Cover Image
close this book Handbook for building homes of earth
View the document Table of contents
View the document Foreword
View the document Chapter 1: Introduction - Types of earth houses
View the document Chapter 2: Soils - And what can be done with them
View the document Chapter 3: Stabilization of soils
View the document Chapter 4: Where to build
View the document Chapter 5: Foundations
View the document Chapter 6: Lightweight roofs
View the document Chapter 7: Preparing the soil
View the document Chapter 8: Making adobe blocks
View the document Chapter 9: Making pressed earth blocks
View the document Chapter 10: Making walls with earth blocks
View the document Chapter 11: Making rammed earth walls
View the document Chapter 12: Roofs for earth houses
View the document Chapter 13: Floors
View the document Chapter 14: Surface coatings
View the document Suggested references
Open this folder and view contents Appendix

Chapter 8: Making adobe blocks

The art of making adobe earth blocks is ancient - and the old hand-mixed methods work as well today as they ever did. But, if you have some machinery to help mix the soil and move it, you can make good blocks a lot faster.

MANUAL (OR OLD) METHOD OF MAKING ADOBE BLOCKS - When a lot of cheap labor is available, use it. All you need are forms for molding the blocks and simple tools for mixing and moving the earth.

The forms may be made for single blocks, but two- or four-block forms are better. Strong, long-lasting forms can be made from 2-inch thick planks. When the forms do not have to last a long time, I-inch thick planks or ¾-inch thick plywood can be used. The forms will receive a lot of wear, so make strong ones. Use plenty of nails or bolts or, better yet, reinforce them with strips of metal at the corners. Since they will be wet much of the time, it helps to soak the forms in oil for a while before you start to use them. Some people line the insides with metal so the forms will last longer and the blocks can come out easier.

Adobe blocks can be made almost any size, but they should be kept small enough so that one man can lift them al! day long without tiring too much. An average workman can do this if the blocks weigh about 50 pounds or less.

Adobe blocks are commonly made 4 to 6 inches thick. The width of the block matches the desired thickness of the- wall, between 9 and 18 inches. The length, then, is controlled by the weight of the block. Two typical sizes of blocks that have been used are 5x10x20 inches (weight about 55 pounds) and 4x12x18 inches (weight about 50 pounds).

Figure 38. Preparing the soil mix.

Figure 39. Charging the forms

The manual method is most efficient when four workers are used. Two men prepare and mix the soil while the other two mold and remove the blocks, then, clean the forms. The dry soil is prepared as explained in Chapter 7. Then it is mixed with water as shown in Figure 38 until the soil barely flows when lightly kneaded. A good way to estimate whether enough water has been added is to use a stick to make a 3-inch deep, V-shaped groove in the soil mix. The mix is wet enough if the sides of the groove bulge and just begin to flow together.

Bituminous emulsions or other liquid stabilizers are added to the soil mix at the same time as the water. Dry or powdered stabilizers are mixed in before the water is added.

Figure 40. Kneading and leveling the blocks

Figure 41. Lilting the forms from freshly made blocks.

After thorough mixing, the adobe mix is placed in the forms as shown in Figure 39. It helps to drop or throw the mix in the forms so it packs tightly. The mix is then kneaded by hand as shown in Figure 40 to fill all of the corners and remove all air bubbles. If the kneading job is done well, the adobe blocks will be solid and have strong corners and edges. After kneading, a small board or trowel is used to cut off the extra soil and smooth the top edge of the molded block. A little water sprinkled on top of the block will help in smoothing it off.

As soon as possible the forms are lifted from the freshly made blocks as shown in Figure 41. The only way of knowing when to remove the forms is to try it. If the blocks slump or bulge too much, either the forms are being removed too soon or the mix is too wet. If the mix sticks to the forms when they are removed, it is too dry or the forms have not been oiled enough. With some soils the forms may be removed immediately. With other soils you may have to wait longer.

After removing the forms they are washed as shown in Figure 42 and returned to the casting bed for the next batch.

Figure 42. Washing forms for next cycle.

Figure 43. Front end loader is used to mix soil with water in large sump.

MECHANICAL METHOD OF MAKING ADOBE BLOCKS The mechanical method of making adobe blocks is not much different from the hand method. By using machinery to mix and dump the soil you can make blocks faster, but you need bigger forms and more of them. A larger casting bed is also required.

The size of the adobe blocks is the same as those made by the manual method, but the forms should be big enough to make 12 to 36 blocks at once. For these bigger forms, use 2-inch thick lumber. If enough water and earth are available to keep the mixing process continuous, as many as 40 to 50 forms may be used.

Using the mechanical method, two or three workers may be enough. One man operates a tractor with a front-end loader such as shown in Figure 43. He digs the soil if necessary, mixes the soil and water in a large pit and then scoops the soil mix up and drops it in the forms. This operation is shown in Figure 44. The other men knead the mix into the forms and lift the large forms from the molded blocks at the proper time. This method will vary depending on the type of mechanical equipment that is available. The front-end loader was used only as an example.

Figure 44. Machine transports prepared soil mix to multi-forms.

CURING ADOBE BLOCKS - Adobe blocks must be cured or sun-dried before they can be used. The usual way of curing the blocks is as follows:

1. After the mold is removed from the newly made blocks, leave them in place two to four days without being disturbed. Gang molds may be used with a pallet to speed this operation. The mold can be removed after the soil mass has set sufficiently to hold its shape and the pallet with blocks can be set aside out of the way while curing takes place.

2. As soon as the blocks are strong enough to be picked up without chipping or breaking, place them on edge to finish curing. At this time any loose sand or other material clinging to the block is scraped off with a small stick (see Figure 45). Curing will take about a month, but it depends a lot on the weather and the type of stabilizer in the block.

3. If stabilizers such as lime or cement are used, cover the blocks with wet cloth or straw as soon as the molds are removed. Keep them moist for seven days and then turn them on edge to complete the curing. (See Figure 58, Chapter 9.)

4. At the end of the curing period, the blocks are stacked on edge as shown in Figure 46 so they will take up less room.

Because of the long curing period for adobe blocks, a large curing area must be available. During dry, hot seasons they can cure without a protective roof. But if it is apt to rain during the curing period, a protective covering will be needed The blocks shown in Figure 47 were ruined by rain before they had a chance to cure.

Figure 45. Unstabilized block is turned on edge to dry after two to four days.

Figure 46. Adobe block is stacked after a month of curing.

An example of an inexpensive shed used for curing blocks is shown in Figure 57 in Chapter 9. But all types of covering must allow the air to circulate around the blocks or they will take too long to cure.

Some builders have laid adobe blocks before they have cured completely. This can be done if the blocks do not shrink after being laid and if they are strong enough to be handled without breaking. The savings in time may not justify this procedure, however. It is really better not to lay uncured blocks.

CHECKING QUALITY OF BLOCKS - Controlling the quality of adobe blocks during construction is not as difficult as it is for pressed earth blocks because moisture content is not so important. It still should be done, though. You can make quick tests such as the reaction to shaking, dry strength. etc., explained in Chapter 2 to determine whether your soil mixture has changed. You should also make tests on the cured blocks to determine whether the correct soil is being used and whether the correct amount of stabilizer is being added. The following tests are recommended.

1. Strength tests of the cured blocks should be made often using the procedures given in Chapter 2. From the first 100 blocks made, two or three blocks should be selected for strength tests. Later it is sufficient to check one block out of every 150 to 200 blocks.

2. Spray testing is an easy way of checking the quality of adobe blocks and assuring uniformity. These tests should be performed on cured blocks using the procedure described in Chapter 2. The number of tests to be performed is the same as for the strength tests.

3. Absorption test as described in Chapter 2 is one of the easiest ways of checking uniformity of the blocks. Check the same number of blocks as for the strength test.

All of the control tests should be done on blocks which have cured for the same amount of time. If tests are performed on blocks which have cured for one week and then other tests are performed on blocks that have cured three weeks, you could not expect the results of the tests to be the same.