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These include bitumen (called asphalt in the USA), coal tar and pitch. They are usually dark brown or black and are in general durable materials, resistant to many chemicals. They resist the passage of water and water vapour, especially if they have been applied hot.
Bitumen occurs naturally as rock asphalt or lake asphalt or can be distilled from petroleum. It is used for road paving, paint, damp-proof membranes, joint filler, stabilizer in soil blocks, etc.
Paint preserves, protects and decorates surfaces and enables them to be cleaned easily. All paints contain a binder which hardens. Other ingredients found in various paints include: pigments, strainers, extenders, driers, hardeners, thinners, solvents and gelling agents. Some water-thinned paints contain emulsifiers.
Because of the cost involved, few buildings in the rural areas are painted. When paint can be afforded priority should be given to surfaces likely to rust, rot or decay because of exposure to rain or dampness and to rooms like a kitchen or a dairy where hygiene demands easily cleaned surfaces. White and other light colours reflect more light than dark colours and can be used in rooms like a sitting room or a workshop to make the room lighter.
Painting
Adequate preparation of the surface to be painted is essential. The surface should be smooth (not shiny for this would not give good anchor), clean, dry and stable. Old, loose paint should be brushed off before a new coat is applied. Most commercial paints are supplied with directions for use, which should be read carefully before the work is started. The paint film is usually built up in two or more coats;
Priming paints are used for the first coat to seal and protect the surface and to give a smooth surface for subsequent coats. They are produced for application to wood, metal and plaster.
Undercoating paints are sometimes used to obscure the primer, as a further protective coating and to provide the correct surface for the finishing paint.
Finishing paints are produced with a wide range of colours and finishes (e.g., matt, semi-matt, gloss). Some commonly used types of paint for farm structures are detailed below, but many others are manufactured with special properties like water and chemical resistance, heat resistance, fire retardant, anti-condensation, fungicidal, insecticidal, etc.
Oil-and Resin-based Paints
Oil paints are based on natural drying oils (e.g., linseed oil). They are being gradually replaced by alkyd and emulsion paints.
Alkyd paints are oil-based paints modified by the addition of synthetic resins to improve durability, flexibility, drying and gloss. They are quite expensive.
Synthetic resin paints contain substantial proportions of thermosetting resins, such as acrylics, polyurethane or epoxides, and are often packaged in two parts. They have excellent strength, adhesion and durability, but are very expensive.
Bitumenous paints are used to protect steelwork and iron sheeting from rust and wood from decay. They are black or dark in colour and tend to crack in hot sunlight. They can be overpainted with ordinary paint only after a suitable sealer has been applied.
Varnishes are either oil/ resin or spirit-based and mainly used to protect wood with a transparent finish, but protection is inferior compared to opaque finishes. Spiritbased varnish is only used for interior surfaces.
Water-thinned Paints
Non-washable distemper consists of chalk powder mixed with animal glue dissolved in hot water. It is cheap but easily rubbed or washed off and therefore only suitable for whitening ceilings.
Washable distemper (water paint) consists of drying oil or casein emulsified in water and additions of pigments and extenders. Hardening is slow but after a month it can withstand moderate scrubbing. It weathers fairly well outdoors and is fairly cheap.
White wash (lime wash) consists of lime mixed with water. It can be used on all types of walls including earth walls and is cheap, but its lack of water resistance and its poor weathering properties make it inferior to emulsion paint for outdoor surfaces. However, an addition of tallow or cement gives some degree of durability for external use. White wash can be made in the following way:
Emulsion paints have the pigments and binder (vinyl, acrylic, urethane or styrene polymers) dispersed as small globules in water. They harden quickly, are quite tough and weather resistant and of medium cost. They adhere well to most bac kgrounds, but since they are permeable, an oilbased primer may be required to seal porous exterior surfaces.
Cement-based paints are often used for exteriors and are quite inexpensive. They contain white portland cement, pigments if other colours are desired, and water repellents, and are sold in powder form. Water is added just before use to make a suitable consistency. Paint that has thickened must not be thinned further. It adheres well to brickwork, concrete and renderings but not to timber, metal or paint of other types. Surfaces should be dampened before painting.
Cement slurries make economical surface coatings on masonry and concrete, but earth walls that shrink and swell will cause the coating to peel off. Slurries are mixtures of cement and / or lime, clean fine sand and enough water to make a thick liquid. A good slurry is made from 1 part cement and 1 part lime and up to 4 parts of sand. It is applied on the dampened surface with a large brush or a used bag, hence the name "bag washing".
Barnes M.M., Farm Construction, Buildings, Slough, Cement and Concrete Association, 1971.
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Everett A., Materials, Mitchell's Building Series, London, Batsford Academic and Educational Ltd., 1981.
Fullerton R.L., Building Construction in Warm Climates, Part 1-3, Oxford, Oxford University Press, 1977-1979.
Hodgkinson A., A.J. Handbook of Building Structure, London, The Architectural Press Ltd., 1982.
International Labour Office, Small-Scale Brickmaking, Geneva, International Labour Office, 1984.
Lippsmeier G., Tropenbau-Building in the Tropics, Munchen, Callwey Verlag, 1969.
Lundborg N., To Choose Timberfor Building, Dar-es-Salaam, National Housing and Building Research Unit, 1976.
Lunt M.G., Stabilized Soil Blocks for Building, Overseas Building Notes No. 184, Watford, Building Research Establishment, Overseas Division, 1980.
McKay W.B., Carpentry, London, Longman Group Ltd., 1975.
National Academy of Science, Ferrocement: Applications in Developing Countries, Washington, D.C., National Academy of Science, 1973.
National Vocational Training Institute, Accra, Rural Building:
1 Reference Book, 2 Basic Knowledge, 3 Construction, 4 Drawing Book, Maastricht, Stichting Kongregatie F.l.C.
Paterson D.N., The Strength of Kenya Timbers Their Derivation and Application, Nairobi, Kenya Forest Department, 1971.
Shetty M.S., Concrete Technology, New Delhi, S. Chand & Co. Ltd., 1982.
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Storrs A.E.G., Know Your Trees, Ndola, The Forest Department, 1979.
Storrs A.E.G., More About Trees, Ndola, The Forest Department, 1982.
Stulz R., Appropriate Building Materials, SKAT No. 12, St. Gallen, Swiss Center for Appropriate Technology, 1981.
Swift D.G., Smith R.B.L., The Construction of Corrugated Roofing Sheets Using Sisal-Cement, Nairobi, Kenyatta University College, 1979.
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A structure is designed to perform a certain function. To perform this function satisfactorily it must have sufficient strength and rigidity. Economy, and an attractive appearance are also of importance in structural design.
Structures are subjected to a variety of loads either singly or in combination. These include the self-weight of the materials used for construction as well as the weight of products stored, animals housed or water dammed. The short-term loads due to wind and even earthquakes must also be included. The designer must have an understanding of the nature and significance of these forces and apply this knowledge to the design, materials and methods of construction if the structure is to safely survive all situations. Each of the various elements, such as ties, struts and beams, has a unique purpose in maintaining the integrity of the structure and must be designed to have sufficient strength to withstand the maximum stress to which it may be subjected.
The many building materials available differ greatly in their resistance to loading and in other characteristics that relate to their use in various building elements. They must be selected carefully to be suitable for the type or types of loading which are determined during the structural design procedure.
The analysis of all farm building structures is based on certain fundamental principles which are addressed in this chapter.
Approximations
It is necessary for the designer to fully understand the principles of statics and mechanics of materials as well as the function of structural members. It is as a result of this understanding that he is able to make assumptions and approximations which enable him to reduce complex situations to a level at which simple design techniques produce adequate, although sometimes over-designed, structures.
Design Codes
Many countries have their own structural design codes, codes of practice or technical documents which perform a similar function. It is necessary for a designer to become familiar with local requirements or recommendations in regard to correct practice. In this chapter some examples are given, occasionally in a simplified form, in order to demonstrate procedures. They should not be assumed to apply to all areas or situations.