One of the smallest but most important members of the structure is the block. Almost all walls in Rural Building are erected with blocks, preferable landcrete blocks.
This word comes from the words laterite, land, and concrete.
The land on which we live provides us with the laterite; the first syllable is a combination of the first two letters of laterite and the last two letters of land.
Concrete as well as landcrete contains cement. in order to show this, the last syllable of the word concrete is used, making the word LANDCRETE.
Landcrete is a low cost, long-lasting and attractive building product. This chapter is about making landcrete blocks using a hand operated block press.
This type of soil is found throughout the tropics. Its colour can vary from white-grey to a dark red, depending on the iron content. Laterite consists mainly of fine and coarse sand mixed with clay.
Laterite has been used to make houses for a long time, but such walls break down easily and get washed away by rain. Pressing the soil into blocks makes it easier to build the walls, and they are stronger and more resistant to rain. By adding some cement to the laterite it is stabilized and makes even better blocks. The basic material, laterite, costs nothing and is usually found on the building site. It is easy to find good soil for building or to mix it with sand or clay to make it good.
The basic steps of the operation to make landcrete blocks are fairly simple: first good soil is found and tested; then it is prepared for the block press, with the addition of cement or lime if available. The soil or soil-cement is put into the press and compacted, raised out and removed for curing.
NOTE: If the blocks do not contain cement they are not called landcrete; they are simply called 'laterite blocks".
- a - Laterite soil: composed of sands, silt and clay
- b - Water: to wet the soil; it should be clean
- c - Cement: to stabilize the soil.
NOTE: If you have no cement you can use lime (twice as much as the amount of cement recommended) or else just make plain laterite blocks.
- a - Block press with proper mounting rails and a wooden handle (2,5 m long)
- b - Box for shrinkage test
- c - Headpan, box or bucket for batching
- d - Pick-axes and shovels for digging, mixing and filling.
TESTING AND CHOOSING THE SOIL
Most soil is suitable for making blocks, but it must be tested first to find out how much sand, silt and clay it contains.
Dig a small pit for testing. First remove and set aside the top soil where plants or grass may be growing (25 to 50 cm deep). This soil should not be used for blocks. Dig out the soil under the top soil. The deeper soil may be sandier, which is usually better for making blocks.
Now make three tests:
- a - Drop test
- b- Jar test
- c - Box test.
Take a handful of soil which is wet enough to form a ball, and squeeze it in your hand, but not so tightly that the water is squeezed out.
Drop the ball from about one meter high onto hard ground. If it breaks up into only a few pieces, the block-making quality is good. If it breaks completely up, there is either not enough water in it or not enough clay, and the quality is bad.
USE SOIL THAT CONTAINS: 5 - 30% CLAY AND SILT AND AT LEAST 30% SAND
This test separates the sand from the clay and silt, so that we can measure the quantity of each.
First dig out some soil (not top soil). Fill a glass jar half-full with the soil (Fig. 1).
Fill the jar up with water and add two teaspoons full of salt, to make the particles settle faster (Fig. 2).
Cover and shake or stir the jar for two minutes to mix the water thoroughly with the soil.
Set the jar on a level surface and leave the soil to settle for several hours. Sand and gravel will settle to the bottom, leaving the silt and clay on top (Fig. 3).
Measure the height of each layer to find the total amount of sand and the amount of clay and silt, compared to the total amount of soil.
The soil you use should contain at least 30% sand (about 1/3), and between 5% and 30% clay and silt. If there is not enough clay add more or else find some better soil. You can also add sand if necessary.
This test shows the quality of the soil and allows you to determine the amount of cement you should use with it.
Use an open wooden box with inside measurements of 60 cm by 4 cm by 4 cm (Fig. 1). Oil or grease the inside of the box.
Fill the box with very wet soil. Compact it well, especially in the corners, and level off the top with a stick or the edge of your trowel (Fig. 2).
Put the box in the sun for three days to dry, or in the shade for seven days. It should be protected from rain.
The soil will shrink as it dries. Do not use soil for blocks if it has many cracks in it (Fig. 3) or if it has arched up out of the box. Don't us< soil if it has shrunk more than 5 cm. Either find some better soil, or Improve the soil by adding sand, since it is the clay which causes shrinkage.
Measure the shrinkage by tapping one end of the box on the ground so that ail the soil slides down to one end (Fig. 4). The cracks will close and you can measure the shrinkage space at the top end (Fig. 5).
The amount of shrinkage tells you how much cement you should use. The more shrinkage, the more cement is needed. Use the table below as a guide for the amount of cement to be added to the late rite.
|Shrinkage||Cement to soil|
|0-10 mm||1 :35|
|10 - 20 mm||1 :30|
|20 - 30 mm||1 : 25|
|30 - 40 mm||1 :20|
|40 - 50 mm||1 :15|
PREPARATION OF THE MIXTURE:
After you have found good soil and the correct amount of cement to use with the help of the box test, the soil mixture must be prepared for the block press.
If you have no cement and must make laterite blocks, you follow the same sequence as described below.
a - Remove the top layer of soil (Fig. 1).
b - Dig out the soil you want to use and pile it (Fig. 1).
c - Measure the required proportions of laterite and cement.
d - Make a dry mix of the batch.
e - Add water and make a wet mix.
f - Check the moisture content using the drop test.
If you are making laterite blocks, steps c and d are of course left out.
Before you start batching, the laterite must be broken up so that no lumps remain. This is usually done by beating the soil with the back of a shovel or with a piece of wood. Large stones are removed.
Use flat, hard ground for mixing. If no such place is available, prepare a mixing platform before you start working.
Spread the laterite out until it is about 10 cm thick. Spread the cement evenly over all the soil. Mix the cement and soil with a shovel until the mixture is of an even colour throughout (about 3 times - see Rural Building Materials, page 163).
Spread the heap again, sprinkle a little water over it and mix. At this point, test the mixture for moisture with the drop test. If it is too dry, spread it out again and add more water.
The soil-cement mixture is now ready for the block press. There should be enough mix for about 7 or 8 blocks in one batch at a time.
NOTE: Never prepare more than you can use up within 30 minutes (about three batches). It is better to mix small amounts more often.
PRESSING THE BLOCK:
a - Place the block press in its rails on flat, solid ground near the mixing platform.
b - Open the mould box by swinging the handle down to the lower roller.
c - Half-fill the mould box with the laterite cement mix (Fig. 1).
d - Press the mix firmly into the corners with a piece of wood.
e - Fill the mould to the top and compact the corners again.
f - Add a little more so that the mould is filled flush to its top edge.
g - Swing the handle quickly over to the othjr pide and press the block until the handle has reached a horizontal position (Figs. 2 & 3).
If the mix is too dry, the handle will net go all the way down to the horizontal position. in this case do not force it, as the handle may break.
Instead, eject the unfinished block so that you can refill the mould box after adding a little water to the mix.
On no account should more than one man at a time work the handle!
h - Raise the block out of the mould box by swinging the handlu back against the lower rollers (Fig. 4).
i - Lift the block carefully off the machine and place it for drying.
The freshly made block is not strong yet. If it breaks or cracks very easily, the mixture is not correct. Try a different mixture.
Hold the block in such a way that your fingers are not caught under it when you put it down, so as not to crumble the edges.
This is the term originally used to describe the chemical change in glues when they set, meaning when they become strong and hard.
As far as cement products are concerned, curing simply means the after-treatment of any of those products.
If the blocks contain cement, we talk about "curing". If the blocks don't contain cement, we talk about "sun-drying".
The blocks with cement must now be cured for about two weeks while the cement sets, a is important to follow the directions for curing. If you do not, the blocks may be weak and full of cracks, and therefore unusable for building.
a - Remove the block from the block press, holding it carefully.
b - Place it on leaves, grass or boards on flat ground. The block should not touch the ground (Fig. 1).
c - The blocks should be under shelter or covered with something so that they are out of tht rain and sun for at least the first day.
d - Let the blocks dry like this for one day.
e - After one day the blocks are a little stronger so that they can be stacked for further curing.
Stack the blocks on boards or on very flat, hard ground up to five blocks high. Place them so that they touch each other. Make the stacks under a cover if possible, to keep them out of the sun (Fig. 2).
f - The blocks must be kept moist by sprinkling water on them twice a day. Put grass or leaves on top to help keep them moist (Fig. 2).
g - After two weeks of watering the cement has set properly and the blocks can dry completely. They are now ready for use.
Blocks without cement simply need to be dried in the sun. Let the blocks dry in the sun for two weeks; then they are ready for building (Fig. 3).
PLANNING THE WORK
PLAN OF OPERATIONS:
Good planning can make the work of block-making go faster and easier.
The places where the different steps are carried out should be as close to each other as possible, so that there is a continuous step by step flow of laterite from the soil pit to the finished wall. The soil and blocks should be transported as little as possible.
If there is more than one building to erect, some operations can be moved.
There should be a smooth flow (Fig. 1) of:
- a - digging
- b - transporting
- c - batching
- d - mixing
- e - testing
- f - filling
- g - pressing
- h - raising
- i - removing/drying
- j - stacking/curing
- k - walling up
You can make a layout like the one in the picture (Fig. 1) or you can lay out the operations in a straight line; or anything in between, whatever suits you and the situation best.
Several factors can be important in deciding what sort of digging pits you will have. If the good soil goes deep, all the soil can come from one pit. However such a large pit might be ugly and undesirable; several small pits could be a better solution in soma cases.
The possible future uses of the pits should also be considered. They could form a part of a drainage system, a water storage tank, a sewage pit, a soak-away, and so on.This of course, provided that the planning is done beforehand.
Stacking, curing and transporting the blocks tc the actual place of building can be done every morning by the soil diggers.
They can do this only if they have dug some soil in advance during the previous day, so that the rest of the workers have the materials to continue their operations of batching, mixing etc.
From one to fifteen men can work on block-making. If there are plenty of workers, they should be organized to keep the block press going constantly, so it is used to maximum efficiency,
To do this, there must be a steady supply of soil-cement mix ready to put into the machine. Make the mixing platform big enough so that there is room for one pile of already mixed landcrete and one pile which is being mixed.
You can use the rough table below as a planning guide to divide the labour, but expedience will be the best guide.
|NUMBER OF MEN||KIND OF ACTIVITY|
| O |
|1||2||2||2||1||BATCHING AND TESTING|
|1||1||1||FILLING AND OILING|
|1||2||REMOVING AND PLACING TO DRY|
In any case the work should be divided so that everyone is busy all the time. If the block press filling worker has to stop and wait for prepared landcrete, he or another man should be switched to doing soil preparation.
Workers should relieve one another in their jobs every few hours to prevent boredom with the work. After a few days of such rotations, the workers will each become skilled and efficient at three or four of the different steps of block-making.
It is important to share the work fairly to keep up the morale and enthusiasm among the workers.
NUMBER OF BLOCKS:
You should know from the start approximately the number of blocks that will be needed for the building. This is necessary to be able to schedule the block-making, curing and building.
To find the approximate number of blocks to be made, you must know the size and plan of the building. Take measurements of the lengths of the walls and add these up to get a total. Multiply this wall-length by the total height from the plinth course to the top of the wall; this gives you the total wall area in square meters. This number, multiplied by 13,5 (the approximate number of clocks per square meter) gives you the total number of blocks to be made.
-REMEMBER: Wall area in square meters x 13,5 = Number of blocks.
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