| Handbook for building homes of earth |
Earth blocks. whether adobe or pressed, are laid in level layers (or courses) just like ordinary burnt bricks. Each course is offset one-hale of the width of the block as shown in Figure 59 so that the vertical joints are not continuous. The blocks are joined together with a mortar to give a uniformly strong wall. Some block-making machines produce blocks with interlocking grooves and ridges so the blocks bond together without using mortar. This type of bond is not very satisfactory, particularly for areas subject to earthquakes or high winds.
The selection of a correct mortar is an important step, and the information given below should be closely studied before a mortar is selected.
MORTARS FOR PRESSED EARTH BLOCKS - The mortar used for pressed earth blocks may be one of high-quality like that used for burnt bricks, or it may be made from the same soil mix used for the earth blocks. Here is one high quality mortar that has been successfully used with many different types of earth blocks:
1 part portland cement.
1 part lime.
6 parts clean sand (particle size less than 1/8").
Sufficient clean water to make a workable mix.
If masonry cement is available, use one part masonry cement, three parts sand and enough water to form a workable mix.
If soil mix is used for the mortar it should be one low in clay content to avoid shrinkage cracks. Since the mortar is not pressed like the blocks, it will require some sort of stabilizer - usually portland cement or lime - so it will have approximately the same strength as the blocks. Research has shown that mortar for stabilized pressed blocks should contain about twice as much stabilizer as the blocks to be as strong as the blocks. In other words, double the amount of stabilizer you used for your blocks when making a stabilized soil mortar.
The soil mix or sand to be used in the mortar should be sieved through a small screen.
SELECTING THE BEST MORTAR FOR PRESSED EARTH BLOCKS - There are some simple tests that can be used in selecting a good mortar. Before making these tests, it is first necessary to know something about laying blocks. Since pressed earth blocks are usually dried before they are laid, they are likely to absorb some of the water from the wet mortar. If they absorb too much water from it, the mortar will dry out too fast. Then it will be weak and will not form a good bond with the block.
A simple way to determine if this will happen is to draw a 1-inch diameter circle on the surface of a block with a wax pencil or crayon. Using some sort of dropper, rapidly place 20 drops of water inside the circle. If the block completely absorbs the water in less than l ½ minutes, the blocks need to be wetted. This can be done by completely immersing stabilized blocks in water for a minute or two. Unstabilized blocks can be thoroughly sprinkled with water. Allow the surface water to be absorbed into the blocks before the mortar is applied.
After the blocks have been properly wetted, two or more of them are joined together with each of the proposed mortar mixes using the same thickness of mortar joint that will be used in the wall. It is important that the consistency ("wetness") of the mortar mix be the same as if it were being used in a wall. The excess mortar is removed or "struck off" and the mortar joint allowed to dry at least one day. The mixes are judged from the appearance of cracks in the mortar. A mortar is unsuitable if it contains open cracks that will allow surface water to enter the joint. Fine hairline cracks in the surface are usually not damaging. Mixes that have damaging cracks need either more sand or more stabilizer.
If the test joint you made shows no cracks, set the tat blocks aside in a protected area for at least 7 days. If portland cement or lime is used as a stabilizing agent they should receive a wet cure for the first day. At the end of the 7-day drying period, drop the joined blocks on a hard surface from shoulder height so as to land on the corner of one of the blocks. The fragments are inspected to determine if the cracks follow the pints or cross the joints through the blocks. Any mortar that is strong enough to hold the blocks together so that some of the cracks are through the blocks is a strong, durable mortar.
If the cracks are all in the mortar joints, the mortar may be suspected as being too weak for successful use. Some blocks are extremely strong and may not crack through the block. In such cases, the only thing to do is to estimate by some other means the force required to pull the blocks apart.
MORTARS FOR ADOBE BLOCKS - Tests have shown that the mortar for adobe blocks should have about the same strength as the blocks themselves. If the mortar is much stronger than the blocks, it could cause the blocks to crack when it shrinks. For this reason, it is usually best to use the same soil mix (including stabilizer) as was used for the blocks. This mix should be put through a fine screen to remove large particles which are never desirable in the mortar.
If the mortar does not satisfactorily bond the blocks, it may help to wet the blocks first by sprinkling them. Adding a small amount of stabilizer might also help. However, as a general rule, very little trouble is experienced with mortars for adobe blocks.
SETTING DOOR FRAMES - Before the block-laying begins, all door openings should be accurately located and marked on top of the foundation wall. Sometimes the door frames are set in place before laying the blocks.
Another satisfactory method, often used with adobe construction, is to leave an accurately measured opening for the door frame to be placed after the block laying is completed. If the door frames are placed first, great care should be taken to make sure that they are in the correct position. They should be perfectly plumb and then solidly braced. The frames should be perfectly square (do this by measuring across the diagonals) and then braced diagonally also.
TERMITE PROTECTION - In certain areas of the world, especially in tropical climates, termites or white ants are so bad that they will even burrow in earth blocks. If the house is being built in such an area, special precautions must be taken. The common method is to use a thin metal shield which is placed on top of the foundation wall and have it project out and down from the wall at least three inches. It may also be advisable to treat the soil and lumber with suitable insecticides to minimize attack by termites.
LAYING THE BLOCKS - After the correct mortar mix has been selected and the termite shields and door frames are in place, the blocks are ready to be laid. The usual procedure is to place the first layer of blocks around at least most of the house as shown in Figure 60b. Leave space for the mortar in the joints, but do not put it in. This step provides the correct block spacing for the remaining courses. When proper spacing has been obtained by trial the blocks are then laid with mortar.
Then several courses of blocks are carefully laid at the corners and at the door frames as shown in Figure 60c. These provide a guide for laying the remaining blocks, so they should be rayed level and correctly spaced. String lines are then stretched between corners of the building as shown in Figure 61 or between a corner and a door frame. The string line should be placed so that it will just be level with the top of the next row of blocks to be laid and checked to see if it is level. If an adjustment is needed, it should be made. It is best spaced a small distance out from the outside edge of the wall (usually about 1 inch). In this manner, the person laying the blocks knows exactly how far the block should be from the string line to give a straight wall. (It may be handy for him to have a small block of wood of just the correct thickness so that he can measure the distance from the string line to the face of the block.) The string line is moved up to the next course after a course is completed. In this manner the courses are brought up to the level of the window sills. Then the block laying is stopped while the window frames are set in place.
Adobe blocks have rougher surfaces and it will be a little harder to lay them to a straight line but it can be done.
A house with crooked walls will not look as good, so check often to be sure that your walls are going up plumb (vertical). Use a level or plumb bob to check them.
The block layers (or masons) should have helpers to keep enough mortar mixed, to soak the blocks (if needed) and to keep a supply of blocks easily available to the mason at all times. The mason needs the correct tools for his job, a large and small trowel, a jointing tool, hammer, and level.
The mortar is placed on top of the blocks with the trowel. Sufficient mortar is used to give a joint somewhat thicker than desired. The mason then puts mortar on each end of the block and sets it in place. Using the trowel handle, he lightly taps the blocks into the exact position and then strikes off the excess mortar which has been squeezed out of the joints. After the mortar has become slightly hardened (in about 30 minutes) the mason finishes the mortar joints. This is done with a rounded tool called a jointing tool, which slightly indents the joints and removes rough edges. This serves to pack the mortar tightly in the joints and also makes the joint more waterproof. The masons helper should occasionally sprinkle the fresh joints with water to keep them from drying too fast. When the mason needs half blocks at door frames and windows he can cut blocks by giving them a sharp tap with the edge of his metal trowel.
If the block wall is going to be covered with a thick surface coating, (as explained in Chapter 13) it is not necessary to finish the mortar joints with the jointing tool. Instead the mortar is left in a rough condition. either protruding out from the blocks or deeply indented. Some builders even omit the mortar from the vertical joints at the ends of the block. This, however, may not be a good idea but if it is done and if this space is later filled with the surface coating it provides a very strong bond between the surface coating and the wall. Rock chips set in the wet mortar, also provide a good bond for surface coatings. This method is shown in Figure 63 for an adobe house.
WINDOWS - When the block wall has been brought up to the correct height, the window sill should be placed. This is one of the most common places for annoying leaks to occur and great care should be taken at this stage of the construction. The sill should be made of a sturdy material that will not rot. A good quality mortar can be used or preferably lightly reinforced portland cement concrete should be formed into a sill as shown in Figures 64 anti 6a. When a wood frame window is to be used, the masonry, sill should not at any place extend above the. bottom of the wooden sill or leaks will occur. The masonry sill should not stick out beyond its finished outside wall. An overhanging sill will collect water running down the face of the wall and cause damaging erosion. If an overhanging sill is used, it should have a drip groove underneath to prevent water from running down the face of the wall.
After the masonry sill is in place the window frame can be set and braced in the same mariner as the doors. Then, the block-laying can continue. Of course, you can also leave an accurately measured opening for the window and place it after the blocks have been raved. A trick often used by adobe builders when the walls will be covered with a surface coating is shown in Figure 67. Wooden nailer blocks - exactly the size of an adobe block - are placed at several points adjacent to door and window openings. The nailer blocks are mortared in place and their center is filled with mortar, also. Window and door frames can be nailed directly to these without fear of cracking adobe blocks. This method can also be used with pressed block walls that will receive thick surface coatings. Another and perhaps better method is to attach sheet metal strips to the frame which will extend into the mortar joints.
LINTELS - The weakest spot in any house is over the doors and windows. A beam or other reinforcing cross piece called a lintel should always be used above these openings as shown in Figure 68. The lintel must be strong enough to support construction loads as well as the weight of the blocks above. A large timber or a reinforced concrete beam makes a good lintel. (Fig. 95 in Appendix A gives some thicknesses of lintels that can be used for various conditions.) The lintel should be at least as thick as the wall so that blocks laid on top of it will be fully supported. They should also project out on either side of the door or window for a distance at least equal to the thickness of the wall so they will have plenty of surface to rest on.
A drip barrier should be cut in all lintels. This barrier is a small groove (about ¼" deep and ¼" across) on the underside of the lintel about one inch from the outside edge. It will keep water from seeping back into the wall, itself. Figure 65 shows the location of the drip barrier.
This method was commonly used over a hundred years ago when this building was constructed, and it's still a good one. Railroad ties also make good lintels because they are strong and usually are treated to resist termites.
BOND BEAMS - Bond beams (sometimes called tie beams or perimeter beams) are used to reinforce the top of the earth wall against loads from the roof. (See Figure 68.) The beam distributes the roof load and ties the top of the walls together. If a low-ceiling house is being built, the bond beam can also serve as the lintel as shown in Figure 70. Bond beams should be the same thickness as the wall and preferably be tied to the wall with bolts or some other type of projection. The beams may either be wood or reinforced concrete. Wooden bond beams should be placed over a thin layer of mortar on top of the last course of blocks. Concrete bond beams (and lintels also) can be poured in place on top of the completed earth wall as shown in Figure 71. Make the forms strong so they will not bulge. If you are using a timber frame roof, set bolts in the fresh concrete for holding the roof down.
REINFORCEMENT - The best made homes use extra reinforcing. In earthquake or high wind areas, reinforcing should always be used. The most common type of reinforcing used in a block wall consists of reinforcing rods or wire mesh laid in occasional horizontal joints. These rods or mesh are embedded in the mortar between blocks and also serve to reduce shrinkage cracking. Usually this reinforcing is placed in at least one out of every six horizontal joints.
Additional reinforcing is normally required in the horizontal joints immediately under window sills. Since a window represents a weak place in the wall, most structural cracks occur at the corner of the window. Reinforcing is not required above the window if a sturdy lintel is used. Reinforcing rods are sometimes placed in the mortar joints at wall corners and where an inside wall joins into an outside wall. This reinforcing can be made from steel bars that are bent to form the shape of the corner of the wall junction.
Additional strength, particularly for a two-story building, is obtained with vertical reinforcing rods. For most block walls this means drilling a hole through the blocks and grouting the rod into the block. Vertical reinforcing is normally used only in areas subjected to severe earthquake shocks.
EARTHQUAKE DESIGNS - In areas subjected to frequent earthquake shocks, the following rules should be followed:
1. The plan of the building should be compact and rectangular in order to minimize the effects of shaking.
2. A continuous reinforced concrete foundation laid on compacted or solid ground is necessary.
3. One-story walls should not be less than 12 inches thick for exterior walls and 8 inches thick for interior walls. A two. story house should have 18- and 12-inch thickness exterior and interior walls for the first story and wood framing for the second story. The lighter construction for the second floor minimizes the hazards resulting from earthquakes.
4. Reinforce mortared joints. Reinforcing rods or wire mesh should be placed in every fourth or fifth horizontal mortar joint and lapped over at corners. When surface coatings are used this method can be improved by using a four- to six-foot width of one-inch wire mesh in every sixth joint. The extra width is turned down against the face of wall and fastened to it. This provides a good bond for plaster.
INSIDE WALLS - The homebuilder has several choices of materials from which to construct the interior walls of the building. They may be made from the same soil that was used in the exterior walls. Normally they do not have to be as thick nor as strong. Also, they do not need to be weather proof. They should, however, be resistant to abrasion, and a hard plaster coating may be desirable.