 |  | Compressed Earth Block - Volume I. Manual of production (GTZ, 1995, 104 p.) | ||
 |  | Quality controls | ||
|  | (introduction...) | ||
| | Controlling raw materials | ||
| | Manufacturing controls | ||
| | Product control | ||
| | Organizational and equipment control | ||
| | How to carry out control operations | ||
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GENERAL OBSERVATIONS
The objective of production controls is to check that production conforms to set objectives. This does not necessarily mean obtaining a precise measurement, but rather checking that the end result is correct due to comparisons with levels set during the running-in production phase (for tolerances, see chapter on FINAL PRE-PRODUCTION OPERATIONS).
RAW MATERIALS
Soil
As soils will have been identified and selected beforehand, it is only necessary to check that a delivery of soil is more or less identical to the soil originally selected. The cigar test can be used, checking that the length of the cigar obtained with the tested soil does not differ from the measurements obtained with the original soil.
The jar test can also be carried out, checking that the proportions of different soil fractions are similar, by using identical containers and placing the original soil alongside the tested sample.
Water
The presence of salts in the water is to be avoided, particularly if the blocks are stabilized with cement or lime. Check that the water is clear and that it is not salty. Let it evaporate completely and check for any deposits. These could include organic matter, which are acceptable only in very small quantities, or salt crystals which are totally unacceptable (see sketch).
FIGURE
Sand or gravel
These are used as additives to lower the proportion of fines, and must therefore be checked to ensure that they do not contain too much clay, using the jar test. If the water above the sand and gravel is cloudy, this suggests the presence of clay, which should not exceed 10 to 15%. The evenness of the particle size distribution can be checked at the same time by comparing the sample tested with a standard sample.
Cement
Cement must be checked to ensure that it has not started to react on contact with moisture. A little cement is sieved using a 1 or 2 mm mesh; if tiny balls of cement which cannot be crushed between the thumb and forefinger are left behind, then the cement is bad. If the delivery cannot be refused, it may still be possible to use the cement by passing it through a 0.5 mm sieve and then mixing it with 50% good quality cement.
FIGURE
The strength of the cement can also be checked. Make a mortar using a 1:3 mix of cement sand and mould it into short bars. Wrap up the bars and the moulds in a plastic bag.
24 hours later, remove the bars from the moulds and allow them to cure, either submerged in water, or in a hermetically sealed package. Subject a bar to a tensile force after 24 hours, and another after 28 days: they should withstand minimum 100 g and 500 g respectively (see below).
FIGURE
Hydrated lime
Lime should not be too impure. It can be tested in exactly the
same way as cement by comparing the sample bars with control bars made from lime
of known impurity.
For large-scale projects, it is advisable to obtain the
professional opinion of a
technician
PROCESSING OPERATIONS
Soil preparation
Proceed as described before for controlling raw materials, i.e. using the cigar or the jar test and comparing results with those of control samples.
Measuring out
This is controlled by weighing or counting, allowing a tolerance of ± 5%:
- the number of blocks obtained per mix;
- the number of mixes made per sack of cement used;
- the number of blocks obtained per sack of cement used.
The accuracy of the counting can be checked by cross-referencing the various results (see below).
Mixing
- Manual mixing: count the number of times the pile is turned. This should be at least twice for each mix (dry and wet).- Mechanical mixing: check the time taken for dry mixing and wet mixing 3 to 5 times: the tolerance is 5 to 10%.
- Dry mix: Visual examination of the evenness of the mix (texture, colour).
- Wet mix: Check that the moisture content is at the optimum using the drop test:
1. take a fistful of moist material and shape it into a ball in the hand;2. drop the ball from a height of -1 m onto a hard surface;
3. observe the result: if the ball has completely disintegrated, the mix is too dry; if it has broken up into 4 or 5 pieces, the moisture content is right; if it has flattened out without breaking, or broken into 2 pieces, the mix is too wet.
Retention time (see above)
FIGURE
Time the delay between mixing and compression. For cement stabilization, this should not exceed 5 to 10 minutes. For lime, the delay should be no less than 2 hours and preferably 8 to 16 hours.
Compression
Check the freshly moulded block for:
- weight, which should not differ from the optimum weight by more than 5 to 10%;- appearance (tolerance criteria are detailed on p. 74);
- dimensions, and also any deviation from right angles: these should not vary by more than 1 to 3 mm. Measurements should be taken using a ruler and a mason's set-square (see below).
FIGURE
An orthogonal gauge can also be made from wood or metal and used to measure dimensions and parallelism at the same time, with small nicks at the ends showing acceptable tolerances (see below).
FIGURE
The strength of the block can be checked with a pocket penetrometer. Carry out at least 5 tests on each side; tolerances will be set according to the performance levels to be attained (see below).
Curing
Examine the quality of wet curing, notably check for condensation (drops of water) (see below) on the inside the tarpaulin or polythene sheets.
The moisture content of blocks while they are curing can also be
measured, and compared to their moisture content when freshly moulded. The
moisture content of the block during curing should not vary by more than 1 or 2%
from the moisture content on compaction.
GENERAL OBSERVATIONS
The minimum number of blocks tested is approximately 5, but this will depend on productivity. Time taken to carry out control procedures should not interrupt the normal working of the brickworks. Tests must therefore be listed in order of importance and made to suit the availability of the person responsible for carrying them out, so that the most important ones (weight, dimensions, breaking point) can be regularly carried out. Blocks should be checked during curing and without fail after regular curing periods for results to be comparable.
WEIGHT, APPEARANCE, DIMENSIONS AND PARALLELISM
Tests and tolerances are identical to those previously described. At this stage the object is to check for any changes in the blocks compared with their state when freshly moulded: a variation in moisture content can be observed in the block's lower weight, and shrinkage in smaller dimensions or in the appearance of cracks. During wet curing, the moisture content should not fall by more than 1 or 2%, i.e. a lowering in weight of approximately 150 to 200 grams maximum for a standard block. Shrinkage should not be more than 1%. For the acceptability of cracks, see chapter on FINAL PRE-PRODUCTION OPERATIONS.
RUPTURE TEST
This test enables one to find the bending strength of the block and by extrapolation its compressive strength. This is an important criterion as it is the one which is most commonly used for most materials. The test is carried out using a site block-breaking apparatus (see below and annexes). This gives a satisfactory estimate of bending strength.
The block is placed on its lower face, compression side downwards, perpendicularly across two tubes laid 20 cm apart. A third tube is placed across the middle of the upper face, parallel to the first two, and a plate attached to it is loaded with blocks or sacks of cement at a rate of approximately 250 kg/minute. Bending strength (of) is calculated from the load required to break the block as follows:
D = the distance between the two tubes in cm (here 20 cm)
L =
the load in kg (including the loading plate)
w = the width of the block in cm
h = the height of the block in cm
k is the coefficient used for hollow blocks, for full blocks k = 1.
This formula is used only for measuring the strength of the block, but a minimum acceptable load can also be set. For example, the tested block should withstand a load of 15 blocks to be acceptable.
The compressive strength (rc) can be deduced by multiplying (rf) by a factor K which will depend on the nature of the soil; this if often between 5 and 8 but must be determined more precisely by laboratory tests for bending and compressive strengths.
rc = K × rf
This test gives a good general idea of the performance of the block.
INTERNAL TEXTURE
Examine the internal texture of the broken blocks. Different soil fractions should be evenly distributed. If they are not, generally as a result of poor mixing, there will be concentrations of gravel or of coarse sand, or stained or lumpy areas. In all cases, such blocks should be rejected.
FIGURE
BRUSHING AND PIERCING
Each broken block is divided up into two samples, one kept as a control and the other used for testing. A styles is jabbed into the block faces. For brushing, a metal brush is applied to two or three sides, using equal force and an equal number of backwards and forwards movements.
These tests give an idea of the surface strength of the blocks. Acceptability limits will be set according to the intended use of the blocks and whether they will be exposed to abrasion and knocks.
IMMERSION
Use the half blocks which have been subjected to brushing and piercing in order to obtain a worst case result.
The half-blocks are completely immersed in water for 6 hours, then allowed to dry for 42 hours, giving a 48 hour total dry-wet cycle. The cycles can be repeated several times. Immersion can be replaced by fine sprinkling using a spray head (see photo below).
LABELLING THE BLOCKS
When the blocks have passed all the tests, they must be
labelled. The date of manufacture should be specified, as well as any
classification of the block. Writing should be easy to read, even during wet
curing where the labels will also appear on the outside of the tarpaulins.
ORGANIZATIONAL CONTROL
This is to check if the best organizational methods are being used, checking if staff are correctly allocated around the production site and their skills being correctly used, and that they have the tools they require.
For each task, targets should tee set teased on the equipment available, on the results obtained during the running-in period of production, etc. These target figures - for outputs, numbers employed, quantities of materials consumed - are then compared with actual results. If shortfalls between targets and results are too great, the reasons for these must be analyzed, once the original target estimates have been double-checked.
For each operation, one should ask oneself:
- if it is really necessary (why?);
- if those carrying it out have the right skills (who by?);
- if the resources and methods used are adequate (how?);
- if the layout is right (where?);
- if the operation is permanent or temporary (when?) (see top right).
Before answering each question, run through all the possible responses and the reasons for them. The same approach can be used to check the cost price by quantifying each element (wages, investments, running costs, etc.)
EQUIPMENT CONTROL
It is crucial to know the state of machines in order to be able to maintain them (cleaning, oiling, replacing parts, setting etc.) at-convenient moments and also to be able to spot the causes of possible breakdowns and alert the manufacturer. Technical incidents can be due to defects in the machine, but are most often due to poor work practices, bad settings or insufficient maintenance.
All maintenance tasks should be listed (number of oiling points, levels or settings, etc.) and their frequency set (replacing disposable parts every so many blocks, complete cleaning every so many days, etc.) (see table on the right).
Frequencies specified by the manufacturer should be checked as they can vary depending on the context.
All repairs should be described and filed in order to be able to analyze them and to take avoiding action in the future.
FIGURE
FIGURE
FIGURE
FIGURE
"RESPONSIBILITY TREE" (see below)
It is crucial to know set levels of responsibility for each person involved in control procedures.
- The head of the brickworks analyses results and makes decisions on fundamental changes.- The team leader or supervisor carries out tests, collects data and takes decisions on minor adjustments.
- The labour force quantify operations (counting etc.) and strive to carry them out in the way which has been decided upon.
FREQUENCY
Each control procedure or check should be carried out at a given rate of frequency depending on the level of the analysis and the way the brickworks operates (see table on the right).
CONTROL TOOLS
Wherever possible, operations should be quantified (numbers of blocks and amounts of raw materials, number of mixes, transport operations, etc.).
Counting should be done by the operators (the labour force), and recorded and checked by the team leader or supervisor. Blackboards are the best way of counting during production. Figures are then copied down onto control cards which are then analysed and classified on data sheets. Each of these support documents corresponds to given levels of responsibility and frequency.
FIGURE
FIGURE
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