 |  | Compressed Earth Block - Volume I. Manual of production (GTZ, 1995, 104 p.) | ||
 |  | Equipment | ||
|  | Selection criteria | ||
| | Products | ||
| | Uses | ||
| | Choosing a press | ||
| | Choosing soil preparation equipment | ||
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GENERAL CONSIDERATIONS
There is a wide choice of machinery and equipment available. The quality of the equipment used is important, but the quality of the soil remains of paramount importance.
GENERAL CRITERIA
Design
The ways in which pieces of equipment can be physically located will influence productivity. This includes, for example, their adaptability, their capacity to be combined together and the extent to which they are integrated into a production line. Working conditions should also be taken into account.
Safety measures must be considered, for example, protection against projected material, marking out dangerous areas, installing fuses or emergency switches for disabling automatic machinery.
Maintenance is indispensable, but should not be excessive. Sophisticated machines will allow high productivity, but if time spent on maintenance is too high, output will fall. Spare parts which wear out quickly should be readily available (in terms of cost and delivery) or easily repairable locally.
Manufacturer
It is important to find out how much experience the manufacturer has in international business and whether he has a distribution network, as this can have a major impact on supply times (of machines and parts) as well as on maintenance, if problems occur.
Manufacturers should be in a position to allow visits to their production unit and to provide the names and addresses of brickworks where their equipment is being used, as recommendations of their products, but also to encourage exchanges of experience between different users.
Conditions of sale
- Prices: The price indications given in catalogues or in proforma invoices should be carefully studied. Prices may be quoted in three different ways: EXW (Ex Works): the machine as it leaves the factory, excluding packing and transport; FOB (free on board), which includes packing, transport and insurance until the order leaves the country of origin (the seller), and to which must be added transport and insurance costs as far as the country of destination; and CIF (cost, insurance and freight), which covers all costs as far as the country of destination, with the exception of local taxes. FOB or CIF prices can sometimes be artificially raised to compensate for discounts given on ex-factory prices.- Expenses: Banking charges (such as letters of credit, exchange or transfer commissions) and charges incurred by intermediaries must be taken into account and it should be clearly stated whether responsibility for these lies with the seller or the buyer. A letter of credit accepted by the seller, who then has to submit an invoice, will cost approximately 5% of the CIF price. In addition, a letter of credit taken out by the buyer will cost him approximately 5 to 7%. Letters of credit for "sensitive" countries can cost up to 14%. Any intermediaries used to transfer a letter of credit from the buyer to the seller will ask for a minimum of 10 to 12%. They will also require a commission of at least 15%. All this can add up to over 25 to 30 % of the CIF price.
- Contracts: it is advisable to include a penalty clause for delivery delays. In the case of an after-sales service contract, repair and maintenance times must be stipulated. A detailed manual including parts specifications and a maintenance plan, and stating which operations need to be carried out and at what intervals, must be provided.
CONSTRAINTS
Budget available
A low budget will considerably restrict the choices available.
Productivity
This depends on market projections. It may be more advantageous to allow for extensions than to start out too big.
Power supply
Power supply availability may be more of a problem than cost.
Power supply
Power supply availability may be a greater obstacle than its cost.
Maintenance
If no qualified staff or no parts are available, machines which are simple to maintain should be selected.
SPECIFIC CRITERIA
Preparation equipment
These are not necessarily indispensable, but they will greatly improve the quality of the end-products. Their purchase may be viable, for example, if they enable one to lower the amount of stabilizer used. The type of soil will be the determining factor in choosing preparation equipment (such as grinders, crusher, screens), but will be less so for the type of mixer chosen.
Presses
Productivity and the size of the production unit will be determined by the presses used. The following are particularly important: the implications of the market survey (on productivity and on product type), the budget available and the production team (cost and degree of specialization of the labour-force).
Technical criteria are addressed below.
FIGURE
MOULDS AND PRODUCTS
The variety of possible shapes of CEBs is illustrated in the table below.
In principle, they are produced using a specific mould for each shape, but it is also possible to integrate frogs into a mould to modify the way the force is applied (see chapter on PRODUCTION: special block moulds).
A wide variety will require interchangeable moulds, which is not always possible on all machines.
PRODUCT DIVERSIFICATION
It is certainly an advantage to produce several types of block, providing one has a use for them. Diversification should be based on a thorough understanding of the context (the market, the building systems, peoples) needs, etc.).
DECIDING ON SHAPES AND SIZES
The dimensions of a block are determined by :
- Its weight: it should be easy to handle, (max 10 kg).- The compressive force which will be applied: if the height of the block is too great, compression will not be evenly applied.
- Bonding patterns: the ratios of length (1) to width (w) are determined by the bonding pattern to be used (headers and/ or stretchers, plus the thickness of the joint). In order to avoid having to cut blocks, it is useful to have sub-multiples of the standard blocks, (e.g. 3/4 or 1/2 blocks). For partially hollow blocks, in addition to these constraints, the building systems to be used will be the determining factor.
FIGURE
DIFFERENT USES
It is important to understand the various possible building systems before launching a production.
Loadbearing and reinforced masonry
This most commonly requires full standard blocks, which are laid according to classic bonding patterns, and which have the following dimensions:
length (I) = 29.5 cm;
width (w) = 14 cm; height (h) = 9 cm.
For correct bonding, it is useful to have blocks varying in length. Thus 3/4 blocks, I =21.75 cm, and 1/2 blocks, I = 14cm, are in common use. The width of the block may also vary to obtain wall thicknesses ranging between 14 and 29.5 cm.
Differences in volume can also be introduced: horizontal channels to allow for cables or rods in the walls, or sometimes vertical channels (perforations) to lighten the blocks and/or to allow for cables or vertical reinforcement, e.g. in earthquake regions. The reduced strengths which perforations entail and the fact that they will be partially or totally filled with mortar which will generally have to be more highly stabilized than the blocks, giving a possible higher global stabilization cost (of blocks + mortar), should not be overlooked.
Chamfered blocks (with a straight or rounded edge), can also be useful, as they prevent wearing at the corners of walls.
Infill masonry
Conditions of use are more or less identical to those of loadbearing masonry, apart from the fact that much lower strengths can be used, since the blocks are not loadbearing. In this case, using hollow blocks (with vertical perforations) can be advantageous.
Special applications
Blocks can be used for facing floors and walls, for decoration, for claustra work, for interlocking building systems, etc. These are, however, fairly unusual building systems and require preliminary market and technical surveys.
FIGURE
FACTORS TO TAKE INTO ACCOUNT
Once the main building systems have been determined, which main types of block will have to be made?
As a general rule, the following approximate order of priorities will apply: various sizes to facilitate bonding patterns, horizontal voids (to allow for cables or ring beams), vertical voids (indispensable in earthquake regions), and finally non-rectangular blocks, the manufacture of which will depend more heavily on commercial than on building criteria.
Horizontal voids are easy to produce using frogs without having to change the moulds. Frogs can also be used to change the size of the block, but here changing moulds is preferable. For vertical voids, special moulds have to be used.
When choosing a press, these considerations will enable one to
decide on whether other moulds will need to be used or not, i.e. the degree of
interchangeability required.
TECHNICAL CRITERIA
Power supply
Manually operated presses will rely on the strength and endurance of the operator and this is why differences in the quality of products can often be observed over several hours.
Motorised presses significantly reduce irregular quality and allow higher forces to be applied. The motor should be easily accessible. For production units where the motor is used to power several interdependent work stations, accessibility and repairs often pose a problem.
Transmission systems exist in a number of forms, but fall into two main groups: mechanical systems and hydraulic systems. Mechanical system machines are in general fairly simple, but also rather heavy, unless special alloys, which have the disadvantage of being rare in some areas, are used. Hydraulic systems are vulnerable to environmental factors (dust and temperature). Under certain difficult conditions, the hydraulic fluid has to be changed frequently (once a month) and machines which are fitted with cooling systems require skilful maintenance.
Moulds
These have to be able to withstand strong forces for long periods and therefore have to be very robust. Mould interchangeability is a bonus if it is easy and simple. With presses equipped with several moulds (e.g. rotating tables), changing moulds is more difficult and arduous.
Products
Double compression applied simultaneously to 2 opposite sides has certain advantages: the least well compressed part of the block will be in the central area of the block rather than on the more critical external edge. Blocks of up to nearly 20 cm in height can be produced.
The ejection of the block from the mould, which is generally in the same direction as the compression, should not put any undue pressure on the still fragile blocks.
Dimensions: if the blocks are too small, the walls will take too long to build; if they are too large, their weight will make them difficult to handle.
The most common size (29.5 × 14 × 9 cm) is a good compromise between these two extremes.
The regular appearance of the block will depend above all on the quality of the soil used.
Productivity
Theoretical production cycles (as quoted by manufacturers) fail to take account of the many factors which are independent of the capacity of the machines (pauses, organisation of the labour-force, etc.)
Actual productivity often works out at nearly half of theoretical productivity. For manual presses, the theoretical cycle (filling, compressing, removing the block) takes between 30 and 60 seconds. For motorized presses, it takes approximately 15 seconds. For automated units, even in the best circumstances, it will never take less than 5 to 7 seconds.
Working conditions
There are bound to be three operations linked to the use of a press: filling, compressing, and removing the block. Whatever mechanism is used (manual or motorized) care must be taken to ensure easy access and well-located work stations for each of these.
Using a manual press is tiring and must be made as easy as possible (e.g. using a long lever arm).
Risks increase when a motor is introduced, so particular attention must be paid to safety measures (protective grills, ensuring manual operations are carried out at some distance from pistons, using circuit-breakers, etc.).
FIGURE
PULVERIZERS
These should enable one to obtain an earth where at least 50% of the particles held together by clay (lumps or nodules) will be less than 5 mm in diameter.
There are two main kinds of pulverizers:
- Blade grinders or cutters consisting of steel blades oscillating around an axis, which in turn rotates around a central, main axis.- Crushers which consist of two "treadmills" turning close to one another in opposite directions.
Grinders will break up not only lumps of earth, but also gravel and small stones, making them closer in particle size to sand. They work well with gravelly, or even stony, soils.
Crushers are more suited to finer soils, which contain no gravel or stones, and where it is mostly lumps of clay which need to be broken down.
Technical criteria
All pulverizers enable one to prepare dry soil but only rarely do they perform well with wet soils which require high speed impact. The choice here will be dictated by the kind of soil and the climatic conditions.
For less efficient pulverizers, it can be useful to place a screen which removes the coarsest particles as they are ejected. The height of the feeding hopper should not be too great to allow easy loading. The height at which the soil is ejected can also usefully be set in such a way as to allow a wheelbarrow to be filled directly.
Apart from for fixed units, the machine must be easy to move around (on wheels) as stocks of untreated or pre-prepared soil vary, so that ejection of earth from the machine is not hampered.
The pulverization system used should be easy to clean, as it can become coated in wet, compacted soil.
Delicate parts should be protected from dust.
SCREENS
Screening is indispensable if the pulverization is imperfect or with soils which contain too many large stones, but no lumps of clay. Diameters vary between 5 and 20 mm depending on the earth and the type of blocks to be produced (e.g. thin-sided, hollow blocks, etc.).
The system which gives the least rejected material should be chosen.
There are two main types of screen (not counting fixed manual sieves):
Rotating screens: a cylindrical grate or cylinder made of extended metal rotates around a horizontal, mechanically or manually driven axis.
Vibrating screens: these are motor-driven and consist of one or several superimposed grates in an almost horizontal position.
Technical criteria
Cylindrical grate screens have the advantage of being robust, but are generally more expensive and difficult to replace than screens fitted with extended metal. Replacing the screen grid itself must be easy. They should be designed in such a way as to allow direct filling of a wheelbarrow or a conveyor belt. For non-integrated units, wheels making them easier to transport can also be useful.
MIXERS
The extent to which materials are evenly mixed is fundamental to the quality of the finished block. Ordinary cement mixers are not suitable, as earth is highly cohesive when wet. Unlike concrete, earth forms lumps of compacted matter which cannot subsequently be compressed.
Some agricultural machinery such as motor cultivators, can be suitable. Here we consider more specifically designed equipment:
Planetary mixers: blades are fixed to a vertical shaft which turns inside a tank.
Linear mixers: a series of blades fixed to a shaft in the form of helical screws which turns inside a horizontal or vertical cylindrical tank. This system is more complex and more cumbersome.
Horizontal shaft mixers: here a series of blades are fixed to a horizontal shaft which turns within a cylindrical tank. The principle is similar to that of the linear system, but without the material being gradually pushed along.
Technical criteria
Earth which is mixed when wet is cohesive and requires more motor-power than concrete.
The shape of the mixer-blades should be examined to ensure that they penetrate well into the mix causing as little abrasion as possible, but moving it along as much as possible. Earth is highly abrasive, particularly when lateritic. The movement of the blades should on no account produce lumps of compacted material in the form of balls of earth. Usable capacities quoted are often calculated for concrete and should be adjusted downwards for earth. Operational capacity is generally about half of the volume of the tank.
Filling access should be easy and it should be possible to empty out into a wheelbarrow.
The tank should be equipped with a protective grill.
If the mixer is equipped with a sprinkling system, water will be added in a fine spray.
FIGURE
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