The problem of durability in building design

The problem of durability in building design G Soronis*

Abstract - Durability is an integral part of modern building design. The demands for innovative building techniques and the inclusion of materials and components with lower life-cycle costs test the knowledge and skills of building designers. The pressing need for reliable information about the role of durability in the building design process has led to the formation of a working commission on 'Design for Durability' (CIB W94) by the International Council for Building Research, Studies and Documentation (CIB). This paper is a state-of-the-art review of this area based largely on the proceedings of the International Conferences on the Durability of Building Materials and Components held in 1978, 1981, 1984, 1987 and 1990. The paper emphasizes the importance of the development and utilization of an internationally accepted systematic approach to the durability problem in building design, and of the development of computerized expert systems to support designers with decision making information.

Design begins with a pre-planning process which includes the selection of materials and the determination of their relative positions in a construction to produce a building or a part of a building. Durability, as a major element in building design, demands extensive knowledge and understanding of the properties of materials and their in-service environment.

Up until the 1960s, building designers were concerned mainly with architectural aspects or with the load bearing capacity of structures. Durability was determined by the use of 'traditional' designs which had been proved suitable for specific conditions. Building structures were massive compared with modern multilayer lightweight constructions. Their substantial mass resulted in a high capacity to resist deterioration. Buildings were durable to deterioration in spite of small faults or defects in the construction. By contrast, minor defects in a modern construction may lead to severe degradation.

Over the past three decades, with the introduction of new building materials and components and new building techniques, the situation has changed. Many traditional materials have either disappeared from the market or are being used in new, non-traditional combinations. The building design process has developed towards specialization. Therefore the total performance of a building greatly depends on cooperation between different designers.

Most durability failures depend on the fact that knowledge of the performance of certain components or materials is only available in research and scientific papers. In practice where problems must be solved 'now', designers do not have the time required to study these voluminous documents. It is important therefore that scientists in the area of durability assist designers by providing information on the behaviour of materials, on their interaction with the environment and on mechanisms of deterioration, in a systematic and concise format. Another way to spread existing knowledge is improvement of teaching for various levels of education.

Lack of reliable information about the role of durability in the building design process has long been a barrier to effective selection, use and maintenance of building and construction materials. The need to address this barrier was recognized in 1991 when CIB (International Council for Building Research, Studies and Documentation) formed working commission CIB W94 Design for Durability. The present paper is a state-of-the-art review of the area. It is mainly based on the proceedings documentation of the International Conferences on the Durability of Building Materials and Components held in 1978, 1981, 1984, 1987 and 1990.

The main aims in writing this paper were:

• To give a state-of-the-art report of durability design which will serve as a basis for planning further research work within CIB W94.

* Building Engineering, Department of Architecture, The Royal Institute of Technology, Stockholm, Sweden

• To identify and summarize the technical barriers and research needs.

• To initiate research to develop a design methodology which facilitates adequate consideration of durability in the design procedure.

• To form a theoretical basis for the development of information systems for the durability design process.

• To provide guidelines for the presentation of research results in scientific publications to improve communication between researchers and practitioners.

The concepts of durability and design No uniform terminology exists for reports on durability research. For this reason the following descriptions have been written to explain the concepts of durability and design and some related expressions.

Durability is the capability of a building, assembly, component, product or construction to maintain serviceability over at least a specified time [1]. Durability of a building material or component should not be seen as something absolute. Instead it is more appropriate to consider the time period during which the required functions will not fall below certain prescribed limits.

Design has been formally defined as [2]: 'Design establishes and defines solutions to and pertinent structures for problems not solved before, or new solutions to problems which have previously been solved in a different way'.

Deterioration is the process of becoming impaired in quality or value [1].

Environment covers all conditions external to a building material or component which influence its performance [3]. Durability cannot be considered rationally except in the context of the environment in which the building material or component operates. Thus, it is the interaction of elements of the environment with a material which determines its durability. The time aspects of durability are accounted for by the expressions 'performance over time' and service life.

Performance over time is the function which describes how the measured values of some chosen properties vary with time [1]. Service life of a building material or component is the time period after installation during which all essential properties meet or exceed minimum acceptable values, when routinely maintained [1].

The design fife [6] of a building or a part of a building can be defined according to:

• The designer's interpretation of the building owner's requirements when a performance specification containing requirements for durability is prepared.

• Discussions between the building owner and the designer (or in the case of contractor-designed items, between the project designer and the contractor).

If the building owner has not specified the conditions that will apply to the building, the designer should record for the owner what conditions have been assumed. It is common that in

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Durability in building designs

such cases the designer utilizes some empirically-estimated values for the design life.

The economic life of a building material can be defined as the period of time over which the material is able to meet the objectives at the minimum of life-cycle costs [4].

Maintenance can be defined as the total of action and measures to keep a structure at least at a desired level [5].

The initial costs for some material alternative include all costs that can be referred to the new building by using the chosen material [6].

The maintenance costs for some material alternative include all present values of planned maintenance, remedy maintenance and operating maintenance that can be referred to the material [6].

People concerned with durability design It is important for all designers who are taking part in the building design process to know the terminology commonly used to describe all groups or categories of people who are concerned with durability. Six groups or categories can be described [7,8]:

• Owners - including tenants. • Designers - including architects and engineers. • Innovators - such as material scientists and researchers. • Contractors - including manufacturers and suppliers of

materials. • Property managers - including maintainers and security

firms. • Users - including lessees.

The owner has the responsibility for maintenance and upkeep of the principal structural, architectural, mechanical, and electrical items which make up the skeleton and fabric of the building. The term tenant refers only to occupants of buildings who are responsible for maintenance and upkeep. A tenant may be an individual or a group employed by the building's owner or by an organization which administers the structure. Many such tenants in the private sector are also responsible for the costs of owning the asset and thereby become involved in the design process in the same way that conventional building owners do.

A designer can be an architect, or an engineer when a special engineering-oriented facility is required. In essence, architects are those who deal in both applied art and science, as opposed to engineers who deal in applied science. Innovators, material scientists and researchers offer their aid to the designers in determining and developing the durability standards of the materials and the components. A contractor bids on a contract for a new building, with price information from manufacturers and suppliers, in competition with other contractors.

At take-over, the building is occupied and administered by a property manager who engages maintainers to be responsible for proper maintenance inspections or to carry out the necessary maintenance. The security firms supervise inspections and other activities concerning the security of buildings.

The users or lessees often have no financial interest in the completed building and are only interested in the physiological, psychological, or sociological aspects of the building performance.

Historical viewpoints Historians and archaeologists have been able to describe our world and its inhabitants as far back as ten million years [9], although it is only during the last 500 000 years that the greater part of human progress has occurred. This knowledge gives relevant information about man's concern for durability in building through the ages and indicates clearly that the survival of materials depends greatly on the environment to

Analysis

f Specific -~' information

Function analysis Environmental data

f General information

Service life of materials Economical data J

,NO

Education, skill,

YES

Synthesis

C O

t.. O Q . O

c" Oh .m u l O E3

Outcome

Fig 1

Go to the next step

A statement of a durability problem

which they are exposed. Throughout history [10], concern for durability of buildings

and structures has been evident. It is evident in the Biblical instructions given to Moses on Mount Sinai for construction of the Tabernacle in Jerusalem, it is evident in the pyramids in Egypt, the construction of the Coloseum in Rome and the construction of thousands of ancient buildings and structures throughout the world. It is obvious that some kind of testing procedures for building materials was used thousands of years ago [9]. The Roman architect Marcus Vitruvius Pollio described a two year weather test of building stones in about 25 sc.

Many historical structures have aged with time, but for some the ruins remain to show us examples of both understanding and mistakes made by our forefathers in using building materials. Even though the construction processes and the materials of construction have changed with technological advances since ancient times it is the classic traditional styles of masonry, brick and timber construction that still give inspiration and knowledge to the modern builder or architect.

The problem statement The critical step in all design processes is the definition of the problem. The result of the design work greatly depends upon how the problem is defined [2]. It is important to define the problem as broadly as possible, because with a broad definition the designer will be less likely to overlook unusual or unconventional solutions. The definition of the problem can be given by the problem statement, which expresses specifically the aims of the design. It includes objectives, goals, definitions, the constraints of the design, and the evaluation criteria.

A statement for a durability problem (Fig 1) demands both analysis and synthesis. Analysis usually involves description of the real world through models and is mostly concerned with separation of the problem into manageable parts. Synthesis is concerned with assembling the elements into a workable unit.

206 CONSTRUCTION & BUILDING MATERIALS Vol. 6 No. 4 1992

Table 1 Performance requirements [3]

1. Stability 2. Fire safety 3. Safety in use 4. Permeability 5. Hydrothermal 6. Atmospheric 7. Acoustic 8. Visual 9. Tactile

10. Anthropodynamic 11. Hygiene 12. Suitability of spaces 13. Durability 14. Economic

Table 2 Stages of the building [3]

Design Before end-use conditions

Manufacture Storage Transport Construction, including new erected

structures

Use conditions Operation Maintenance Repair Replacement

Demolition Re-use

Table 3 Levels of the building [3]

Group of buildings Buildings Spaces Part of buildings Building element Interfaces

Building elements - Product interfaces - Products

Material interfaces -

Materials

Internal structures -

Walls'foundations, joints between prefabricated wall and horizontal space divider elements, window frames, walls. Walls, roofs, floors, foundations. Bricks/mortars, paints/substrates, nails/wood. Bricks, mortars, paints, wall panels, foils, insulation materials. Cements/aggregates, resin/glass fibres, lime/earth. Cements, pozzolans, earths, plastics, resins, sand, fibres. Chemical compositions, pore properties, cohesion

To involve durability in the building design process means that materials (and their relative positions in a construction or product) are selected and positioned to give predictable performance [11]. As no material in itself is durable, it is the interaction of the environment and the materials that determines durability. Durability is ensured by the selection of effective and compatible materials and components. This is especially true for the exterior building envelope, and also for all interior materials to prevent wear-out by human use. Maintenance and economic considerations must also be a part of the designer's durability thinking.

There are four factors which must all be taken into account in designing the optimum solution:

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• Design and functions • Design and environment • Design and materials • Design and economy

Design and functions Function analysis is the initial step of building design. All functions must be described in detail according to some performance requirements list, Performance in use and life to first maintenance are commonly a matter of correct specification of materials for the particular situation where they are to be used [10]. Incorrect application of materials and misunderstanding of the whole performance characteristics have caused many durability failures in buildings.

Information on durability, standards and various levels of official requirements may often be insufficient. The designer often lacks the basic information to make a proper selection with regard to durability, Standards and codes give only a limited degree of durability information. In Table 1 the performance requirements are expressed in 'human terms' [3] which is a typical feature of performance thinking. To obtain the performance under conditions of use, all stages of the construction process should be taken into account (Table 2). The performance may be evaluated at different 'levels' of building, as described in Table 3.

Design and environment

Environmental control The function of a building material or component is to maintain separation of two non-identical environments [11]. Because there are many components in an environment, the general function consists of a multitude of subfunctions. Their number and nature depend on differences between the two environments and the degree of separation required. Where several materials are required to perform a complex overall function, it is obvious that each material will have a different environment on each side. The location of a material in a building component is therefore an important factor in determining its environment and function as well as those of all other materials in the system. The designer must try to minimize the critical nature of the function imposed on each material and also to improve the environment in which most of the materials must perform.

It is important therefore to quantify the physical and chemical characteristics of the materials and to examine their compatibility. Where the environment is particularly aggressive it may be necessary to diminish the stresses due to the environment by architectural and structural design as follows [12,13]:

• By reconsidering the location of the building, as well as shape and height. This may influence at least the amount of wind and rain which will reach the surface of the building.

• By re-examining the material properties in detail. • By using structural details to alter the local environment

in the component.

Degradation factors The exposure environment may be categorized using various weathering (meteorological), biological, stress, compatibility, and use factors as described in Table 4. For a specific material only some of the environmental factors identified by the designer are degrading factors. In the same way only a part of all the properties which characterize a material are performance characteristics.

Climate classification A commonly-used base for classification of climate is division into macro-, meso- and micro-climate [16]. These terms

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Table 4 Environmental agents affecting building materials and components [3]

Weathering factors Radiation

Solar Nuclear Thermal

Temperature Elevated Depressed Cycles

Water Solid, as snow and ice Liquid, as rain, condensation, standing water Vapour, as high relative humidity

Normal air constituents Oxygen and ozone Carbon dioxide

Air contaminants Gases as oxides of nitrogen and sulphur Mists, aerosols, salt, acids, alkalis Particulates, as sand, dust, dirt

Freeze-thaw Wind

Biological factors Microorganisms Fungi Bacteria

Stress factors Stress, sustained Stress, periodic Stress, random

Physical action of water, as rain, hail, sleet and snow Physical action of wind Combined physical action of water and wind Movement due to other factors, such as settlement or vehicles

Incompatibility factors Chemical Physical

Use factors Design of system Installation and maintenance procedures Normal wear and tear Abuse by the user

indicate different scales for the description of variations in meteorological variables. There exist no exact and common definitions of the different scales.

Macro-climate normally refers to the gross climate, described using one of the following terms [7]: arctic, extreme, temperate, tropical, equatorial, desert. These descriptions are based on measurements of meteorological factors such as air temperature and precipitation.

When describing meso-climate the effects of the terrain and of the building's nearest surroundings are taken into account [14]. The climatological description is still based on the standard meteorological measurements.

The micro-climate describes the meteorological variables in the absolute proximity of a material's surface [14]. The micro-climate or the micro-environment is decisive for the material's degradation. The most important meteorological factors describing micro-climate are relative humidity, surface moisture, surface temperature, uv-radiation and deposition of air pollutants.

/nterna/ climate The main emphasis in durability design lies commonly in the climatic/weathering factors (Table 4), as they are most promi- nent in the design of exterior constructions. Nevertheless it is relevant to consider how the inside climate influences the durability of the structure. The humidity rate and the presence of aggressive chemicals in the inside atmosphere are significant factors in durability design. Two examples are given here of the significance of the inside climate.

The first concerns durability in shower-room walls and floors [15]. Before the 1970s Europeans generally took baths

rather than showering. Since then people have become more interested in personal hygiene. They use showers with strong water streams running on the walls and floors. The relative humidity can be over 90% for several hours a day when the water films are drying. The designer must consider all these climatic conditions to avoid severe durability failures such as rotten and mouldy organic materials, weakening gypsum boards, loosening PVC wall-coverings.

The other example concerns a building where the masonry envelope failed after only 17 years of service because of severe deterioration of some materials within the masonry walls [16]. The building had been designed for use as an office block but was actually used for the display, servicing and storage of art treasures. The inside atmosphere was maintained at a temperature of 21°C and 50% relative humidity, year round. The external walls had a facade facing consisting of fine stone and brickwork which was supported or 'backed up' with rough masonry of stone, bricks and tiles. Three types of failure have been encountered, each related to water carried by leakage of the inside air towards the external wall environment.

The first type of failure concerns the back up bricks. These bricks spalled and broke as they were exterior to the building insulation and exposed to freezing temperatures for most of the winter. The second type of failure, displacement of the facing stone and bricks, was caused by debris from deterioration of back up masonry falling into the cavity between the facing and the back up. The third type of failure was caused by the deterioration of the metal ties used to hold the stone to the masonry back up.

Design and materials In many buildings, the performance of materials can be observed under a variety of exposure conditions. It is obvious that most materials cannot themselves be classified as durable - unless they can withstand any conceivable climate - or non-durable. A systematic methodology for service life prediction of building materials and components is of particular importance since it makes it possible to conceptualize deterioration [1]. Conceptualization is a vital part of synthesis as in most cases the degree of deterioration with time is not only a function of the individual materials, but also a function of their interaction with other materials, environmental elements and location in the structure.

A systematic methodology for service life prediction of building materials and components is outlined by Masters and Brandt [1]. It includes the identification of required information, the selection or development of tests, the interpretation of data, and the reporting of results. Another relevant methodology for service life prediction of building materials and components is outlined by Lucchini [21]. This methodology introduces a simple model which allows identification of the significant variables and a model for the process of service life prediction. Reliability evaluation of functional models is also important using methodologies for service life prediction. Proper reliability value referred to the service life simplifies the optimization of design choices [22].

The service life of a material can be predicted in one or more of the following ways [17]:

• By reference to previous experience with the same, or a similar, construction and in similar occupation or climatic circumstances.

• By measuring the natural rate of deterioration over a short period of use or exposure and estimating from the measurement when the durability limit will be reached.

• By interpolation from accelerated tests that have been devised to shorten the response time to the action of an agent. The science of accelerated testing is complex; care should be taken not to produce additional effects by changing the natural intensity of the environmental agents.


Table 5 Average fife expectancy of roofing membranes [18]

Description of built up roofs Years

One ply base sheet and three plies of No. 15 saturated organic felt applied with asphalt 18.8

One ply base sheet and three plies of No. 15 saturated asbestos felt applied with asphalt 17.0

Four plies of No. 15 organic felt applied in asphalt 17.0

Four plies of No. 15 asbestos felt applied in asphalt 16.2

One ply base sheet and three plies of No. 15 tarred felt applied in coal tar pitch 15.8

One ply base sheet and two plies of No. 15 asbestos felt applied with asphalt 11.6

Two plies of heavy weight saturated asbestos felt applied in asphalt 8.2

Two plies of coated felt applied in asphalt 7.1

Whatever method is used, the predicted service life is unlikely to be a precise figure because the effect of an action in any building material is not likely to be accurately predictable [17]. More reliable predictions can be made when there is a correlation between the results of different assessments. Despite the availability of the above-mentioned methods for asessing 'relative durabilities', the data obtained from these methods are seldom adequate for a reliable service life prediction of building materials [10].

It must be emphasized that when a period of service life for a material is given, it is essential at the same time to specify all conditions on which this time period is based. These conditions can be the material's interaction with other materials, data on the construction geometry and detailing, maintenance information and the quality of craftsmanship.

Deterioration analysis of building materials shows that the causes of deterioration are often interrelated and complex. For this reason it is important that the designer should have the opportunity of using all available information on the deterioration mechanisms according to some systematic procedure.

A good example of service life information is the following case concerning built up bituminous roofing membranes [18]. In the the winter of 1977-1978 a questionnaire was sent to 184 individuals within the roofing industry throughout the USA and Canada. These individuals were roofers, employees of roofing materials manufacturers, consultants in roofing technology, government employees concerned with roofing materials. The results of the investigation are given in Table 5.

The analysis shows that none of these types of roofing membrane has more than a 50% probability of lasting 20 years; membrane durability increases as the number of plies increase; membranes composed of organic felt have greater durability than membranes composed of asbestos felt. No detectable differences in membrane performance were found based on geographic differences. The values in Table 5 were valid only when the workmanship, flashings, surfacing, insulation, supporting structure, drainage, and other details were adequate.

It is rare that the results of some service life prediction methods are as simple to interpret as those in the example above. Often the results are too complex to be easily interpreted by the practical minded designer and can serve

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Present value of / / . t o t a l l ife cycle costs f J

~ / P n r c ~ s e n ; I v a e l u e °tf croepair

L

Durab i l i t y

Fig 2 Relationship between durability of materials and costs [19]

only as a basis to following research in the area. Researchers need to give greater attention to presenting results in a way that designers can readily use.

Design and economy The durability of building materials may be associated with service life [4], but neither durability nor service life are necessarily synonymous with economic life. Materials may for example remain in service for a long time after they have been judged economically unattractive. In economic terms durability is expressed in life-cycle costs. This requires the materials competing in a particular application to be ranked using initial, operating and repair costs.

Rational, management-oriented materials selection means that the designer chooses materials from a horizon of costs which corresponds to the design life of the building. The costs of a new construction and the future costs for maintenance and replacement, that are strongly depending on the durability properties of these materials, must be considered together through some rational decision making methodology. The most common decision methodology is LCC computation (life- cycle cost). The LCC methodology can be used by the designer to find the material with the optimum level of durability according to a management-oriented construction perspective [6]. The LCC methodology can be summarized as follows.

Let w denote the factor multiplied by a future construction cost or series of costs (such as production costs, operation and maintenance costs, replacement costs, etc) throughout the design life of the building. It will give a present equivalent cost (present value) according to the formula [6]:

Cp = Cf x w

where w = [11(1+d)] t, Cp is the cost in present value, Cf is the cost in year t, and d is the discount rate. If there exist some materials that satisfy a given objective, it is possible to find the material with the optimum durability by assuming that there are technical possibilities to increase durability by using more resources [19]. Thus the optimum level of durability will be reached at a point where the present value of total life- cycle costs is a minimum (point L in Fig 2). The curve representing the present value of total life-cycle costs is the vertical summation of the curves for the initial costs and for the present value of repair and replacement cost. If a material is made more durable then the initial cost rises while the present value of repair cost falls. Thus, the total cost curve will first decline and then rise, as the rise in initial costs outweighs the fall in repair and replacement costs.

Although LCC computation can work as a tool to provide the designer with a priority list of a group of materials, there exist disadvantages that can obstruct an effective building design:



• LCC computations are usually very complex calculation procedures which demand that the designer (who is usually an architect or an engineer and not an economist) has a deep knowledge of the various investment philosophies.

• It is often difficult for the designer, especially for an inexperienced one, to find reliable information about the time intervals for acquisition, planned maintenance, remedy maintenance and material replacement which are included in the LCC computations. Different designers will make different operational definitions about these time intervals.

• LCC computations demand more time than the traditional materials selection methods based on the designer's previous experience. For this reason it is common that the designer will consider the initial cost of the construction as the only criterion to the materials selection procedure.

Research needs

Systematic methodology At present design for durability is quite difficult for an ordinary designer [13]. This is not due to a lack of knowledge on most of the deterioration phenomenona. The main problem with durability in design arises from the fact that information on durability and deterioration is given only in scientific publications, and then mainly in a non-systematic form. The necessary information is quite difficult to extract from these publications and they typically do not include any clear guidelines for the designer.

Thus, the most urgent need for advancing the state of the art in this area is the development of a systematic, functional and performance-oriented methodology to facilitate the design process. The lack of an international accepted methodology for systematic treatment of the problem leads to a number of other problems [10]. It makes it difficult for participants in joint research projects to communicate; it hinders the ability of researchers to link various segments of their research in a clear and adequate manner. The importance of developing and utilizing an internationally-accepted systematic approach to the design of durability is emphasized in the work of CIB W94 Design for Durability [23].

The main aims of a systematic methodology must be to provide instructions on the combination of factors which cause durability problems. These factors are (cf Reference 3):

• The durability properties of individual materials under different exposure and maintenance conditions.

• The intensity of typical deterioration factors, the building type and its environment.

• The location of the materials in the building, with regard to their exposure to various deterioration factors.

• The location of the materials and components in the building with regard to the possible detection of deterioration, and to the application of preventative maintenance or remedial measures.

• The linkages that exist between initial costs, maintenance costs and the property of durability in building materials and components.

There are several main advantages of having a systematic methodology to treat the durability problem in the design process [20]. The optimum construction material is often a sensitive function of the initial construction costs and of later costs depending on the material durability. A systematic methodology increases the probability that the best material will be selected. Traditionally, information on performance over time is based on the designer's earlier construction experience. With a systematic methodology the materials selection process becomes easier for inexperienced and experienced building designers alike. In addition, it is known

that most building designers are familiar with only a limited number of materials and construction systems. With a systematic methodology, alternative materials and new construction systems with suitable durability properties can more easily be considered.

Computerized systems With the development of a systematic methodology, an important step will have been taken; but much more is needed to overcome all the difficulties with durability problems in the design process. These difficulties can be overcome by taking full advantage of recent advances in computer technology. The development of powerful computers and CAD/CAM systems offers opportunities to support decision making. Some of the advantages of using these computerized systems can be summarized as follows [20]:

• Information on building materials can be taken from many sources and stored according to a specific area of interest. Lists with relevant material properties - for example, data regarding the material durabilities in different environments - can be collected.

• Stored information can easily be updated. • The materials selection procedure becomes easier to deal

with for the building designer. Materials with certain functions/durability requirements can easily be found in the material data base systems.

• Decision making methodologies can easily be adapted in these computerized systems. For example, economic aspects and LCC costs for different materials can easily be stored and can later be utilized by the designer as a basis for material selection.

Although tllese advantages are obvious there are also some disadvantages that have to be taken into consideration when discussing the near future [18]. Effective computerized design systems often require high costs and many years of work in development. This requires considerable financial support of the necessary research. In addition, the communication programs are usually difficult for non-specialists to use. Expert system technology has, in recent years, been proposed as a solution to this problem.

It should be emphasized that these disadvantages should not be considered as intrinsic but rather as an illustration of the difficulties that can be involved in developing computerized design systems.

Conclusion Decisions on durability are explicitly or implicitly a vital part of modern building design. The demands for more innovative building techniques and the inclusion of new materials and components with lower life-cycle costs set requirements on the knowledge and skills of designers as they have to design and choose the right solution from a large variety of possible solutions. The most urgent needs for advancing the state of the art in this area are:

• To develop an internationally-accepted, systematic, functional and performance-oriented methodology to facilitate the design process.

• To develop computerized systems aimed at giving designers decision making information.

• To develop practical guidelines for authors of scientific publications so that they might interpret their scientific results in a way which more closely meets the needs of building designers.

The following general guidelines summarize state-of-the-art knowledge as described in this paper and can help authors of scientific papers more usefully to convey information for durability to building designers (cf Reference 10).


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(1) Define the problem explicitly before attempting to describe it. (2) Define the concept of durability such that (a) it can be measured, and (b) it can be related to the specified in-service performance. (3) Specify all possible degradation factors. (4) Give explicit data for service life of the materials according to specified degradation factors and in-service conditions. (5) Give information on all deterioration mechanisms (and the actual combinations of degradation factors) to avoid failures concerning installation, design and detailing, maintenance, craftsmanship, user's behaviour. (6) Use simple and systematic design procedures having as their basis logic, common sense, and materials science. (7) Be aware that durability is closely connected with economy. Cost/benefit considerations must always be given in connection with the results of durability research. (8) Think always that there is a designer who will use the research results in practical applications. Describe the formulation of the problem, all assumptions made and all research results in a simple, systematic and explicit form. (9) Always give information and guidelines to designers concerned with specific durability problems, in all research publications. If the researcher is without adequate design experience, it may be necessary to have the assistance of an experienced designer when interpreting the research results into practical design guidelines.

A c k n o w l e d g e m e n t s The author appreciates the many helpful suggestions by and discussions with Professor Sture Samuelsson, coordinator for CIB's W94 'Design for Durability' and Dr Birgitta H&ssler at the Royal Institute of Technology, Department of Architecture, during the preparation of this paper. Thanks are also due to Dr Christer SjSstrSm at the National Swedish Institute for Building Research who has supported and followed this work closely and to the referee for his insight and comments.

References 1 Masters L W and Brandt E. Systematic methodology for service

life prediction of building materials and components. Materials and Structures, 1989, 22, pp385-392

2 Dieter E G. Engineering Design, ,4 materials and processing approach. McGraw-Hill Inc, (ISBN 0-07-016906-3), New York, 1991, ppl-5

3 Sneck T. General Report 1. Service life of building materials and components. Third International Conference on Durability of Building Materials and Components, Volume 4, Technical Research Centre of Finland, Espoo, August 1984, pp12-40

4 Rakhra S A. Economic aspects of durability of building materials: an exploratory analysis. Second International Conference on Durability of Building Materials and Components, US Department of Commerce, National Bureau of Standards, Washington, DC 20234, September 1981, pp13-21

5 Rey F W A and Toorn A. A decision model based on minimum life cycle cost. Fourth International Conference on Durability of Building Materials and Components, Pergamon Press, Singapore, November 1987, pp463-470

6 Westin T. Kostnadskalkylering led LCC-modelI. Till&mpning f(3r byggprocessens olika skeden, (English: Cost Calculations with LCC-model. Applications for Various Steps of The Building Process). Report R27:1989 (ISBN 91-540-5016-2), Swedish Council for Building Research, Stockholm 1989, pp33-46

7 McGregor S K. Architectural overview on durability - by W5 & HOW. Fourth International Conference on Durability of Building Materials and Components, Pergamon Press, Singapore, November 1987, pp486-490

8 Van Court D R The owner/tenants interest in unifying durability data. First International Conference on Durability of Building Materials and Components, ASTM STP 691, (P J Sereda and G G Litvan, Eds), American Society for Testing and Materials, 1978, pp~-76

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10 Mastem W L. Service life prediction: A state of the art. Fourth International Conference on Durability of Building Materials and Components, Pergamon Press, Singapore, November 1987, ppl-2

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23 Soronis G. CIB-94, Design for Durability, State of the Art. Report 32, ISRN KTH/KLA/R-32-SE (ISSN 1101-9468), The Royal Institute of Technology, Department of Architecture, Building Engineering, Stockholm 1991, pp22-23


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The problem of durability in building design