Building Failures

Diagnosis and avoidance

W.H. Ransom

• Publisher: Routledge
• Year: 2002
• ISBN:
  9781135920791
• Edition: 2
• Topic: Architektur
• Subtopic: Architektur Methoden & Materialien

In recent years building failures and the resulting lawsuits and awards for damages have frequently been in the news. The biggest headlines may have been reserved for structural failures and complete collapses, but we should not forget the less newsworthy failures such as leaky roofs, damp walls, dropped foundations and rotted timber. This book gives practical guidance on the prevention of failure by describing the nature and cause of the most common defects in buildings, and then shows how they should be avoided in design and construction.

1
Introduction

Most building defects are avoidable: they occur, in general, not through a lack of basic knowledge but by non-application or misapplication of it. Knowledge seems to become mislaid from time to time. Those with long memories, and those whose business it is to make a particular study of building defects, are often struck by the reemergence of problems which have been well researched and documented. Certain basic properties of materials, such as their ability to move through changes in temperature and moisture, seem to be overlooked and a rash of difficulties occurs. A call goes out for more research but, in truth, all that is usually needed is a good system for the retrieval of information, a better procedure for its dissemination and, most important, the realization that an information search is desirable.

Current training in design tends to concentrate on what to do rather than what not to do. A similar situation exists in training in constructional techniques, where the craftsman is instructed how best to undertake a particular operation but, to a lesser extent, in the dangers of deviation from an accepted technique. Understanding of the likelihood of defects through inadequate design or construction is taught implicitly rather than explicitly. The level and nature of defects in building construction currently encountered suggest that more guidance is required on the avoidance of failures. A need is seen, too, for such guidance to be a positive part of a training curriculum. Indeed there are good arguments for suggesting that, as the first essential in design and construction is to ensure that the structure provided is stable and durable, specific education in the avoidance of failure should be a major part of any design and construction syllabus. The purpose of this book is to provide this positive guidance in a suitably compressed form. It does not set out to describe every possible way in which a building may become defective: such a task would scarcely be possible and certainly would not be particularly helpful. It seems better to aim at identifying the principal defects and their causes which, if wholly eliminated, would prevent the great majority of the defects which currently occur, save occupants of buildings much annoyance and discomfort, and reduce the national bill on maintenance and repair by scores and, possibly, by hundreds, of millions of pounds annually.

The book aims to identify the nature and cause of important defects occurring in buildings, with emphasis on those affecting the fabric of a building and its associated services. It does not deal with issues of aesthetics, lighting, or thermal or acoustical comfort. While concerned primarily with the avoidance of defects, the text also gives guidance to aid in their correct diagnosis when, unfortunately, the situation demands cure rather than prevention. Except in a general way, the repair of such defects is not covered. Any one specific failure needs a detailed examination to decide on the most appropriate repair, for this depends not only upon technical considerations but also upon the type of building and its age, and upon related economic and social considerations. There are few standard solutions to problems of repair.

Most defects occur through the effects of external agencies on building materials and the two succeeding chapters consider in some detail the nature of these and their effects on the materials commonly used in building. These agencies include the principal components of the weather, namely, solar radiation, moisture and air and its solid and gaseous contaminants; biological agencies, in particular fungi and insects; ground salts and waters; and manufactured products used in conjunction with building materials, for example, calcium chloride. Moisture occupies a central role, as the villain. Work initiated by the Joint Working Party on Heating and Energy Conservation in Local Authority Housing has shown that there may be as many as two and a half million dwellings in the UK suffering from dampness and in two-thirds of these the dampness is caused by condensation. The main sources of moisture and ways in which the amounts present may be minimized are dealt with in Chapter 4. Special emphasis is given to the cause and effects of condensation, and how the risks may be avoided or reduced. Condensation, particularly in local-authority dwellings, can truly be said to have been the greatest single cause of human discomfort in dwellings since the end of the Second World War. The elements of building structure are then dealt with, starting with foundations and progressing logically upwards to roofs and parapets, passing on the way, floors, walls, cladding and external joinery. The avoidance of defects in building services has a chapter to itself. The book concludes with a more speculative chapter dealing with failure patterns and control. This attempts to relate defects to problems associated with the structure of the industry, to the dissemination of information and to particular difficulties which result from rapid innovation. Current control methods are outlined and a possible strategy is suggested for improving control, quality and reliability.

The intention and hope is that this book will provide positive guidance to the student designer and builder on how to avoid the principal building defects. It includes no complex scientific concepts and requires no special knowledge of science. Though concerned more with normal building than with major civil engineering construction, much of the text is of relevance to structural engineers also, particularly those parts dealing with the properties of the structural materials, with foundations and with cladding. The point was made at the beginning that knowledge gets mislaid: a further aim of this book is to serve as an aide-mémoire for practising designers and builders. For this reason, it has been kept concise, and is illustrated to give visual emphasis to some of the more important defects which can occur. These illustrations and parts of the text which describe the likely appearance of failures may assist surveyors and maintenance personnel, too, by steering them towards the probable cause of a failure. Though the essential aim is to avoid failure, once it has occurred and maintenance is needed, it is hoped the book will help both in identifying the cause and in preventing the adoption of the wrong remedial action. It may also help the maintenance engineer and surveyor by putting the severity of a failure and its consequences into a reasonable perspective and so prevent over-reaction to the event, which is not uncommon, particularly with foundation problems. If the book succeeds only partly in these ambitions it will, nevertheless, save both money and misery.

2
Agencies causing deterioration

When building materials and components are transported, stored on site and used in a structure, they are subjected to the effects of a number of agencies, some of which may influence adversely their durability and performance and, thereby, have a major bearing on the possibility of their premature failure. The action of the weather, or the external climate, is foremost amongst these. It is often taken to affect only those materials exposed externally. However, the distinction between the external and internal environments in a building is not always clear-cut. There are partially protected areas still open to the weather to some extent. Outside air enters a room through an open window and sunlight is filtered through window glass. Weather changes in protected or internal environments are usually the same in type but slower in action than those taking place outside. Buildings themselves cause modifications to the weather and to the microclimate, considerable differences in which may occur in quite short distances. The micro-climate is, indeed, of particular significance and, because of the large number of individual circumstances which can cause its modification, is still an area of much uncertainty and a fruitful one for detailed investigation.

The principal components of the weather include:

1. Radiation—from the sun and from the rest of the sky; varying in amount, frequency, direction, intensity and spectral composition.
2. Rain—varying in direction, droplet size, quantity, intensity, duration, temperature and distribution.
3. Solidified water—snow or hail; varying in frequency, direction, shape, size, amount, terminal velocity.
4. Air and its gaseous constituents—in particular, water vapour, oxides of sulphur, oxygen and carbon dioxide.
5. Solid and liquid contaminants of air—dirt, tar and oil particles, salts; varying in composition, distribution, ease of attachment and detachment, chemical effects. (Salt spray near the coast is an important example.)

Durability and performance are also affected by biological agencies (of which moulds, fungi, bacteria and insects are the most important), ground waters and salts, and manufactured products, for example, calcium chloride. It is worth making the point that durability is not an inherent property of a material: different materials have different effective durabilities due in part to each of the following—their physical and chemical properties, the function each has to perform and their position on, or in, a building. In practice, each building material or combination of materials tends to respond differently to the influences outlined above, many of which may be active at any one time. These influences are often inter-related in a complex way and may reinforce or oppose one another in different materials. Thus, coincident strong sunlight and dew have a particularly damaging effect upon paint films. On the other hand, unobstructed sunlight following dew deposited on metal will assist evaporation and so reduce the likelihood of corrosion. One agency alone may exert very different effects, depending on its form or intensity. For example, water in the form of rain washing over a surface can retard or prevent mould growth, but moisture in the form of repeated condensation can be highly conducive to its formation. For any particular situation, it is necessary to assess the likely combination of agencies and their effects upon durability and performance, and succeeding chapters seek to make this assessment for the most common building situations. As a basis to considering these more complex inter-relationships, it is, nevertheless, useful to consider separately the agencies mentioned and their general effects.

2.1 SOLAR RADIATION

Solar radiation is received at the Earth’s surface both directly and as long-wave diffuse sky radiation. The proportion of diffuse sky radiation to total radiation received is considerable and cannot be neglected: indeed, it can exceed direct radiation. Solar radiation is absorbed when it strikes an opaque surface. Most building materials are opaque and their absorptivity (the ratio of the radiation absorbed to the incident radiation received) varies, depending upon the nature and colour of the surface. Black non-metallic surfaces have high absorption. Some values for building materials are shown in Table 2.1. The absorption by some materials of bands of short-wave solar radiation (referred to generally as the ultra-violet) can lead to degradation. Such degradation is confined to organic materials, in particular, to plastics, some paints and bituminous-based materials.

Table 2.1 Absorptivity to solar radiation of some common materials

This table was originally published in Section A6 of the CIBSE Guide by permission of the Chartered Institution of Building Services Engineers.

2.1.1 Temperature effects

The absorption by surfaces of solar radiation is accompanied by a rise in temperature. Building surfaces can also emit long-wavelength radiation and, in so doing, cool. The drop in temperature can be considerable, particularly on clear nights when radiation from black surfaces, such as asphalt roofs, can cause surface temperatures to fall well below shade air temperature. A rise in temperature leads to an increase in the rate of reactions and can accelerate many degradation processes. (An increase of 10°C doubles the rate of many chemical reactions.) High temperatures, in themselves, also lead to high rates of evaporation and volatilization. Loss of volatiles from bituminous compositions, some plastics, mastics and sealing compounds can cause shrinkage and brittleness. Evaporation of water from cement mixes can lead to early weakness, poor adhesion and cracking. Phase changes may occur, the best-known example being that which occurs in high-alumina cement, the change being closely associated with a loss in strength. Some building materials, for example bitumen, soften or melt with high temperatures. In contrast, temperatures that are permanently below freezing can be highly favourable, which is often not recognized, for they ensure the absence of all cyclic thaw-freeze effects, of leaching by rain and of liquid moisture migration. Many degradation processes, too, are slowed down.

Temperature changes cause dimensional changes in materials, particularly when the coefficient of expansion is high as, for example, with aluminium and some plastics. These changes cause stresses which, if not accommodated, can exceed the strength of some materials and cause distortion or rupture. Temperature changes can be quite sudden. Sunlight breaking through a frost-laden fog can heat up a surface very rapidly. Rain falling on a sun-heated surface applies a severe quenching shock. Brittle coatings and joints between dissimilar materials can then undergo the first initial breakdown, leading to subsequent deterioration. In the United Kingdom, air temperatures can change by over 20°C between night and day, and by over 50°C between maximum summer temperatures and minimum winter ones. The maximum temperature changes on the surface of building materials and the rates of temperature change are often even greater. Black surfaces which are good absorbers of solar radiation and powerful emitters of lower-temperature radiation can show the maximum changes in temperature and, particularly so, if insulated behind the black surface. The range of temperature changes for such surfaces may be double that of the air-temperature changes mentioned above. The coefficients of thermal expansion of some typical building materials are shown in Table 2.2, together with an indication of the unrestrained movements consequent upon a change in temperature of 50°C—an annual change which can be expected as a minimum for many materials.

2.2 MOISTURE

Moisture in solid, liquid or vapour form can be regarded as the principal agent causing deterioration. It is always present in the atmosphere and, when surface temperatures of materials fall sufficiently, condensation can occur, which may be heavy and prolonged. Even under cover, surfaces can become thoroughly wetted, metals may corrode and be brought into aqueous contact with other metals or materials which may lead to electrolytic attack, and glass may be etched. Conditions are particularly conducive to deterioration when moisture condenses in relatively inaccessible crevices from which subsequent evaporation is slow. Rain, particularly when blown by strong winds, can erode soft materials and, washing over a surface, may remove part of it in solution. Water has a high heat of vaporization and is, therefore, slow to evaporate. High precipitation, consequently, implies not only a more complete, but a more prolonged, wetting of materials. When water freezes in the pores of materials, such as brick, stone and concrete, stresses are produced which may cause spalling of the surface, general cracking or disintegration. Water frozen in the form of hail can cause pitting of some surfaces and, as snow, has to be allowed for in structural design.

Table 2.2 Thermal expansion of some common building materials

Changes in relative humidity can lead to dimensional change in materials, with deformation, crazing or cracking. Prolonged low humidities can cause the dehydration of gypsum products (though such humidities do not occur naturally in the UK); prolonged high humidities aid fungal growth and the subsequent decay of organic materials. Moisture also stimulates biological activity and acts as a medium or catalyst through which reactions occur which could not otherwise take place. Because of its major role in causing or assisting failures in materials, components and structures, moisture is dealt with separately and at greater length in Chapter 4.

2.3 BIOLOGICAL AGENCIES

Attacks by fungi and insects are principally upon timber, though other materials, generally organic, can be affected. In recent years, a good deal of timber decay has been caused by the several varieties of fungi which are covered by the term ‘wet rot’. Wet-rot fungi require markedly damp conditions to germinate and a continuing source of moisture for their existence. Sapwood at a moisture content of around 30% and a temperature around 20°C provides an ideal abode. Timber with a moisture content not greater than 20% is not endangered. Once the source of moisture is removed, the fungi will die and do not have the ability to spread to dry timber, or to penetrate plaster and brickwork, as does the fungus Serpula lacrymans (dry rot). Spores of the dry-rot fungus are generally present in the air and, given the right conditions, will germinate. Though they are too small to be seen individually by the unaided eye they can be spread by moving air in a building to settle as a rust-coloured dust. The conditions favoured are dark stagnant ones with timber of moisture content above 20% and temperatures around 20°C. Poorly ventilated sub-floor areas and situations where timber is in prolonged contact with damp materials are those where the danger of attack is greatest. The fungus grows most readily on unsaturated wood which has a moisture content of 30% or more. Once germination has occurred filaments called hyphae spread over the surface to form whitish fluffy growths or sheets known as the mycelium. The hyphae can penetrate cracks in materials such as plaster, brick and block, which in themselves do not provide nourishment, in search of further wood. The growth of the fungus over these inorganic materials is ma...