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close this bookGlobal Overview of Construction Technology Trends: Energy Efficiency in Construction (HABITAT, 1995, 210 p.)
close this folder3. Innovative technologies related to the increased utilization of low-energy building materials
View the document3.1. Soil construction
View the document3.2. Building stone, sand and aggregates
View the document3.3. Low-cost binders
View the document3.4. Timber and bamboo

3.1. Soil construction

Unlike energy-dependent building materials which consume predominantly thermal energy resources, the energy requirements for modern soil construction are mainly in the form of electrical and mechanical energy to run a variety of mixing, extrusion, ramming and compaction of soil blocks. In this context, it could be argued that soil construction, even in its most technologically-sophisticated form, is capital-intensive and hardly energy-demanding. There is a wide range of soil construction technologies that utilize capital items in one form or another. One important asset in these technological trends is that there are different scales of capital intensity and, by implication, energy expenditure, including technologies with no energy requirement. In fact, in most small-scale operations, every stage of the soil construction process is manual, utilizing simple tools, and the only energy requirement is related to transportation of materials. However, where appropriate soils are obtainable near the construction site, there is hardly any expense in transportation and, perhaps, this is the greatest potential in soil construction as an indigenous low-cost material (18).

The selection of building materials, in general should aim at providing the most desirable levels of climatic comfort. In countries with harsh weather conditions, especially where there are extreme seasonal or daily variation in temperature, climatic comfort and, by implication, the thermal characteristics of the building materials used, are as important as their durability.

Climate conditions of countries such as the Middle East and the Arabian Peninsula although, generally, hot and dry, tend to vary sharply between summer period and winter months, thus making climatic conditions of utmost importance in the choice of materials for low-cost housing.

In this connection, it should be mentioned that soil, in comparison to several other materials, has a relative advantage due to its good capacity to store heat. Soil has a latent inertia related to its absorption capacity and has a significant ability to delay thermal variations and external thermal inflows. These properties are valuable in areas, characterized by highly variable climatic and atmospheric conditions. Perhaps the greatest asset of soil in relation to its thermal performance, is the ease with which its specific gravity can be altered for construction. This is particularly noticeable when soil blocks are made of clay and straw. With the soil-straw technique, it is possible to produce blocks with a bulk density as low as 300 kg/m3 and a corresponding high insulation value of 2.80 m2.°C/W, while solid concrete blocks have a bulk density of 2,400 kg/m3 and an insulation value of 0.24 m2.°C/W (19).

A brief review of soil construction technologies

The most predominant examples of soil construction are the outcome of traditional practices but, unfortunately, technological errors are common in these constructions. The underlying principles of modern soil technology are a response to the defects of the traditional practices. A great deal of technological achievements in soil construction are replicas of other building materials technologies, notably fired-clay bricks and concrete. The three most common modern soil construction technologies are rammed earth, adobe construction and compressed-block technology. In several aspects, rammed earth technology is similar to monolithic concrete techniques, adobe is close to unfired-clay brick, while the production of compressed-soil blocks follow the same principles of concrete-block manufacturing.

(a) Rammed earth

The quality of soil is an important criteria for good quality rammed earth construction. The suitable soils should have adequate cohesiveness and should contain sandy aggregates to the point that they attain characteristics similar to those of lean concrete. When the available soils are clayey (have high plasticity index-PI), they should be mixed with soils containing a high percentage of gravel and sandy fraction. In the preparation of soil for rammed earth construction, depending on the location and size of construction projects, both manual and mechanical methods can be applied. The mechanical process, which obviously, would require some energy input, vary from simple technologies to sophisticated automated equipment. One important advantage of rammed earth technology is that its use in movability wall construction allows for a variety of forms such as round corners. In temperate climates, it is not advisable to carry out rammed earth construction less than three months before or during the frost period. In hot-humid climates, the rainy periods should be avoided, while in hot-dry climates, the hottest season should be avoided.

Owing to the similarity between rammed earth construction and in situ concrete technology, most of the machinery used in concrete works are directly applicable to rammed earth. However, the two most distinguishable and important activities in rammed earth construction are formwork and ramming.

One weakness of rammed earth construction is that the walls are liable to crack after a period of time if not properly constructed. These negative aspects can be minimized by means of putting the walls on strong foundation, using soil stabilizers, surface protection or patching up the cracks as a remedial measure.


Figure 1. Different types of formwork for rammed earth construction

Formwork

Formwork for rammed earth must be solid and stable in order to resist the pressure and vibrations resulting from ramming. In addition, it should be made of lightweight material and it must be easy to assemble and dismantle. The formwork must also be capable of accommodating changes for the height, length and thickness of walls. A broad range of materials, such as timber, glass fibre, steel and aluminium, can be used for manufacture of formwork. Depending on the type of building, available workforce and equipment, formwork can be organized as either a system consisting of small units or an integral formwork. Integral formwork is normally used when the design to be executed is relatively simple and it is presented in a simple format adopting modular dimensions.

Movement of formwork on site can be done by gantry formwork. This technique is best suited for the erection of columns or wall sections. Such formwork should be light, consisting of simple planks, plywood panels or even billets, which should be kept in position by wooden supports driven into the ground and secured at the top. Other techniques for movement of formwork on site are the use of rollers and sliding formwork as in modern concrete practice.

Rammers

The performance of a building made of rammed earth depends largely on the efficiency of the ramming technique. Ramming is the most tedious process in the entire operation and, depending on the choice of technique, there could be some minimal energy required. There are two types of rammers - manual and mechanical. The key elements for manual ramming are:

(a) an effective striking face: a striking angle of 60° or more with detailing in the shape to cater for difficult angles in the formwork;

(b) a striking area of not more than 225 sq cm;

(c) a durable material for the striking head, preferably solid metal;

(d) a handle of about 1.4 m; and

(e) the total weight of the rammer should range between five and nine kilograms.

There are two types of mechanical ramming, i.e. impact and vibrating ramming. Impact ramming consists of pneumatic rammers which have been adapted from the foundry industry. Their function is similar to that of manual rammers except for a higher impact frequency (up to 700 strokes per minute). Pneumatic rammers must not exceed 15 kg in weight and should not be too powerful, otherwise they tend to destabilize the formwork causing the earth to bulge or rather the rammer might penetrate the soil. Vibrating rammers are powered by combustion engines or electric motors and are heavy, cumbersome and expensive yet not proven to be effective particularly, in remote areas.

(b) Adobe technology

Adobe technology is equivalent to that of fired-clay bricks except for the fact that while fired-clay technology relies on a kiln process with high energy requirement, adobe technology replaces this process with open-air sun-drying. The soils used for adobe should be plastic or cohesive in texture. Clayey soils are thus the most suitable for adobe manufacture, but it is important to select those clays which are not generally expansive. Sometimes sandy soils have to be added to the clay to achieve the right quality of soil.

The first step of adobe blocks is pugging the clay. Because of the cohesive nature of soils, the pugging of clays could sometimes be a long operation. Traditionally, the clayey soil is mixed with water and sometimes with bare feet until a consistent mixture is obtained. Animals are sometimes used to knead the soil with their hooves. The manual processes of kneading clay are extremely labourious and thus make mechanized options easily attractive. Mixing is the most important operation in the production process if the soils used require an admixture such as sandy soils or stabilizers. Standard concrete mixers can be adapted to this practice but there are other technologies which are particularly suited for soil mixing. Vertical mixers can be fabricated on the site using planks, timber sections, ropes and steel wire with animal-driven power. There are also standard vertical mixers on a commercial scale capable of handling 10 cu m per day. Similar linear mixers adapted from the ceramic industry are available with rated capacities of up to 50 cu m per day.

Manual moulding

To produce good quality, dense, and resistant adobe bricks, it is advisable to use a clay mixture that is semi-solid in texture which implies the use of the sand-moulding technique. Small-scale moulding tables from the brick industry can be adapted to this technique. A typical example is the ITDG moulding table with an output of about 500 bricks per day (20). The manual technique of adobe production can be adapted to large-scale production. The key item is the multiple mould. The clay mixture should be liquid in consistency so that it can be easily spread manually to fill all the moulds in one single operation. It is important to ensure that the multiple mould can be handled by two persons and that blocks can be easily demoulded. With this technique it is possible to produce 8,000-10,000 blocks per day using a team of five or six workers.


Figure 2. A heavy manual earth block press, developed by Appro-Techno, TERSTARAM, Belgium

Mechanized presses

The simplest form of mechanized production is an adaptation of the manually-operated multiple-mould production. The moulds are mounted on wheels with or without a mobile hopper to feed the clay mixture into the moulds. An output of 7,000-10,000 blocks per day can be expected from such a technique. There is a similar technology in which the mechanized multiple mould is replaced by a manually-operated set of cutter discs. The latter technique can produce 15,000 blocks per day. The extrusion technology is the most popular form of mechanized production. The vertical extruder consists of a vertical mixer fitted with an extrusion nozzle. The system can be motorized or animal-driven. Small vertical extruders are estimated to have an output of 1,500 blocks per day. Horizontal extruders, as used in the ceramic industry, require a high level of investment but are more efficient and versatile than the vertical ones. Owing to the fact that the soils used for adobe have to be sandier than those for fired-clay bricks, a high degree of wear and friction occurs in the extruding machines, hence the importance of versatile extruders.

The energy content of unstabilized manually-made soil blocks is so low as to be almost negligible, by comparison with the energy content of fired bricks or of concrete blocks. When machine compaction is used, the compaction energy required is still very low, but, when the energy cost of the block press used is included the total energy consumption rises to 50-100 MJ/ton (14).


Figure 3. A motorized earth block press, developed by CERATEC, Belgium

(c) Stabilized soil blocks

Unlike rammed earth and adobe technology, the production of compressed soil blocks allows the use of a more flexible range of soil types because adjustments can be made in the production process to counteract any deficiencies in the soil characteristics. However, good quality compressed soil blocks require the use of soils containing fine gravel and sand as well as some clay and silt to bind the sand together. At least 50 per cent of the soil should have a grain size of less than five mm.

The production process involves pulverization of soil, screening, mixing with stabilizer and compaction. Pulverization is required for lumpy soils and can be done manually or with machines. Screening is essential for removing large particles and organic matter, especially if no pulverization has been done. The pulverized and screened soil - with or without additives - should be mixed in the dry state before mixing it with water. A conventional concrete mixer may not be useful for mixing wet soils because of the tendency of composite materials to stick on the inside of the rotating drum and because of the formation of lumps in the soil.

Based on extensive studies and experiments, it has been found out that blocks stabilized with 5 per cent cement have a strength and durability comparable with ordinary clay bricks and concrete blocks. The energy requirements for this type of blocks with only 5 per cent cement is about 150 to 300 MJ/ton. However, blocks having 10 per cent cement, which would have adequate strength and durability, would increase the energy requirement of soil blocks to only 300 to 500 MJ/ton, still substantially less than the figure for any other type of alternative walling materials (4).


Figure 4. A CINVA-RAM block-press. Courtesy IBEC Housing Corporation, New York

Soil block presses

The mode of compaction is the key to a durable and cost-efficient compressed soil block. The strength of a block increases with the intensity of compaction to a limit of about 4 to 10 MN/sq m, beyond which the effect is reversed. Even though the strength of a block depends on the compressive capacity of the machine, it is equally important to ensure the appropriate use of the machine. Blocks with different strengths could be produced by the same machine if the production parameters are changed. Compaction machines can be classified by the moulding pressure - measured in MN/sq m - they generate, namely (a) low pressure: 0 to 4, (b) average pressure: 4 to 6, (c) high pressure: 6 to 10, (d) hyper-pressure: 10 to 20, (e) mega-pressure: 20 to 40.

Some of the compaction machines available in the market were developed especially for soil construction but others were adapted from the concrete block industry. Soil compaction machines can be further divided into manual and motorized units. In all cases, presses can be distinguished by their weight and their transmission principles-mechanical or hydraulic presses.

Appropriate soil construction technology

Few programmes formulated in response to the shelter needs of low-income populations have considered soil construction as a viable option. The legacy of unsafe and non-durable construction, reminiscent of most existing low-income houses built of soil, has made soil construction technology unpopular and difficult to promote, while modern soil construction, in spite of all its improvements and advancements, poses problems of inappropriateness in the choice of technology. In fact, in a number of cases low-income housing constructed with modern soil technology has failed. These failures were often due to excessive cost of production or breakdown of machinery and led to the abandonment of entire projects. A successful soil construction programme for low-income housing should be based, first and foremost, on sound technical criteria. Thus, the following aspects need to be taken into account:

(a) Understanding basic soil science;
(b) General design and construction considerations;
(c) Surface protection of exposed walls;
(d) Machinery for the production of soil materials;
(e) Government promotional activities.

Figure 5. Soil identification and performance tests

Soil construction requires an ability to identify soil types.

Whilst a full laboratory report would be the Ideal to aim at, this Is not always possible.

The tests outlined here can be conducted in the field with the minimum amount of equipment. They will not give accurate proportions of the ingredients of a soil sample, but that is not always necessary. They will give an objective result whereby it will be possible for someone to say... “that is a sandy clay”, or... “that is an expansive clay”, etc. The ideal soil for soil construction will contain about one third clay, one third silt and one third sand. The proportions can be slightly different, and even totally unsuitable soil can be used, If proper precautions are taken.

Therefore, we start with a review of the equipment needed

1. EQUIPMENT


With this equipment, the following tests are designed to help you decide what are the most important materials in your soils.

Simple field tests can be carried out using:


Figure


Figure


Figure


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Figure

2. SIEVING


Figure

One worthwhile precaution to take is to sieve all soils which are to be used in construction. This will ensure large lumps and stones are removed, and will make remaining soil easy to use.

A useful sieve can be made from a simple wooden frame with a metal grid giving holes approximately 3 mm to 5 mm in size.

It is, incidentally, easier to let the soil fall through the sieve than to pick the sieve up with the soil on it!

3. CLASSIFICATION

An ideal soil type for soil construction will be in the proportions shown by the shaded area.

However, construction can go forward, even if the soil type falls outside the shaded area. It just requires extra effort or extra precautions.


Figure

Basic soil science

Often issues related to soil science are taken for granted or even ignored. An understanding of basic soil science, however, is essential for a successful soil construction practice. Decisions as to whether a particular soil is appropriate for construction can only be made if the characteristics of the soil are known. This knowledge will also determine the different aspects of the production process - whether to use adobe, rammed earth or compressed block technology, whether stabilization is needed and what type of stabilizer is to be used, which, if any, precautionary measures need to be taken. These considerations are of particular importance when soil construction is used for low-income housing because they have far reaching implications for the final cost of production. For instance, to use excessive quantities of a costly stabilizer or even to use one which is inappropriate for a given soil composition may result in an unnecessary increase in the cost of production.

For the purpose of low-cost construction, simple field tests can be undertaken to determine the characteristics and suitability of the chosen soils such as the presence of organic matter, type of soil (sandy, silty or clayey), sedimentation and shrinkage properties.

An important component of soil science is soil stabilization. Building with soil implies a choice between three main approaches:

(a) using the soil available on the site and adapting, as much as possible, the functional requirements of the construction to the quality of the soil;

(b) using another soil already suitable for the requirements of project but which has to be brought to the site from another source;

(c) modifying the local soil so that it is suited to the functional requirements of the project. This last option implies the use of stabilizers. There are three basic types of stabilization:

(i) Mechanical

By compacting the soil, it is possible to change its density, mechanical strength, compressibility, permeability and porosity;

(ii) Physical

The properties of the soil can be modified by acting on its texture through, for example, the controlled mixing of different grain fractions.

(iii) Chemical

Materials or chemicals can be added to the soil to modify its properties either by a physical and chemical reaction between grains and the additive or by creating an impermeable layer which binds or coats the grains.

The most common stabilizers are fibers, cement, bitumen emulsion and hydrated lime.

Design and construction considerations

Appropriate design and construction practices are a prerequisite to the durability and cost efficiency of housing for low-income groups. Low-cost soil blocks produced by rudimentary techniques could prove durable and satisfactory if the correct design and construction principles are adopted. Conversely, the use of high-technology and expensive soil blocks could lead to premature deterioration and eventual failure. In this context, it could be argued that most of the existing soil housing in low-income settlements have high levels of deterioration mainly because of errors in design and construction procedures rather than because of any inherent weakness of soil as a building material. The design and construction of low-cost houses should take, at least, two items into consideration:

(a) foundations and base courses;
(b) walls and openings.

Soil buildings are vulnerable to water penetration, especially at the ground level. Foundations not only bear structural loads but also act as barriers to ground-water penetration and thus prolong the existence of the building. It is important, therefore, to provide a damp-proof course layer between the foundation and the soil block work. For purposes of low-cost housing, simple strip foundations are suitable but stepped foundations are more effective where sloppy grounds are encountered. Foundations can be made of ordinary concrete rubble, concrete block masonry, stone masonry and even stabilized soil-block masonry. The base of a soil building is also vulnerable to rain water even when a foundation is provided. Water penetration can be controlled, in areas with high rainfall conditions, by a short-base course of impermeable material - a base course of about two or three layers of concrete blocks, stone or fired-bricks should be adequate for this purpose. An inexpensive alternative is to use soil blocks for the base course and protect the exposed surface at the base of the course with a rendering such as bitumen emulsion or cement mortar.

Rain water erosion and drainage shrinkage caused by sunshine can also affect exposed soil walls. It is advisable, therefore, to have overhangs of at least one meter in areas of high rainfall. Lintels are necessary above doors and windows and so is a wall plate to distribute loads on the soil wall. The corners of soil walls are particularly prone to deterioration from a combined effect of rain and wind. In fact, soil erosion is more intensive at corners than at the main body of the wall. Soil erosion at comers can be minimized by either a water-resistant rendering on the corners or by simply rounding the corners of unprotected walls. Window sills should project a sufficient distance from the wall to prevent the eroding effect of running water. The position of openings affects the strength and durability of soil walls, especially where the ground is unstable. High concentration of openings and openings near corners should be avoided.

Figure 6. Foundations

1. CONCRETE - RAFT TYPE


Figure

On soil types which are soft, unstable or of changing character, or are subject to heaving, houses can be built on a footing which floats on top of the soil like a raft on water. The raft acts as a footing for the walls and the floor. This footing type should be surrounded by drains which will lead water away from the house.

Mesh reinforcement for houses of different widths:

width up to 2.5 m - use 2.22 kg/m2 mesh
width 2.5 to 3.6 m - use 3.02 kg/m2 mesh
width over 3.6 m - use 3.95 kg/m2 mesh

Use ordinary concrete of 1:2:4 cement - sand - aggregates


Figure

2. CONCRETE - STRIP TYPE


Figure

Concrete mix 1:2:4 strip 150 mm thick all around under all walls. Concrete must be pored between timber shutters. Top concrete struck off level.


Figure

Trench to be backfilled with soil which must be well rammed in layers not more than 200 mm thick




The width (W) of the concrete depends on the soil type on which the footing sits.

SOIL TYPE

WIDTH (W) mm


SOFT

600


LOOSE, EASILY MOULDED

500


STIFF

400

3. BROKEN ROCK TYPE


Figure

USE THIS TYPE ONLY IN STILL OR LOOSE SOIL TYPES

Rocks approximately 100-150 mm size. Each layer must be laid and rammed, and then sand must be swept into holes. Top off with 30 mm 1:3 cement-sand mortar.

Ramming the rocks down firm Each layer of rocks must be placed carefully to fit tightly. Brush sand over the surface.


Figure

4. STEPPED-BLOCK TYPE


Figure

In situation where concrete footings are too expansive, stabilized-soil blocks could SERVE as materials for footing.



Blockwork to be stepped out in three equal steps to width of foundation. Width of foundation (W) to be the same as for concrete footing. Backfill to be brought up equally both sides of the wall. Trench to be filled with soil which must be well rammed in layers not more than 200 mm thick.

When making a stepped blockwork footing, it is most important that each layer be staggered in its jointing from the layer below. Below damp-proof course, blocks must be stabilized types only, or dressed hard building stones and burnt clay bricks may be used.




Figure


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Figure 7. Walls - Type of damp-proofing and termite-proofing

1. CONVENTIONAL BITUMINOUS FELT DAMP-PROOF COURSE

2. ASPHALT LAYER DAMP-PROOF COURSE

1. CONTINUOUS METAL-BARRIER TERMITE PROOFING

3. CEMENT-SANDTILE TERMITE-PROOFING


Figure


Figure


Figure


Figure






3. DENSE REINFORCED MORTAR LAYER DAMP-PROOF COURSE

2. STRIP METAL-BARRIER TERMITE-PROOFING

4. ASBESTOS-CEMENT TILE TERMITE PROOFING


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Figure 8. Foundations - Special situations

1. BUILDING ON THE SIDE OF A HILL

2. BUILDING WITH A STEPPED FOUNDATION


Figure

If the depth on the uphill side is more than 1.0 m either:

i) cut back and put in the drainage, or

ii) use stepped foundations.


Figure


Figure

Vertical face of step must be cut straight and clean.

Concrete strip footing must be used in stepped foundations. Make 10 cm wider than normal footings.


Figure

This is how the finished house and ground will fit into the hillside.


Figure

Stretcher brick must span over point from concrete footing on to wall on lower step.

Backfill must than be brought up equally on both sides of wall.

Height of step must be limited to a maximum of three courses of bricks.


Figure

Figure 9. Floors - In-situ construction

1. ADOBE-TYPE FLOORS



Figure

If an adobe-type floor is to be laid, it must be laid in at least two layers, each not more than 5 cm thick. Immediately after laying, the floor should be cut into regular blocks not more than 60 cm square. This will reduce the risk of unwanted cracks. The cuts in higher layers should be staggered from those in LOWER layers.


Figure

Cuts in final layer must be filled with a fine adobe mortar. One week must pass before laying successive layers. After final drying out, the surface can be treated with a mixture of turpentine and linseed oil, then waxed after that has dried out.

2. REINFORCED-TYPE FLOORS



Figure

Some soils where houses must be built may be unstable. They may heave, shrink or slip. Floors of houses in these areas must, therefore, be reinforced. Reinforcement can be done by steel bars.


Figure

However, bamboo poles up to 2 cm thick can be used and can be built into the floor as it is being laid. They should be spaced 15 cm apart and should be placed in two layers right angles to each other. Poles in one layer must be tied to poles in the other layer.


Figure

Alternatively, the poles may be laid before the floor, and the floor mix may then be placed on to and around the reinforcement. There should be at least 8 cm cover over the top of the reinforcement.

3. STONE-FLAG FLOOR



Figure

In situations where stones are readily available as low-cost materials they can be used for floors. Stone should be carefully cut to size.


Figure

Alternatively, the poles may be laid before the floor, and the floor mix may then be placed on to and around the reinforcement. There should be at least 8 cm cover over the top of the reinforcement.


Figure

Stones used should be find-grained type.


Figure

Bedding and joints should be made with soil-cement or soil-lime-cement mortar.


Figure

All joints should be carefully filled with 1:3 cement-sand mortar.



4. BRICK-ON-FLAT FLOORS


Similarly, in situations where bricks are readily available as low-cost materials, they can be used for floors


Figure


Figure



5. BRICK-ON-EDGE FLOORS




Figure


Figure



Figure

NOTES

BRICKS MADE WITH A FROG MUST BE LAID WITH THE FROG UNDERNEATH.

CEMENT-SOIL OR CEMENT-LIME-SOIL MAY BE USED FOR THE MORTAR BED.

BRICKS MUST BE LAID WHILE THE MORTAR BED IS STILL WET.


Figure

Surface protection of buildings

Where low-strength soil blocks or unstabilized soil construction are used, the performance of an otherwise non-durable structure can be improved by applying a surface protection. Similarly, surface protection can be used as a technique for maintenance or upgrading existing deteriorated soil-based dwellings. Unprotected soil walls have a fair chance of durability in temperate climates, especially if good quality soils have been used and good foundations and base courses have been provided. In rainy and humid regions, however, surface protection is a basic requirement. The range of surface protection techniques for soil construction allows for several low-cost options. There are two basic techniques for surface protection which are relevant for low-cost soil construction: paints and rendering.

Conventional paints, distempers, slurries of lime and cement, as well as bitumen in the form of liquid cut-back, can all be used as protective coatings on soil walls. They are normally effective, however, when the soil walls have been carefully prepared prior to application of the paint. For example, if lime or cement slurry is to be applied, the soil walls must be sprayed, in advance, with water. Soil is easily the cheapest material for rendering. For this purpose, soil particle size should be two millimeters or less and composed of one part clay and three parts sand. Soils for rendering can be stabilized with cement, lime and bitumen. A variety of fibers, such as vegetable fibers, animal fur and even synthetic fibers, can also be used as stabilizers.

In preparing stabilized soil renderings, cement is an effective stabilizer for sandy soils. Mix proportions vary between 2 and 10 per cent of cement. It is also possible to improve the water resistance of a soil-cement rendering by adding a small amount of bitumen, i.e. 2 to 4 per cent. Lime stabilizes clayey soils when at least 10 per cent is added. By adding animal urine to the soil-lime mixture, the rendering improves shrinkage, hardness and permeability of soil walls. Bitumen should be used for soils which are both clayey and sandy. The proportion of bitumen is usually 2 to 6 per cent. If the bitumen is in the form of cut-back, it should be heated to a temperature not exceeding 100°C. Alternatively, bitumen emulsions can be used.

Machinery for production of soil materials

As indicated above, soil construction is liable to be a capital-intensive technology rather than an energy-demanding one. The choice of soil construction technology should be consistent with the levels of capital affordability of the respective countries. For low-cost housing, one has to bear in mind that most developing countries operate with scarce capital and may not have access to capital items from imported sources, even for seemingly basic ones. Therefore, a low-cost housing scheme should aim, as much as possible, at operating with the minimum of capital inputs.

In modern soil construction, capital items are indispensable for quality control, even in low-cost projects. Pulverization of soils, screening, mixing, compaction and testing of raw materials, as well as finished products, all require some form of equipment. The key issue is to select the appropriate machinery, consistent with low-cost criteria, bearing in mind possibilities of improvizing and fabricating local equipment. For instance, in the area of soil compaction machinery, blocks with the appropriate compressive strength for simple low-cost houses can be produced using low technology-machines. Moreover, most of the low-technology compaction machines can be locally fabricated in several developing countries. This is also the case of item such as wheel barrows, measuring cans, sieves and a host of simple field test equipment.

The scale of the technology used is a determinant of cost and appropriateness in low-cost housing construction. Most large-scale technologies need to be imported and are subject to uncontrollable cost escalations. In addition, repair and maintenance may present major difficulties. Furthermore, the bulkiness of most large-scale machinery results in additional transport costs. It is important to promote small-scale technology to counteract these deficiencies. It must be noted that there is a distinction between the scale of output and scale of technology and that, with appropriate organization and management, large-scale output can be achieved with small-scale technology.


Figure 10. A mobile block press machine, developed by “Hydroform” in South Africa (a)


Figure 10. A mobile block press machine, developed by “Hydroform” in South Africa (b)

Government promotional activities

The wide-scale adoption of soil construction for low-income housing will require certain institutional support to facilitate adequate production capacity of soil materials and to ensure a sustained market for them. It is essential to provide an initial activity of technology demonstration, independently of the modern soil construction technology chosen. In order to have an impact on builders in low-income communities, technology demonstration should take the form of sustained periods of on-the-job training.

Similar to other local building materials in developing countries, there are as yet no standards and specifications for soil construction. This could be an important factor hindering the wide-scale adoption of this type of technology. In this context, standards and specifications should adopt pragmatic and workable mechanisms. In particular, they should be complemented by training in quality-control procedures and access to simple operations for testing raw materials and finished products.

There are inhibitions against the use of soil construction for low-cost houses, partly as a result of technical errors in most existing traditional soil dwellings. Unfortunately, this stigma cannot be overcome by simply promoting technical solutions. The main approach to this problem is for governments to actually use soil construction in on-going shelter programmes.

Note 1.

For further details on soil construction see UNCHS (Habitat), “Earth Construction Technology”, reference No. 21 and UNCHS (Habitat), “Journal of the Network of African Countries on Local Building Materials and Technologies, Volume 1 No. 4 and Volume 2 No. 2”, reference No. 22.



Note 2.

This section has been produced based on a study prepared by UNCHS (Habitat).

3.2. Building stone, sand and aggregates

Building stone, sand and aggregates are the oldest, widely available and durable building materials. These important and widely used materials, by themselves, could be considered as no-energy materials, however, for the purpose of their processing and transportation, some energy would be required which will make them low-energy materials. The energy used is, principally, for mining, crushing and transporting. In the case of dimension stone, in addition to energy requirements for quarrying and transportation, some energy is required for cutting and polishing stones which are done in stone cutting factories with sophisticated machinery and the use of electrical energy. In the case of sand and aggregates, which are the main ingredients for concrete and mortar making additional energy is used for screening them, thus, making them suitable for the purpose of their use.

In most developing countries and depending on the local circumstances most operations related to the use of stone in building construction are carried out manually. Some values for the energy requirements from different sources are given in table 1 below. These values are exclusive of transporting of materials to the site and do not include the energy of manual work involved.

Table 1. Energy requirements of some stone-based materials

Material

Energy requirement
(GJ/ton)

Source

Sand and aggregate:


Sand, the United Kingdom

0.03-0.3

Gartner and Rankin


Crushed aggregate, India

0.22

Rai


Building sand, India

0.015

Rai


Stone rubble, India

0.1

Rai

Building Stone:


Building stone, Kenya

0.1

Spence

3.3. Low-cost binders

Binders are important materials for any type of construction activity. They are used for plastering walls of buildings, for making mortars in masonry and foundations for soil stabilization, etc. Among the various types of binding materials used in construction, Portland cement is the most attractive one because of its superior characteristics. However, because of its high energy content and, consequently, high cost, is not accessible to many low-income house builders. Low-cost binders, on the contrary, have great potentiality and adequate technical characteristics to meet the requirements of low-cost construction. The most important low-cost binders include: lime gypsum, pozzolanas and blended cements. Lime was already discussed in the previous sections, therefore, in the following parts, only gypsum, pozzolanas and blended cements will be briefly discussed.

(a) Gypsum

Gypsum is a traditional material made from heating mineral gypsum stones. It is used for plastering walls, manufacturing gypsum-based plaster-boards and many other applications. The energy requirement for manufacturing building gypsum, compared to cement and lime is very low.

Depending on the quality of gypsum stones the heat required for gypsum kiln ranges between 150° and 170°C, much lower than for cement (1450°C) and lime (900°C) production.

Similar to lime production, a high proportion of total energy for gypsum production is used in the kiln. The raw material, is calcined directly either by mixing it with the fuel, or it is burnt indirectly by burners. Indirect heating offers better control, produces better quality gypsum and is suitable for small-scale-operations. Most production processes in developing countries are intermittent and not energy-efficient. Rotary kilns, however, offer more energy efficiency but require more capital.

Comprehensive data on the energy consumption of gypsum plaster production at all scales are not available. Some examples of the energy consumption using different kiln types and scales of production are given in table 1. Even the least efficient processes use substantially less energy than either cement or lime production. However, a direct comparison between the three materials is not possible, because they all have slightly different properties (1).


Figure 1. A walled kiln for burning gypsum. Courtesy IT, U.K., Gypsum and Plaster


Figure 2. A shaft kiln with masonary cylindrical wall. Courtesy IT, U.K., Gypsum and Plaster


Figure 3. Hopper-fed furnace-heated batch kiln. Courtesy IT, U.K., Gypsum and Plaster

Table 1. Comparative energy requirements for gypsum production

Process

Energy requirement
(GJ/ton)

Source of data

Large-scale production, United Kingdom

0.8 to 1.0

Coburn and others

Calcined gypsum, India

1.5

Rai

Plaster of paris, Germany

1.5

Rai

Small-scale production, North Africa

2.7 to 4.6

Coburn and others

(b) Pozzolanas

Pozzolanas are siliceous materials which can be either natural or artificial. Pozzolanas on their own have little or no binding property, but when finely ground and mixed with lime will set and harden, in the presence of water.

The most commonly occurrences of natural pozzolanas are volcanic ashes and pumice powder. These pozzolanas, depending on the expected strength of the mortars, can reduce the use of lime by 20 to 30 per cent, but if mixed with cement, the substitution to cement can even be higher. Thus, considerable amount of energy saving can be achieved through using less cement or lime while obtaining an acceptable result.

Artificial pozzolanas are produced from a number of industrial and agricultural wastes which are highly siliceous. These include:

(i) fly-ash obtained from coal burning power plants

(ii) ground blast furnace slags resulting from iron plants

(iii) burnt clay powder and

(iv) rice-husk ash from burning the rice husks in specially designed incinerators. As in the case of natural pozzolanas, the artificial pozzolanas are processed and mixed either with lime or cement to be used as binding material in low-cost construction or soil stabilization.


Figure 4. Lime sludge/rice husk balls laid out to dry. Courtesy IT, UK, Rice Husk Ash Cement

Even though processing pozzolanas and producing them in a usable form, to be mixed with lime or cement, requires some energy, experiment and extensive research work have shown that the sum of energy used in this process is less than the energy used in the substituted cement or lime. Needless to emphasis that the environmental impact of the use of industrial and agricultural wastes is another reason which makes the use of pozzolanas attractive.

(c) Blended cement

Manufacturing blended cement (also called pozzolanic cement) is one of the promising trends in the reduction of energy use in construction. Blended cements are either produced directly in cement plants (e.g. by mixing certain types of pozzolanas with cement clinkers and grinding them together) or they can be produced on site by mixing ordinary Portland cement (OPC) with finely ground pozzolanas. In producing blended cements, almost all types of common pozzolanas can be used. However, among the different types of pozzolanas, furnace slag has been widely used in the process of producing blended cement, which is very popular in both developed and developing countries and for which relevant standards have already been established. The replacement percentages of pozzolanas vary from 15 to 50 per cent resulting in an energy saving of up to 40 percent. Table 2 shows some energy saving examples from India.

Blended cements can be used not only for making mortars for masonry or plastering purposes but also for structural concrete, particularly for foundation and/or mass concreting purposes. It can be also used in block-making for use in load and non-load bearing walls.

Table 2. Energy-saving opportunities using pozzolanas as replacement of cement

Type of blended cement

Composition
(percentage)

Energy
(kj/kg)

Saving
(percentage)

Portland burnt clay pozzolanas cement

Portland cement: 75

5945


Burnt clay pozzolanas: 25

335

20

Portland fly-ash pozzolanas cement

Portland cement: 75

5945


Fly-ash: 25

Nil

25

Portland blast furnace slag cement

Portland cement: 60

4773


BFS: 40

Nil

40

Masonry cement

Portland cement: 50

3970

Mineral tailing: 50

Nil

50

Source: Government of India, “Use of energy by household and in production of building materials”. Unpublished paper prepared for the 13th session of the Commission on Human Settlement, Harare, May 1991.

Note:

For a detailed treatment of low-cost binders, see UNCHS (Habitat), “Endogenous capacity-building for the production of binding materials in the construction industry-Selected case studies”, reference No. 23 and UNCHS (Habitat), Journal of the Network of African Countries on Local Building Materials and Technologies volume 1, No. 1, 1989 and volume 2 No. 1, 1992.


Figure 5. Calcination kiln for powdery lime sludge to be used in production of blended cement

3.4. Timber and bamboo

3.4.1 TIMBER

Timber is the most important and widely accepted organic material used for centuries in building construction and if extracted from managed forests and used in a sustainable manner it can be used for ever. It can be used at any level of technology for a wide range of purposes ranging from its uses in rural areas to being processed in high-tech factories for manufacturing structural and non-structural building elements as well as furniture.

As a natural and renewable resource, it is also a very low-energy material with superior characteristics which makes it quite attractive for many construction purposes. Various studies have shown that even though using timber as a building material involves the use of energy for logging, transportation, seasoning, cutting, processing etc., the sum of all energy used in all these processes per weight basis is much less than any other energy-intensive structural material such as steel, aluminium, concrete, bricks etc. Table 1 shows some examples of the primary energy requirements for some timber products.

Table 1. Energy requirements fur timber production

Product

Energy requirement
(GJ/ton)

(GJ/m3)

Source of data

Softwood framing, USAa

0.7

0.34

Stein

Timber at site, Australiaa

2.0


Lawson

Timber processing, Argentina

0.4


Rai

Timber, Germany

1.04


Rai

Timber (concrete formwork), UK

1.6


Haseltine

Timber products, UK

5.4


Gartner and Rankin

Particle hoard, India

3.1


Rai

Particle hoard. USAa

9.2

4.6

Stein

Plywood. USAa

13


Stein

a Includes transport to site

The advantages of timber as a low-cost construction material are first and foremost technical. Timber, after certain processing, can be used for almost the entire structure of a house as load bearing and non-load bearing elements. Timber has a unique combination of both thermal efficiency and superb sound insulation qualities. It has a high ratio between structural resistance, resilience and weight and is, therefore an excellent material to be used in building structures in earthquake-prone areas. For example, in Southern California, about 80 per cent of all residential buildings of up to four storey high are constructed with timber which include: walls, floors, roofs and even outside cladding of the buildings.

In principle, production of commercially accepted timber from tropical forests can be made sustainable, but in practice, a combination of commercial interests, lack of information, population pressure and mismanagement is leading to a rapid loss of forests in most worlds tropical regions.

One way of tackling the problems associated with deforestation is encouraging the use of commercially less-accepted species (CLAS) and industrial tree plantation species (ITPS). If properly managed and exploited, these species can serve as abundant and renewable resources of building materials that can be afforded by the vast majority of the population. There are currently no significant examples of use of CLAS and ITPS as a walling material or roof-cladding material in developing countries. In some cases, the wood elements are only restricted to internal partitioning (24).

The use of CLAS and ITPS for construction, especially for walling purposes and as shingles for roofing, is slowly showing potential in industrially-processed wood products where the CLAS and ITPS serve as raw material. The use of wood chips, pulps and excelsior for composite boards in some countries relies on timber species which are less suitable as sawn wood due mainly to their irregular form. There is potential for using chips and excelsior from CLAS and ITPS to manufacture wood-cement boards. In some countries, where these products are on commercial sale, there is an unfavourable market trend probably due to the unattractiveness of the finish of the boards.

The Second Consultation on the Wood and Wood Products Industry, organized jointly by UNCHS (Habitat) and UNIDO and held in Vienna in January 1991, underscored the importance of greater utilization, on a sustainable basis, of wood, including CLAS and ITPS, as a renewable source of indigenous building materials in housing and construction. The Consultation, while focusing on environmentally-sound management of forests, devised a set of recommendations addressing the industry, governments and the international community on ways and means for popularizing the use of CLAS and ITPS in the construction sector (24).

Seasoning timber

Although timber is considered as being “no-energy” material, for processing purposes, there is a need for certain amount of energy. Among the various stages of processing, seasoning the timber, particularly, when it is done in heated kilns, requires the highest amount of energy compared to other processing stages such as cutting, shaping, transporting, etc.

The purpose of seasoning timber is to remove the extra moisture of green timber under controlled conditions. Timber has a tendency to change dimensions in response to changes in moisture content, therefore, a proper seasoning bringing the timber to its final dimensions and making it stable for use in structures is one of the most important and energy demanding stages of timber processing.

For structural work, timber should be dried to within 5 per cent of equilibrium moisture content (EMC). This value refers to the moisture content which would be attained in service. EMC depends upon the relative humidity and the temperature of the surroundings and the following may be considered a rough guide (25).

Table 2 shows some examples of EMC for various climates.

Table 2. Equivalent moisture content of timber

Region

Equilibrium moisture content
(percentage)

Hot dry regions (desert, semi-desert and savannah)

10-12

Tropical highlands (above 1500m)

12-14

Tropical lowlands, rain forests

14-18

(Other climate conditions are not included)
Source: UNCHS (Habitat), reference No. 25.

Seasoning timber can be carried out in two ways, air drying and kiln seasoning.

Air drying

Green timber is stacked in open sheds using spacers made of wooden sticks for air circulation. The thickness of spaces vary between 15 mm to 40 mm depending on the timber species and outside temperature. The stacks would be put on solid-base and raised above the ground.

The time required for seasoning depends on the species and the climate (humidity and temperature) of the surroundings. In tropical climates some soft woods require at least six weeks, while denser hardwood would need 25 weeks or more.

Kiln seasoning

Green timber is placed in a kiln or a chamber in which temperature, humidity and flow of air can be controlled. Kiln seasoning allows timber to be dried to any desired moisture content appropriate to its end use. The technology of kiln seasoning is well advanced in both developed and developing countries and there exist quite a number of types and methods including special treatments of timber such as reconditioning and sterilization.

Solar seasoning

Solar seasoning is the most energy efficient method for drying timber. This method uses the sun radiation to heat the air inside a chamber where timber is stacked. Over the past decade, considerable research into this method has been carried out in some developing countries such as India and Sri Lanka and in some cases it has been introduced in commercial use. Figure 1 shows a solar kiln designed in the Central Building Research Institute in Roorkee, India and figure 2 shows a Bangladesh kiln designed by M. A. Sattar. It is made up of three main parts namely:

(a) solar-energy collector;
(b) seasoning chamber; and
(c) chimney.

Experiments on this kiln has shown that the time taken in seasoning three different types of wood, e.g. mango (Man giferaindica), Jamu (Eugenis jambolana) and haldu (Adina cardifolia) from green stage to 10 per cent moisture content for a thickness of 3.75 cm would require 17, 27 and 18 days respectively, while seasoning the same specimen under a shed in open air would need 35, 62 and 40 days (25). The number of different designs of solar kilns has increased rapidly in the last few years with a tendency towards better insulated structures and more sophisticated control of humidity, venting and air circulation (26), (27) and (28).


Figure 1. Solar kiln designed in Central Building Research Institute (CBRI), Roorkee, India.

The attached Annex presents, in a summary from, a research project carried out on solar timber seasoning in Sri Lanka. It demonstrates the advantages and technical feasibility for drying some secondary species of wood including the rubber and coconut wood.


Figure 2. Bangladesh kiln. Courtesy TRADA and ODA, reference No. 29

Timber protection (25)

In order to protect timber against hazardous attacks such as fungi, insects and marine borers, timber used in structural work should be treated by special preservatives. A successful treatment of timber depends upon the type of chemical used, as well as the type of timber and treatment. With the exception of diffusion treatment, the moisture content of the timber to be treated should not exceed 30 per cent. The maximum permiceable moisture content depends also to some extent upon the process, type of preservative chemical and the kind of timber.

In general, there are three types of wood preservatives: tar oil; water-borne; and organic solvents.

Tar-oil compounds or creosotes are suitable for exterior work, in water or in ground contact. The principal advantages of creosotes are their high toxicity against fungi, insect and marine borers. Tar-oil preservatives may be applied by vacuum-pressure, hot and cold open tank, brushes, spraying or immersion. For exterior work the recommended methods are either vacuum-pressure or hot and cold open tank.

Water-borne preservatives require special methods of application to ensure deep penetration and are not suitable for brush treatment. Contrary to tar-oil treatment the treated timber is odourless and can be painted over when it is dry. Water-borne preservatives are applied to seasoned timber by vacuum-pressure methods, whilst boron compounds are applied to green timber by diffusion. They can be used in wet and dry conditions, including ground contact.

Organic solvent preservatives are readily absorbed by the timber and so may be applied by brush, spray or immersion. For a deeper penetration, methods such as double vacuum may be used. They do not cause the timber to swell. Treated timber may be painted and is not corrosive to metals.

Extensive research over the past few years, has revealed that most of the common and commercially-produced preservatives are toxic, and hazardous to human health, even though they are the most effective means to protect timber from all types of destructive agents. In fact the higher the toxicity of preservatives the higher their effectiveness. As a result of new experiments, the manufacturers of timber preservatives have developed alternative products some of which are also suspected to be harmful and the use of a series of preservative substances (such as aldrin, chlordane, dieldrin and some arsenic compounds, to name a few) is officially prohibited in several countries. The use of substances like pentachlorophenol (PCP) and lindane (gamma-hexachloro-cyclohexane or just y-HCH) is also greatly restricted, and likely to be forbidden in due course (30).

Unfortunately, despite their high toxicity, most of these chemicals are still being officially recommended and widely used in almost all the developing countries, a serious problem that requires urgent attention. The main drawbacks of chemical treatment are: (30)

- Fungicides and insecticides have to be sufficiently toxic to be effective, and cannot differentiate between harmful and harmless organisms, which are destroyed likewise. Some destructive organisms develop resistance to toxic chemicals, which thus fail to serve their purpose, but destroy useful creatures (e.g. bees, spiders, birds).

- The chemicals affect animals and humans by way of inhalation, skin contacts or through contaminated food, causing various health problems, ranging from headaches, nausea, dizziness, aggressions, depressions, rash, etc. to diseases of the lungs, heart, liver, kidneys and other organs, malformations, paralysis and even cancer.

- The production, application and disposal of biocides all contribute to serious environmental pollution. The toxic chemicals and their poisonous by-products, many of which are extremely persistent (the most well-known example being DDT), enter the food chain and accumulate in ever increasing concentrations in the bodies of all living organisms, most of all in human beings. The production and handling of biocides endangers workers and users of treated products. Solar radiation, high temperatures and humidity, atmospheric pollutants and other factors can transform certain preservatives into other, more dangerous substances (for instance, PCP produces dioxin, a so-called ultra-toxin).


Figure 3. Preservation of timber by dip diffusion

3.4.2 BAMBOO

After timber, bamboo is the second important organic product which requires much less energy for its processing and seasoning than timber. In fact, many researchers and professionals call bamboo as a “no-energy” material with very good structural property to be used in low-cost construction, particularly, in bamboo growing countries. Bamboo is a very fast growing plant and some species can reach their full height within 6 months (up to 35m height). However, it takes 3 to 6 years to develop adequate strength for use in structures. There are, in principle, two main types of bamboo, namely: “Sympodial” or clump forming bamboo, found mainly in warmer regions, and “Monopodial” or running bamboo, found in the cooler areas.

Well-matured culms have greater resistance to deterioration than younger ones, however, a proper preservative treatment can double or triple the resistance and the durability of bamboo. Like the treatment of timber, chemical preservation of bamboo is very important to reduce the attacks of termites, but again most of the available chemicals are toxic and harmful to human health and the environment. Based on extensive research, it has been found out that preservatives derived from borax, soda, potash, wood tar, beeswax and in seed oil are less harmful than other commercial chemicals. The effectiveness of these preservatives are, normally, less than those poisonous chemicals, but can be equally effective in conjunction with good building design and construction (for example, avoidance of contact with soil, exclusion of moisture, good ventilation, accessibility for regular checks and maintenance, etc.) (31).

The areas of application of bamboo include: frames (beams and columns), trusses for roof, grids (spare trusses), seaffolding, fencing, bridges, etc. Sometimes, for special purposes, full culms are halved to produce two U-shaped cross-section for using in gutters, walls, purlins, etc. Split bamboo strips can also be used for matting and woven panels, fencing, ornamental screens, etc. Even the bamboo fibres and chips can be used for manufacturing fibre boards, particle boards and fibre concrete.

Even though use of bamboo, as a very low-energy and low-cost material, has considerable advantages, it has a number of setbacks such as: (31)

· Low durability, especially, in moist conditions, as it is easily attacked by biological agents, such as insects and fungus;

· Bamboo catches fire easily;

· The low compressive strength and impact resistance limit its application in construction. Wrong handling, bad workmanship and incorrect design of bamboo structure can lead to cracking and splitting which weaken the material and make it more vulnerable to attack by insects and fungus. Nails cause splitting;

· The irregular distances between nodes, the round shape and the slight tapering of the culms towards the top end makes tight-fitting construction impossible, and therefore, cannot replace timber in many applications;

· Bamboo causes greater tool wear than timber;

· Bamboo preservative treatments are not sufficiently well-known, especially the high toxicity of some chemical preservatives recommended by suppliers and official bodies.

If compared with timber, bamboo has quite a number of advantages. It is an abundantly available material which is low-cost and renewable. Its handling and treatment can be done with specific tools and traditional methods. Its strength characteristics, particularly its high tensile strength makes it an ideal material for frames and roof trusses. Having very low weight and high ductility, structure made of bamboo, if well designed and constructed are earthquake resistant.

For example, in Costa Rica, the powerful earthquake of April 1991 which damaged many buildings, did not affect those houses which were built in bamboo. In fact, the emergency reconstruction programmes supported by the Government of the Netherlands after the earthquake focussed mainly on the use of bamboo for new housing schemes. (32)

Furthermore, it is worth mentioning that based on a UNCHS (Habitat) supported project in Costa Rica it is expected to construct some 7000 houses per year after 1995 using bamboo as structural elements. For this purpose necessary planning and arrangements have been put in place to plant and harvest sufficient quantities of bamboo in the country. The expertise and experience gained by this project is now widely sought in the Latin American region where interest in bamboo technology is steadily growing (32).


Figure 4. Column placement in continuous foundation. Courtesy UNDP/UNIDO, RENAS-BMTCS, Manila


Figure 5. Column placement in square footing. Courtesy UNDP/UNIDO, RENAS-BMTCS, Manila


Figure 6. Bamboo roof trusses. Courtesy UNDP/UNIDO, RENAS-BMTCS, Manila