground parts of the building, leaks or accidents of water supply and sewerage pipes and mains, poor drainage of the terrain, insufficient ventilation, etc. (masonry moisture in dry state is 1%– 3% by weight) [38,39]. In many cases of historic brick buildings, there is no damp proofing, or the damp proofing, ventilation systems, etc. have deteriorated and degraded [40].

**Figure 7.** Graphical representation of the course of cracks and failures of vertical masonry - characteristic tensile cracks on the side (**a**), resp. in the middle (**b**) parts of the building, which arose as a result of the decrease of the affected part and thus the change in the shape of the foundation joint, e.g., an uneven foundation conditions (sands – rock), uneven compaction of the substrate or waterlogging of the soil, etc. (arrows indicate the place of decline of part of the foundations) [41].

The increased moisture levels of underground and partly overground masonry are usually accompanied by chemical and biological degradation processes, whose intensity depends on the value of increased moisture levels and its aggressiveness. The knowledge and assessment of salinity, especially the content of sulphates, chlorides and nitrates in masonry, are essential for the evaluation of the severity of increased moisture levels and the design of an effective remediation method (Figure 8) [42,43]. Salts with hygroscopic properties and the presence of bacteria and mould in the pore system of masonry, which increase the masonry hygroscopicity, can significantly affect the equilibrium moisture content. Degradation processes usually reduce the content of binder components in the masonry. For these reasons, the assessment of the condition of building structures should include, inter alia, an analysis of the change in total porosity and pore distribution and an assessment of the content of water-soluble salts (Figure 9) [44,45].

**Figure 8.** Graphical expression of the dependence of the (**a**) total porosity and modulus of elasticity and ultimate strength, (**b**) wt. moisture and modulus elasticity and ultimate strength of bricks from the end of the 19th century [46].

**Figure 9.** Examples of non-structural failures caused by the effects of the external environment, especially the synergic effect of increased moisture levels and temperature, which are accompanied by chemical, mineralogical and biological processes in structures from the second half of 19th century and the start of 20th century: (**a**) damage of the ledge and its surface finishes, visible salt efflorescence; (**b**) closed and open blisters in the surface layers due to their low diffusion permeability; (**c**) extensive damage to the plinth part caused by the long-term effect of humidity and salts.

#### *3.2. Horizontal Structures*

Among the defects of wooden floor structures preceding the occurrence of failures, we might mention, above all, the following: wood infested by wood-destroying fungi or insects, the inappropriate mounting or poor quality workmanship of joints of wooden elements, installation of wood with high moisture levels (whm 12%), impermeable closed-cell wood with initial moisture contents, or closure of high moisture levels in the structure [47].

Other major factors leading to the failures of wooden parts of horizontal load-bearing structures are, among others, the following:

**Figure 10.** Examples of failures of wooden parts of horizontal load-bearing structures caused by mounting wood without a natural possibility of drying and with insufficient permeability of the structure: (**a**) rotting head of a wooden ceiling beam; (**b**) yellow patch of wet rot on ceiling wooden panel.

The reserves in the stability of massive longitudinal walls in the transverse direction are insufficient and longitudinal load-bearing walls on their own usually cannot ensure the stability of a building with more than 2 overground storeys in relation to cross wind effects and other effects exerting transverse horizontal forces (e.g., effect of differential settlement, dynamic traffic effects, eccentricity of vertical loads transmitted e.g., by beam floors, temperature differences). This problem is often neglected during rehabilitations when the structural safety (spatial stiffness) may be seriously impaired by the demolition of internal transverse, primarily non-load-bearing walls and partitions.

The reinforcement of a wooden floor structure must be preceded by a detailed survey of wooden structures—particularly a mycological and construction and technical survey, removal of damaged or infested wood and basic chemical protection of wood by a suitable coating, or the sterilization of wood at an elevated temperature [50,51]. The most reliable protection measures of wood against biotic wood-destroying agents are preventive measures against their appearance and development, mainly the protection of wood against increased moisture levels (above 15%).

**Figure 11.** Example of laboratory evaluation of bioderiogents in cultures of a sample of a damaged ceiling structure in Prague 7—confirmed presence of bacteria and fungi. In all cases, massive development and growth of microscopic fibrous fungi was noted. In cultures of samples were identified: (**a**) *Trichoderma* sp. and bacterias; (**b**) *Trichoderma* sp., *Mucor* sp., wood-destroying fungus and bacterias; (**c**) Mucor sp. and wood-destroying fungus; (**d**) *Penicillium* sp., *Acremonium* sp., *Aspergillus* sp. and bacterias [52].

#### **4. Conclusions**

The article presents the most common defects and failures of vertical and horizontal structures of apartment buildings from the second half of the 19th century and the beginning of the 20th century. The construction practice of this period was influenced by three successively issued waves of building regulations, which reflected the building boom of this period.

The issues related to defects and failures, degradation processes and the design of rehabilitation measures of buildings from the second half of the 19th and the start of the 20th century include a wide range of topics of interdisciplinary nature, from the natural sciences, materials engineering, mechanics, elasticity and structural theory of building structures to the knowledge of historical materials, structures and technologies. The knowledge of historical structures, materials and construction methods used helps us to prevent errors in the rehabilitation and restoration of prominent, mainly heritage buildings to the original state.

A precise analysis of the causes and consequences of defects and failures is the basic prerequisite for the reliability and long-term durability of rehabilitation and remediation measures. An integral part of the reconstruction and renovation of buildings must be the elimination of the causes of failures, remediation of all defects that reduce their structural safety, health safety and energy efficiency. To ensure the durability of reconstructed buildings, degradation and corrosive processes caused mainly by chemical and biochemical processes and non-force effects of temperature and moisture must be prevented and adequate protective measures must be taken. In this respect, the protection of buildings and their individual parts against the effects of increased moisture levels is of primary importance.

**Funding:** This research was funded by the Ministry of Culture of the Czech Republic.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available in a publicly accessible repository.

**Acknowledgments:** This article was written as part of the NAKI DG18P02OVV038 research project "Traditional City Building Engineering and Crafts at the Turn of 19th and 20th Centuries (2018–2022, MK0/DG)".

**Conflicts of Interest:** The author declare no conflict of interest.

#### **References**

## *Article* **Critical Analysis about Emerging Technologies for Building's Façade Inspection**

**Ilídio S. Dias 1,\*, Inês Flores-Colen 1,2 and Ana Silva <sup>1</sup>**

**Abstract:** The diagnosis of the building's façades pathology is extremely important to support rational and technically informed decisions regarding maintenance and rehabilitation actions. With a reliable diagnosis, the probable causes of the anomalies can be correctly identified, and the correction measures adopted can be more compatible with the existing elements, promoting the durability of the façades. Visual inspection is the most common approach to identify anomalies in a building's façade and, in many cases, this technique is sufficient to support the decision to intervene. However, the pathological phenomenon is complex, and the anomalies observed may indicate the presence of other defects, or some anomalies may not be visible in a simple visual observation. This study intends to discuss the application of emerging technologies on the diagnosis and anamneses of building's façade, in order to automatise the collection of reliable on-site data and, thus, reduce the uncertainty of the diagnosis. The use of these techniques can help existing inspection methodologies, already tested, based mainly on the visual assessment of the buildings' elements degradation condition.

**Keywords:** diagnosis; advanced technologies; façade; claddings; building inspections

**Citation:** Dias, I.S.; Flores-Colen, I.; Silva, A. Critical Analysis about Emerging Technologies for Building's Façade Inspection. *Buildings* **2021**, *11*, 53. https://doi.org/10.3390/ buildings11020053

Academic Editor: Alessandra Aprile Received: 13 January 2021 Accepted: 1 February 2021 Published: 4 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

In recent years, the international scientific community has gained more attention in the field of inspection, diagnosis, maintenance, and rehabilitation of buildings. The maintenance and rehabilitation are considered key factors in buildings' sustainability since these interventions increase the service life of buildings [1]. Inspection and diagnosis of building elements are a very important task to support decision and efficiency of building façade maintenance from the building facades [2,3]. However, there is some uncertainty during these procedures associated with the observation of the degradation phenomenon and the diagnosis process [4].

Collecting information from fieldwork about natural degradation of building elements, is an alternative to laboratory tests like artificial weathering cycles. The fieldwork survey could be very useful to compare with the laboratory tests, since it allows identifying the real-life effects of weathering on the performance of building elements, considering the simultaneous effects of different climatic degradation agents [5–8]. In facades, the degradation condition could be assessed from visual inspections, which involve a nondestructive inspection method. This type of analysis is a natural way to judge the service life of these constructive elements since the maintenance decisions are made based on this type of assessment [9,10]. In that context, several works have been developed, concerning the degradation of buildings and their elements, based on visual inspections [11–14]. These studies can be complemented by some expedient tools, like binoculars to get a closer observation, a tape measure to obtain some dimensional information, a crack with a ruler, and a colour system sample [2]. Other additional diagnosis techniques can also be applied, whether they are non-destructive or destructive techniques. However, visual inspections intend to be easily applicable in practice and without high costs.

Visual inspection is the first method used to evaluate the condition of many infrastructures and buildings [15–17]. Despite the limitations of visual inspections, this method can immediately provide general information about the condition of the building elements [9,18]. This task is usually done by an expert, which performs a survey looking for the damage in order to determine the areas that need intervention. The results of that visual inspection are reliable when dealing with easily visible parts of the façade. However, usually it is not easy, for a surveyor, to analyse the top of the façade or assess anomalies that are in deeper locations without appropriate means of access, and with unfavourable weather conditions. In that context, a reliable and rapid assessment of the defect areas on a large façade may be a difficult work for the surveyor [4,19–23]. Moreover, a visual inspection is highly dependent on the surveyor's expertise [2,24]. Hence, a visual assessment can be considered subjective, human-dependent, time-consuming, and can have low accuracy in defects' measurements in certain situations [16,24]. However, based on the knowledge regarding the condition of the building's elements, is crucial for the adoption of adequate maintenance strategies and it is fundamental for life cycle analysis, considering the economic and environmental costs of the building components over their life cycle [23,25,26].

The limitations of visual inspections motivated the use of systemized alternatives such as remote sensing for mapping the defective areas on a surface of building façades [19,22]. In recent years, the interest about advanced technologies to automate the inspection of building façades has emerged. Some studies have been developed in the field of building's façade inspection, mainly in cultural heritage buildings, by focusing on building an energy audit [21,23,27–31]. Novel technologies for a reality capture have been applied, mainly terrestrial laser scanning, photogrammetry, infrared thermography, photogrammetry, and drones. The application of these technologies can improve the traditional survey, producing more objective results in a faster way [23,32]. They could automate and improve the visual inspection, reducing the subjectivity associated with the inspector's survey [17,24]. Nevertheless, automation as a way to detect and measure a given defect is still a great challenge, which needs several technologies combined for accurately mapping all the anomalies that can occur in a given building component. More research is needed to put into practice these technologies to support the facades' inspections.

Most of these studies only focus on a single technology. Different defects could require different technologies to support and automate building's facades' diagnosis. In this sense, this study intends to discuss the suitability of these advanced technologies to automate a visual assessment during field inspection and the new trends for inspection of buildings' facades with a focus in current buildings. Furthermore, the most suitable technology to support the diagnosis of different kinds of defects is identified. These technologies can bring some automation for the inspection and diagnosis of buildings and their components, reducing the inspection time in the field, avoiding several visits to the building's location (because all the necessary information is not collected during the first time), and producing more reliable information about the anomalies present in the component under analysis.

#### **2. Materials and Methods**

In order to discuss the most suitable technologies to automate a visual assessment of several defects in buildings' facades, this research follows a specific methodology. To better understand the needs of advanced technologies, two case studies are presented, to illustrate the application of a building inspection system, based on a visual assessment of the degrading building components. In this inspection system, the parameters involved in the calculation of the facades' degradation condition are explained in detail and some improvements are suggested. In addition, to illustrate some uncertainty associated with the visual inspection, the same 47 natural stone claddings are inspected by two surveyors in different moments. Then, an extensive literature review, with about 50 references, is performed to systematise the technologies able to automate the visual façade inspection in buildings. In the end, the suitability of previous technologies is presented, in the sense of an automated visual assessment, according to different defects present in facades. Some recommendations are made in this topic. In the next sections, these topics will be developed and discussed in detail.

#### **3. Global Inspection System for the Buildings' Envelope Elements: 2 Case Studies**

The adoption of adequate maintenance strategies is the most efficient way of increasing the durability of buildings and their elements. However, an adequate intervention needs an accurate and reliable diagnosis of the defects and possible causes. Each building is unique and presents different types of defects. Nevertheless, certain patterns can be identified when analysing a large sample of buildings. A systematic analysis of the data collected during the building inspections can provide a reliable database for a guidance to prevent the occurrence of defects and to repair the existing ones. For these reasons, several inspection systems have been developed by different authors. Ferraz [1] presented and discussed 10 building pathology databases, developed between 1982 and 2013. More recently, Lee [33] proposed an inspection methodology to introduce information about building defects into BIM (Building Information Model). Bortolini and Forcada [26] proposed a building inspection system for the assessment of whole building performance [34]. Gonçalves [35] analysed the existing methods of inspection and diagnosis for ancient buildings, based on visual inspections.

In fact, it is almost consensual that building inspection benefits from the standardisation of inspection and diagnosis procedures [36,37]. The adoption of inspection systems helps surveyors to have harmonised procedures, which makes inspection reports more consistent and simpler to understand [2,3,38].

In recent years, different methodologies for the inspection and diagnosis of nonstructural elements of the building envelope have been established, in order to create a reliable database and systemise procedures for the on-site assessment of defects [39–41]. Moreover, several studies in service life prediction methodologies have been developed, with the aim to evaluate when it is necessary to intervene based on the element's degradation condition assessment [42,43].

Under the SLP for BMS (Service Life Prediction for a risk-based Building Management System) research project, a global inspection system was developed [44]. This system embraces 12 building elements, namely: external claddings of pitched roofs, flat roofs, adhesive ceramic tiling, natural stone claddings, wood floorings, door and window frames, epoxy resin floorings, vinyl and linoleum floorings, wall renders, external thermal insulation composite systems (ETICS), painted façades, and architectural concrete surfaces. The unification of these partial inspection systems implied the harmonisation of the classification of defects, probable causes, diagnosis methods, and repair techniques in a single system. This global inspection system intends to provide a solid basis for surveyors to inspect the building envelope through standardisation of procedures [3,34].

This global system is based on visual and physical scales to characterize the building envelope elements during the inspections. Gaspar & de Brito [45] begin this work with the development of a qualitative scale, based on the assessment of the physical and visual degradation of rendered façades, analysed during a fieldwork survey, which can be associated with a quantitative index. This describes the global performance of the façades [42]. This numerical index, called severity of degradation (Sw), expresses the global degradation of a given façade through the ratio between a weighted degraded area and a reference area, equivalent to the total cladding with the highest possible level of degradation (Equation (1)). The weighted degraded area is a product of the façade area affected by different groups of anomalies with a weighted factor related to the severity of each anomaly detected (kn) and a weighted factor associated with relative weight of each anomaly on the overall degradation of façade (ka,n).

$$S\_{\text{av}} = \frac{\sum (A\_{\text{n}} \times k\_{\text{n}} \times k\_{\text{a},\text{n}})}{A \times \sum (k\_{\text{max}})} \tag{1}$$

where Sw is a severity of degradation of the cladding, expressed as a percentage. kn is the multiplying factor of anomaly n, as a function of their degradation level, within the range K = {0, 1, 2, 3, 4}. ka,n is a weighting factor corresponding to the relative weight of the anomaly detected. An is the area of cladding affected by an anomaly n. A is the total area of the cladding, and kmax is the multiplying factor corresponding to the highest degradation level of the cladding [42]. This methodology was developed to external renders and applied to the building elements previously mentioned.

To discuss the reliability of this severity of the degradation (Sw) index, two case studies are presented in this paper: (i) a continuous element (a rendered wall) and (ii) a discontinuous element (a natural stone cladding). These two claddings represent different challenges in the calculation of severity of the degradation index, which will be presented later. Figure 1 describes a rendered wall inspected according to the global inspection system. The case study is a building located in Tavira, Algarve (South of Portugal). The façade is oriented toward the west and present an average level of protection against the wind and rain. In terms of environmental factors, the inspected building is located at less than 3 km from the sea in a rural environment and exposed to normal conditions of air temperature and relative humidity. The total area of the facade analysed is 105.52 m2 and the render is 10 years old.

**Figure 1.** First case study: rendered façade (data sourced from Reference [46]).

Table 1 presents the defects in the rendered façade (Figure 1) observed during the visual inspection carried out during the fieldwork survey. The surveyor recorded the area of rendering affected by each group of defects (An). The degradation condition level of each defect found (kn), according to its condition (taking into account the classification of defects proposed in Table 2), and the factor corresponding to the relative weight of each defect in the overall degradation of the façade (ka,n), according to Table 3. In summary, Table 1 presents the weighted degraded area (*An* × *kn* × *ka*,*n*), taking into account the defects observed. In order to calculate the severity of the degradation (Sw) index of this cladding, Equations (2) and (3) are used.

**Table 1.** Identification of anomalies observed in a rendered façade, after an inspection.

**Table 2.** Classification of the degradation condition of rendered facades to the level of the defect (kn) 1.

<sup>1</sup> Data sourced from References [42,47].

**Table 3.** Weighted factor for render facades (ka,n) 1.

\* 1.00 in the situations of occurrence of ice / thaw cycle. <sup>1</sup> Data sourced from References [42,47].

In Equation (3), the calculation of the severity of degradation (Sw) for this case study is presented. The result is a Sw index of 13.39%, which means a slight degradation of rendered façade, according to Table 4.

$$S\_{w} = \frac{\sum (A\_{\text{stains}} \times k\_{\text{n}} \times k\_{a,\text{n}}) + \sum \left(A\_{\text{cracking}} \times k\_{\text{n}} \times k\_{a,\text{n}}\right) + \sum (A\_{\text{adhesion}} \times k\_{\text{n}} \times k\_{a,\text{n}})}{A \times k\_{\text{max.}}} \tag{2}$$

$$S\_w = \frac{\overbrace{1.762 + 5.909 + 6.553}^{A\_{\text{slim}}} + \overbrace{6.940 + 5.730 + 25.800}^{A\_{\text{cruking}}} + \overbrace{3.825}^{A\_{\text{alvusion}}}}^{A\_{\text{alvusion}}} \times 100 = \frac{56.519}{422.08} \times 100 = 13.39\% \tag{3}$$

**Table 4.** Correspondence between the degradation condition and severity of degradation of rendered facades 1.

<sup>1</sup> data sourced from References [42,47].

Figure 2 describes another case study, a natural stone cladding, directly adhered to the substrate, which was inspected using the global inspection system. This case study refers to a building located in Parque das Nações, Lisbon (Centre of Portugal). The façade is oriented toward the east and presents an average level of protection against the combined action of wind and rain. In terms of environmental factors, the inspected building is located at less than 3 km from the sea, in an urban environment, and exposed to unfavourable conditions of air temperature and relative humidity. The total area of the facade analysed is 75 m<sup>2</sup> and the natural stone cladding is 12 years old.

The defects observed in the natural stone cladding inspected (Figure 2) are synthetized in Table 5. The surveyor recorded the area of natural stone affected by each group of the defect (An): Av—visual anomalies, Aj—joint anomalies, Af—bond-to-substrate anomalies, and Ai—loss-of-integrity anomalies. The degradation condition level of each defect found (kn) and the factor corresponding to the relative weight of each defect on the overall degradation of façade (ka,n) are assigned according to the condition classification in Table 6. In summary, Table 5 presents the weighted degraded area (*An* × *kn* × *ka*,*n*) in order to calculate the severity of the degradation (Sw) index (Equations (4) and (5)).

**Figure 2.** Second case study: natural stone cladding.

**Table 5.** Identification of anomalies observed in natural stone cladding after performing façade inspection.

\* joint anomalies only with material degradation. \*\* joint anomalies with material loss—open joint.

**Table 6.** Classification system for natural stone claddings 1.

#### **Table 6.** *Cont.*

\* Material degradation is meant to be every anomaly that involves loss of volume of the stone material. <sup>1</sup> data sourced from Reference [42].

In Equation (5), the calculation of the severity of degradation (Sw) of the second case study is presented. The result is a Sw index of 10.43%, which means a slight degradation (Condition C) of the natural stone cladding analysed, according to Table 7.

$$S\_w = \frac{\sum (A\_v \times k\_n \times k\_{a,n}) + \sum (A\_j \times k\_n \times k\_{a,n}) + \sum \left(A\_f \times k\_n \times k\_{a,n}\right) + \sum (A\_i \times k\_n \times k\_{a,n})}{A \times k\_{\text{max.}}} \tag{4}$$

$$S\_w = \frac{\overbrace{7.862 + 7.5 + 15}^{A\_f} + \overbrace{18 + 16.2 + 45}^{A\_f}}{75 \times 14} \times 100 = \frac{109.562}{1050} \times 100 = 10.43\% \tag{5}$$

**Table 7.** Correspondence between the degradation condition and the severity of degradation of natural stone claddings 1.

<sup>1</sup> data sourced from Reference [48].

In these two case studies, the visual inspection was aided by a crack ruler to measure the thickness of the cracks and a tape measure to get dimensions to support the quantification of defected areas (An) in the stone cladding and rendered façades. These procedures strongly influence the parameters used to calculate the severity of the degradation (Sw) index. Furthermore, these inspections are a time-consuming process. Only the natural stone cladding in the bottom wall was analysed, and, in some areas, a ladder was needed to evaluate the defects present in the façade in more detail at a higher level. These examples confirm that the reliability of this global inspection system depends on the accuracy of the data collection, and it is intrinsically related to the surveyor's expertise and the inspection conditions. Some automation in data collection could help to obtain more reliable and standardised results and reduce the acquisition time of the data.

#### **4. The Uncertainty Associated with the Building Inspection Based on Fieldwork**