Estimating the Service Life of Exterior Stone Claddings Subjected to Regular and Marine Service Conditions

Featured Application

Exterior Stone cladding.

Abstract

The attractiveness and durability of stone claddings make them a common exterior finish. However, comparative research of the performance of long-lasting “dry fixed” stone claddings and those made using the “wet fixed” method has not yet been carried out. The objectives of this study are to (1) characterize the deterioration patterns of exterior dry- and wet-fixed stone claddings exposed to normal and marine environments; (2) evaluate the life expectancy limiting coefficient (modifying factors), and (3) quantify the influence of a marine environment on both cladding methods. The research methods integrate laboratory tests with the field survey and probabilistic service life prediction methods. The results of laboratory tests indicate that the compliance of the stone with the requirements of the standard does not assure the durability of the stone when used as an exterior cladding material in a marine environment. The findings of the probabilistic methods show that, in both normal and marine environment service conditions, the fixing method has a significant impact on the rate of the stone deterioration process and the long-term durability of dry-fixed stone claddings.

1. Introduction

The exterior claddings of building facades are gradually degraded from the moment of construction due to various environmental damages. This process can be significantly accelerated by harsh service circumstances, such as a marine environment and exposure to polluted air, which are well recognized to have a detrimental impact on the durability of building components [1]. The salinity of marine aerosols makes this environment particularly hostile since the salt particles can enter porous construction materials and crystallize there [2]. Because of the enormous crystallization pressure that may ruin even the most resilient materials, this process can induce volume changes inside building materials and result in cracking and spalling [2,3,4].

Due to their rich aesthetic quality and exceptional durability, stone claddings provide a desired exterior finish [5]. Nevertheless, even if Emidio claims that the method of fixing plays an essential role in their durability [5], an investigation of the long-term toughness of “dry fixed” stone claddings has not yet been conducted, despite the rising interest in research into new methodologies to analyze the service life of various types of exterior cladding (Double-Skin Facades). A relatively new building technique could be dry-fixing in natural stone claddings, where the old practice of attached stone cladding (wet-fixed) is replaced mainly with dry-fixed stone cladding [6,7]. This construction approach is commonplace in current building practice [8]. According to Israeli Standards [9,10], Figure 1 shows a typical wet- and dry-mounting technique system. According to Shameri [6], dry-fixed stone cladding includes a cavity space between the facade layers, making it more attractive than standard skin facades because of the resulting acoustic and energy efficiency and its adaptability for tall structures. The main drawback is the substantial increase in building costs over single facades.

The objectives of the current study were set using the method created by Shohet et al. [11,12] and the findings of a prior investigation of wet-fixed stone cladding subjected to typical service conditions [13]. In order to assess the potential impact of a marine environment on both stone cladding techniques, it is essential to (1) characterize the deterioration patterns of exterior dry- and wet-fixed stone claddings exposed to either standard or marine environment service conditions and (2) measure the life expectancy limiting coefficient (modifying factors).

When selecting between the two alternative anchoring systems for exterior stone claddings, designers, construction managers, and facility managers will consider the study’s findings very helpful. They serve as an example of the significance of assessing environmental factors, whether standard or marine, when making design and construction decisions. They also serve as a critical component for maintenance managers when formulating an efficient preventive maintenance strategy.

2. Background

The estimated service life that a building or its components are expected to withstand under specific service conditions is reflected in the Reference Service Life (RSL) of building components. The methodology for determining RSL was developed by ISO 15686-1 [14]. As indicated by the associated international rules and regulations [15,16], there is significant interest in this subject on a global scale.

The performance of exterior finishing components subjected to various service conditions has been the subject of substantial fieldwork based on visual observations (empirical technique) in previous research [13,17]. Furthermore, over the past decade, several studies have used accelerated laboratory tests (mineralogical–petrographic and chemical analyses, physical tests of porosity, hydric tests and durability, mechanical tests, and so on) to imitate the natural weathering of stone claddings in normal conditions or a marine environment [18,19,20]. These analyses are time-consuming and demand high resources, but they shed some light on the correlations between the micro- and macrostructure of stone and its degradation. To address this issue, statistical and probabilistic service life prediction methods offer a way to assess the data gathered during fieldwork at a higher degree of complexity due to the simultaneous examination of several degradation elements [17,21,22].

To determine the degradation patterns that most closely resemble the data points gathered in the field and calculate the average change over time, simple regression analysis has been employed extensively. The deterioration degree of each observed building component may be determined by averaging the visual and physical performance values, according to a prior study by Shohet et al. [11,12]. A prediction interval is applied for the statistical analysis of errors related to estimating the anticipated service life of a building component at a specific age. The service life of several types of facades, including wet-fixed stone cladding exposed to typical circumstances, was investigated by Shohet et al. [13] using this approach. Typically, the deteriorating paths led to linear deterioration patterns. Regression analysis was also used by other researchers, such as Gaspar and de Brito [23], Silva et al. [24], Galbusera [25], and Bordalo [26]. The only distinction in these cases was that the intensity of the overall degradation of the different types of building claddings was determined by dividing the ratio between the size of the degraded area, which was weighted according to the severity of the anomalies, and the degree of degradation by the maximum amount of degradation of the total cladding area. Silva et al. [24] used this technique to assess the deterioration of 142 structures with natural stone cladding that was directly adhered to the substrate. A third-degree polynomial line was used to correct the point distribution. The report by Silva et al. [24], in contrast to Shohet and Paciuk [13], did not classify their study buildings according to the service conditions to which they were exposed.

Silva et al. [17,27] applied multiple regression analysis based on the same idea as simple regression analysis to estimate the service life of building components by analyzing the relationship between more than two independent variables [28]. According to Silva et al. [28], the following factors are to be held responsible for the deterioration of stone claddings: (a) less than 5 km from the coast, which was the least favorable condition; (b) carved finishings, which were more susceptible to degradation than smooth and polished surfaces; and (c) larger stone plates, which were more susceptible to mechanical deformation and, as a result, to degradation because of their larger dimensions and weight.

Another cutting-edge analytical technique, artificial neural networks (ANNs), has recently been used for the same purpose [17,28,29] and shows tremendous potential for handling challenging civil engineering problems with several independent variables. The capability of an ANN to learn from experience and examples and then adapt to changing circumstances is one of its outstanding features [30].

Stone claddings are recognized for their durability and function level under normal conditions, as well as when exposed to harsh environments [31,32,33]. In an effort to maximize their service life performance, the degradation patterns of stone claddings are consequently the subject of extensive research [34,35]. In the design stage of exterior stone claddings, factors such as the exterior envelope geometry and the exposure conditions are given considerable emphasis [36]. Reliable models of the degradation of stone claddings along the service life are required for the optimization of the maintenance and service life expenses of the external stone cladding [37]. A key factor in maintaining the durability and service life of the stone claddings is the external claddings’ maintainability [38]. Recent models have made it possible to forecast the service life and maintenance of exterior stone cladding facades by introducing the condition-based, functional, and physical performance of natural stone claddings [39,40]. The development of service life prediction models for stone claddings based on condition evaluations revealed non-linear patterns with moderately significant levels (R2~0.7–0.8), showing substantial variability in both the degradation patterns and the forecast precision [27]. The need for risk management tools drives the need for more research and model development to anticipate and evaluate the service life of stone claddings in both standard and offshore marine contexts [27,41].

The introduction makes it clear that more research is urgently needed to provide accurate service life assessment tools, service life prediction models, and design and maintenance management systems for natural stone claddings.

3. Methods

The research methods followed three phases: (1) laboratory tests of six specimens representing typical natural stone cladding materials, (2) development of probabilistic method for service life prediction in standard conditions, and (3) development of the expected service life in intensive service conditions.

3.1. Laboratory Tests

Laboratory tests were conducted on the durability of six types of limestone used in the exterior cladding of high-rise buildings and structures in the coastal zone of Israel from 1997–2016. The coastal zone of Israel is characterized by high average temperatures for most of the year, combined with high humidity and strong sea breezes affecting buildings and structures. XRD (X-ray diffraction) analysis and standard tests of the capillary water absorption and the resistance to immersion of stone samples (120 × 120 × 20 mm) for 24 h in a sodium sulfate salt solution (concentration 14%, by mass) were carried out to assess the durability of the stone samples, followed by drying to a constant weight in a laboratory drying oven at a temperature of 102 °C ± 2 °C. The objectives of the tests were to examine the compliance of the stones with the standard requirements and to assess if compliance with the standard requirements guarantees the performance of the stone cladding along time. The standard (I.S. 2378 parts 1–4) defines the performance requirements, as well as the detailed technical laboratory tests required prior to the selection of an appropriate natural stone according to the fixing method and the service regime. The technical laboratory tests encompass 13 tests, amongst which are total water absorption, capillary water absorption, resistance in sulfate crystallization, freeze and thaw resistance, sulfate resistance, initial compressive strength, and degraded compressive strength (after the accelerated deterioration test of salt crystallization).

3.2. Standard Service Conditions

The methodology introduced here for exterior claddings deterioration assessment was developed by Shohet et.al., [10]. Empirical data gathering was implemented to determine the deterioration pattern of building components and their expected service life. The evaluation is based on the direct observation of the building exterior claddings exposed to the same service conditions at different times along the cladding service life.

The first step consists of the assessment of the aesthetical and physical condition of building claddings according to systematic physical, aesthetic, and performance scales, as illustrated in

Table 1. Physical rating score, Shohet and Paciuk [12].

Rating Score	Features Observed
1	Detachment or falling off of significant portions of the cladding have been observed. Cracking wider than 5 mm has been observed.
2	Cracking wider than 1 mm has observed on at least 5% of the cladding area. Stone cladding portion has detached.
3	Cracking 0.5 mm wide observed on less than 5% of the cladding surface. Up to 3% of cladding hasdetached.
4	Capillary cracking has been observed on portions of the cladding surface. Sole cladding elements have fallen off.
5	Cladding elements have not fallen off, cladding is undamaged. Minor capillary cracking may be observed.

and

Table 2. Visual rating score, Shohet and Paciuk [12].

Rating Score	Features Observed
1	Significant fractions of the cladding are incomplete or missing. Cracks have been observed on the cladding surface.
2	Localized damage observed. Colonization of microorganisms in over one third or more of the cladding.
3	Physical damage and/or discoloring of the cladding surface. Cladding surface is not uniform.
4	Minor cracking results in cladding surface lack of uniformity, detached tiles, microorganisms, or alterations in cladding color.
5	Cladding surface is uniform and undamaged. No visible deterioration, such as cracking, missing elements, or discoloration, observed.

. This procedure yields a value for the exterior cladding component performance (CP) that ranges between 0 and 100 percent, depending on the mean of the aesthetic and the physical performance scores. A score level of 100% (5 on a 5-point rating scale) indicates the absence of any defect or sign of failure, a score of 80% (4 on a 5-point rating scale) indicates emerging deterioration, a score of 60% (3 on a 5-point rating scale) indicates escalating deterioration, and 40% (2 on a 5-point rating scale) represents comprehensive failure. The scoring indicates the symptomatic implications of degradation caused by the interrelationship of several factors (e.g., duration of exposure and service regime) with the service conditions and the durability of the components. The scoring represents the survey score of all facades of the building, and the deterioration was documented by photos and visually surveying from the ground level and from the roof level, as well as by analyzing the photos (10–12 per case study).

Fine tuning of the physical and visual grading is carried out according to the ratio between the scope of each failure detected and the entire cladding area in the building [10,11,12,30]; see Equation (1).

$$CP={\displaystyle \sum \frac{a_i}{A}}$$

(1)

where

CP—component performance

a_i—scope of the particular degradation, e.g., cracks, joint filling decay, steel anchor corrosion, staining of stone plates, etc., between two adjacent grading scales, i.e., 100/80, 60/40, etc. (in sq.m.).

A—Total area of exterior stone cladding in [sq.m.].

The typical deterioration patterns (TDPs) of the cladding components can be deduced by regression analysis of the observed CP of the claddings sample (dependent variable) against the age of the cladding components (independent variable). The outcomes represent the average degradation of the cladding component over time for the particular deterioration mechanism analyzed. In addition, the prediction interval allows an evaluation of the uncertainty associated with the assessment of the service life expectancy of future observations for a given significance level.

Analysis of typical deterioration patterns enables the prediction of service life of building components at different levels of required performance. The Life Expectancy (LE) of a component can be deduced from the interception between the typical deterioration patterns curve and the minimum required component performance (MRCP) [12]. This method is valid in an ideal case when only one specific agent of degradation is affecting the building components. The above procedure must be reiterated in order to assess the life expectancy yielded by multiple degradation agents.

The lower and upper margins of the computed deterioration pattern prediction interval yield the predicted service life interval (PSLI). This represents the time interval for which a (1-α) probability exists, and a future LE value for the required CP can be established.

The (1-α) predicted component performance interval (PCPI) can be calculated similarly. The PCPI reflects the future component performance level for a given service age.

3.3. Prediction of the Sevice Life in Intensive Service Conditions

The Life Expectancy of building components under standard service conditions indicates the life cycle of building components, subjected to standard weathering conditions, excluding severe exterior service conditions and excluding intrinsic defects, e.g., poor or inappropriate materials, poor or lack of design details, or poor quality of work. An intensive service regime and inherent failure service conditions yield typical deterioration patterns, triggering the premature obsolescence of the exterior claddings. A marine environment is considered severe service conditions in this context. The typical deterioration path for the degradation mechanism of exterior stone claddings situated in marine environment service conditions is calculated for wet- and dry-fixed stone claddings samples. The prediction of the effect of each particular mechanism on the estimated service life of a building component (ESLC) can be deduced from the life expectancy limited coefficient (LELC) [10,11,30], as presented in Equation (2):

$$LELC=1-\left(\frac{SLE-LEDP}{SLE}\right)\ast IC$$

(2)

where

-

LELC expresses the life expectancy limiting coefficient for a particular deterioration mechanism (e.g., air pollution, marine environment);

-

SLE expresses the life expectancy for standard service conditions;

-

LEDP represents the life expectancy resulting from the effects of a particular degradation mechanism;

-

IC expresses the influence coefficient, indicating the relative significance of the degradation mechanism on the overall performance of the cladding component. IC ranges from a value of 0, for decay agents with no effect on the ESLC, to a value of 1, for agents that have a comprehensive impact on the performance and durability of the cladding. IC is determined empirically by experts, based on literature data and long-term observations of stone claddings in seashore zones [31]. The present research focuses on premature degradation of exterior stone claddings caused by exposure to severe marine service conditions. IC was taken to be 1, reflecting the high significance of the vulnerability of stone cladding to marine environment degradation agents, such as exposure to high humidity and high chlorides.

4. Findings

4.1. Laboratory Test—Natural Stone Specimens

Laboratory tests were conducted on the durability of six types of limestone used in the exterior cladding of high-rise buildings and structures in the coastal zone of Israel from 1997–2016. The coastal zone of Israel is characterized by high average temperatures for most of the year, combined with high humidity and strong sea breezes affecting buildings and structures. In the exterior cladding made from five types of stones analyzed in this article, there was significant wear and tear within the first 2–4 years of building construction. As a result, property owners filed lawsuits against the contractors. The authors of this article prepared expert opinions on the compliance of all stones with Israeli standard SI 2378 Part 1 and the most likely reasons for the rapid wear and tear of the stones. The sixth type of stone was chosen as a reference since the exterior cladding of a 16-story building in the coastal zone proved to be highly durable.

Three types of stones (marble, such as limestones no. 1, 2, and 6) were used as exterior cladding in pre-polished factory slabs. Three other types of stones were used as unpolished cladding stones with a rough natural texture. The physical and mechanical properties of four types of stones (three with a polished texture and one with a rough natural texture) fully complied with all requirements of Israeli standard SI 2378 Part 1. Two of the tested types of stones with a rough natural texture had water absorption exceeding the standard requirements; however, all other physical and mechanical properties fully complied with the standard requirements.

In this article, we report only the results of the qualitative mineralogical composition, determined by XRD (X-ray diffraction) analysis, and standard tests of the capillary water absorption and the resistance to immersion of the stone samples (120 × 120 × 20 mm) for 24 h in a sodium sulfate salt solution (concentration 14%, by mass), followed by drying to a constant weight in a laboratory drying oven at a temperature of 102 °C ± 2 °C. The samples were weighed on analytical scales before the start of the first cycle and at the end of each subsequent cycle. The stone test was stopped as soon as the weight of the sample decreased after drying at the end of the next cycle. Based on the changes in the sample weight, the critical content of crystallized salt causing stone crumbling was calculated. The test results are presented in

Table 3. Properties of limestones used in high-rise buildings in coastal zone in Israel.

Stone Type and Code	Origin	Main Mineral (More than 98% (% of Mass))	Secondary Minerals	Stone Finishing	Capillary Water Absorption Coefficient, Gram per m2 per Hour½	Number of Cycles during the Crystallization Test, before Beginning of Stone Crumbling	Critical Sulfate Content Causing Stone Crumbling, % of Mass
Limestone #1 (Hard)	Turkey	Calcite	Biotite, Quartz, Grossular	Industrially polished surface	137	1	0.19%
Limestone #2 (Hard)	Turkey	Calcite	Biotite, Quartz, Grossular	Industrially polished surface	245	2	0.47%
Limestone #3	Morocco	Calcite	Biotite, Grossular	Coarse natural texture	1422	1	1.08%
Limestone #4	Morocco	Calcite	Biotite, Grossular	Coarse natural texture	1013	2	0.99%
Limestone #5 (Hard)	Jordan	Calcite	Grossular, Quartz	Coarse natural texture	311	3	0.39%
Limestone #6 (Hard)—reference sample	Israel	Calcite	Grossular, Quartz	Industrially polished surface	41	3	0.09%

and in Figure 2, Figure 3 and Figure 4. The results of the tests clearly demonstrate that the greater the capillary water absorption coefficient of limestone, the greater the critical sulfate content causing stone crumbling. However, no relationship was established between the capillary water absorption coefficient and the number of cycles during the crystallization test before the beginning of stone crumbling. For example, Limestone #1 (Hard) and Limestone #2 (Hard) began to crumble after the first and second cycles, respectively, just like Limestone #3 and Limestone #4. Although the capillary water absorption coefficient of Limestone #3 and #4 is twice as high as the maximum value allowed by the standard, the capillary water absorption coefficient of Limestone #1 and #2 is much lower than the maximum allowed by the standard. The reference stone (Limestone #6 (Hard)), which has the minimum capillary water absorption coefficient, began to crumble after the end of three cycles of testing. Similarly, Limestone #5 (Hard), whose capillary water absorption coefficient is 1.5 times lower than the maximum allowed by the standard, but almost 8 times higher than the reference stone (Limestone #6 (Hard)), began to crumble after the third testing cycle.

Thus, the results of laboratory tests suggest that the compliance of stone with the requirements of the standard for water absorption and resistance to crystallization testing is not a guarantee of the durability of the stone when used as an exterior cladding material. Therefore, the probabilistic methods proposed in this article may be a reliable method for predicting the durability of wall cladding.

4.2. Standard and Marine Service Conditions

This stage of the empirical research was based on comprehensive fieldwork on stone cladding deterioration over time. The exterior claddings of 87 tall and high-rise (more than 4 floors) buildings were sampled by visual observations at all heights from the ground level and the roof level, as well as by analysis of 10–12 photos. The claddings were graded according to a pre-tested template [30] (see Appendix A). Degradation sources, such as marine environment, air-polluted environment, poor details (e.g., lack of joints and expansion joints, poor fixing details, and poor materials), were rigorously considered. The data gathering referred to factors such as the wind direction, the orientation of the large facade towards the wind direction, and the size of the natural stone panels. Observations with an evident failure degradation mechanism were excluded from the survey. The sample was further categorized according to the fixing method of the stone and the exterior service conditions.

Table 4. Characteristics of three wet-fixed stone claddings observed in marine environment.

Samples	Service Conditions	Type of Cladding Technique	Age [years]	Physical/Visual Grade 1
	Marine environment	Wet-fixing	15	5; 4
	Marine environment	Wet-fixing	28	3; 4
	Marine environment	Wet-fixing	40	3; 3

¹ According to Table 1 and Table 2 and Equation (1).

and

Table 5. Characteristics of three dry-fixed stone claddings observed in marine environment.

Samples	Service Conditions	Type of Cladding Technique	Age [years]	Physical/Visual Grade 2
	Marine environment	Dry-fixing	17	5; 4
	Marine environment	Dry-fixing	21	4; 4
	Marine environment	Dry-fixing	40	3; 3

² According to Table 1 and Table 2 and Equation (1).

delineate examples of various physical and visual degradation levels in three wet- and dry-fixed stone claddings taken from the sample inspected. Three different scatter diagrams were prepared from the results, where standard conditions reflect the normal conditions for which the building was designed, constructed, and maintained, and a marine environment reflects harsh environmental conditions: (1) dry-fixing exterior stone cladding under standard service conditions, (2) dry-fixing exterior stone cladding exposed to marine environment service conditions, and (3) wet fixing exposed to a marine environment. The buildings in the sample exposed to marine conditions were located in close proximity to the seashore (less than 400 m) on the central coast-line trip of Israel, considering the exposure to local sea water breeze regime conditions. This region is characterized by a mild rain regime (annual rain fall of 400–700 mm), 30–60 annual rain days, and a light wind regime (mainly south–west). The orientation of the large façade was found to be west in the majority of the sample, exposing the façade to the effects of the south–west wind regime. The dry-fixed stone cladding sample was limited to claddings aged up to 35 years, as this cladding is a relatively new method for exterior finishing in Israel and has only been implemented on a large scale since approximately 1985. The performance grading indicated that as a result of all facades, in a marine environment where the wind regime is dominantly a west wind, deterioration appears mainly in western facades (facing the seashore). Observations were taken at all height levels by observations at a roof level and a ground level, and by analysis of 10–12 photos.

As discussed above, the predicted life expectancy of a component is determined by the typical deterioration path intersection with the MRCP. Two threshold levels of minimum required component performance were considered [10,11,12,30], as follows:

(1) MRCP = 60%, for a high service standard of CP, for instance, in public or corporate buildings.

(2) MRCP = 40%, for moderated or low service standards, where the owner prefers to minimize maintenance costs while compromising the service standard of the exterior cladding.

Thus, the LE for MRCP 60 and 40 may be derived from the intercepts between the typical deterioration path curve and the 60 and 40 lines of performance levels, respectively. The confidence limit of the service life prediction was established at 80%, as recommended by ISO 15686 (2011) [13].

The standard life expectancy (SLE) for wet-fixed stone cladding under standard service conditions was previously discussed by Shohet and Paciuk et.al. [12], who reported values for MRCP_60% and MRCP_40% of 44 and 64 years, respectively. In this study, the reference service life of exterior dry-fixed stone cladding under standard service conditions was investigated in order to compare the values to the standard life expectancy of wet-fixed stone cladding. In addition, the typical deterioration patterns of both cladding methods in marine environmental conditions were examined in order to estimate the LELC.

The typical deterioration curve for dry-fixed cladding under standard service conditions is shown in Figure 5. It expresses the regression analysis of 24 data points in a linear degradation process. The moderate decline in cladding performance reaches MRCP_60% and MRCP_40% levels after 59.5 and 88 years, respectively. It should be emphasized once more that the service life cycle expectancy at MRCP_40% is marginal with respect to the component performance of exterior stone cladding and involves inherent hazardous defects, i.e., delamination of stone plates, cracks in cladding, and detachment of complete stone plates.

Figure 6 shows the typical deterioration progression for wet-fixed stone claddings subjected to a marine environment. The life expectancies (LEFCs) for MRCP60% and MRCP40% were found to be 35 and 54 years, respectively. Calculating the life expectancy limited coefficient according to Equation (1) gave LELC = 0.79–0.84, indicating the strong impact of marine environment service conditions on the service life expectancy of the wet-fixed exterior stone cladding.

Table 6. Service life expectancy, PSLI, and predicted component performance interval, PCPI, of wet-fixed stone cladding.

Predicted Service Life and Performance Parameters	Service Conditions
	Standard	Marine Environment
Service life prediction MRCP60%	44	35
Service life prediction MRCP40%	64	54
Expected service life interval * MRCP60%	39–50	30–40
Expected service life interval * MRCP40%	59–70	49–59
Component performance interval expected * at life expectancy of MRCP60%	52–69	55–65
Component performance interval * expected at life expectancy of MRCP40%	32–49	35–45

* p = 0.80 (p—statistical probability).

summarizes the typical service life expectancy of wet-fixed exterior stone claddings exposed to the various service conditions. These findings are valid if no differences in the weather conditions occurred, compared to the characteristics at the time of analysis or if a constant increase of the sunny days number occurs, or the temperature, or the number of freeze–thaw cycles per year—and the weathering conditions, such as the salinity of water, increase (and accordingly the salinity of the air and thus the degree of influence on the anchorage system of the exterior stone cladding wall). It should be emphasized that the predicted service life intervals, MRCP_60% and MRCP_40%, were deduced by extrapolation of the linear regression lines of the predicted life expectancy, as well as the lower and upper prediction intervals lines. Therefore, further research with an extended range of service life is recommended in order to validate the results.

Finally, the typical deterioration pattern for dry-fixed stone cladding exposed to marine environment is presented in Figure 4. The relatively moderate gradient of the average regression line yields life expectancies (LEFC) of 55 and 85 years for MRCP_60% and MRCP_40%, respectively. In this case, calculation of the life expectancy limited coefficient according to Equation (1) gave values for the LELC = 0.94–0.97, indicating the moderate impact of the marine environment service regime on the service life expectancy of dry fixed exterior stone cladding.

The service life expectancy of dry-fixed stone claddings subjected to standard conditions and marine environment are summarized in

Table 7. Service life expectancy, PSLI, and component performance prediction interval, PCPI, of exterior dry-fixed stone claddings.

Service Conditions	Standard	Marine Environment
Service life expectancy MRCP60%	59	55
Service life expectancy MRCP40%	88	85
Service life prediction interval * MRCP60%	55–63	47–63
Service life prediction interval * MRCP40%	84–92	76–93
Component performance prediction interval * at life expectancy of MRCP60%	57–63	55–66
Component performance prediction interval l * at life expectancy of MRCP40%	38–43	34–46

* p = 0.80 (p—statistical probability).

.

The LELCs express the effects of the marine environment service conditions on the reference service life of wet- and dry-fixed stone claddings are presented in

Table 8. Life expectancy limited coefficient, LELC of stone cladding exposed to marine environment, according to Equation (1).

Stone Cladding System	LELC
	MRCP60%	MRCP40%
Wet-fixed stone cladding	0.79	0.84
Dry-fixed stone cladding	0.94	0.97

. These findings reveal the main differences in the impact of marine environment on the performance of the two fixing techniques under discussion, i.e., the dry-fixed stone cladding method is less vulnerable to the marine environment service conditions. This conclusion is derived from the finding that the LE of Dry-fixed cladding in marine environment (for MRCP = 60) is 55, whereas the wet-fixed stone cladding is 35, and the prediction intervals of the at probability of 0.8 are distinctive (47–63 years for dry-fixed stone and 30–40 years for wet fixed stone—see Figure 6 and Figure 7 and Table 6 and Table 7). Nevertheless, the durability of the dry-fixed cladding is highly dependent on the performance of the stone and the fixing details and, therefore is highly dependent on the adherence of the design and construction to the technical parameters of the cladding systems, such as anchors resistance, suitability between the stone and joints materials and boundary details, such as the façade cornice and drip-edge).

Based on the data in Table 7, the degradation impact due to the exposure to marine environment conditions can be calculated according to Equation (3):

$$E{F}_{A-B}=\frac{1-LEL{C}_A}{1-LEL{C}_B}$$

(3)

where,

EF_A−B—Coefficient expressing environmental conditions impact for alternative exterior cladding fixing techniques A (Wet-fixed) and B (Dry-fixed);

LELC_A and LELC_B—Coefficients expressing the Life Expectancy Limiting ratio for Wet and Dry fixed stone claddings, respectively. From values presented in Table 7, EI_A−B for MRCP60% and MRCP40% can be calculated as 3.50 and 5.33, respectively. These findings reveal that the impact of marine environment service conditions on the deterioration of wet-fixed stone cladding is 3.5 to 5 times higher than on dry-fixed stone cladding.

5. Discussion and Conclusions

This research integrates empirical laboratory tests with field work for the assessment of the expected service life of natural stone claddings in standard and marine service conditions. The results of the laboratory tests indicate that the compliance of stone with the requirements of the standard for water absorption and resistance to crystallization testing is not a guarantee of the durability of the stone when used as an exterior cladding material in coastal zones. Therefore, a probabilistic method proposed in this article may be a reliable method for predicting the durability of exterior stone cladding.

The analysis of the cladding techniques revealed that all of the degradation patterns fit linear patterns with a range of deterioration rates and regression coefficients R² between 0.74 and 0.90, indicating a high degree of reliability.

The findings in Table 5 and Table 6 demonstrate unequivocally that the fixing technique significantly impacted the rate of stone cladding degradation. Dry-fixed stone claddings were distinguished by longevity of service life in a marine environment, as well as under normal service conditions. Wet-fixed claddings, on the other hand, demonstrated a stronger influence of the marine environment on the reference service life, while already exhibiting a greater rate of deterioration than the dry-fixed counterparts in both settings. The environmental factor coefficient derived using Equation (3) serves as validation for this finding.

The usage of industry standard anchors in dry-fixed stone claddings may be the cause of the discrepancies. The quality and durability of the dry-fixed connection in the exterior stone cladding may be improved by the stainless-steel anchors specifically developed and regulated during the production process. Additionally, this method of fixing enables larger size and weight flexibility for the stone plates and/or panels, which makes it better suited for tough limestone, marble, and granite. These materials are less porous than porous sedimentary stones, which makes them more resistant to a marine environment [4,42,43,44]. Nevertheless, large-size marble panels (height larger than 1.0 m) in a marine environment, in high temperatures, and in the presence of a moisture gradient suffer from the bowing failure mechanism; therefore, they are not recommended, especially under the influence of a marine environment [45,46,47,48]. The main cause of the deterioration mechanism in wet-fixed stone cladding is the effect of cement mortar on the cladding stone. The high pH of the mortar (more than 12) and the high humidity level maintained at the back surface of stone plates have been linked to previously published claims that natural stones are extremely sensitive to the influence of Portland cement mortars (PCMs). As a result of the production of limonite (rust) crystals caused by the high pH levels in PCM, veins open up, and sedimentary stones (limestones and sandstones) containing secondary iron minerals begin to split [49,50]. Because there is no airflow behind the stones due to high humidity, there are cycles of hygric expansion and shrinkage [51].

The following aspects were revealed to be the most frequent deterioration-promoting problems for both types of stone cladding exposed to a marine environment throughout the fieldwork: (1) Using stones with excessive (high) porosity (water absorption coefficient larger than 1), which allows salt solutions in a marine environment to penetrate stones and crystallize. When the pressure inside a stone builds due to crystallization within the microscopic capillary holes, flexure strains exceeding the stone’s capacity result, which finally leads to fracture. (2) Using improper joint materials, which chemically react with the stone’s minerals and result in visual deterioration from filler leaks. (3) Using insufficiently strong steel, which causes intense steel corrosion and, as a result, early visual and physical deterioration of stone cladding. Future research should look more closely at the three degradation patterns that are typically seen.

The current study advances knowledge of how marine environmental factors affect the performance of exterior stone claddings. The main factors that lead to cladding degradation were investigated, and its Predicted Service Life was evaluated in years. These findings could be applied to inform decision making about the design of single- and double-skinned stone facades, as well as maintenance planning. By using prediction intervals and life expectancy limiting coefficients for maintenance management, the results may improve the maintenance, safety, and durability of exterior claddings [52] and prevent failure scenarios by exact maintenance intervals [53].

The research limitations include the limited size of the field survey samples. Larger sizes may improve the significance of the research findings and allow the use of ML (Machine Learning) methods. Further research is also recommended on degradation mechanisms, such as air-polluted environments, and lack of or poor design details, such as anchoring fixtures.