Dirty Building Exteriors

Nano Technology

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Dirty Building Exteriors

 Dirty Building Exteriors

After a period of time the exterior surface of buildings like roofs and facades will become dirty, uneven discoloration will appear, often in patterns revealing how rain washes off the surfaces. Depending on the kind of dirt and the risk of deterioration of the surface by the dirt, the building owner after some time might choose to clean the surface. Cleaning, however, can be expensive and can introduce other problems to the surface.

There are two main causes of why building exteriors becomes dirty:

1. Dust and dirt accumulation

Floating dust is usually under one micron particle size and is generated by combustion of fossil fuel burning engines. Particles smaller than 0.01um are not stable in the atmosphere; they will either react with oxygen or coagulate to larger particles. Emission of gases and particles by industry and cars generate particles up to 1um. Coarser dust, with particles larger than 1um, is generally generated by abrasion. The particle size distribution varies with the environment, but generally the bulk of the particles are < 1um, both measured on mass, volume and on numbers.

Sedimentation of larger particles occurs quite soon because of their weight, smaller particles < 1um float in the air for days or weeks. During that time, they can be transported over 1000 km before they deposit. The particles are washed out in clouds or rain. A high content of water-soluble salts, especially in “urban” particles, lead to wet deposition. Sedimentation of smaller particles does not occur in the same way; instead the particles are drawn by forces of adsorption of oil content in hydrocarbon particles and electrostatics.

2. Biological Growth

A fast indicator of whether discoloration is caused by biological growth or environmental particles is the smudging pattern. Environmental particles are transported by water and to some extent also washed away by rain, leaving exposed surfaces cleaner than other parts. Biological growth, on the other hand, is typically seen on exposed surfaces that for some reasons are moist.

Consequences of Dirty Buildings
Dirt on surfaces is generally not seen as a major problem because it does not involve safety consequences unless it covers signs of deterioration. But the dirt is visible, and it is, therefore, natural to ask what effect the dirt has on the building.

Aside from some romantic ideas on how old houses should look, a too visible dirt layer on a building is an aesthetical failure, regardless of its origin. This kind of aesthetical problem is to some building owners unacceptable, as it can be seen as neglect and ignorance. Dirt on surfaces can probably not cause this alone, but is an important factor. Although there is probably no other damage than aesthetical, the reason for the growth must be determined and the facades will have to be cleaned.

Deteriorated areas are often covered with biological growth, but this does not mean that the biological growth has caused the deterioration; it is more likely that the cause is water damage. It is likely that heavy biological growth may enhance deterioration; biological growth keeps the surface moist, increasing the risk of frost damage.

Depending on the surface and the composition of environmental particles, a dirt layer can prevent the surface from deteriorating or reacting with the surface in an irreversible way, e.g. limestone and concrete can react with sulphate, forming a gypsum layer, which will appear as a crust, sometimes just visible to the naked eye as a discoloration.


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