 |  | Compressed Earth Block - Volume I. Manual of production (GTZ, 1995, 104 p.) | ||
 |  | Final pre-production operations | ||
|  | ''Running in'' production period | ||
| | Production tests | ||
| | Interpreting results and tolerances | ||
| | Training production staff | ||
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GENERAL OBSERVATIONS
During the setting up stage of the project, various estimates of quantities, methods, etc. may have been made. Products' performance requirements and consumption of materials, and thus their production cost, will therefore have been able to be evaluated. At the actual start-up of production, all these estimates will have to be finalized and examples of the material will have to be produced by manufacturing various blocks and evaluating the results, both in terms of their performance and their cost. Adjustments are made with the help of various tests and analyses carried out specifically at this stage, which enable first the quality standard values (or tolerances) to be set and then regular manufacturing checks to be carried out, taking account of the context and the means of production of the unit.
PRODUCT QUALITY
Quality is an important notion which is defined in relation to the needs of the end-user. Criteria used to assess quality vary greatly and will depend on the context. It is most important to identify them if possible during the setting up stage of the project (market survey, etc.) but at all events before the start-up of production. They should subsequently be reconsidered, as they may change.
Blocks are not always put to identical use. Blocks which are not going to be exposed to water, for example, do not necessarily need to be stabilized; buildings of one or several storeys height will not have the same block strength requirements; rendered blocks will not inevitably have to have a smooth finish, etc.
Quality can be defined in sum as the capacity to produce blocks to the standard which has been set in the light of their future use, no better, and no worse.
Each use will have a corresponding class of block: the primary objective is to produce the best possible blocks at the best possible price, which means satisfying both the end-user (the blocks he needs, at a price he can afford) as well as the producer (meeting his clients' requirements at a production cost which leaves him a profit margin).
QUALITY CONTROL
The objective of control procedures is both to know the performance levels of the blocks and to manage production costs. To know how the blocks perform, objective control procedures and tests, enabling one to measure quality indicators, need to be defined. The value of these tests will depend not on how complex they are, but on how rigourously they are carried out, however simple they may be.
Once this procedure has been defined, acceptable tolerances for the various measures must be decided. The higher the quality requirement, the lower the tolerance. This notion of tolerance is important, as during production, tests are no longer needed to give quantifiable results, but only to confirm the acceptability of the product or not. This has the advantage of being simple and cheap, whereas when precise quantifiable results are required, expensive and sophisticated equipment, often not within the means of most brickworks, is usually needed.
CONTROL PROCEDURES
There are several levels of control:
The first, product control (the blocks), allows the final result of the production unit to be evaluated. This deals mainly with the blocks' performances and little with production costs.
The second, manufacturing control, evaluates the resources and methods used for each operation. It enables one to find out if all the quantities and manual actions are correct and can detect the causes of any potential defects in the blocks before they are produced.
The third, organisational control, is essentially concerned with production costs. It enables labour outputs to be assessed and indications of the quality of the blocks to be given, if the resources available prove insufficient.
For each of these levels of control, procedures which suit the skills and resources of the brickworks have to be decided upon.
First product control is carried out until the blocks achieve the required standard. The manufacturing methods due to which this standard has been achieved are then checked to make sure that they are operational and can be reproduced, and target results for each operation are then set. Finally, the resources made available for each operation are checked to ensure that they guarantee optimum financial viability.
PRODUCT CONTROL
Once quality standards have been set, a way of measuring them must be found. The main parameters to examine are: dimensions, weight, appearance (smooth, rough, brittle etc.), water-resistance (immersion, capillary absorption, sprinkling, etc.).
MANUFACTURING CONTROL
The quality of the materials and the methods being used are checked to ensure that they enable the blocks required to be produced (analysis of constituent ingredients, quantities and proportions used, time taken, measurements of size, weighing out, temperature, humidity, visual observation, etc.)
ORGANISATIONAL CONTROL
Gaps between initial estimates and actual results are assessed. If the gap is too wide, corrective action will need to be taken, reviewing all the possible responses and justifying the choices made.
AN EXAMPLE OF THE CLASSIFICATION OF
MECHANICAL PERFORMANCES
These figures are indicative only. For actual figures, it is important to distinguish between minimum and average strength values. Test procedures should be described or refer to a norm (e.g. Belgian norms NBN B/24/201 etc.)
GENERAL OBSERVATIONS
The product must first be developed in the light of the materials and the equipment available. If the scale of the project and the means at its disposal permit, various laboratory analyses can be carried out (particle size analysis by sedimentation, Atterberg limits, Proctor test, chemical and mineralogical analyses, etc.) Clearly, it is rarely possible to carry out all of these analyses, either because they are very expensive, or because not all laboratories are sufficiently well-equipped. Very satisfactory results can, however, be obtained by using production tests the empirical nature of which can be limited by certain field tests and by ensuring that they are fairly rigourously carried out. Common laboratory analyses, such as particle size analysis by sedimentation and Atterberg limits, can also be useful. During production testing, blocks must be produced following all processing operations. At this stage, economic viability is not relevant, and the time taken will be the time needed to carry out each operation best (see chapter on PRODUCTION); only the quantity and quality of the constituent ingredients will vary.
ANALYSIS OF THE CONSTITUENTS
Testing methods for soil analysis have been detailed in the chapter on SOIL. Field tests relating to water, sand or gravel will be detailed in the chapter on QUALITY CONTROL.
Acceptability will depend on results, but in the first instance the objective is to know the exact nature of the material in order to be able to observe any variation in quality between deliveries.
SOIL
If several soils which seem suitable are available, several blocks (10 to 20) will have to be produced and then examined in a wet and a dry state before making a final choice.
MODIFYING THE PARTICLE SIZE DISTRIBUTION
Soil with a high stone or gravel content: if the soil contains too many coarse particles, but the fine particles are evenly graded, passing it through a 10 mm or a 20 mm sieve will be enough to obtain a good soil.
Soil with a high clay content: if the soil has too many fines, it will need coarser grains to be added, sand and/or gravel, or even another soil containing mainly sand and gravel but very little clay. Clay can also be washed out, but the procedure is a fairly delicate one.
Procedure: whether a particle fraction has been removed by screening or added by mixing, the procedure for determining the proportions is almost identical. First a rough estimate of the proportions is found, e.g. by using the jar test. Then several series of blocks are manufactured using different proportions, and finally the proportions resulting in the best blocks are retained.
Testing: approximately 20 blocks are made for each differing proportion, making sure that the moisture content is at the optimum (using the drop test). The freshly moulded blocks are examined: if they are cracked when the moisture content is optimum, the proportions used are rejected. Blocks which are in good condition when freshly moulded are allowed 2 to 3 days of wet curing and one day's drying. At that point, the proportions resulting in the best blocks are retained.
FIGURE
TESTING FOR THE AMOUNT OF STABILIZER AND FOR CURING TIMES
Proceed as for particle size modification, gradually increasing the quantity used and the length of the curing time, and then examine the dry blocks in order to determine the results which are the closest to those required.
Economic considerations should not be neglected. The cost of cement can, for example, represent up to 50%, generally 20 to 40%, of the total cost of production. The amounts of cement used generally fluctuate between 4 and 10% (see sketch below). Approximately 20 blocks are produced for each different amount used, and for each sample of ± 20 blocks, i.e. for each amount used, 6 or 7 blocks are given, for example, 3 days wet curing + 11 days drying out; 6 or 7 more blocks: 5 days wet curing + 9 days drying out; and the final 6 or 7 blocks, 7 days wet curing + 7 days drying out (see sketch below).
The dry blocks are then examined (for appearance, weight, measurements, breaking point, immersion etc.), see chapter on QUALITY CONTROLS).
The lowest amounts and shortest curing/ drying times giving the results required at the lowest production cost will be retained.
CHECKING THE OPTIMUM MOISTURE CONTENT (OMC)
This means carrying out a static Proctor test, where compaction takes place in a press. The objective is to find out the amount of moisture giving the highest density, or in other words the heaviest blocks.
Once again, successive tests are carried out, with the drop test giving a rough approximation of the OMC. A first mix close to the OMC is then produced, assuming, for example, that 7 litres of water will be needed for 60 litres of dry material (soil, cement, sand, etc.) Some ten blocks are produced using this amount of water and these are then all weighed to obtain an average weight.
A second mix is prepared, but this time, with for example, 1 litre more water, i.e. 8 litres for 60 litres of dry material. All the blocks are again weighed. If the average weight is higher than the preceding average, the operation is repeated, adding more water until the average weight falls. If the average weight of the second mix blocks is lower that than of the first, a third mix using less water than the first is prepared, e.g. 6 litres instead of 7, until the average weight falls.
MEASURING MOISTURE CONTENT
To determine the exact percentage moisture content for each mix prepared, take a small quantity of the wet mix, weigh it, dry it out (in a kiln, on a stove, in the sun, etc.) and weigh it when dry. The moisture content is calculated as follows:
Note
Knowing the quantity (in litres) and the OMC (%) is useful, but as these are calculated using dry soil, they can differ widely for moist soil (due to climate, rain etc.) Soil is in fact almost never completely dry in most "normal" ambient conditions.
FIGURE
FIGURE
TESTING
The various procedures are detailed in the chapters on SOIL and QUALITY CONTROLS as well as on the previous pages. The importance of tests must not be overlooked; it is thanks to them that the best blocks can be produced at the lowest cost. It is also due to them that the credibility of building with earth will increase and thus encourage the emergence of new markets for this building material.
INTERPRETATION
The principle of all production tests is to use a structured step-by-step approach to arrive at the best results. This is certainly empirical, requires not very much equipment and undeniably gives good results.
In interpreting results, one must consider first the performances required but also current production, checking that the resources, methods and personnel being used during the running-in period are realistic and economically viable.
Economic aspects must not be overlooked when deciding on manufacturing procedures. Results can vary very widely depending on whether labour cash, or on the contrary materials and mechanization costs are high.
In choosing a soil, for example, it is important to know if transporting a very good soil requiring no modification and no preparation works out cheaper than using an average soil, extracted on site, but which requires preparing and modifying to achieve the same results. One must also bear in mind that modifying a soil by repeatedly adding measured quantities of material needs to be controlled by a skilled person and that any losses due to inaccurate measuring out also need to be evaluated.
Mechanizing preparation and mixing operations can often enable the rate of stabilization to be reduced, provided that it is well done.
As can be seen, a good analysis of the overall context is crucial before making final decisions. It can sometimes be preferable to start out with a simple brickworks in line with the skills and financial resources available, rather than find oneself out of one's depth in the management of a complex brickworks.
TOLERANCE LEVELS
Raw materials
- Soil: tests should give results which are positive or close to recommendations so that any modifications which are necessary are not too difficult.- Sand or gravel: these are used to correct soils with a high clay content. They must therefore not contain too much clay. Generally, they cost more than soil and therefore should be used as little as possible. It can sometimes work out cheaper to use small quantities of clean (river) sand than large quantities of sand with a high clay content.
- Cement: ordinary Portland Cement (CPA 250 or possibly 350) or the equivalent is highly suitable. Care must be taken that it is not past its usable date, nor damp.
- Lime: excellent results are obtained with non-hydraulic slaked lime (or quicklime). The degree of impurity varies greatly, and therefore several tests for the amount to be used will need to be carried out.
Checking measuring container volumes
Knowing the volumes of the measuring containers used (wheelbarrows, buckets, etc.) is crucial for controlling costs and quality. Volume precision should be in the order of + 5%, i.e. a margin of error of i 3 litres for a 60 litre capacity wheelbarrow, or + 0.5 litre for a 10 litre capacity bucket. An error of 1 kg of cement per 120 litre mix can represent an additional cost of 2 to 5% of the total cost price.
Particle size distribution modifications
Sand (or gravel) is generally more expensive than soil, making it very important to calculate the correct proportion needed accurately. An error of 1/2 a bucket of sand, for example, for a proportion of 30% sand can represent an additional cost of 2 to 5% of the total cost price, i.e. approximately $5 to 10 per thousand standard blocks.
Amount of stabilizer to be used
As cement costs often account for 20 to 50% of the cost price, a difference of 1% in the rate of stabilization can have an approximate effect of 4 to 8% on the cost price, i.e. $10 to 20 per thousand blocks. One can check that the correct amount is being used by calculating the quantity of cement required for the manufacture of a given number of blocks. For example, for 6% cement, one 50 kg sack should be enough for 115 to 120 blocks (the calculation should be done using dry densities). Approximately + 4% tolerance is acceptable.
Curing and drying time
The duration of the wet curing and drying out periods is crucial to the ultimate quality of the blocks. A drop in quality due to poor wet curing for cement or lime stabilized blocks can lead to the need for a sometimes significantly higher rate of stabilization.
On the other hand, a prolonged wet curing stage demands large covered areas and storage areas. Accurately determining the curing time needed will ensure a balance between these two constraints (see chapter on STABILIZATION).
Optimum moisture content for moulding
A 1 to 2% error in the moisture content can lead to a drop of 10 to 50% in the dry density of the blocks.
The same operator should always be responsible for measuring out and adding water so that he can rapidly gain sufficient experience to determine the OMC.
Duration of mechanical mixing
Mixing for too long will slow down the rhythm of production, allow lumps to form and cause the machines to wear; mixing for too short a time will give an uneven mix. The optimum duration is generally 1 to 2 minutes for each mix (dry and wet) i.e. 3 to 4 minutes in total. Blocks can be produced after mixing times of 3 to 5 minutes and examined when dry (for appearance, strength, internal texture, etc.).
Compression
Once the amounts used have been checked, the way the press is being filled and the rate of compression are checked by counting how many blocks are obtained for each mix; the number should not vary by more than 5%. The moulds and the regularity with which they are filled can be checked by examining the dimensions and the parallelism of the blocks. Deviations should not exceed 2 to 3 mm. Variations in the direction of compression (height) may be due to irregular filling.
Appearance
Cracks are not acceptable on faces to be exposed to water. On other faces, they should not exceed 1 mm in width or depth or 10 mm in length and there should be no more than 3 on each face.
Lamination (due to too much clay or too high a moisture content)
is not acceptable. Chipped corners should not exceed 10 mm. Holes or scratches
should not exceed 1% of the surface area for smooth blocks, but can be as high
as 10-15% of the surface area for rough blocks.
GENERAL OBSERVATIONS
The production of compressed earth blocks does not require very high skills. But good initial training will enable good productivity to be achieved, whilst at the same time maintaining good product quality. CEBs are not a traditional building material. This is an important distinction: adobe blocks, for example, require too wet a mix for CEBs; fired bricks, too high a clay content.
Training should cover manual proficiency, quality standards, work coordination, safety regulations, and maintenance of equipment and of infrastructure. Essentially it will take the form of practical exercises and demonstrations.
QUALITY STANDARDS (see chapter on QUALITY CONTROLS)
Quality control procedures are crucial for obtaining quality products. If they are to be efficient, these procedures must have the commitment of the staff who must understand their importance. This means encouraging them to take pride in their work, so that each of them can feel involved in the final product. Control procedures which suit the context and the size of the production unit should be set up.
Exaggerating the effect of each standard and then offering an explanation is generally an effective way of proceeding.
Soil quality
Blocks are made using extremes of soil type (too much clay, too much sand, etc.) and the results examined.
Moisture content for compression
Blocks are produced too dry, too wet and finally with the correct moisture content.
Mixing
Blocks are made using untreated barely mixed soil, and then with a well mixed and prepared soil.
Amounts of stabilizers used
Blocks produced using different amounts of stabilizers are broken.
Retention time
The delay between mixing and compression is made to vary.
Compression
Blocks are first made from a badly filled mould and then from a correctly filled one.
Curing
Blocks correctly cured are compared with others which have been exposed to direct sun and wind immediately after removal from the mould.
For all these exercises, once the blocks have been produced, tests enabling the operation to be controlled are introduced. One should also try to illustrate the effect which badly carried out operations have on cost price by using telling images such as the number of blocks produced for each sack of cement, placing an empty sack in front of the corresponding pile of blocks, etc.
MANUAL PROFICIENCY (see chapter on PRODUCTION)
Manual proficiency will come with practice but explaining a few "tips" can speed up the learning process. Dangerous or tiring work positions must also be avoided.
It is also important that all the members of the team should have been trained for each task. This enables them to stand in for each other but above all to appreciate the difficulties of each task and not to blame each other unjustly.
At the same time some flexibility should be allowed initially so that each worker can gradually find the job which suits him best.
WORK COORDINATION
Each worker must realize that the production of a block is a series of interdependent operations and that everyone has therefore some shared responsibility for the final result. Here too examples of poor coordination can help understanding, but initial recruiting, remuneration principles, whether the management style is participatory or not, also play a part. The organigramme of production and responsibility must be carefully drawn up and reassessed.
SAFETY
There is always a danger of accidents occurring even with minimal equipment (crushed hands, the compression lever springing back, screened soil being projected, etc.). For each task, a list of potential accidents should be drawn up and simulated, and safety rules decided on in the light of these.
MAINTENANCE OF EQUIPMENT AND OF INFRASTRUCTURE
The degree of maintenance will directly influence the products and therefore indirectly the financial return. Each person is therefore involved and should be capable of carrying out routine maintenance with the necessary means. Adjustments or more delicate repairs on the other hand should be carried out only by designated skilled staff.
For the first few weeks, great attention must be paid to the staff and there should be no hesitation in devoting the time needed for each member of staff to understand exactly what he has to do.
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