12.08 Environments

Categories: Finishes & Corrosion

Introduction
The durability of façade materials is affected by the environment to which they are exposed. The external environment is normally more aggressive but the internal environment should also be considered. This section describes the principal environmental factors and the way they affect different materials.
 

Temperature
Temperature may affect durability directly by affecting the rate of deterioration reactions or indirectly by affecting the properties of materials and hence their ability to perform as required. Most deterioration reactions, such as corrosion, will take place more rapidly at higher temperatures.

The external temperature will normally be the governing factor but the internal temperature should also be considered. Local sources of heat such as central heating radiators, convection heaters, hot water pipes and machinery should be considered in addition to general temperatures.
The surface temperatures after allowing for radiation effects should be considered not simply air temperatures.

Properties that can be altered by changes in temperature include:

• Shape or dimensions
• Strength
• Rheological properties
• Resistance to solvents
• Softening or hardening of polymer materials
• At temperatures greater than 70^oC steel may corrode preferentially to zinc
• Temperature gradients across components can cause additional problems such as distortion and interstitial condensation

Materials used must be suitable for the expected temperature range and the design should make allowance for anticipated thermal movements. Where proprietary materials are used the manufacturer should be consulted.
 

Water/Humidity
Wetting of the facade is always expected during rainstorms.  Although there is a temptation to shelter parts of the facade, using overhanging elements, it should be noted that rain washing is essential in maintaining the durability of some materials (e.g. stainless steel), and the lack of rain washing can lead to dirt build-up on some surfaces (e.g. stone); horizontal surfaces which accumulate water can also lead to problems.  Sheltering may occur inadvertently, for example by using brise soleil to provide solar shading, which then prevents rain washing. Alternatively it may accumulate dirt which is washed onto the facade. Outdoor humidity (water vapour) may reach 100% in periods after heavy rainfall. This can cause problems in preventing the drying out of porous materials and driving moisture into the building.

The most likely sources of water on the inside of cladding are rain penetration and condensation. Most forms of deterioration will be more severe where liquid water is present, however high humidity will also increase the risk of some forms of deterioration. For example corrosion of unprotected steel can occur at a relative humidity of 80%.

The presence of moisture will affect many forms of deterioration as follows:

• Corrosion of metals
• Rotting of timber
• Breakdown of seals to glazing units
• Interlayer of laminated glass or wire of wired glass may be attacked
• Staining of stone
• Mould growth on many surfaces
• Pure water in the form of condensation can cause corrosion of lead

Changes in moisture content of materials will create volume changes. Cementitious materials shrink as they dry to service conditions whereas clay bricks take up moisture and expand. These movements are irreversible but there are also reversible movements in response to changes in ambient conditions. Timber is subject to high reversible moisture movements. Excessive movements may lead to breakdown of sealants and finishes and restrained movements can cause overloading of components. Allowance for moisture movement should be made in design.

Deterioration may be avoided by preventing damp conditions and treating materials to increase their resistance to deterioration; for example preservative treatment or painting timber, use of corrosion resistant metals or use of suitable protective treatments. Where the occurrence of water cannot be prevented suitable means of draining the water or other means of removal of moisture should be provided.
 

Atmosphere
The atmosphere outside a building may carry chemicals and particulates that can damage facade surfaces as follows

• Salt in coastal areas increases the risk of corrosion of metals and may adversely affect the durability of stone. Salt may also penetrate concrete and cause corrosion of the reinforcement,
• Acid rain may cause deterioration of stone and may attack metals,
• Ozone can cause hardening and cracking of polymer materials,
• Particulates can stain porous materials,
• Carbon dioxide may neutralise the alkaline conditions in concrete which normally prevent corrosion of the reinforcement,
• Accumulation of dirt can retain moisture on the surface. The moisture can then attack the surface directly (as in the case of glass) or in combination with the dirt.
• Kerosene near airports can attack plastisol.

Deterioration can be minimised by selecting materials resistant to the prevailing environment and by use of protective coatings where appropriate. If protective coatings are used maintenance may be required during the life of the structure.

The atmosphere inside a building may also carry chemicals and particulates that can damage facade surfaces, such as in cigarette smoke or process gases.  Whilst good ventilation is always important it is not always used. Where the use of the building produces chemical contamination of the atmosphere which could cause deterioration, the suitability of the proposed facade materials should be checked.
 

Solar radiation
Solar radiation affects durability by raising the temperature of the facade but ultraviolet radiation can also affects finishes and polymers directly. Ultraviolet radiation is the principal destructive agent affecting many polymer materials. Atomic bonds may be attacked causing additional cross linking of the polymer chains or reversal of the polymerisation process.

The resistance of polymers can be increased by the use of pigments. White pigments reflect radiation protecting the body of the material but the surface may be more severely affected as the radiation passes through twice. Black pigments allow complete absorption of the radiation but will be subject to higher temperatures. Materials which are totally transparent to UV light such as perspex are unaffected.

The lignin in unprotected wood is attacked by UV light and becomes soluble in water.
Wood should be protected against UV light by the use of stains or paints. Clear varnishes do not give satisfactory performance.
 

Frost
Due to the expansion of water on freezing, porous materials may be damaged if subject to freezing whilst wet. The use of thin cladding in highly insulated wall constructions may give more severe conditions than traditional construction.

Frost damage only occurs when materials are saturated or nearly saturated. Damage may therefore be prevented by keeping the structure dry. This can be achieved for most cladding situations by attention to detailing however where this cannot be achieved frost resistant materials must be used.
 

Salt
Exposure to chloride in marine atmospheres, swimming pools or from road de-icing salts is a major cause of corrosion. Chloride may increase the risk of corrosion by breaking down protective oxide films and the rate of corrosion may be increased due to the increased electrical conductivity of the water.

The effects on different metals are generally as described for pollution above.

Stainless steel can corrode when in contact with aluminium in the presence of chloride.
 

Chemicals
The commonest group of chemicals that will come into contact with the surface of facades, other than atmospheric pollutants, are cleaning agents.  Bleaches may cause damage to some surfaces, but even the acids associated with the human skin can cause problems. Polymers can be seriously affected by chemicals through a process known as environmental stress cracking. In this process, cracks can be induced when a component under stress is in contact with particular chemicals. The stresses may be due to applied loads or locked in during the manufacturing process.

Materials suppliers should be consulted for possible aggressive agents. Good communication between the designer and the operator of the structure is essential where sensitive materials are used.

Where the structure is to be used for processes involving particular chemicals, the suitability of the proposed materials should be checked.
 

Flora
Large plants and trees growing close to or against the surface of buildings may increase the moisture content by restricting drying out thus increasing deterioration mechanisms dependent on moisture. There are also several situations in which flora may affect the durability more directly as follows:

• Lichen, algae and mould growth may occur, particularly where porous materials are allowed to remain damp for prolonged periods. This is unlikely to cause serious damage although some stones and mortars can be attacked by acid metabolic products.
• Plants and shrubs grown in close proximity to the structure, particularly climbing plants may penetrate small gaps in the structure and cause disruption.
• If dirt is allowed to accumulate on ledges or in gutters wind blown seeds can germinate and the resulting plants can result in disruption of the structure either directly or by blocking drains which then lead to water flowing over the façade.

Most problems can be avoided by designing the structure to ensure that surfaces do not remain damp for long periods, ensuring that plants are kept under control and ensuring that dirt does not accumulate on ledges or in gutters.
 

Fauna
Animals within buildings can lead to damage to facades, ranging from scratching of finishes to spraying of surfaces with various noxious chemicals - this may be due to uninvited vermin or household pets. Animals are less of a problem outside buildings, although the effect of birds should always be considered.  Birds have been known to extract sealant materials in the search for insects, and bird droppings are particularly corrosive if allowed to accumulate.

Insects can infest facades, and the point of entry may be from within the building, where the internal environment is more conducive to breeding. The use of vents to permit drying out of cavities or to allow pressure equalisation may provide an easy access for small insects.

Timber can be attacked by various insects, principally beetles for buildings in the UK. The adults lay their eggs on the surface and the larvae eat their way into the wood forming tunnels. Various beetles may be involved but not all are likely to lead to structurally significant damage. Some beetles are more likely to attack damp and rotting wood but this is not true in all cases. The sapwood of all species is affected but the heartwood of many species is resistant to attack. House longhorn beetle is generally confined to areas of Surrey and Hampshire and there are requirements in the Building Regulations for preservative trearment for some timber used in these areas.

Many materials are resistant to attack by pests but can affect the durability of the facade as a whole by being designed and detailed to reduce the risk of pests entering the structure and attacking other components. Further guidance is given in BRE Digests 415 and 418.
 

Fittings
Objects may be fixed to both the inside and outside of facades. Masonry facades have always been used to hang objects internally, but fixings now exist whereby objects can be fixed to thin sheet materials. Inappropriate use of fixings can cause damage especially to thin materials and finishes but if this leads to the puncturing of a vapour barrier or vapour-tight layer then serious interstitial condensation problems can ensue. Such damage may not be noticed by the occupier of the building until more serious deterioration occurs due to the presence of the condensation.
 

Abrasion
Abrasion of the external surface may be caused by wind-blown debris (scouring), however, this is unlikely to cause significant damage in UK conditions. Damage may also be caused during cleaning and maintenance work.
 

Electricity
Lightning strikes are a risk on all buildings.  Generally lightning protection is afforded by the use of rods and conductors, but damage can still occur if these are poorly placed, or damaged, or if the fixings used are not compatible with the conductor or substrate. BRE Digest 428 and BS 6651:1985 give further guidance.