13.02 Component life

Categories: Durability

Introduction
The life to maintenance, repair or replacement of a component depends on the mateials used, the use to which the component is put and the environment in which it is used.  The use to which a component is put, its service environment, and the need for repair or maintenance is covered in Section 13.04.  This Section describes the different materials used, factors affecting their durability and their impact on the life expectancy of facade components..
 

Metals
Metals include all the common metals and alloys, together with some materials which are comparatively new to the facade industry:

• Iron and steel, including stainless steel
• Aluminium and its alloys
• Copper and its alloys (brasses and bronzes)
• Lead and its alloys
• Titanium and its alloys
• Zinc and its alloys

It should be noted that whilst zinc is often used as a coating, particularly as galvanising of steel it could also be used as a material in its own right.

Although there is potentially an infinite number of combinations of different metal elements, it is a feature of metal alloys that for each base metal there are only a few proven alloys which are frequently used.

Some materials which had fallen out of fashion, for example bronze, have recently shown a resurgence in use. Similarly an increase in the complexity of modern facades has introduced new interactions between ‘old world’ metals and ‘new world’ polymers and the need to limit weight and improve cost effectiveness has lead to a reduction in the thickness of sheet metal components.
 

Main forms of deterioration
The main form of deterioration of metals is corrosion. It may be avoided by choosing a corrosion resistant metal or use of a protective coating. Deterioration mechanisms are generally well understood and the main uncertainties relate to the durability of finishes and protective coatings.
 

Life expectancy and maintenance requirements
The life expectancy and maintenance requirements for metals vary greatly. Corrosion resistant metals such as copper, lead and stainless steel may last for more than 100 years with little maintenance whereas plain carbon steel will have a very short life in external conditions unless protected and will generally require regular maintenance to ensure continued durability. The frequency of maintenance will depend on the protective system adopted see Section 12.03.
 

Timber
There are many species of timber which find their way into the facade industry, both softwoods and hardwoods. Whilst some timbers are particularly durable in their natural state, many are treated with preservatives and surface coatings to improve their durability. The use of green timber is increasing, and more new timbers are being introduced from tropical sources.

Timber may be used as a material in its own right but is also used in manufactured products including plywood, particleboard and fibreboard.
 

Main forms of deterioration
The main forms off deterioration are fungal decay and attack by wood boring insects. The heartwood of some species is resistant to fungal attack but the sapwood of all species is vulnerable. Fungal decay can be prevented by keeping the moisture content below 20 per cent and preservatives may be used to provide additional protection where this cannot be achieved.

The risk of insect attack is less predictable than for fungal attack. Generally only the sapwood is affected and painted timber is rarely attacked. Preservatives may be used to provide protection to unpainted timber.

Unprotected timber will deteriorate if exposed to UV light.

Manufactured board products may be subject to breakdown of adhesives particularly in the presence of moisture. This problem can be overcome by using a grade of material appropriate to the exposure conditions.
 

Life expectancy and maintenance requirements
Where timber remains dry it may last for over 100 years without maintenance. However there are examples of window joinery that have shown evidence of decay after five to ten years.

BS 5589 gives guidance on preservative treatments for external timber intended to achieve design lives of 30 or 60 years and BS 5268 gives treatments for structural timber intended to give a design life of 60 years. These treatments assume that good practice will be used in the design and construction to reduce the risk of decay. The preservative treatments are primarily designed to give protection against fungal attack but some treatments will also protect against insect attack. Externally exposed timber will also require protection against UV light.

Apart from durability considerations, exposed timber will normally require regular treatment to restrict moisture movement and maintain appearance.
 

Glass and ceramic
The principal type of glass that is used is soda-lime silicate glass, but borosilicate glass is also used where reaction to fire is important.  A number of other types of glass may be encountered, as may other ceramic materials with similar properties.
In addition to its use as a glazing material, glass is also used in the form of glass fibres for reinforcement of cement and plastic which are considered under cementitious materials and plastics respectively. Glassfibre is also used for thermal insulation.
 

Main forms of deterioration
Glass is a very durable material but it can be attacked by alkalis or prolonged contact with water and is vulnerable to fracture.

Procedures to be adopted to ensure durability are covered by British Standards and if this guidance is followed the durability of plain glass is unlikely to govern the life of the structure.

Durability problems are more likely to occur in components containing glass, such as double glazing units where failure of the unit may result from failure of other materials used in the component. There may also be durability problems associated with the use of coatings applied to glass.
 

Life expectancy and maintenance requirements
For plain glass, maintenance should be limited to regular cleaning and replacement of breakages. Double glazing units may need wholesale replacement after 20 to 30 years. Maintenance is likely to be required for associated components such as seals and frames.
 

Cement and concrete
A wide range of cementitious materials may be used in claddings as follows:

• Precast concrete cladding panels
• Cast stone, either as masonry units or cladding panels
• Concrete bricks and blocks
• Concrete tiles
• Mortars
• Renders
• Glass reinforced cement
• Fibre cement boards

Cladding panels made of concrete and cast stone will normally contain steel reinforcement and for the durability considerations of this report the composite material will be considered under this material type.
 

Main forms of deterioration
Cementitious materials may suffer from frost attack, sulfate attack and alkali aggregate attack. Selection of appropriate mix proportions in accordance with codes of practice and British Standards should overcome these problems. Carbonation and chloride contamination of reinforced concrete may lead to corrosion of the reinforcement but, for most cladding situations in the UK, these problems can also be prevented by correct choice of mix proportions. It may, however, be necessary to use stainless steel to ensure the durability of reinforcement in cast stone.

Renders may fail due to loss of bond to the substrate and may be susceptible to sulfate attack when applied to clay brickwork.
The durability of glass reinforced cement is affected by the reaction between the alkali in the cement and the glass fibres and there have also been problems due to failure to make sufficient provision for movement. Cem-Fil fibres for glass reinforced cement were introduced in 1971 and Cem-Fil 2 fibres with improved durability were introduced in 1979. There is, therefore, limited experience to gauge the long-term durability.
 

Life expectancy and maintenance requirements
Well designed concrete structures should not require regular maintenance however there have been problems in the past arising from poor design and workmanship and the use of unsuitable materials such as chloride based admixtures. No clear reference to design life is given in the codes of practice for concrete structures but it is generally assumed that a life of at least 60 years is expected.

Concrete masonry may require repointing during the life of the structure.

Renders can provide maintenance free service for 50 years or more but may require patching of debonded areas and where they are painted will require repainting at intervals.
 

Brick and terracotta
Clay based components are key elements in domestic and commercial buildings. Clay bricks are a staple component in low-rise wall construction and clay tiles are widely used in roofing. Brick slips can be used to decorate pre-cast concrete panels and terracotta tiles are now being used as the outer element in vertical cladding systems
 

Main forms of deterioration
Although fired clay products can be very durable, some materials are susceptible to frost attack when wet. There is no established relationship between measurable properties of the brick and frost resistance and the best guide to durability is performance in service. British Standards give guidance on the suitability of different types of brick for various exposure conditions.

Problems can also arise due to differential movement particularly where brickwork is used in conjunction with concrete. The shrinkage of the concrete and moisture induced expansion of the brickwork can lead to severe problems if the design does not allow for the movement.

Tiles may fail due to breakdown of the bond to the substrate.

Clay bricks contain sulfates which can cause sulfate attack of cementitious mortars.
 

Life expectancy and maintenance requirements
Brickwork is generally a very durable material capable of lasting over 100 years. It should not require regular maintenance but may require repointing during the life of the structure.
 

Stone
Natural stone has been widely used as a building material in the form of large blocks but is increasingly being used in thinner layers and in combination with other materials in composite panels. The decorative effect that is available with natural stone has often been allowed to overshadow the engineering issues, and concerns about durability are increasing. The huge natural variability of this material often makes it difficult to select appropriate materials for use on facades and the frequent presence of internal pores and fractures complicates selection.

Reconstituted stone is also used on facades and can generally be relied upon to be more regular in terms of properties and uniformity than natural stone. Reconstituted stone is considered as a cementitious material in this report.
 

Main forms of deterioration
The main forms of stone deterioration are frost attack, erosion by acid rain and salt crystallisation. The dense cladding stones are generally more durable than some stones used in masonry however for thin panels the effects of movements and fixings are important. For prestige structures staining may be a major factor.
 

Life expectancy and maintenance requirements
Guidance on the durability of stone is given in British Standards and other documents but these recommendations require interpretation and there is a degree of uncertainty particularly in respect of novel uses of stone.

Stonework may perform satisfactorily for many years with little or no maintenance although regular washing is recommended for some stones to maintain their appearance.
 

Finishes
Finishes is a very diverse group of materials, some for general use, others limited to use on specific backgrounds. Traditional finishes such as paints and wood stains have been joined by flexible polymer coatings suitable for use on metals and plastics. Coloured foil coatings have also been developed, and galvanising and anodising are widely used for corrosion protection of steel and colouring of aluminium, respectively.
 

Main forms of deterioration
The main forms of deterioration of finishes are loss of adhesion to the substrate and breakdown due to weathering particularly due to exposure to UV light. Durability can be improved by selecting a material which is compatible with the substrate and environmental conditions and by good site practice.
 

Life expectancy and maintenance requirements
The life of many finishes will be less than the life of the structure and they will therefore require replacement at intervals.

Formulations are constantly being changed to improve performance and the life of materials currently on the market is therefore uncertain.

BS 6150 suggests that site applied paints are generally expected to give a life of 5 to 10 years depending on type of paint and exposure.

BS 5427 suggests that factory applied polymer coatings on profiled metal sheet may give a life of 10 to 30 years depending on the type and colour of coating and the exposure conditions.

Anodising on aluminium may give a life of 50 years or more.

Regular cleaning is recommended for some finishes such as anodising of aluminium. Some finishes may not be replaceable and it may be necessary to replace the components affected.
 

Rubber and plastic
Whilst natural rubber is rarely used in cladding, synthetic rubbers find their way into facade components ranging from gaskets and weatherseals to damp-proof membranes. The large number of different chemicals that can be incorporated into a typical rubber make this probably the largest group of materials that is encountered. However it is generally possible to blend a rubber that is durable under given conditions.

Plastics are also diverse in the number of basic types, but there tends to be more standardisation in terms of those which are used.

• Unplasticised polyvinylchloride (PVC-U)is widely used in glazing frames and sheet claddings
• Polycarbonate (PC), polymethylmethacrylate (PMMA, acrylic) and ethylenetetraflouroethylene (ETFE) are used as glazing materials
• Polyamide (PA, nylon) is used for the manufacture of thermal and electrical isolators
• Glass reinforced polyester (GRP) is used for cladding panels and as a glazing material

Many of the agents that affect rubbers also affect plastics due to the close relationships between the two groups of materials. Many of the materials in these groups can exist as either rubber or plastic depending on the formulation and processing.

Note that there are also a number of polymer foam insulation materials which fall into this category.
 

Main forms of deterioration
The main form of deterioration is degradation due to exposure to UV light which may result in loss of flexibility, yellowing or chalking.
Plastics have a high coefficient of thermal expansion and allowance must be made for the resulting movements.
 

Life expectancy and maintenance requirements
Plastics generally require little maintenance. Cleaning may be required to maintain appearance but dirt can be beneficial in blocking out UV radiation. Many materials will have a life less than that of the structure and will therefore need to be replaced.

Some PVC-U window frames have been in service for 30 years but there is insufficient experience to know whether this is representative of the general standard of material in use.

BS 6093 suggests that most gasket materials have a life expectancy of up to 20 years.

BS 5427 Suggests that profiled cladding made from PVC-U, polycarbonate and GRP has an expected service life of up to 20 years.
 

Sealants and adhesives
Sealants and adhesives could be grouped together with rubbers and plastics under a ‘polymer’ material type. However, rubbers and plastics are predominately used in preformed components whilst the majority of sealants and adhesives are applied in liquid form and allowed to cure. For this reason fabrication and installation agents play a much larger role in the durability of these materials.

Sealants covers a wide range of materials with different movement characteristics and life expectancies as follows:
 

Some sealants are also used as adhesives, for example in double glazing units and structural silicone glazing, however, adhesives also covers a wide range of other materials ranging from animal and vegetable glues to synthetic resins. Uses of adhesives in cladding include the manufacture of board products, joinery and fixing ceramic tiles.
 

Main forms of deterioration
The main form of deterioration of sealant materials is weathering however the causes of failure of sealant joints are often related to the choice of the sealant material, geometry of the joint, poor workmanship during installation and stresses and strains imposed by joint movement. Prediction of service life is difficult due to the many factors affecting joint life and the fact that material formulations are developing.
 

Life expectancy and maintenance requirements
With correct design and installation, sealants should require little maintenance other than possible removal of mould growth and replacement of sections with minor defects. However the life of the sealant will often be less than the life of the building and complete replacement of the sealant will therefore be necessary. BS 6093 and BS 6213 give guidance on the design life to be expected for different materials and typical life expectancy for various materials is given in the table above.
 

Bitumen
Bituminous materials are generally used for waterproofing, for example in the form of bitumen-impregnated damp proof membranes, bitumen-mastic backed aluminium foil for use as flashings and bituminous paints.  Bituminous materials also include mastic asphalt, pitch and coal tar, and each of these products has different uses.  Whilst bituminous materials are often used to protect materials, acidic by-products are formed by exposure to UV radiation and these can cause problems.
 

Main forms of deterioration
The main form of deterioration is embrittlement due to loss of volatiles, oxidation and the effect of UV light.

Many materials such as dpcs are built into the structure where they are not accessible to affecting agents and do not require maintenance. Where bituminous materials are exposed on the outside of the facade they may need replacing during the life of the structure.
 

Fabrics
Fabrics are being used as cladding materials in a variety of applications.  The basic fabrics are often treated with various chemicals to make them weather-resistant (water-repellent, draught-proof) and to reduce dirt retention. The most commonly used fabrics are as follows:

• PVC coated polyester
• PTFE coated glass fibre
• Silicone coated glass fibre

Research into new materials is continuing and PVDF coating has been introduced in an attempt to overcome problems with PVC and PTFE
 

Main forms of deterioration
PVC is subject to degradation by UV light and with ageing the surface becomes sticky and attracts dirt. Polyester fibres creep and may need re-tensioning during the life of the structure.

Glass fibres are attacked by water and rely on the coating for protection. The fibres are also brittle and can be damaged by rough handling or flapping in the wind during construction. Glass fibre materials are also vulnerable to damage from sharp objects.
 

Life expectancy and maintenance requirements
The anticipated lifespan of PVC coated polyester is approximately 15 years whereas glass fibre materials are expected to have a life of approximately 50 years.