Analysis of red stains on building facades in Bahia, Brazil

The red stains on facades are a type of damage resulting from the accumulation of microorganisms on the surface of sealants, which can lead to the deterioration of the paint film, as well as negatively affect the aesthetics of the constructions. The literature has a gap regarding studies on red stains on facades, therefore research conducted in this theme becomes a valuable addition to the scientific knowledge base. Thus, the objective of this article is to analyze the red stains on facades of buildings in relation to the geographic orientation, vertical distribution of stains on the facade surface, distance from the sea, and the surroundings of the buildings. For the research development, data was collected from nine buildings in the city of Salvador, Bahia, and the Metropolitan Region. In terms of geographic orientation, the facades with less solar radiation exposure were the most affected by the red stains, due to their higher humidity compared to other orientations, which favors the development of microorganisms. Regarding the distribution of stains on facades, there was a tendency for concentration in the higher regions of the buildings. With regard to the surroundings of the buildings, it was possible to verify that the higher the incidence of vegetation cover, the greater the proliferation of stains. However, in this study, no correlation was found between the distance from the sea and the red stains. Finally, it was noticed that humidity, height, rain, wind, and vegetation are factors that, according to the studied data, have an influence on the development of microorganisms responsible for the red stains.


Introduction
The first impression of a building is defined by the aesthetics of its facade, from which it is possible to infer characteristics about the occupants, such as their socioeconomic level [1].However, the function of the facade goes beyond aesthetics; it plays an important role in sealing the building against adverse external conditions, providing hygrothermal, acoustic, and security comfort to users [2].
As in any other segment of the construction industry, facades must also comply with technical standards during design and execution.In Brazil, for facades with decorative finishes in painting, it is essential that the project meets the performance standard NBR 15,575 [3], as well as the standard for paints in civil construction, NBR 13,245 [4], in order to guarantee the durability and performance of the system.NBR 15575-5 [3] states that for external sealing systems with painted finishes, the minimum required design service life is 8 years.However, in practice, current building facades show early damage, resulting in economic impacts for the property owners with maintenance and repainting processes before the designated design service life for this type of cladding.It is also worth mentioning the environmental impact, as there will be material replacement before the design service life.
In the city of Salvador and its metropolitan region, red stains (Fig. 1) are a type of damage that is affecting the aesthetic integrity of facades, and within less than 2 years 82 Page 2 of 8 of painting the buildings, the emergence of these stains on external seals has been noticeable, resulting in the early need for maintenance and repainting processes.These stains originate from the accumulation of live microorganisms, which find favorable conditions in the substrate for their development, giving rise to the formation of the red biofilm observed through the red stains.According to Breitbach [5], in facades with decorative paint finishes, the presence of the biofilm can lead to the deterioration of the paint film, with algae and fungi being the main causes.
The biofilm is an extremely complex microbial ecosystem that can consist of varying amounts of different microorganisms along with a matrix of organic and inorganic nutrients and extracellular polysaccharide substances secreted by the cells [6].This ecosystem can be visualized in various colors, with the most common being black, brown, green, and red.According to the literature, the red biofilm is derived from a green alga that contains a large amount of carotenoids, which are chemical substances responsible for the orange, yellow, and red colors in fruits, vegetables, flowers, algae, bacteria, fungi, yeast and animals [7].
Barberousse et al. [8] and Labres [9] discuss that the development of biofilm on buildings depends on local characteristics (geographical orientation and vegetation), regional climate (humidity, temperature, precipitation), and nutrient sources.Therefore, the location where the building is situated can be a determining factor in the proliferation of microorganisms.In this context, this article aims to analyze red stains on building facades in relation to four factors: geographical orientation, vertical distribution of stains on the surface of facades, distance from the sea, and the surroundings of the buildings.To achieve this goal, nine buildings located in Salvador, Bahia, and its metropolitan region, which exhibit red stains on their facades, were selected.Subsequently, possible correlations between the studied factors and red stains were examined.
It is important to note that studies on red stains on facades are still limited in the literature.Therefore, understanding how the studied factors influence the emergence of red stains is a significant addition to the scientific knowledge base and can contribute to the development of future studies aiming to find effective solutions in mitigating the occurrence of this type of damage, thereby extending the lifespan of facade paintings.

Methodology
For this study, nine buildings in the city of Salvador and its metropolitan region were examined, which had red stains on their facades.The number of buildings was limited due to the bureaucracy and delay in obtaining authorization for the study in the buildings.The release of data from the study subjects was granted by the responsible parties of the development, and the research period occurred between the months of June and August 2022.The buildings were named A to I, with A to D belonging to Condominium 1 (C1), E to H belonging to Condominium 2 (C2), and I belonging to Condominium 3 (C3).Figure 2 depicts a map with the location of the condominiums.It is worth noting that all facades have decorative finishes in acrylic paint.
The city of Salvador and its metropolitan region are located in the state of Bahia on the Northeastern coast of Brazil, bordered to the east by the Atlantic Ocean.It is a region with a humid tropical climate, with a high average annual precipitation of 1871.1 mm and 2400 h of sunshine per year.The maximum temperatures reach 34 °C, with an average of 25 °C and minimums of 17 °C.The average relative humidity is 81% [10].As for the prevailing wind in the region, it is Southeast, with an average speed of 3.2 m/s in winter and 2.8 m/s in summer [11].Figure 3 presents a scheme of the methodological procedure of the work.For data collection, visits were made to the condominiums to acquire photographs and videos of the facades.A 12 MP camera from an iPhone 8 Plus was used for this purpose.It is worth noting that the best time for capturing photographs and videos is when there is direct sunlight on the facade, as this improves visibility of the stains.The facade designs were obtained from the company responsible for the construction works and through sketches created by the authors.Geographical orientations, distance from the sea, and images of the surrounding areas of the buildings were acquired using the free software Google Earth.During the visits, information regarding the year of the last facade painting was also obtained.
The damage map was developed using Autodesk AutoCAD ® 2023.The facade damages were marked based on visual analysis aided by the collected photographs and videos during the field visits.As such, the extent of the area affected by the stains on the facades was estimated through visual inspection.
The creation of the map was highly relevant for quantifying the area of the facade affected by the damage, which facilitated the analysis of geographical orientation, distance from the sea, and the surroundings of the buildings.Additionally, it aided in understanding the distribution of the stains across the facade plane.It is important to note that for the analysis of the vertical distribution of the stains, the facades were divided into three regions: lower, middle, and upper sections (Fig. 4).
In the process of data analysis, the obtained data was organized and treated, followed by the following analyses: For the analysis of the distance from the sea, the following data was used: distance from the sea to the studied buildings in kilometers (km); the number of years since the last facade painting (P) the total area of the facades of each building (F t ) ; and the total area of the facade affected by the red stains (S t ) .With the data of S t and F t , it was possible to obtain the ratio of the area affected by the stains to the total area of the facades (S) according to Eq. 1. From the obtained value of S , the estimate of stain progression per year E was determined according to Eq. 2.

S ∶ Is obtained in percentage [%] E ∶ Is obtained in percentage per year [%∕y]
It is important to highlight that studying the relationship of the annual progression estimation of the stains is crucial, as each passing year exposes the coating to the action of external agents, weakening it and consequently making it more susceptible to the development of damages.
For the analysis of the characteristics of the surrounding areas of the buildings, the following data were used: the estimate of stain progression per year E ; the area of veg- etation as well as the area of urbanization surrounding the buildings within a radius of 0.5 km.To obtain the last two data points, a circle with a radius of 0.5 km was drawn on Google Earth, centered on the buildings, and the areas with vegetation cover and urbanization were counted, as shown in Fig. 5.
It is important to mention that in the analysis related to the distance from the sea and the characteristics of the surroundings, only three buildings were studied, one from each condominium, as in condominium 1 and 2, the distance between the sea and the buildings has very similar values, and the same occurs with the area surrounding each building due to the proximity of the constructions.Buildings A from C1 and G from C2 were considered, as their facades were more affected by the red stains.

Analysis of geographical orientation in relation to red stains
The development of the damage map of the facades affected by red stains allowed for quantifying the area affected by the damage in each facade orientation (north, south, east, and west).Based on this, the graph in Fig. 6 was constructed.
According to the data presented in the graph of Fig. 6, of the nine buildings studied, 8 (88.89%) showed the south facade to be the most affected by the stains.This can be explained by the fact that these enclosures receive less solar incidence compared to the other orientations.Quadrado [12] states that in Brazil, during winter, radiation on south facades is minimal, while in summer it only occurs in the early morning and late afternoon.Consequently, southfacing facades become enveloping structures with higher humidity levels, favorable for the growth of microorganisms.According to TERRA [13], algae, lichens, and fungi require habitual moisture conditions and a supply of mineral salts for their development, with the latter being derived from construction materials.
Unlike the other buildings, Building I only presented red stains on the east facade.Analyzing the surroundings of the building (Fig. 7), it can be observed that there is a similarly tall building very close to the façade in question, resulting in the projection of a shadow throughout the entire morning period.Consequently, there is low solar incidence on the enclosure and higher levels of humidity.The affected facade is highlighted in red, and by observing its orientation, it can

Analysis of the vertical distribution of red stains on the facades
By dividing the facades of the buildings into three regions (bottom, center and top), the area of the stains in each segment was calculated, and the graph in Fig. 8 was generated.Observing this graph, it can be noted that there is a tendency for red stains to concentrate on upper floors.This can be explained by the fact that rainwater first reaches the upper part of the facades, which are characterized as areas with higher levels of humidity.Coupled with low solar incidence on the facade, these conditions create favorable environments for the development of microorganisms that cause red stains.Another point to be highlighted is the intensity of wind at higher locations, which can act as a transport mechanism for pollutant particles and/or fragments of microorganisms present in the surrounding environment.Pollutant particles can serve as a source of nutrients for microorganisms already present on the facade substrate.As for fragments of microorganisms, when they find favorable conditions for their development, they can proliferate along the extent of the enclosures.Since the upper regions tend to be the most humid, proliferation may occur from top to bottom.

% Occurrence of stains by orientaƟon
Fig. 7 Building surroundings I Bersch et al. [14], in their study, discuss that the higher incidence of damage in the upper regions of buildings is likely related to sources of moisture, heavy rainfall, and wind exposure.
In order to improve the understanding of the data obtained in the graph of Fig. 8 and determine if the values are statistically different, an analysis of variance (ANOVA) was applied.The hypothesis tested was: H o : There is no significant difference in the variation between the samples H 1 : There is a significant difference in the variation between the samples The values obtained through ANOVA are presented in Table 1.It is known that the critical p value to reject the null hypothesis (H o ) is 0.05 or 5%.Therefore, based on the p value obtained in Table 1, approximately 0.02 or 2%, it can be observed that the null hypothesis is rejected, indicating a significant difference in the variation between the samples.Thus, it can be concluded that the upper region of the facades is significantly more affected than the others.

Analysis of the influence of distance from the sea on the red stains
Table 2 presents the data used for this analysis, and according to the obtained information, it was not possible to find a correlation that characterizes the proximity to the sea as a predominant factor in the spread of stains on facades.This is because there is no consistent behavior between the distance from the sea and the estimated evolution of stains per year (E) .According to Zanini [15], stains are the most com- mon type of damage found on facades of buildings located in coastal areas.However, all the buildings studied are located on the coast of Bahia.Therefore, for better analysis, it would be interesting to also study buildings outside this region.
It is worth noting that the proximity of the E values, approximately 3% per year, may indicate the progression of the phenomenon.However, more samples are needed to confirm this behavior.

Analysis of the characteristics of the surroundings of the buildings in relation to the red stains
The analysis of the surroundings becomes important as there may be elements near the buildings that influence the local microclimate and promote the proliferation of microorganisms and stains on facades.By analyzing the surroundings of the buildings within a radius of 0.5 km, values for urbanized areas and vegetation coverage were obtained.The data is presented in Table 3.
In Table 3, the third column is related to the area of the analyzed surroundings of the buildings, with a radius of 0.5 km.The total area of the surroundings is approximately 0.785 km 2 .By observing the other data in the table, it can be noted that there is extensive vegetation coverage in the vicinity of Building A, the building most affected by stains, accounting for approximately 59% of the total examined area.On the other hand, the urbanized area is smaller, comprising approximately 41%.Building G is located in an area with minimal vegetation coverage, occupying 10% of the total area, and has a larger urbanized area, accounting for 90%.This building is the least affected by stains.Building I, on the other hand, has a vegetation coverage area of 28%, higher than that of Building G, making it the second most affected by stains.Thus, the data suggests a possible relationship between vegetation area, urbanized area, and the spread of stains.
Breitbach (5) discusses that high moisture content promotes the growth of microorganisms, and as vegetation coverage impacts the increase in local humidity, the facades' substrates are prone to higher humidity levels, leading to the development of microorganisms that cause red stains.

Conclusions
According to the generated analyses, the following conclusions are reached: • Facades with orientations that have lower solar radiation incidence tend to be more affected by red stains.This is due to their higher humidity compared to other orientations, which promotes the growth and development of microorganisms, leading to the deterioration of the facade.• The collected data suggest the importance of analyzing shadow projections from other structures on the planned buildings, as these projections can hinder the passage of sunlight and make the substrates of these facades more susceptible to the attack of microorganisms that cause stains.• There is a significant tendency for the concentration of red stains in the upper region of the facades.This can be explained by the intensity of rainfall and winds in this region, which increases the humidity in these areas.Winds can also transport polluting particles that become a nutrient source for existing microorganisms on the facade substrate, as well as transport fragments of microorganisms that can be carried to the facades and, under favorable conditions, develop and proliferate.• Based on the data obtained regarding the distance from the sea, no correlation was found to characterize the proximity of the sea as an influential factor in the dissemination of red stains.• Analyzing the surroundings of the buildings, it was observed that the higher the incidence of vegetation area in the vicinity of the buildings, the greater the proliferation of red stains.This can be explained by the larger vegetated area causing an increase in local humidity, which in turn promotes higher humidity in the facade substrate and the development of microorganisms.• From the analysis of the data, it is evident that factors such as humidity, height, rainfall, wind, and vegetation influence the development of microorganisms responsible for red stains.
However, further research is needed to confirm the observations presented here and identify other factors that influence the incidence of these stains.Continuous monitoring is also necessary to detect the emergence of stains, assess their progression, and develop effective mitigation solutions aimed at extending the durability and lifespan of painted facade coatings.

Fig. 5
Fig. 5 Surroundingsof the Buildings.a Building A, b building G, c building I

Fig
Fig. 6 Occurrence of stains by orientation inrelation to the facade area

Fig
Fig. 8 Occurrence of stains in the bottom, center and top bands 66Occurrence of stains by orientation inrelation to the facade area

Table 2
88Occurrence of stains in the bottom, center and top bands Data from buildings A, G and I for sea distance analysis

Table 3
Data related to vegetation area and urbanized area