Case 1

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Cause of Defects

Dirt particles of all sizes are able to settle and collect on the sloping surface of the concrete window sill (Table 1).

Pollutants	Examples	Sizes	Sources	Possible effects
Aerosols	Sulphur dioxide (SO2)	Gaseous	Burning of fuels	Formation of acid rain. Reaction with water and other materials to form sulphates and chlorides to cause destruction to façade materials.
Soot or black smoke	Smoke from tobacco, coal, fuel oil, metallurgical industry	<1 pm	Imperfect combustion	Transported by air and deposited on facades to cause stains
Coarse particulate matter	Ash, dust, rock debris, mineral dust.	>1 pm	Unburnt fuel and dust from roads and industries	Transported by air for short distances and deposited more easily on horizontal or sloping surfaces to form stains.

Table 1: Air pollutants that may cause staining to facades.

In times of heavy rainfall, rainwater that impacts on the window sill will pick up the dirt particles and redeposit them as it flows over the sill and onto the brickwall.

Stain marks are longer and more intense at either end of the sill.

The window sill does not project far from the wall and thus runoff is still able to flow back along the wall surface. The runoff is also able to flow off the sides of the window (Figure 1a to c). Incompatibility of materials can also lead to staining.

Due to smoother surface texture of concrete, rainwater will run down concrete, bringing along any dirt particles and depositing the dirt particles on brickwall which has a rougher surface texture.

Possible sources of water:

Moisture from condensation: Condensation problems can occur in air-con buildings when warm, moist outdoor air is driven towards cooler, drier interior spaces.
Water mainly originate from rainwater and manifest into many forms which include dampness, deposition of foreign matters, solution of salts etc.

Consequences

If left unchecked, the staining would spread to the whole facade and affects the aesthetics of the building.

Staining on a facade has a lot to do with the pattern of rainwater runoff, which in turn is dependent on the facade configuration, colour, texture, as well as the “roughness” and “water absorption” properties of the materials used: