At the very least, the site must be big enough for the building being planned plus at least some room around the building for the storage of construction materials during the construction. Generally, about 2 meters along each side of the building is the minimum builders will need. In urban areas, extra land may be required for health, safety, transportation, or other needs.

A major note of caution here: a site can only be truly adequate if it leaves enough room for present and future needs beyond the space taken by the main building being planned. These needs will be different for different types of buildings.

Here are some of the things planners might want to leave space for:

Schools

- a shaded outside area for assemblies, lunches, outdoor classes

- recreation area for sports, play

- future classrooms

- other additions such as offices, library, theatre, school garden

- compost pile

- livestock/poultry pens

- two or more latrines at least 17 meters from water supply

- cistern/well

- lodging for teachers

- storage for school and maintenance supplies

Health Clinics

- future additions for extra reception and treatment rooms

- cooking area for patients who must stay in clinic

- latrines

- well

- lodging for health workers/ doctors

- separate building for maternity care and recuperation

Homes

- outdoor latrine

- garden

- livestock/poultry pens

- future additions to the home

- well

Many of these needs may appear farfetched to the family or community when they first plan their building project. However. the field worker should urge them to allow room for as much future expansion as possible. It is always easier to add to a present building and site than to start over again because there is no room at the first site.

Even in cases where the family or community cannot afford a large enough site for all future needs, they con consider what land next to their site might be obtained for later expansion.

In addition to the actual size of the site, planners need to consider other conditions at the site that might block future expansions:

Whether these obstacles are on the projected site itself, or on the land next to it, care should be taken that they will not block the builders' present or future needs.

A building can only be easy to build and easy to use if the site is accessible; that is, if it is conveniently close to other people and places in the community and if it can be approached easily.

At the same time, a building can only be comfortable for those who use it if the site provides enough privacy to satisfy their needs. Privacy can also be important for health: for example, dirt, dust, and smells can cause serious problems if a building is too close to a heavily travelled road.

Because the need for accessibility often conflicts with the need for privacy, planners often have to compromise between them.

Here are the major considerations that may have to be balanced:

Access for Construction. Unless the materials for construction are available on the site itself, some or all of them will have to be brought to the site in large amounts. Generally, this means the site should be near a road, or in an area where a road to the site can be built. In addition, there must be space on the site to store materials during construction.

Access for Use. To serve its purpose well a building must be easy for people and supplies to get to. For different kinds of buildings different considerations should be kept in mind. For example: Schools should be

- within reasonable travel distance (by foot and/ or vehicle);

- close to clean water;

- close to fuel supplies in cold climates;

Clinics should be

- centrally located so that the community can reach them easily;

- close to clean water and fuel supplies;

- next to a roadway suitable for vehicles;

Homes should be

- close to clean water for cooking and washing;

- close to neighbors;

- close to fuel supplies;

- close to the family's fields or other places where the family earns income;

- close to markets;

- close to community facilities.

Privacy, Comfort, Health, and Safety. All building sites should be:

- away from rivers and streams heavily infested with mosquitoes, or other sources of disease;

- away from forest, bush, or jungle (all these cut off breezes, are dangerous in case of fire, and provide homes for snakes, rats, ants, and other pests);

- away from major roads and other sources of noise, dust, dirt, distraction, smells (this is especially important for schools and clinics);

- away from flood-prone areas;

- away from fault lines in earthquake areas (see section on earthquake areas, page 195.

Possibly the most important consideration in choosing any site is that the soil must be able to support any building(s) erected on it. Almost any ground can be built on (or over), but the nature of the site chosen will affect the simplicity or complexity of the foundation, the cost of construction in labor and money, and the durability of the building.

Each type of soil has very definite limits to the weight it will support. Generally speaking, 1-story buildings will be well within those limits except in the case of buildings with rock walls on the weaker soils.

When a tentative site has been selected, it's a good idea to double check that the soil at the site will support the planned building safely. Quite often this can be done simply by looking at similar nearby buildings (2 or more years old) on the same kind of soil. If they show signs of uneven settling such as cracks in the walls or foundations, the side of the new building may have to be dug out and filled with firmer soil, or another site may have to be selected.

In cases where there are no similar buildings nearby, or no similar soils, an estimate of the planned building's weight per square meter should be made and compared with the weight-bearing capacity of the site. Detailed directions for calculating the weight of a building, plus a guide to the weight-bearing capacity of various soils will be found in Appendix 1.

Almost as important as the type of soil at the site is the uniformity of the soil. If the ground at the site contains a mixture of different soils, it must be dug out and filled in until it is uniform in order to provide a stable support for any building.

If the ground at the site cannot be made uniform, or if the weight of the planned building will be close to the weight-bearing capacity of the site, it may be necessary to use reinforced concrete in the foundation.

In addition to the weight the site can support, builders need to consider how well the land is drained. Water seepage can destroy even the strongest foundation if it is not controlled or planned for. Wet or damp floors can make a building uncomfortable and unhealthful.

Whenever possible, marshland or any ground that lies underwater for extended periods of time should be avoided for construction purposes. In some areas, people have developed methods for construction on stilts because they have no access to dry land, or because they depend on access to the water for economic support. Buildings in these areas should be made of light materials and should be replaced often.

Low-lying land, and land that may be exposed to short-term flooding is also less than desirable for construction purposes. When there is no choice, however, this kind of land can sometimes be improved by digging a trench about 25 cms. wide and 10-15 cms. deep around those sides of the building site that won't drain. These trenches should lead far enough from the planned position of the building to keep water from rising to the foundation depth.

If the dark topsoil in low-lying areas is more than 30-45 cms. deep, every effort to find a different site should be made, since removing this topsoil for a firm foundation will be expensive and can make effective drainage much more difficult.

High, dry ground is usually the best for building sites since it is not subject to the ill-effects of water seepage. However, ground can be too high. Planners need to avoid or guard against these dangers:

The ideal site for any building is on dry soil safe from flooding and sheltered against ground slippage, strong winds, and storms.

After selecting a site, the builder needs to decide where on the site the building or buildings planned will be. The position that is chosen should ideally:

The most important factors community groups and families should consider in choosing a site for their building are:

In most cases, builders have to compromise between these concerns. Choosing the best site among several less-than-ideal possibilities is often difficult and complex. It is always best to get experienced advice, whenever there is any doubt. Contractors, engineers, or architects in the area can help make the options clear, especially in terms of cost and labor.

One Final Caution. In many areas, local regulations will influence what can be built, where it can be built, and how it can be built. Building and zoning codes, permits, licenses, and accepted practices vary widely from country to country and locality to locality. To avoid wasted or illegal effort, the field worker should help family or community members learn their restrictions and obligations during the basic planning and design process. This should always be done before settling on a definite site for construction.

Rammed earth is a combination of sand, silt and clay that when properly pressed and dried is suitable for walls in dry climates. (wet climates, too, when protected with lime plaster.) If the sand, silt, and clay are locally available, rammed earth and adobe blocks are two of the least expensive materials to use.

Basically, the ingredients are mixed with water in this ratio by volume:

The amount of water needed varies considerably and must be determined by making several tests. There should be enough to make a mud that holds its shape but can still be molded,

Once the ingredients are mixed thoroughly, they are pounded with a thin pole into a portable mold (form) 2/3 to 1 meter high until they are packed solid.

The mixture is then allowed to cure (dry) thoroughly. When one section is ready, the mold is removed and used to form the next section

Adobe bricks are individual solid blocks made from the same mixture of ingredients as rammed earth.

A simple form - or mold - is used to shape the bricks, which must be dry for at least a month before use. Because of the drying time needed, adobe bricks can only be used where there is a long season of hot dry weather.

Construction with either rammed earth or adobe is inexpensive and highly labor intensive. This makes both materials especially good in low-income communities that can contribute volunteer labor

Rammed earth and adobe should never be used underground for foundations, since underground moisture will eventually weaken them. Both materials are excellent for walls, however, and can withstand rainfall if protected with lime plaster,

In earthquake areas, rammed earth and walls must be reinforced with wood, wire, or bamboo.

Finally, rammed earth and adobe structures cannot support heavy roof loads without strong wood or iron reinforcement. Light roof materials, such as thatch or bamboo, are the best to use with either rammed earth or adobe brick walls.

Wood is one of the most versatile and durable construction materials. It is easy to work with and can be used to build almost any part of a structure. Due to deforestation of many countries, however, wood is too scarce and expensive for use by most low-income communities as their main construction material. Field workers in countries like Thailand or Malaysia where wood is still plentiful may wish to consult one of the many excellent books describing wood frame construction listed in Appendix 7.

Over the long-term, projects to re-plant forest areas as a replenishable source of wood for construction (and fuel) are highly desirable. But most communities now will only be able to afford wood as a secondary material for:

The best timbers to use are generally those that are hard and have some resistance to decay, rot, and termite attack, such as:

Some softer woods, such as Eucalyptus Pine, are also quite good. When in doubt, a survey of nearby buildings may help to determine which local timbers have proven successful.

Bamboo is an excellent material for almost any part of a building except the foundation. Where available it is usually very inexpensive. Its light weight makes it very easy to work with-generally easier than wood.

Bamboo must be used with care, however, because it cannot support great loads or excessive dampness or rain. Because it decays in water, it cannot be used underground for a foundation.

Since construction with bamboo is very different from construction with other materials, it is covered in a separate section beginning on page 181.

Piles of stone and rock can be used to make strong foundations and walls, especially if used with a cement-based mortar to hold them firmly together.

Stone and rock have several disadvantages, however:

• if not locally abundant, they are expensive to obtain;

• walls made with rock or stone have many uneven surfaces. Filling in the large cavities in order to made inside walls smooth is time consuming and expensive;

• walls of rock/stone must be thicker than cement block walls if they are to be equally strong;

• building to precise measurements is difficult or impossible; builders planning to use rock/stone should plan slightly larger rooms and construction sites to ensure that the finished building will be large enough.

Cement is an adhesive material that "bonds" or glues objects such as rocks or grains of sand together so that they form a strong permanent piece. A useful cement can be made locally from finely ground limestone. But commercial cements also contain silica, alumunum, and iron oxide. These materials ensure that the mixture will contain a variety of grain sizes. The more different grain sizes there are' the stronger the cement will be.

Because of its bonding action, cement is used as an ingredient in many construction materials including:

Each of these materials is discussed below.

Generally, cement is used two ways:

- as an ingredient in mortars, (pastes used to bond bricks, blocks, or stones, and to plaster walls);

- as an ingredient in concrete.

Mortar is a general term for any mixture of cement with sand and either lime or clay. The ingredients are mixed in varying proportions with water to form a paste that can be used:

- to bond bricks, blocks, or rocks;

- as a wall finish (plaster, stucco).

The tables in Appendix 5 give recommended proportions for different mortar uses.

Concrete is a mixture of sand, gravel, and water that is held together and given strength by cement.

The strongest concrete has particles ranging from very fine sand to gravel of 3.75 meters across. Some builders and manufacturers refer to the sand as fine aggregate, and the gravel as coarse aggregate.

Before concrete can be poured, wood forms must be carefully built to hold it in the exact size and shape desired. When the formwork is ready, plain wet concrete is mixed, poured in the forms, and allowed to cure (dry) for a few days to a few weeks (depending on the weather and type of construction!.

Plain poured concrete can be used to build:

• foundation footings (pads of concrete that distribute a building's weight over a wide surface);

• blocks for foundations and walls;

• floors.

The proportions of cement, sand, and gravel vary according to the planned use of the concrete. But these proportions are always referred to in the same way by builders. For example, a formula often used for foundations, 1:2¼:3, means concrete composed of 1 part cement, 2¼ parts sand, and 3 parts gravel. A builder using this formula would need 2¼ wheelbarrows of sand and 3 wheelbarrows of gravel for every wheelbarrow of cement. In addition, he/she would need approximately 23 liters of water to be added to the mixture.

Reinforced concrete combines two materials with opposite characteristics:

• plain, poured concrete: resists downward pressure (compression), but will not bend; it cracks instead;

• iron rods: will bend (they have tension strength), but they will buckle under compression,

Reinforced concrete is prepared and handled as plain concrete is, except that an iron rod, or a series of rods, is fastened inside the form before the concrete mixture is poured in. (In some cases, bamboo stalks can be used for reinforcement in place of iron rods. See page 188).

Reinforcing concrete either with iron or bamboo is fairly easy and can multiply the strength of the concrete 2 to 5 times in:

A variety of blocks can be made by combining cement with other ingredients. All these blocks are useful for foundations and walls and can be produced locally if the proper ingredients are available. They can either be made by hand or with a simple press. A press is generally more efficient and produces a stronger, more tightly-packed block than can be made by hand.

Whether made by hand or press, the blocks can be solid or hollow, depending on the mold chosen. Hollow blocks use considerably less material, reduce weight, and improve insulation. They are not as strong as solid blocks. But in sections of a building where additional strength is required, the hollow parts can easily be filled with poured concrete or other reinforcing materials.

TWO BASIC BLOCK-MAKING PROCEDURES

By hand:

#1 Gather and Mix Ingredients

#2 Pour Mixture in a Mold

#3 Pack Mixture Tightly in Mold with Shovel

#4 Add More Mixture and Pack Again

#5 Remove Mold

#6 Cure (Dry)

By Hand-press.

#1 Gather and Mix Ingredients

#2 Pour Mixture in a Mold

#3 Pack Mixture Tightly in Mold with Press

#4 Add More and Pack Again

#5 Eject Block from Press

#6 Cure (Dry)

Concrete blocks are excellent for foundations and walls. They are made from a mixture of 1 part cement, 2 parts sand and 4 parts gravel. As manufactured commercially they:

• are usually among the strongest blocks available (in fact, they are stronger than necessary for 1-story buildings);

• have excellent insulating qualities;

• are the most uniform blocks (important for accurate, durable construction).

However, they are by far the most expensive block a community could use.

Concrete blocks can be made locally if limestone (or commercial cement) sand, and gravel are available in large quantities near the site. The process is simple, but timeconsuming. Generally, the blocks must be made about a month before they can be used for construction.

Sand-cement blocks are strong blocks made of 1 part cement to 6-8 parts sand. They cannot be used to support roof loads without some reinforcement, but are more than adequate for 1-story buildings.

Because of the high proportion of sand needed, sand-cement blocks are only practical for construction where there is a lot of sand (sand-cement blocks are not generally made commercially). But if the sand is available, sandcrete block-making is a quick and efficient process. Sandcrete blocks only take 12 days to cure (dry), less than half the time required for concrete blocks.

These are blocks in which clay soil, rather than sand, is mixed with either cement or lime. The amounts of cement and soil needed for a good mixture can only be determined by testing how much the soil shrinks when dried (the procedure is simple and can be followed easily by any community group).

The blocks are formed in the same way as sand-cement blocks and can be used for all the same purposes. They are especially suited for low-cost construction in areas where sand is not available in large-enough quantities for sandcrete. However, great care must be taken to keep organic material out of the clay soil used in the mixture. Otherwise the mixture will not form a strong bond. (This too can be done with a simple test of local soil samples. See page 146).

Stabilized earth blocks can be used after only 8 days of curing.

In surveying locally available materials, it is useful to keep in mind that:

The orientation of a building is the direction its front walls face: north, east, south, west or something in between.

To mark the orientation on the site, first find north. This is easiest with a compass. But if no compass is available, finding north is still fairly simple.

The builder should stand at the building position so that his/her right arm points at the spot where the sun rose in the morning while the left arm points to where the sun set the last night. In this position the builder will be facing north.

Standing in the same position, if the builder moves his/her arms so they form a straight line to the sides, they will be pointing due west (left arm) and east (right arm).

Once the main compass points have been established, use a stick to mark out the shape and position of the building. Add about 2 meters to the dimensions of the building on all sides: the extra space will be needed for supplies and work space during construction.

Next, clear the ground inside this area of trees, shrubs, and loose plant growth. If the topsoil is loose, clear away the top 15cm or so to get down to hard earth: loose soil will not support a building.

If clearing the land at the position outlined proves too difficult, or if it would require felling trees that are desired for their shade or beauty, consider moving the building a short distance before clearing the land.

The next step is to mark the outline of the foundation on the site using string and pegs.

Three measures are very important:

* the length of each wall must be marked exactly;

* the string must be exactly level;

* the corners must be square: exactly 90°

The length of each wall is easy to set out on level ground. Simply measure it with a tape measure, making sure to pull the measure tight.

When the site is on uneven ground, care must be taken to measure the length of the wall along a level line: following the slope of the ground will throw off the measurements.

To prevent errors, begin at the highest end of the first wall's length and attach the string to a peg planted at the end-point. Set a new peg every 2 meters to prevent the string from sagging, and test to make sure the string is level by using a plumb bob and mason's square as shown: when the plumb bob stops swinging it will be plumb (straight up and down); the string will be exactly level when it is a 90° to the bob string.

Repeat this process until the length of string set out is equal to the planned length of the first wall.

If the ground is sloped very steeply, it may be easier and more accurate to measure the wall's length in steps or stages. To do this run a new length of string from the bottom of every second peg (that is, every 4 meters). Test with a plumb bob to be sure that every peg is straight up and down. In addition, to be sure there are no gaps or overlaps in the measurement of each new stage, plant a small block with a circle on it right next to every other peg: then in measuring the next stage (or step) along the wall, hold the tape measure to the point in the circle that marks the end of the last stage.

Once the first wall has been set out, the second wall should be set out at right angles to it (90°). There are several ways to make sure that this angle, and the angles between all the walls set out are square:

• A mason's square is good for distances up to 3 meters. Use it to get started, but don't use it to check the entire outline.

• One of the easiest and most accurate methods of checking large distances for squareness is to compare the diagonals. Simply measure the diagonal lines from opposite corners of the foundation. When they are exactly equal in length, all the angles will be 90°. On uneven ground, be sure to use the tape measure along a level line'

• If measuring the diagonals is inconvenient, or if the building is not a simple rectangle, another method is the 3-4-5 calculation:

If the wall on one side of a right angle is 3 units long and the wall on the other side is 4 units long, a line drawn between their ends will always be 5 units long. Here's how to use this rule to test whether the angle between two walls is square: Along the string set out for one wall, measure and mark off 1.5 meters (3 x .5m). Then, along the string set out for the second wall, measure and mark off 2 meters (4 x .5m). Next, measure and cut a piece of string 2.5 meters long (5 x .5m) and hold its ends to the points marked off along each wall. When the 2.5 meter string just touches both marks with no slack left over, the angle between the two wall strings will be exactly square. If the 2.5 meter string is too short or long, adjust either one of the wall strings until it fits exactly.

MARKING THE INSIDE FOUNDATION LINES

Once the outside walls of the foundation have been set out with pegs and string, the next step is to set out a second set of lines to mark the inside of the foundation walls. Use the same procedures to keep them level, accurate in length, and square.

The easiest way to begin is to measure the width of the foundation wall and mark it along the strings set out for two opposite walls. Then set out a string between these two points and fasten the ends with pegs. Use a plumb bob to be sure each peg is directly beneath the outside foundation line.

Next, repeat this process for each of the remaining walls. Place a peg at every point where the new lines cross: these are the inside corners of the foundation walls.

Batter boards are like a single rail fence placed around the building position. They should be placed after the foundation lines have been set out and before any digging for construction begins. Batter boards are essential for two reasons:

• They provide permanent reference points for the position of the foundation walls: these will be needed once the string markings have been removed during construction;

• They can be used to measure and mark off the exact position of doors, windows, and floors, thus making it much easier to build the walls accurately.

To be useful, batter boards must be exactly level and should be set about 1 meter back from the string marking the outside foundation line.

PLACING BATTER BOARDS

To place batter boards around the building postion, first find the highest point on the site about 1 meter outside the outer foundation line and place a wooden stake firmly in the ground.

Nail the first board to this stake so that the top of the board is at least as high as the top of the foundation walls will be. Since the foundation walls must be as high as the floor, this will be at least 20-3Ocm. above the ground.

Next, place a second stake 2 meters from the first and 1 meter outside the outer foundation line. When this stake is secure, nail the other end of the first batter board to it, taking care that the batter board is level. Nail a second board to this same stake and repeat the process of placing new stakes, levelling, and nailing the boards, until the batter boards form a fence that goes completely around the building position.

HOW TO LEVEL BATTER BOARDS

A commercially made "spirit level" is the easiest levelling device to use. Place it on the surface - in this case the top edge of the batter board - and keep adjusting the height of the board at the opposite end until the bubble is in the center.

When joining boards at stakes, place the level over the seam of the board to maintain a continuous level line.

An additional check on level can be made at the corners: use an extra board to provide a surface for the level.

Another method is to use a water level: this is a clear plastic hose that is filled with water, Adjust the first end of the hose to a known level mark. When the other end of the hose is placed at the next stake, the water inside will be at the same level as at the first end. To prevent spillage the ends may be plugged until the hose is in position, But all plugs must be removed to get an accurate measurement.

TRANSFERRING MARKS TO BATTER BOARDS

Once the batter boards are complete, all the measurements shown on the written foundation plans should be marked on the boards.

First, transfer the foundation lines marked by the strings set out earlier: run a string from one board to the opposite board directly over each foundation line. Use a plumb bob to make sure this new string is over the corner pegs.

Next, place nails or saw cuts in the tops of the boards where they intersect the strings.

A good system is to use different marks for the foundation lines than those to be used for other important measures such as wall lines and the positions of doors and windows. For example, use small notches cut with a saw to indicate the foundation lines. Then use nails to indicate the wall lines.

Once the foundation lines and wall lines have been marked on the batter boards, the builder is ready to start digging the foundation trenches. Note: the digging will be easier if the strings are left in place until the trenches have been well started.

Before the concrete footing can be poured, the builder must dig a trench for it. The depth of the trench depends on characteristics of the site and building that should have been determined in the basic planning stage (see pages 82-88),

However, the width of the trench, and the care with which it must be dug depend on whether the concrete footing will be poured directly in the trench or into wooden forms.

TRENCHES FOR FOOTINGS POURED WITHOUT FORMS

When wooden forms will not be used, the trenches must be dug very carefully:

• the walls of the trench must be plumb (straight up and down);

• the earth forming the trench walls and bottom must be very firm so that there is no danger of the trench crumbling when the concrete is poured;

• the earth must be removed carefully so that the earth at the bottom of the trench is smooth, clear of all trash or debris, and undisturbed (undisturbed means that the earth has not been moved, loosened, or dislodged by the digging);

• the bottom of the trench must be absolutely level: 6-8 measurements with a straight board and level should be made along the length of the trench to be certain of the level;

• the trench must be exactly as wide as the concrete footing;

• the trench must be free of standing water.

TRENCHES FOR FOOTINGS POURED WITH FORMS

It is much more common to use wooden forms when pouring the concrete footing because the ground at building sites is rarely hard enough to stay in place when the concrete is poured directly in the trench.

Like the trenches for non-form footings, trenches in which forms will be used must be level at the bottom, and hard. The earth at the bottom must also be left smooth, clear of debris, and undisturbed.

However, there is no need to make the side walls of the trench plumb. The only requirement is that the trench be 30-45 centimeters wider than the footing will be, so that there will be room in the trench for the wooden forms and work space for the builders.

A FINAL NOTE ON DIGGING TRENCHES

When digging the trenches, any loose soil, or soil with organic material should be shovelled outside the foundation lines and discarded.

Hard earth that is dug up should be spread evenly inside the foundation lines: it will be very useful as part of the fill under the floor.

The formwork should be made of boards measuring about 5cm x 15cm or 5cm x 30cm. The boards should be supported by 5cm x 10cm braces called studs. Allow about twice the width of the footing between the studs (but never more than 1.3 - 1,6 meters); the formwork must be strong because concrete weighs 2.5 metric tons per cubic meter.

In addition:

• the joints and corners of the formwork must be tight: neither the concrete, nor the water should be able to leak out. If water leaks out it will weaken the mixture;

• the walls of the forms must be exactly plumb (straight up and down), and the tops of the forms must be level: any inaccuracies will weaken the foundation;

• small boards, called spacers, should be nailed to the tops of the formwork to keep the sides of the forms from leaning toward each other before the concrete is poured;

• the formwork should be built so that it may be removed easily without damage: most forms can be re-used;

• if part of the footing will be visible and the builder would like its surface to be smooth, the boards of the formwork must also be smooth: any irregularities in the wood - burrs, bends, chips, and so on - will leave an impression in the concrete.

FORMWORK FOR A STEPPED FOUNDATION

The formwork for a stepped foundation is constructed in the same manner described above. However, if the walls will be made of block or brick, the height of each step of the footing must be equal to the height of the blocks used, or to a whole number multiple of their height. For example, if the blocks used in the foundation are 20cm high,: then the steps must be 20cm or 40cm or 60cm high, and so on.

To prepare a strong, durable concrete mixture, the builder must:

SELECTING INGREDIENTS

Cement

The commercial cement most commonly used for 1-story buildings is Portland cement. It is available from most manufacturers. Portland cement, and most locally made limestone cements, can be stored for up to 6 months in a cool, shaded, dry , area. If the cement has hard lumps that are difficult to break or crumble, don't use it.

In humid tropical areas, or areas with heavy rainfall, it may be advisable to purchase "hydrophobic" cement, since this kind of cement can be stored in damp conditions for long periods of time.

Sand

The sand is possibly the most important ingredient in the concrete mix, since it provides most of the plasticity: that is, it makes the mixture easy to work with. There should never be more than 3 parts sand for every 1 part cement. Too little sand in a mixture may result in shrinkage cracks when the concrete dries; too much sand will produce a harsh mortar that will be difficult to work with and may lead to leaks.

Never use sand from the ocean: the grains in ocean sand are too uniform in size and will not make a useful concrete. Any other source of sand is fine. But it is crucial that the sand used be clean.

To test whether sand is clean enough to be used:

Gravel

Gravel adds strength to concrete because it increases the range of grain sizes in the mixture. Generally it is also the easiest ingredient to find and prepare locally.

Gravel should be clean, and should range in size from 6-18mm.

MIXING THE BEST PROPORTIONS OF INGREDIENTS

The concrete mixture required for a foundation footing may be very different from the mixture needed for concrete used for another purpose.

For example, the average mixture for foundation footings is 1:2 3/4: 4 (that is, 1 part cement, 2 3/4 parts sand, and 4 parts gravel).

But a typical mixture for fence posts is 1:1:1½. The higher proportions of sand and gravel in the footing mixture provide maximum strength and resistance to water. Since fence posts do not need to be as strong or as water resistant as footings, they can be made of a weaker and less expensive mixture.

Table 4 in Appendix 5 recommends average mixtures for different uses of concrete. Following these recommendations will assure builders that their concrete will be well-suited to their needs.

MIXING PROCEDURE

Mixing Area

Concrete should be mixed on a flat surface that will not absorb water. Mix it on a wooden platform about 2 meters by 2 meters. Or, mix it on a cement-sand platform.

Cement-sand platforms must be made a week before concrete can be mixed on them. To build a platform, mix cement and sand on the ground in a 1:6 ratio, cement to sand. Add water, and spread the mixture in a circle about 2 meters in diameter.

Choosing A Measuring Unit

Since the dry ingredients of concrete, cement, sand, and gravel, are mixed by volume, the easiest way to be sure that the proportions are right is to decide on a standard measuring unit such as a box or wheelbarrow.

Once the unit is chosen, simply fill it to the appropriate level with each ingredient in turn. For example, to make the recommended mix for foundation footings, 1:2 3/4; 4, for each unit of cement, add 2 3/4 units of sand, and 4 units of -travel.

Generally, the easiest measuringunit to use is a gauge box or wheelbarrow that holds 1 bag of cement. The box or wheelbarrow should be approximately 40cm x 40cm x 40cm. Note: to avoid confusion, once the size of the measuring unit has been chosen, do not use different size boxes or wheelbarrows on the site for any purpose.

When using a box:

Mixing the Ingredients

The ingredients in concrete should always be mixed in the same order:

FIRST, spread the correct amount of sand on the platform with a shovel.

SECOND, dump the required amount of cement on the sand, and mix the sand and cement until the color of the pile is uniform: there should be no streaks of color.

THIRD, add the required amount of gravel and mix it with the sand and cement until the gravel is thoroughly distributed and there are no streaks of color.

FOURTH, hollow out a hole in the center of the mix. Add water slowly, pushing the ingredients toward the center while turning them over with a shovel.

A good test for the correct amount of water is to walk on the mixture:

• if you sink to your ankles or higher, there is too much water;

• if you sink about 5-6cm, you have the right mixture.

The correct amount of water is important because too much or too little will weaken the mixture:

- if there is too little water, air spaces (weak spots) will form in the concrete;

- if there is too much water, a cement-water paste will rise to the surface, weakening the mixture underneath.

Fresh concrete mix has to be carried to the footing trench in water-tight containers such as wheelbarrows: no water should seep out of the mixture:

The strength and water-tightness of concrete improve with age. Proper "curing. is important to get the aging process started well.

As concrete hardens, the water in the mix gradually evaporates. ' Prolonging that process increases the strength and water-tightness of concrete.

Immediately after the concrete has set, put a wet covering over it. The covering may be made with wet burlap bags, or a thin layer of wet sand, or wet straw. The covering should be sprinkled with water 2 or 3 times a day for at least 7 days.

Reinforced concrete footings require the same formwork as unreinforced footings (see page 129). Steel rods are attached to the form before pouring. The rods should be made of steel about 9mm in diameter. They must be clear abd free of rust.

When placing the rods:

The recommended concrete mixture for reinforced foundation footings is 1:2½:4.

Footings may also be reinforced with bamboo. See the separate section on Bamboo, page 188.

Earth floors are easy to build. For drainage, simply place a layer of dry fill inside the area bounded by the foundation. The fill can be made of any material that will not hold water, such as gravel, rock, broken brick, or rubble. The top of the dry fill layer should be about 5cm below the top of the foundation walls.

The fill will be packed down as people walk over it during the rest of the construction process.

When the walls are complete, add a 10-15cm deep layer of hard earth and pack it down either with a shovel, or by walking on it. When the hard earth is well-packed, wet it down slightly, and sweep it out to complete the floor.

FILL

Like earth floors, concrete floors need a level of dry fill underneath for protection against ground moisture. Dump the fill inside the foundation walls so that it will be packed down by rain and people walking over it during construction of the walls. The fill should be composed of two layers: a bottom layer of dry fill: rock, gravel, broken brick, or rubble; and a top layer of hard earth. If dampness in the area is excessive, a 5-7 cm. layer of sand with a sheet of plastic (or other waterproof material on top) should be laid between the dry fill and hard earth.

The top of the hard earth should be about 1P cm. below the top of the foundation walls.

PREPARING TO POUR THE CONCRETE

Concrete floors can be poured at two stages in the construction process:

There is an advantage to the second alternative: since the floor and walls will very likely settle at different rates, the floor may develop cracks if it is connected to the walls. Another advantage is that the walls and doors provide easy protection for the floor while it is curing.

If the fill has been in place some time, check that the hard earth is at the correct level--about 10 cm. below the top of the foundation walls. If it has settled, add more hard earth and pack it down hard. Wet it and then pound it with the end of a steel rod.

Use a straight edge and a level to check that the surface is reasonably level before the concrete is poured.

Pound 6 or 8 wood stakes (about 5 x 10 cm.) into the fill so that their tops will be even with the level of the finished floor (the same as the top of the foundation walls). These stakes will guide you during the pouring, but they must be removed before the concrete settles.

POURING THE CONCRETE

The best concrete mixture for floors is a 1:2½:3 mixture (1 part cement: 2½ parts send: 3 parts gravel). Mix the cement as described on pages 132-135.

Then, begin pouring in the corner farthest from where you will finish and work back toward a door so you can get out without walking on the fresh concrete. Pour the concrete at the full thickness-10 cm - don't layer it.

As the area covered grows, begin to level the concrete off with a "screed"--a board about 5 x 10 cm and several feet long that is swept across the surface to keep the mix even with the tops of the guide-stakes.,

Then, before the concrete sets:

Using thin boards to kneel on or walk across the surface without marring the fresh concrete, "finish" the floor by sweeping a trowel or wooden "float" across the surface. A rectangular float is best--approximately 15 x 25 cm. The sweeping action will bring a smooth cement-water paste to the surface of the floor. Be careful not to bring up too much water: if water collects in small pools, the surface of the concrete will be too weak when dry.

After the floor has started to harden, but while it is still workable, sweep the surface with a broom. This will give a rough texture to the finished floor and create a safe, non-slip, surface.

When the floor is finished and is left to cure, close the doors and windows to keep children and animals from walking on the surface.

The floor should cure for at least 4 days, and preferably 10. Keep the surface moist by sprinkling frequently Since so much area is exposed, the concrete can easily dry out too quickly.

Rammed earth walls are built in layers by packing a damp earth mixture into temporary forms, allowing each layer to dry slightly, and then moving the forms to repeat the process until the walls are as high as planned.