Some building materials are likely to last as long as their building does whereas others will require periodic replacement. In life-cycle costing, the cost of such replacement is taken into account, but it is a matter of opinion whether the choice of a material should be based on its initial cost or its life-cycle cost.

Some materials for example, glazed tile and stainless steel sheet--are extremely durable; once installed in a new building, they may not require renewal during its entire life. Other materials, such as paint and galvanized steel sheet, will need renewal several times during its life. Is it then cheaper to use galvanized steel or stainless steel for the rain gutters? Is it cheaper to use glazed tiles or painted plaster?

In estimating a building on present cost, the maintenance is ignored, or at least not formally considered. In life-cycle costing, the capitalized cost of the maintenance during the entire life of the building is estimated to obtain the total cost. Many materials that require frequent renewal or maintenance have a far higher life-cycle cost than others that cost much more initially, but require no maintenance. Thus, it is likely that stainless-steel gutters and glazed tiles will have a lower life-cycle cost but galvanized-steel gutters and painted plaster the lower initial cost.

High or low temperatures and high or low humidities in themselves do not cause significant damage to building materials, but changes in temperature and humidity do. Thermal expansion and?contraction can cause cracks in brittle materials and so can moisture movement. The alternate wetting and drying that takes place during and after rain can also cause appreciable damage, particularly to materials that absorb some of the water.

Some bricks and natural stones contain soluble salts that rain water can bring to the surface, where they form efflorescence. This can usually be removed with a brush. Efflorescence may also occur on concrete containing unsuitable aggregates.

Most climatic zones experience a substantial number of days when the temperature at night drops below the freezing point (0°C or 32 F) and rises above it during daytime. Since ice has a greater volume than water, these cycles of freezing and thawing can be very damaging to materials that are both porous and brittle.

The spectrum of solar radiation includes both infrared (heat) radiation with a longer wavelength and ultraviolet radiation with a shorter wavelength; both overlap with visible light. Heat radiation can produce overheating of some materials. Thus, bituminous flat roofs are usually covered either with a thin layer of light-colored paint or white stone chips to reflect some of the radiation or with a thicker layer of gravel. Ultraviolet radiation is a major cause of deterioration of certain plastics, and some of these cannot be used if they are to be exposed to sunlight.

Sunlight fades certain pigments, mostly those of organic origin, and these should not be used externally either in paints or for coloring anodized aluminum. Sunlight is also a major cause of the breakdown of paint film, which is further accelerated by the thermal movement that occurs.

Iron and steel corrode in the presence of moisture by forming rust, the common term for hydrated iron oxide ( ?),?unless the climate is extremely dry.

The reinforcement in concrete must be protected by an adequate cover of concrete; if this is insufficient, spalling may occur.

Erosion of external walls is caused by wind-driven particles of sand. It occurs comparatively rarely since it requires high wind velocities or the formation of eddies, a supply of sand or dust, and soft material in the wall. The main damage is to old buildings that contain deteriorating stone or brick. Abrasion is the damage caused by fine solid particles to floor surfaces.?Wear is a more complex?phenomenon that is caused partly by abrasion, but also by compression and by impact. Carpeting, for example, is frequently damaged more by the permanent impression made by heavy pieces of furniture than by abrasion. It is relatively simple to test abrasion resistance, however, and many machines for testing accelerated wear are simply abrasion-testing machines.

Concrete is one of the best floor surfaces for industrial use, and it can be further improved by using a hard aggregate at least for the surface layer. Cast iron tiles may be appropriate for conditions of extreme wear.

Quarry tiles, which are hard-burned, unglazed clay tiles, provide one of the hardest wearing surfaces for commercial and domestic buildings. Tile and stone floors provide heat storage for passive solar design.

Carpeting is used increasingly, both for offices and for homes, because its relative cost has been greatly reduced by modern manufacturing processes that enable the entire floor to be covered wall to wall. Carpet provides a comfortable walking surface; it is easier to clean than a hard surface; and it is a good thermal insulator and an excellent absorber of impact sound.

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