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View the document Making blocks
View the document Laying block foundations
View the document Finishing the mortar

Making blocks

There are three types of block suitable for foundation walls: concrete blocks, sand-cement blocks, and stabilized earth blocks (not adobe). Each requires different procedures and ingredients.

MAKING CONCRETE BLOCKS

Ingredients

The ingredients required for concrete blocks are cement, sand, and gravel in a 1:2:4 mix. The concrete is mixed using the method described on page 132-136.

Forms

The most common nominal size of concrete blocks is 20cm x 20cm x 40cm.


Block mold

 

The nominal size of a block is the actual size of the block + the thickness of mortar in which it will be set. Planning for 5mm of mortar between blocks, this means the actual size of the most common concrete block is 19.5cm x 19.5cm x 39.5cm. Thus, the forms used for these blocks should measure 19.5cm x 19.5cm x 39.5cm on the inside.

The forms should be made of wood 2.5cm thick, with removable cores (or dowels) to create holes in the block.

Procedure

The mixture should have enough water to make the mix plastic but not watery. The mix is placed in the form and hand-spaded or vibrated mechanically to remove air pockets.

If the mix is spaded, the forms should not be removed for at least 12 hours (or 2-3 days in cold weather). If the mix is vibrated mechanically, the forms may be removed 30 seconds after vibration.

The cores should be removed first: tap them gently and pull them out slowly. Then slowly remove the outside form. Avoid breaking the corners and edges.

The blocks should be kept damp for 7-10 days and should be sheltered from wind and sun for 28 days before use.

MAKING SAND-CEMENT BLOCKS

Ingredients

The usual mixture of cement-to-sand for sand-cement blocks is 1:6. A mixture of 1:8 can be used for block walls that will not carry roof loads, but this is not sufficient for foundation walls.

The best sand is clean river-bed sand composed of many different size particles. If it is dirty, the sand should be washed, or sifted through a fine screen (4-5mm square), since dirt will weaken the blocks. Do not use ocean sand.


Ingredients

 

Mixing Site

The site for making sand-cement blocks should be flat and shaded: if the blocks rest on an uneven surface, they will crack before they harden; and direct sun will dry blocks out before they can cure properly. If the mixing site must be in the open sun, the blocks should be moved to a flat, shaded area for curing.

Mixing

Following the mixing procedure for concrete (page 132), measure out the proportion of sand and then mix the cement in thoroughly. Add clean water in the center of the pile and turn the ingredients over at least 3 times until all streaks disappear and the mixture is even. The mix should have the consistency of concrete: wet. but not watery.

Molding Sand-Cement Blocks

Sand-cement blocks may be molded by hand or with a simple hand-press. Blocks made by press are generally stronger and more durable. But blocks made by either method are acceptable.

Hand Molding. If a commercially made mold is not available, a mold' can be made out of 2.5-5cm lumber in the dimensions desired. Typically, these blocks are:

Actual SIZE, cm

Nominal SIZE,

19.5 x 19.5 x 39.5

20 x 20 x 40

14.5 x 19.5 x 29.5

15 x 20 x 30

9.5 x 19.5 x 29.5

10 x 20 x 30


Retainer plate

 

Once the molds of the correct size have been built, shovel the wet mixture into the mold and shake the mold to settle the contents.

Then re-fill the mold slightly over the top and pack the mixture down with a spade or shovel. Scrape any excess off after packing thoroughly.


Pack with mix

 

 

Next, carry the full mold to the curing area.

Turn it upside down gently, and place it a few centimeters away from any previous blocks.

Then, lift the mold off: first the core;


First the core

 

then, the casing. Repeat the process until enough blocks have been made. Using several molds simultaneously will greatly speed the process.


Using several molds

 

Molding With A Hand Press. There are several types of hand press available for making strong sandcement blocks. Cinva-Ram and TekBlock presses are the most common, but any similar press will work as well.

There are three basic steps in the operation of most hand presses:

• loading the mold box;


Loading the mold box

• compressing the mix;


Compressing the mix

• ejecting the block.


Ejecting the block

With any hand press, it is a good idea to make a few test blocks before starting production in order to determine the correct amount of mix needed to make a strong block.

Curing Sand-Cement Blocks

Sand-cement blocks should be sprinkled with water after they have set for about 12 hours. They should be dampened at least once a day for 5 days. They should not be laid up in a wall for at least 12 days after being molded.

MAKING STABILIZED EARTH BLOCKS

Ingredients and Soil Selection

The selection of a suitable soil for mixture with cement is crucial in making strong stabilized earth blocks.

There are, roughly, 5 kinds of soil:

• gravel - rock ranging from 5mm to 7.5cm;

• sand - rock particles 5mm and smaller;

• silt - very finely ground rock, spongy when wet, but not sticky;

• clay - very fine grained earth, sticky when wet, hard when dry;

• organic soils - spongy, stringy appearance, odor of decaying, wet wood.

The best natural soil for stabilized earth blocks is sandy clay. As with plain concrete, the strongest blocks have a combination of fine, medium, and coarse particles. Earth that has only one particle size, or has mostly organic and/or coarse particles is unacceptable.


Stabilized earth block form

 

Testing Soil Acceptability

To determine whether local soil can be used to make stabilized earth blocks, use the following test, called the "jar test":

• Fill a glass jar about 1/3 full of soil.

• Add water to the 2/3 mark.

• Shake vigorously for 1 minute.

• Let the jar set for an hour. When the soil has settled there should be 3-4 distinct layers in the jar. If there are not, reject the soil.


Testing soil acceptability

 

Deciding How Much Cement to Mix With the Soil

Once an acceptable soil has been found, you need to determine how much cement to mix with it to make strong stabilized earth blocks. This can be done easily with a "shrinkage test":

• Make a box 4cm deep, 4cm wide, and 60cm long. Grease or oil the insides thoroughly.

• Fill the box with damp, but not muddy, soil.

• Pack the soil with a shovel or spade, especially at the corners.

• Level the surface with a straight edge or ruler.

• Place the box in the sun for 3 days, or in the shade for 7 days, and protect it from rain.

The soil should shrink and develop 3 or 4 cracks. If 6 or more cracks appear, or if the dried soil arches out of the box, reject the soil. It will not make good blocks. As long as there are less than 6 cracks and the soil lies flat in the box, measure the shrinkage by tapping the box and sliding the soil to one end:

If the shrinkage is:

use a cement: soil ratio of:

1 cm or less

1-15

1 cm to 2cm

1-12

2cm to 4cm

1-10

4cm to 5cm

1- 9

more than 5cm

reject the soil, or add sand and repeat the test


Measure shrinkage

 

Mixing the Ingredients

Once a suitable soil has been selected, it should be screened through a wire mesh with holes no larger than 5mm.

After the soil has been screened, measure out the proportions of earth, cement, and any sand to be added.

On a platform or other flat area, mix the ingredients thoroughly as for concrete.

Add water slowly with a watering can until the mixture is moist but not muddy.


Mixing the ingredients

 

To test whether you have added the correct amount of water, squeeze a handful of the wet mix into a ball:

• If it can be broken in two without crumbling and without leaving moisture in the hand, the amount of water is correct.

• If the mixture crumbles, it is too dry. Add more water and repeat the test.

• If the mixture leaves moisture in your hand, it is too wet. Compress the mixture and allow the excess water to run off. It is best to test the mixture frequently to avoid getting it too wet.


Test

 

Curing Stabilized Earth Blocks

The moisture in stabilized earth blocks should be eliminated slowly, under cover, with the blocks protected from sun and rain. For the first 3-4 days the blocks should be separated in single rows, never stacked upon each other. They should be sprinkled lightly with water twice a day for this initial period. After 4 days, the blocks may be stacked up to 10 layers high in a pyramid fashion with a little space between each brick. On the 8th day, bricks may be laid up in a wall where they will continue to cure and gain full strength in about 30 days.