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The home mechanic should master a few principles of mixing and working with concrete. Not only will this knowledge be necessary in doing many kinds of repairs about the house, such as pointing up masonry and repairing concrete walks, but with a little practice and patience the amateur will soon find himself building many useful contributions to his home and garden, out of concrete. Outdoor fireplaces, garage floors, and garden pools are all within the range of the amateur concrete worker. Concrete is one of the least expensive building materials, and the beginner can experiment without fear of wasting a great deal of money. Experience is necessary before highgrade job can be done, and the beginner should not feel discouraged if his first attempts do not turn out as expected. Portland Cement. Portland cement is the basic ingredient for all concrete and mortar. Cement is the portion of the mixture that the other materials together into a strong, watertight composition. "Portland" is not a trade name. It refers to the type of cement in general use throughout the world. Cement is sold in bags weighing 95 pounds and filling I cubic foot. Cement may be purchased in different colors, and this should be remembered when a job requires several bags. Use the same color cement for the entire project to avoid differences in color in the final result. Cement should be stored in a dry place, particularly after the bag has been opened and moisture can reach the cement inside. Fine Aggregate. The next ingredient used in making concrete is sand, called "fine aggregate." The sand must be sharp, and free of dirt clay, or vegetable matter. Sand from ocean beaches is not satisfactory because it contains salt. Test the sand for purity by placing a small amount ,of it in a glass of water and stirring it. Let the sand settle to the bottom of the glass, and if the water is only slightly discolored, the sand is clean enough to use. Do not expect to obtain good results from concrete if the sand is dirty. Coarse Aggregate. The third ingredient for concrete is crushed stone or pebbles. This material is called the "coarse aggregate," and ranges in size from 1/2 inch to 2 inches, depending upon the job for which it is to be used. When working on a thin wall, the size of the coarse aggregate should not be more than one-third the thickness of the thinnest section of the wall. The coarse aggregate must be as clean and free of any impurities as the sand.
Water. Pure water completes the list of materials needed for making concrete. A good rule to follow is to use only water fit for drinking to mix with cement. Dirt or other matter in the water will produce inferior concrete just as readily as dirt in the fine or coarse aggregate. Salt water should never be used. Cement fine and coarse aggregate, can all be purchased at lumber yards. While it is possible for the home-mechanic to screen his own sand and gravel, this task requires considerable time if it is to be done thoroughly. Clean sand and gravel can be purchased either by the square yard or by the pound, and the cost is comparatively slight. Mixing Concrete. The proportion of cement, sand, gravel, and water used for mixing concrete depends on the class of work to be done and on the condition of the sand and gravel. These two materials are seldom completely free of moisture, and this must be taken into account when adding water. The amount of moisture in the aggregates will vary from a damp condition to very wet. The water required for very wet aggregate is considerably less than that required for damp aggregate. The strength of concrete depends upon the union of the water and cement to form a paste which, when hard, will bind the particles of sand and gravel together. Accordingly, it is very important that the proper portions of water and cement be maintained. The table gives recommended quantities of water for various jobs. Make up a trial batch of concrete, using this table as a guide. If the mixture is too stiff to work, use less sand and gravel in the next batch. If the mixture is too wet, add more sand and gravel. Do not change the amount of water, as this will affect the bonding properties of the mixture. In other words, keep the ratio of, cement to water as set down in the table. Control the workability of the mixture through the use of more or less aggregate. Always make up a trial mixture of concrete, and get it right, before mixing up a large batch for the job. Concrete that has been properly mixed can be poured into a form with ease and packed down until it forms a dense mass. The concrete should be plastic enough-so that the pebbles or gravel will not fall out when the mixture is handled. There should be no space between the elements of the coarse aggregate. The stiffness of the mixture varies with the job, so that the mixture used for a garage floor can be stiffer than that used for small forms. Mixing the various ingredients together should be done with care. To make certain that the right amount of each material is used, have a pail marked in quarts and gallons for measuring the water and a bottomless box for the sand and coarse aggregate. The box should be built to hold exactly one cubic foot. A bag of cement contains one cubic foot. There is no measurement problem except when mixing up a small mass of concrete. All ingredients must be measured out carefully. Do not try to judge the quantity by eye. Almost any flat, clean, waterproof surface can be used for mixing concrete. While machine mixing is always preferred, the home mechanic will seldom find this very practical unless he is doing a job big enough to warrant the renting of a mixer. Excellent concrete can be mixed by hand providing it is done thoroughly. Thorough mixing is essential to good concrete; it is the thoroughness of the machine mixer that makes it so efficient. A garage floor, or a platform built of tongued and grooved lumber, is excellent for mixing concrete. Measure out the quantity of sand in accordance with the type of mixture and spread it on the surface of the mixing platform. Measure out the cement and spread it over the sand. These two materials should be mixed together until they form a mass with a uniform color. There should be no streaks, an indication that the work has not been done thoroughly. After the sand and cement have been mixed, measure out the coarse aggregate and spread it over the mixture. Continue mixing until the pebbles or gravel are well distributed throughout the sand and cement. Measure out the correct amount of water and pour it into a depression formed in the middle of the pile of dry concrete. Start mixing again and continue until there is no question that all the materials have been combined and the water distributed throughout the mixture. The concrete should be placed in the forms, or wherever it is to be used, at least thirty minutes after mixing. This means thirty minutes after the sand and cement are mixed. Remember that sand contains some moisture, and this acts upon the cement when the two are combined. Spade the concrete, when it is put in the forms to make it dense. Too many home concrete jobs have been ruined by not giving the concrete time to dry before stripping off the forms. Concrete does not dry by the evaporation of the water but by a chemical change. It takes approximately ten days for concrete to dry enough so that the forms can be removed. During this period of curing, the concrete should be kept moist and in shade. Cloth, paper, or straw should be placed over it and occasionally sprinkled with water. Concrete should not be mixed in freezing weather.
Coloring Concrete. Concrete may be colored by mixing coloring powders into it while still dry. Paint and hardware stores sell these powders. Outside concrete jobs such as driveways and terraces can be colored best this way, better than by paint because they will not be damaged by moisture and the color will not fade as the surface is worn down. You can buy concrete stains which will give nearly the same result. Dusty Concrete. It is very important, when making a concrete floor, that enough cement is used so that the concrete is not dusty. A floor in this condition can be remedied by using one of the prepared concrete hardeners. Cement mortarMortar is used for laying bricks and concrete blocks. It is also used to repair cracks in concrete and for similar work. Mortar is made by mixing 1 part cement to 2 or 3 parts sand, depending upon how rich the mixture is to be. For average masonry work, I part cement to 3 parts sand will give a very satisfactory mortar. The same general rules apply to mixing mortar as were set down for mixing concrete. As mortar cannot be applied as rapidly as concrete, only small amounts should be mixed at a time. A clean iron wheelbarrow or a solidly-built wooden box can be used for mixing, instead of a floor or platform. After the sand and cement have been well mixed, add water slowly, stirring the mixture as it is added. The finished mortar should be soft enough to work but by no means watery. Do not let the mortar stand after it has been mixed or it will lose its strength. Concrete formsConcrete weighs about 125 pounds per cubic foot, and the forms used for it must be solidly built if they are to stand this strain. Green lumber is the best lumber from which to make the forms because it will not absorb moisture from the concrete and, consequently, is not liable to warp. If well-seasoned lumber is used, it should be coated with oil or tar paper to prevent the wood from soaking up moisture. Plan the form so that very little cutting of the wood is necessary. In this way you will be able to use the same wood for many jobs. Construct the forms so that they can be taken apart easily and without damage to the wood or concrete. In this respect, it is better to use bolts, screws, and clamps, rather than nails. If nails are used, do not drive them in all the way. Leave enough of the head above the surface so it can be pulled out easily and quickly. Tongued and grooved lumber does very well for the sides of the form, and 2 x 4 inch stock is suitable for the studding. Take time to make certain that the form is as nearly perfect as possible before pouring the concrete. After the concrete has been poured, leave forms in place until you are certain that the concrete is hard. Do not make the mistake of ruining a good concrete job by stripping off the forms too soon. The length of time that is necessary to cure concrete varies, but it is wIse to wait at least ten days before removing the forms. Concrete blocksConcrete and cinder blocks have become extremely popular in recent years as a building material. They can be used not only for foundations but for the exterior walls of the house; and there is the advantage that they do not require the upkeep that wood siding does. The home mechanic, looking for materials with which to build a garage, garden wall, or some other project would do well to consider these blocks. They are relatively inexpensive, not difficult to handle or to lay, and a wall made out of a single thickness of blocks will be extremely solid. Concrete blocks may be purchased at most lumber yards. Cinder Blocks. While concrete blocks are made of Portland cement, sand, and gravel, cinder blocks contain cinders in place of the other aggregate and are consequently lighter in weight than concrete blocks and have greater insulating qualities. They contain I part cement to either 6 or 8 parts cindersboth proportions are used. Concrete and cinder blocks come in many different sizes and shapes, but a size usually available is 12 x 5 x 7 3/4 inches. The blocks must be laid on a solid footing of concrete extending below the frost line. If this is not done there is great danger of the wall buckling and heaving because of frost. The mortar joints between each block should be about 1/2 inch thick and should be packed tight to prevent the wall from leaking. The mortar is made with I part cement to 3 parts sand. The blocks should be laid in such a fashion that the vertical mortar joints are staggered. To fasten woodwork to a wall of blocks, bolts should be cemented into the blocks with the end extending far enough above the block so that the wood can be secured by means of a nut. Making Blocks. The home mechanic can realize a very substantial saving in the cost of building materials by making his own concrete blocks. There are special one-block forms to be had for this work and they are so constructed that a block can be made in a matter of minutes. The form does not have to be left on the block until the concrete is hard but can he removed and used to make additional blocks. In this way with a few well-spent winter weekends in a warm basement, the home mechanic can make up a sufficient number of these blocks to build a garage or other small structure when the spring comes. A good concrete mixture for blocks is composed of 1 part cement, 2.5 parts sand and 5 parts gravel. It is important not to allow the mixture to become overwet. Use 5 gallons of water to each bag or cubic foot of cement.
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