Algae

Introduction

Building façade faces a myriad of external stresses in tropical cities such as Singapore, which increases its vulnerability to develop common surface defects such as façade staining. It is well established that façade cleaning of tall buildings is an expensive exercise due to its high cost associated with the provision of access, resources (i.e. energy, water, and chemicals),  and safety risks [1-4]. Façade staining by atmospheric effects may be due to non-biological and/or biological agents. These staining agents may be carried by the wind and deposited directly on the façade or they can be deposited in solution by rainwater. Biological staining agents affect building façades that are in close proximity to vegetation. In Singapore, they may be traced to a number of diverse plant groups such as algae, fungi, mosses, ferns, and figs. The growth of algae, fungus, etc. requires moisture and sunlight to grow. These are abundant in tropical regions. Countries such as Singapore thus have a greater possibility of such attacks.

Algae

Algae are classed as aeroterrestrial as they spread through windborne spores and colonise in a biofilm on a surface, and when the conditions are susceptible, they will grow organically. They are photosynthetic and must have sunlight to continue growing. Although mostly green due to the presence of chlorophyll, black, blue, red, orange and yellow algae (depending on the number of other pigments within it) are found on all types of façade materials, including rendering, glass, aluminium and granite.

Algae thrive best where there is sun, moisture and nutrients. Dirt that is blown off the road and retained on façades becomes a source of nutrients. As such, algae are commonly found mainly outdoors on external façades. Common species of algae that may be found on façades are Trentepohlia odorata, Chlorococcum and algae comprising Scytonema, Schizothrix and Anacystics.

Algae Growth

Algae growth has been characterised by two (a) factors attributed to the environment (b) factors attributed to the building envelope. Environmental factors consist of climate, thermal amplitude, precipitation, hygrometry (humidity), distance from the sea and presence/absence of vegetation. Precipitation and hygrometry directly affect the availability of water on to building facades, which is widely known as one of the key requirements for algae growth. In Singapore, the high humidity and precipitation result in buildings experiencing high amounts of water contact throughout the year. The closeness of the building to the sea can also result in higher atmospheric humidity, leading to a greater chance of algae growth. Building related factors affecting algae growth are high alkalinity (e.g. fresh concrete, high in alkaline helps algae to grow), excess surface moisture, windborne transport (orientation), and rain streaks that carry the algae spores down a façade.

Left unchecked, algae will grow on surfaces to create an aesthetically subjective discolouration with various shades of green, orange, black or blue. Besides being aesthetically unpleasant, biological growth on facades may also cause deterioration and further weathering to the wall. Proper preventive measures may be able to minimize the occurrence of such staining. One proactive approach to reducing the number of cleaning cycles is by incorporating strategies such as self-cleaning façade. Studies have shown that superhydrophobic surface applications can yield a surface to have such self-cleaning properties. Superhydrophobic self-cleaning technologies are increasingly used in commercial products due to its aesthetic, economic and environmental benefits, including water repellency, breathability, prevention of façade blisters, UV protection, resistance to biological agents, reduction of cleaning resource usage, corrosion and pollutant resistance. Photocatalytic coatings such as TiO₂ are also found to be effective on external façade to cut dirt build-up and reduce maintenance as an anti-staining coating for building facades; due to its self-cleaning, anti-bacterial, anti-viral, fungicidal, anti-soiling properties. TiO₂ is also both acid and alkali resistant and is harmless to humans [5-14].

Maintenance Strategy

In order to make informed decisions to combating this maintenance issue, it will do facility managers good to understand how algae propagate and how strategies such as self-cleaning façade technologies can be used to inhibit it. This knowledge can help the facility managers prepare the buildings against algae infestation instead of merely conducting corrective measures. Therefore, determining and understanding the in-service performance information on façade systems is useful in determining proactive maintenance interventions and realistic operational budget estimates. It is apparent that self-cleaning façade coating systems may require a lesser frequency of maintenance interventions which may, in turn, be favourable on a facility’s operating costs [15-17].

Case Study

References

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