ME31B: CHAPTER TWO BUILDING MATERIALS

A wide range of building materials is available for rural building construction. The proper selection of materials to be used in a particular building can influence the original cost, maintenance; ease of cleaning, durability and of course, appearance. Proper selection of material is very important and the following factors are to be considered. 2.1 INTRODUCTION

Properties of Building Materials a) Strength and Durability: This includes tensile and compressive strength, hardness, toughness, shear strength etc. The materials should also be durable ie. able to retain strength and other properties over a considerable period of time. b) Workability: Ease of working with the material eg. Cutting and shaping. This should be considered hand in hand with skilled labour or otherwise available.

Properties of Building Materials Contd. c) Cost: Cheaper materials should be preferred without sacrificing other characteristics. d) Availability: Local materials should be preferred, all other things being equal over imported or transported materials.

Other Properties of Building Materials Other properties to consider include Esthetics, Resistance to corrosion and to fire, Easy transportability and ease of cleaning, Resistance to environmental changes, and to insect attack as well as imperviousness.

2.2 Soil(Earth) This is one of the oldest materials for building especially in rural areas. Advantages of earth as a building material are: i) It is resistant to fire (ii) It is cheap and readily available (iii)It has a high thermal capacity which enables it keep the inside of the building cool when the outside is hot and vice versa. (iv) It is a good noise absorbent

Soil (Earth) Contd. (v) It is easy to work using simple tools and skills. The following properties of earth need improvement for it to be a good building material: Look, Crackness, Weakness, Lack of water proofness.

2.2.1 Methods of Improving Soil Properties Soil can be improved by compaction. Adobe bricks(made from soil) can be used to build. This improves the looks and reduces cracking. a) Cracks: Soil particles range from clay to sand. Clay is strong but cracks, while sand does not crack but is weak. A mixture of clay and sand can be used to avoid cracking.

Improving the Soil Contd. This is the principle of brick making. Cracks can also be controlled by using fibres of certain plants e.g. straw. The fibre reinforces the soil particles by bridging the gap between them. b) Weakness: Soil strength can be improved by adding clay or chemicals eg. cement. Other stabilizers include lime, bitumen, rice husk ash, natural fibres e.g. grass, straw, sisal, saw dust, resins, cow dung and gypsum.

Improving Earth Contd. c) Water Proof-ness: The soil's waterproof qualities can be improved by adding some chemicals e.g. benzene. Emulsified asphalt can also be added but it is very expensive. The earth wall should be protected from rain by extending the roof.

2.3 MASONRY Masonry is an art of building with stones, bricks or concrete blocks. 2.3.1 Stones: Soft stones are preferred for building to ease reshaping. Stones generally show good strength, high durability, low cost(especially if available in site), resistance to fire, insects and environmental changes. Stones have high compressive strength and hence good for walls, columns, foundations etc. 

2.3.2 Clay Bricks Bricks are made from clay; the clay paste is placed in proper moulds, hardened and burnt in kilns (oven) in high heat. Bricks are generally strong, durable, resistant to environmental changes, fire and insects.

2.3.3 Mortar Used to put stone materials together. It is a mixture of sand, water and a binding material like clay, cement or lime. Cement is more expensive that lime which is more costly than clay. Lime is produced by burning calcareous stone (Caco3) in water. CaCo3 + H 2O = Ca(OH)2 + CO2 Cement is a mixture of lime and clay.

2.3.4 Cement or Concrete Blocks Made using mortar of cement and sand with necessary quantity of water. The blocks can also contain low sized gravel also. The blocks are generally made hollow with the core making about 25% of the gross area. These blocks are quite strong, durable and have most other characteristics of a good building material. The core of the block increases the acoustical and thermal properties and reduces the self-weight. 

2.4 CONCRETE Concrete is probably the most important structural material today. It has the following ingredients: i) Cement : Portland cement ii) Fine aggregate: Sand, granite granules etc. iii) Coarse aggregate: Gravel, crushed stone, broken bricks etc. iv) Water.

2.4.1 Ratio of Concrete Normal ratio of concrete is 1: 2: 4 (by volume). This means 1 part cement, 2 parts fine aggregates and 4 parts coarse aggregates. Fine aggregates fill the pores of coarse aggregates, cement fills the pores of fine aggregates. Cement with water forms a very good binding material which binds the concrete into a homogeneous mass.

2.4.2 Water-Cement Ratio (w/c ratio) The best w/c ratio is 0.45 to 0.65 by weight. Too high w/c ratio increases workability of concrete but reduces strength of the paste while Very low values reduce workability and binding ability leading to cracking of concrete.

2.4.3 Processes in Concreting a) Batching: This involves bringing all the ingredients together in a proper ratio. It is normally done by volume but better results are obtained if the ration is taken by weight. For 1 m of concrete, take approximately 1 m3 of coarse aggregate. With 1: 2: 4 ratio, 0.5 m3 of sand and 0.25 m 3 of cement are used. Water is normally added by experience.

Processes in Concreting: Batching Contd. With these mixtures, a little more than 1 m3 of concrete is obtained. For more accurate results, for 1 m3 of concrete, take 0.96 m3 of coarse aggregate and others accordingly using the mixing ratio.

Processes in Concreting: Mixing Mixing can be done manually or mechanically. In manual mixing, cement and sand are mixed thoroughly and then mixed with the coarse aggregate, and then water is added and mixed thoroughly. In mechanical mixers, all aggregates and water are added in the rotating mixer and rotated to mix them thoroughly.

Other Processes in Concreting c) Conveyance or Transport: Done using barrows or mechanical dumpers. Should be done quickly. d) Placing: Before placing concrete in sites, the formwork is to be checked, cleaned and oiled. e) Compaction: Concrete must be compacted adequately to secure maximum density. This is done either manually or using rods or mechanical vibrators.

Processes in Concreting: Curing f) Curing: The setting of cement and hardening of concrete depends on the water content. The placed concrete has to be protected against evaporation. Curing is done by keeping the exposed surface of concrete wet continuosly. It is done by flooding in case of slabs or by continuous spraying in other cases. Curing is done for 7 to 10 days, but 21 days of curing gives maximum strength.

2.4.4 Reinforcement of Concrete Concrete is strong in compression but weak in tension. Reinforcement involves inserting steel bars into concrete to make it resist tension. Beams, slabs etc. need reinforcement in order to make them resist bending which produces tension.

Example Find the required quantities of cement, sand and coarse aggregate required to concrete a slab 10 m long, 5 m wide and 12 cm thick. Calculate the quantity of water required also. Specific gravity of cement is 1.5; ratio of ingredients is 1 : 2 : 4 and water cement ratio is 0.5.

Solution Total volume of concrete needed = 10 m x 5 m x 12/100 m = 6 m3 Assume 0.96 m of coarse aggregates per m3 of concrete Coarse aggregate required is 6 x 0.96 = 5.76 m3 Fine aggregate required is 5.76 x 2/4 = 2.88 m3 Cement required = 5.76 x 1/4 = 1.44 m3

Solution Concluded Note: Bulk density of water is 1000kg/m3 ; with specific gravity of 1.5; bulk density of cement is 1.5 x 1000 = 1500 kg/m3 Wt. of cement = 1.44 m3 x 1500 kg/m 3 = 2160 kg. Assuming 1 bag of cement weighs 40 kg, Cement required = 2160/40 = 54 bags. w/c ratio is 0.5, water required = 0.5 x wt. of cement = 0.5 x 2160 = 1080 kg = 1080 litres Note: 1 litre of water weighs 1 kg.

2.5 WOOD Wood is a traditional building material. The quality of wood varies widely. Wood is generally durable, strong, dependable, workable and possesses many other characteristics. It is, however, highly inflammable and liable to insect attacks if not properly protected. Wood is mainly used for roof skeleton, doors and windows, walls and partitions in small buildings.

Properties of Wood Contd. Wood may be hard or soft. Hard ones include ebony, mahogany, etc. while soft ones include Douglas fur, palm tree etc. Strength of wood increases with decreasing moisture content. By seasoning (drying), the strength can be improved. Wood is highly workable.

Plywood Produced by gluing many layers of split wood together. The grains are arranged alternatively to eliminate distortion. The plywood is a very good modern construction material whose quality depends on type of wood used, glue used and pressure of steam used in gluing.

Shapes of Wood Contd: Block c) Blocks: Very thick eg. 25 x 25 mm(width-depth) or 20 x 25 mm or 30 x 30 mm   BLOCK   Many grades of wood exist depending on the strength. BLOCK

2.6 METALS These are man-made materials whose properties are known. The most widely used metal is steel. 2.6.1 Steel: Steel is very strong, highly durable, resistant against weather, fire and insects and possesses almost all good characteristics. Steel is used in reinforcement in concrete, gates, windows, roof trusses, steel sheets for partitions and tanks etc.

2.7 Other Building Materials (Synthetic Materials) a) Asbestos Cement: Used for making roof sheets, drain pipes and accessories. It is also used for partitions and ceilings. b) Plastics: used in water supply and sanitary systems. Also electric fittings. c) Ceramics, fibre glass etc. are new materials used in some restricted areas.

2.8 PROTECTION OF BUILDING MATERIALS Plastering with cement mortar, lime mortar etc. is common to protect the super structure. Painting is used to protect steel from rusting, timber from insect attack and shrinkage etc. Painting of walls, ceilings etc. are done for light control and beauty. Galvanizing and other protective coverings of certain materials especially metals are common.