Concrete blocks - sand - cement blocks

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It is faster to build with concrete blocks than with bricks and the amount of mortar is reduced to less than half. If face shell bedding is used, in which the mortar is placed only along the edges of the blocks, the consumption of mortar is reduced by a further 50%. However, the total cement required for the blocks and mortar is far greater than that required for the mortar in a brick wall.

Concrete blocks are often made of 1:3:6 concrete with a maximum size aggregate of 10mm or a cement-sand mixture with a ratio of 1:7, 1:8 or 1:9. These mixtures, if properly cured, give concrete blocks a compression strength well above what is required in a one-storey building. The blocks may be solid, cellular or hollow. Cellular blocks have cavities with one end closed while in hollow blocks the cavities pass through. Lightweight aggregate such as cracked pumice stone is sometimes used.

Blocks are made to a number of coordinating sizes, the actual sizes being about 10mm less to allow for the thickness of the mortar.

Block Manufacturing

Blocks can be made by using a simple block-making machine operated by an engine or by hand. They can also be made by using simple wooden moulds on a platform or floor. The mould can be lined with net steel plates to prevent damage during tamping and to reduce wear on the mould. In large-scale production steel moulds are often used. The wooden mould is initially oiled overnight and need not be oiled each time it is filled. It is sufficient to wipe it clean with a cloth. The concrete, of stiff or plastic consistency, is placed in the mould in layers and each layer is compacted with a 3 kg rammer.

The mould in Figure 3.30 has a lid made so that it can pass through the rest of the mould. The slightly tapered sides can be removed by lifting the handles while holding down the lid with one foot.

Figure 3.30 Wooden mould for solid concrete blocks.

The mould illustrated in Figure 3.31 has a steel plate cut to the shape of the block which is put on as a lid and held down as the hollow-making pieces are withdrawn. Bolts are then loosened and the sides of the mould removed with a swift motion. All parts of the mould should be slightly tapered so they can be easily removed from the block.

Starting the day after the blocks have been made, water is sprinkled on them for two weeks during curing. After 48 hours the blocks can be removed for stacking, but the wetting is continued. After curing, the blocks are dried. If damp blocks are put in a wall, they will shrink and cause cracks. To assure maximum drying, the blocks are stacked interspaced, exposed to the prevailing wind and in the case of hollow blocks, with the cavities laid horizontal to form a continuous passage for the circulating air.

Decorative and Ventilating Blocks

Decorative concrete or sand/cement blocks can serve several purposes:

In addition, some are designed to keep out rain while others include mosquito-proofing.

Blocks of simple shape can be made in a wooden mould by inserting pieces of wood to obtain the desired shape, but more complicated designs usually require a professionally made steel mould.

Figure 3.31 Mould for hollow or cellular concrete blocks.

Mortar

Mortar is a plastic mixture of water and binding materials used to join concrete blocks, bricks or other masonry units.

It is desirable for mortar to hold moisture, be plastic enough to stick to the trowel and the blocks or bricks and finally to develop adequate strength without cracking.

Mortar need not be stronger than the units it joins. In fact cracks are more likely to appear in the blocks or bricks if the mortar is excessively strong.

There are several types of mortars each suitable for particular applications and of varying costs. Most of these mortars include sand as an ingredient. In all cases the sand should be clean, free of organic material, be well graded (a variety of sizes) and not exceed 3mm of silt in the sedimentation test. In most cases, particle size should not exceed 3mm as the mortar will be "harsh" and difficult to work with.

Lime mortar is typically mixed 1 part lime to 3 of sand. Two types of lime are available. Hydraulic lime hardens quickly and should be used within an hour. It is suitable for both above and below ground applications. Non-hydraulic lime requires air to harden and can only be used above ground. If smoothed off while standing, a pile of this type of lime mortar can be stored for several days.

Figure 3.32 Ventilating and decorative concrete blocks.

Cement mortar is stronger and more waterproof than line mortar, but is difficult to work with because it is not 'fat' or plastic and falls away from the blocks or bricks during placement. In addition, cement mortar is more costly than other types. Consequently it is used in only a few applications such as a damp-proof course or in some limited areas where heavy loads are expected. A 1:3 mix using fine sand is usually required to get adequate plasticity.

Compo mortar is made with cement, lime and sand. In some localities a 50:50 cement-lime mix is sold as mortar cement. The addition of the lime reduces the cost and improves the workability. A 1:2:9, cement-lime-sand mix is suitable for general purposes, while a 1:1:6 is better for exposed surfaces and a 1:3:12 can be used for interior walls or stone walls where the extra plasticity is helpful.

Mortar can also be made using pozzolana, bitumen, cutback or soil. A 1:2:9 lime-pozzolana-sand mortar about equals a 1:6 cement-sand mortar. Adobe and stabilizedsoil blocks are often laid in a mortar of the same composition as the blocks.

Tables 3.16 and 3.17 provide information on the materials required for a cubic metre of various mortars and the amount of mortar per square meter for several building units.

Starting with cement mortar, strength decreases with each type, although ability to accommodate movement increases.

Finishing Mortar

Table 3.16 Materials Required per Cubic Meter of Mortar

Type Cement bags Lime kg Sand m
Cement mortar 1 :5 6.0 - 1.1
Compo mortar 1:1:6 5.0 100.0 1.1
Compo mortar 1:2:9 3.3 13.5 1.1
Compo mortar 1:8 3.7 - 1.1
Compo mortar 1:3:12 2.5 150.0 1.1
Lime mortar 1:3 - 200.0 1.1

Table 3.17 Mortar Required for Various Types of Walls

Type of wall Amount required per m wall
11.5cm brickwall 0.25m
22.2cm brickwall 0.51 m
10cm sand-cement block wall 0.008m
15cm sand-cement block wall 0.01 1m
20cm sand-cement block wall 0.015m

This is sometimes used on floors and other surfaces to give a smooth finish or as an extremely hard coating to increase the resistance to wear. While such a top coating is prone to cracking, it seldom increases strength and is difficult to apply without causing loose or weak parts. Concrete floors can normally be cast to finished level directly and be given a sufficiently smooth and hard surface without a top coating.

For coating, a mix of 1 part cement and 2 to 4 parts sand is used. The coating is placed in a 1 to 2cm thick layer with a steel trowel. Before application, the surface of the under laying concrete slab should be cleaned and moistened.

Plastering and Rendering

The term plastering is usually applied to interior walls and ceilings to give jointless, hygenic and usually smooth surfaces often over uneven backgrounds. Exterior plastering is usually called exterior rendering.

Cement plaster can be used on most types of walls, except it does not adhere well to soil-block walls as the shrinking and swelling tend to crack the plaster. The mixing ratio is 1 part cement and 5 parts sand, and if the plaster is too harsh, 0.5 to 1 part of lime can be added. The wall is first moistened and then the plaster is applied in two coats of about 5mm each, allowing at least 24 hours between layers. Cement plaster should not be applied on a wall while exposed to the sun.

Dagga plaster is a mixture of clay soil, such as red or brown laterite, stabilizer and water. The plaster is improved by adding lime or cement as a stabilizer and bitumen for waterproofing. A good mixture is 1 part lime or cement, 3 parts clay, 6 parts sand, 0.2 part bitumen and water. Dagga plaster is applied on previously moistened earth or adobe brick walls with a thickness of 10 to 25mm.

Ferrocement

Ferrocement is a highly versatile form of reinforced concrete made with closely spaced light reinforcing rods or wire mesh and a cement and sand mortar. It can be worked with relatively unskilled labour.

The function of the wire mesh and reinforcing rods is first to act as a lath providing the form to support the mortar in its plastic state, while in the hardened state, they absorb the tensile stresses in the structure which the mortar alone is not able to withstand.

The reinforcing can be assembled in any desired shape and the mortar applied in layers to both sides. Simple shapes such as water tanks can be assembled with wooden sticks as support for the reinforcing while the first coat of mortar is applied.

The mortar should have a mixing ratio of 1:2 to 1:4 cement- sand by volume, using the richer mix for the thinnest structures. The water-cement ratio should be below 0.5/1.0. Lime can be added in the proportion 1 part lime to 5 parts cement in order to improve workability.

The mechanical behavior of ferrocement is dependent upon the type, quantity, orientation and strength of the mesh and reinforcing rods. Of the several types of mesh being used, the most common are illustrated in Figure 3.33.

Standard galvanized mesh (galvanized after weaving) is adequate. Non-galvanized wire has adequate strength but the problem of rusting in limits its use.

A construction similar to ferrocement has recently been developed for small watertanks, sheds, huts, etc. It consists of welded 150mm square reinforcement mesh (6mm rods) covered with hessian and plastered in the same way as ferrocement.

Fibre - reinforced concrete

Fibre - reinforced concrete members can be made thinner than those with conventional reinforcement because the corrosion - protective cover over the steel bars is not necessary. The fibres improve flexible strength and resistance to cracking.

Figure 3.33 Reinforcement mesh for ferrocemens.

Commonly used fibres are asbestos, steel (0.25mm diameter), sisal? elephant grass, etc.

Asbestos Cement (A-C)

Asbestos, a silicate of magnesium, occurrs as a rock which can be split into extremely thin fibres from 2 to 900mm long. These have good resistance to alkalis, neutral salts and organic solvents, and the varieties used for building products have good resistance to acids. Asbestos is noncombustible and able to withstand high temperatures without change.

Inhalation of dust causes asbestosis (a disease of the lungs) and asbestos is now used only where no alternative fibre is available. Workers must wear masks and use great care not to inhale any asbestos dust!

The fibres being strong in tension and flexible, are used as reinforcement with Portland cement, lime and bitumen binders, in asbestos-cement and asbestos-silica- lime products, vinyl floor tiles and in bitumen felts. Asbestoscement is used in farm structures for corrugated roofing sheets, ridges and sanitary pipes.

Sisal-Fibre-Reinforced Cement (SFRC)

Sisal and other vegetable fibres have only recently come into use for reinforcement of concrete.

Sisal fibre can be used as short, discontinuous timbres ( 15 to 75mm in length) or as continuous long fibres over 75mm in length. Sometimes both short and long fibres are used together. The manner in which the fibres are incorporated into the matrix affects the properties of the composite both in the fresh state as well as in the hardened state.

Sisal fibres may deteriorate if not treated. Although the alkalinity of the concrete helps to protect the fibres from outside attack, it may itself attack the fibres chemically by decomposing the lignin.

Sisal-fibre reinforcing is used with various cement-sand mixing ratios, depending on the use:

wall plastering 1:3
guttering 1:2
roofing tiles 1:1
corrugated roofing sheets 1:0.5

The sand should be passed through a sieve with 1.5mm to 2mm holes (e.g., mosquito netting). The mixing water must be pure and the mix kept as dry as possible while still being workable.

Between 16g and 17g of short (25mm) dry sisal fibres are added to the mix for each kilogramme of cement. The short fibres are mixed into the dry cement and sand before adding water. Sisal fibres have a high water absorption, and some extra water may have to be added to the mix to compensate for this.

When mixing there is a tendency for the fibres to ball and separate out from the rest of the mix. This tendency will increase with longer fibres, but if fibres shorter than 25mm are used the reinforcing effect will be reduced. In most cases, the mix is then trowelled on to a mesh of fulllength sisal fibres.

Making Corrugated Reinforced Roofing Sheets

Homemade reinforced corrugated roofing is usually cast to standard width, but only one metre long because of its additional weight. Commercial asbestos-cement roofing is heavier than corrugate steel and the home made sheets are still heavier. Thus special attention must be given to rafter or truss sizes to ensure a safe structure.

The casting procedure for SFRC is involved, but once the proper equipment has been assembled and several sheets have been made the process becomes much easier.

A concrete block cast over a 1m length of asbestoscement roofing is needed as a face for the casting of the roof sheets. The block is cast within a form, 100mm high, which will give a block of sufficient strength after a few days curing. Two or more 1m lengths of A-C roofing will be needed as well as a piece of 18mm plywood 1.2m by 1.2m and a sheet of heavy duty polythene 2.25m long and 1m wide. The polythene is folded in the middle and a thin batten 9mm by 15mm is stapled fast at the fold. Strips of 9mm plywood or wood are nailed along two edges of the plywood sheet leaving exactly 1 m between them as shown in Figure 3.34.

Following are the steps in the casting procedure:

Figure 3.34 Plywood casting board and polythene "envelope"

Walls Using Sisal-Cement Plastering Technique

Soil blocks can be used for inexpensive walls with good thermal insulation. However, they are easily damaged by impact and eroded by rain. One way of solving these problems is to plaster the face of the wall. Ordinarily mortar plaster tends to crack and peel off as it does not expand at the same rate as the soil. This can be overcome by letting long sisal fibres pass through the wall to be incorporated into the mortar on each face. The double skin so formed provides sufficient strength and waterproofing to the wall to enable soil blocks to be laid without joining mortar between the blocks.

Metals

Several ferrous metals (those containing iron) are useful in farm building construction. Cast iron is used for making sanitary waste pipe and fittings. Steel consists of iron plus a small percentage of carbon in chemical combination. High-carbon or hard steel is used for tools with cutting edges. Medium-carbon steel is used for structural members such as "I" beams, reinforcing bars and implement frames. Low-carbon or mild steel is used for pipe, nails, screws, wire, screening, fencing and corrugated roof sheets.

Non-ferrous metals such as aluminium and copper are corrosion resistant and are often chosen on that account. Copper is used for electric wire, tubing for water supply and for flashing. Aluminium is most commonly used for corrugated roofing sheets, gutters and the accompanying nails. Using nails of the same material avoids the problem of corrosion due to electrolytic action. Brass is a corrosion resistant alloy of copper and zinc which is used extensively for building hardware.

Figure 3.35 Sisal-cement plastering technique.

Corrosion

Air and moisture accelerate corrosion in ferrous materials unless they are protected. Acids tend to corrode copper while alkalies such as found in animal waste, Portland cement and lime, as well as some soils, will cause rapid corrosion of aluminium and zinc. Electrolytic action caused by slight voltages set up when dissimilar metals are in contact with each other in the presence of water also encourages corrosion in some metals. Aluminium is particularly subject to electrolytic corrosion.

Corrosion can be reduced by carefully selecting metal products for the application; reducing the time that the metal will be wet by preventing condensation and promoting good drainage, avoiding contact between dissimilar metals, and by using corrosion-inhibiting coatings.

Corrosion Inhibiting Coatings

Copper, aluminium, stainless steels and cast iron tend to form oxide coatings that provide a considerable amount of self-protection from corrosion. However, most other steels require protective coatings if they are exposed to moisture and air. Methods used include zinc coating (galvanizing), vitreous-enamel glazing and painting. Painting is the only method practical for field application, although grease and oil will provide temporary protection.

Before painting, the metal surface must be clean, dry and free of oil. Both bituminous and oil-based paints with metallic-oxide pigments offer good protection if they are carefully applied in continuous layers. Two to three coats offer the best protection.

Building hardware

Nails

A nail relies on the grip around its shank and the shear strength of its cross-section to give strength to a joint. It is important to select the right type and size of nail for any particular situation. Nails are specified by their type, length and gauge (the higher the gauge number - the smaller the shank diameter). See Table 3.18. Most nails are made from mild steel wire. In a corrosive environment galvanized, copper-plated, copper or aluminium nails are used. A large number of types and sizes of nails are available on the market. The nails most commonly used in farm building are:

Round plain-headed nails or round wire nails are used for general carpentry work. As they have a tendency to split thin members, the following rule is often used: the diameter of the nail should not exceed 1/7 of the thickness of the timber.

Table 3.18 Dimensions and Approximate Number per Kilo of Commonly Used Sizes of Round Wire Nails

Length

Diameter Approx.
Inches mm mm no/ kg
6 1 50 6.0 29
5 125 5.6 42
4 100 4.5 77
3 75 3.75 154
2.5 65 3.35 230
2 50 2.65 440
1.5 40 2.0 970
1 25 1.8 1 720

Lost-head nails have a smaller head which can be set below the surface of the wood. Their holding power is lower because the head can more easily be pulled through the wood.

Panel pins are fine wire nails with small heads used for fixing plywood and hardboard panels.

Clout or slate nails have large heads and are used for fixing tiles, slates and soft board. Felt nails have even larger heads.

Concrete nails are made from harder steel, which allows them to be driven into concrete or masonry work.

Staples are U-shaped nails with two points and are used mainly to fasten wires.

Roofing nails have a square twisted shank and a washer attached to the head. Roofing felt or rubber may be used under the washer to prevent leakage. The nail and the washer should be galvanized to prevent corrosion. They are used for fixing corrugated sheet materials and must be long enough to go at least 20mm into the wood. Alternatively wire nails with used bottle caps for washers can be used.

Figure 3.36 Types of nails.

Screws and Bolts

Wood screws have a thread which gives them greater holding power and resistance to withdrawal than nails and they can be easily removed without damage to the wood. For a screw to function properly it must be inserted by rotation and not by being driven with a hammer. It is usually necessary to drill a pilot hole for the shank of the screw. Screws made of mild steel are normally preferred because they are stronger. A wide range of finishes, such as galvanized, painted and plated, are available.

Screws are classified according to the shape of their head as countersunk, raised, round or recessed (not slotted across the full width). Coach screws have a square head and are turned with a spanner. They are used for heavy construction work and should have a metal washer under the head to prevent damage to the wood surface. Screws are sold in boxes containing a gross (144 screws) and are specified by their material, finish, type, length and gauge. Unlike the wire gauge used for nails, the larger the screw gauge number, the greater the diameter of the shank.

Bolts provide still stronger joints than either nails or screws. As the joint is secured by tightening the nut onto the bolt, the load in most cases becomes entirely a shear force. Bolts are used for heavy loads such as at the joints in a gantry hoist frame, the corners of a ring beam installed for earthquake protection or to secure the hinges for heavy doors. Most bolts used with wood have a rounded head and a square shank just under the head. Only one spanner is required for these 'coach' bolts. Square head bolts, requiring two spanners, are also available. Washers help to prevent the nuts from sinking into the wood.

Figure 3.37 Types of wood screws and bolts.

Hinges

Hinges are classified by their function, length of nap and the material from which they are made and come in many different types and sizes. Hinges for farm buildings are mainly manufactured of mild steel and provided with a corrosion-inhibiting coating. The most common types are:

Steel butt hinge is commonly used for windows, shutters and small doors, since it is cheap and durable. If the pin can be removed from the outside it is not burglarproof. The flaps are usually set in recesses in the door or window and frame.

The H-hinge is similar to the butt hinge but is usually surface mounted.

The T-hinge is mostly used for hanging match-boarded doors. For security reasons the strap of the T-hinge should be fixed to the door with at least one coach bolt, which can not be easily unscrewed from the outside.

The band-and-hook hinge is a stronger type of Thinge and is used for heavy doors and gates. This type is suitable for fabrication at the site or by the local blacksmith.

Figure 3.38 Types of hinges.

Table 3.19 Conversion of Screw Gauge to Millimetres

Locks and Latches

Any device used to keep a door in the closed position can be classified as a lock or latch. A lock is activated by means of a key whereas a latch is operated by a lever or bar. Locks can be obtained with a latch bolt so that the door can be kept in a closed position without using the key. Locks in doors are usually fixed at a height of 1050mm. Some examples of common locks and latches used in farm buildings are illustrated in Figure 3.39.

Figure 3.39 Types of locks and latches.

Glass

Glass suitable for general window glazing is made mainly from soda, lime and silica. The ingredients are heated in a furnace to about 1500 C and fuse together in the molten state. Sheets are then formed by a process of drawing, floating or rolling. The ordinary glazing quality is manufactured by drawing in thicknesses ranging from 2 to 6mm. It is transparent with 90% light transmission. Because the two surfaces are never perfectly flat or parallel there is always some visual distortion. Plate glass is manufactured with ground and polished surfaces and should be free of imperfections.

Glass in buildings is required to resist loads including wind loads, impact by persons and animals and sometimes thermal and other stresses. Generally the thickness must increase with the area of glass pane. Glass is elastic right up to its breaking point, but is also completely brittle so there is no permanent set or warning of impending failure. The support provided for glass will affect its strength performance. Glass should be cut to give a minimum clearance of 2mm all around the frame to allow for thermal movements.

Plastics

Plastics are among the newest building materials, ranging from material strong enough to replace metal to foam-like products. Plastics are considered to be mainly organic materials derived from petroleum and, to a small extent coal, which at some stage in processing are plastic when heated.

The range of properties is so great that generalizations are difficult to make. However, plastics are usually light in weight and have a good strength to weight ratio, but rigidity is lower than that of virtually all other building materials, and creep is high.

Plastics have low thermal conductivity and thermal capacity, but thermal movement is high. They resist a wide range of chemicals and do not corrode, but they tend to become brittle with age.

Most plastics are combustible and may release poisonous gases in a fire. Some are highly flammable, while others are difficult to burn.

Plastics lend themselves to a wide range of manufacturing techniques, and products are available in many formssolid and cellular, from soft and flexible to rigid, from transparent to opaque. Various textures and colours (many of which fade if used out-of-doors) are available. Plastics are classified as:

Thermoplastics which always soften when heated and harden again on cooling, provided they are not overheated.

Thermosetting plastics which undergo an irreversible chemical change in which the molecular chains crosslink so they cannot subsequently be appreciably softened by heat. Excessive heating causes charring.

Thermoplastics

Polythene is tough, water- and oilproof and can be manufactured in many colours. In buildings it is used for cold water pipes, plumbing and sanitary ware and polythene film (translucent or black). Film should not be unnecessarily subjected to prolonged heat over 50C or to direct sunlight. The translucent film will last only one to two years exposed to sunlight, but the carbon pigmentation of the black film increases resistance to sunlight.

Polyvinyl chloride (PVC) will not burn and can be made in rigid or flexible form. It is used for rainwater goods, drains, pipes, ducts, electric cable insulation, etc.

Acrylics, a group of plastics containing, polymethyl methacrylate, transmit more light than glass, and can be easily moulded or curved to almost any shape.

Thermosetting Plastics

The main use of thermosetting plastics in buildings is as impregnants for paper fabrics, binders for particle boards, adhesives paints and clear finishes . Phenol formaldehyde (bakelite) is used for electrical insulating accessories. Urea formaldehyde is used for particle board manufacture.

Epoxide resins are, for most uses, provided in two parts, a resin and a curing agent. They are extremely tough and stable and adhere well to most materials. Silicone resins are water repellent and used for waterproofing in masonry. Note that fluid plastics can be very toxic.

Rubber

Rubbers are similar to thermosetting plastics. In the manufacturing process a number of substances are mixed with latex, a natural polymer. Carbon black is added to increase strength in tension and to improve wearing properties.

After forming, the product is vulcanised by heating under pressure, usually with sulphur present. In this process the strength and elasticity is increased. Ebonite is a fully vulcanised, hard rubber.

Modified and synthetic rubbers (elastomers) are increasingly being used for building products. For example unlike natural rubbers they often have good resistance to oil and solvents. One of them, butyl is extremely tough, has good weather resistance, excellent resistance to acids and a very low permeability to air. Synthetic rubber fillers and nail washers are used with metal roofing.


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