Even if you live where ready-mix is available, you may wish to know how to make good concrete, and if ready-mix is not available, you must learn how.

Select good clean gravel or crushed stone. If the largest stones are not much larger than 1" in diame­ter and are well graded from fine to coarse, you have a fine start toward good concrete. A few larger stones will do no harm. The most important single ingredi­ent that determines the quality of concrete is the sand. Sand must be clean and free from organic matter. This is a must. The grains should be sharp, not ocean sand that has been rolled until the grains are almost round.

Cement as you buy it from the dealers in the United States is almost certain to be good, unless it has been on hand so long that it has become lumpy, or has been wet and is partially hardened. If it is fine like flour and free from lumps, you can use it with confidence.

The best proportions for good concrete are to use one part cement, two and one-half parts sand, and three and one-half parts gravel. If you find natural river gravel and sand already mixed by nature, and it is clean, use it with one part cement to six parts sand and gravel.

Water for concrete work should be clean; the water that comes out of the faucet, suitable for do­mestic use, should be all right.

The mixing and placing has an effect on the qual­ity of any concrete job equal to that of the quality of the ingredients.

Good concrete is not hard to make if you are willing to follow a few simple rules.

Select good clean gravel for the large aggregate. Ordinary river gravel is good if it is clean; if it is not clean, wash it with a hose, letting the water strike the gravel with considerable force. Arrange it so the water can run away from the rocks after they are washed. Avoid rocks that are active chemically, and avoid ores, quartz, sandstone, and mica-bearing rocks; just ordinary hard rocks are the best.

The quality of the sand has a much larger bear­ing on the quality of the concrete than the gravel. Sand is more liable to carry dirt in varied forms. Avoid sand that contains clay or organic matter. Sand should be well graded from fine to coarse and should contain some very fine particles, but these must not be clay.

Here are two simple tests for sand. Fill a clear glass bottle about half full of sand and add enough water to almost fill the bottle. Shake it well, let it stand a few hours, and observe the layers of sand. The coarse particles will be on the bottom with fine particles above. The line at the top consisting of very fine particles should not be much more than one-eighth inch thick. A little salt added to the water will hasten the settling process and clear up the water much sooner. The water should not look muddy after a few hours of standing.

Below the top layer there should be a gradual in­crease in the size of the particles, until the large grains at the bottom are reached. It is important that the sand should not nearly all be of the same sized grains, but it should be well graded from fine to coarse.

To test for organic matter, fill a bottle two thirds full of sand, add a bit of caustic soda, calcium hy­droxide, or lye, and fill with water. Shake well and let stand overnight; if the water is clear the next day the sand is free from deleterious quantities of organic matter. If the water is dark brown or red, do not use the sand. A slight tinge of pink will prob­ably do no harm.

The amount of water put into concrete is a mat­ter of importance. The less water you use, if you still get a good workable mixture, the better. Concrete should not be so thin that it will run like gruel, and it should not be so stiff that it stands up dry. It must not be so dry that it cannot be spaded into place, but it should not be so wet that it will run ten feet down the form when poured in at one place. Don't make it too soupy. When first put into a wheelbarrow it should not flatten out completely, but after you have wheeled it forty feet, it should be smooth on top. If it will smooth out under a pat of your hand, it is about right.

For the very best concrete do not use more than seven and one-half gallons of water to one sack of cement; a little less water is better, provided you can get a workable mixture. The strongest concrete has been made with about five gallons of water to the sack, but this requires mechanical vibrators and other means of compacting the concrete, not available to the average builder.

If you can make a mix that you can get to fill the forms without leaving voids, using only six gal­lons of water to the sack of cement, you will have better concrete. Even if it takes more spading and working to get it into place, it will be stronger and more waterproof. Perhaps seven gallons per sack is about the best that most people can do. If the mixture is so dry that you can't get it properly into place, you will get a poor job. It is a rather narrow channel that you have to navigate: too much water, weak porous concrete; too little water, honeycombed and porous concrete; just the right amount of water, the aggregates being good and in the right propor­tion, perfect concrete.

A certain limited amount of water is necessary in the setting of concrete. This water enters into chemical composition with the concrete, becoming a per­manent part of it. Since excess water, beyond that which is needed in the chemistry of the concrete, must evaporate, leaving minute pores where it was, concrete that has been mixed with too much water is inclined to be porous. Of course, this porosity leaves the concrete less strong than it would be if more compact. Where water has dried out, it can go back in if given the opportunity.

Concrete must be mixed thoroughly, and al­though it can be done by hand in emergencies, a ma­chine will do the job a lot easier and generally much better. If you have to mix it by hand, mix it thor­oughly about three times dry. Then add water and mix it through twice. It should then be in good con­dition. If you are mixing by machine, let the ma­chine run at least one full minute after all the in­gredients are in the machine. Letting it run two or three minutes will result in better and more workable concrete. The extra mixing will help you to get more benefit out of the cement you use as well as resulting in a denser and more waterproof concrete.

After the concrete is in place in the forms, do not let it dry out for two weeks. It can be kept wet by frequent sprinkling, or by covering it with wet saw­dust or burlap. There are also waterproof materials that can be sprayed on the fresh concrete to form a membrane to keep the water from evaporating out of the fresh concrete. At any rate, some means must be found for keeping it wet for several days, if you want good concrete.

To make a good job, concrete should be encour­aged into place in the forms, by spading or working with some tool to get it to fill all the corners, leav­ing no void places, and no places that look honey­combed when you remove the forms. A tool made by straightening a hoe, so the blade is parallel with the handle, makes a good tool for spading concrete into place. Spade next to the outsides of the wall that is near the forms, and the center will take care of it­self. Thorough spading is important to a good job.

Do not try to pound concrete into place with a blunt stick, but rather slice or spade near the forms on each side. Often tapping the outsides of the forms with a sledge hammer, not too violently, just gently and firmly, will result in much better compaction of the concrete. You can be proud of a good con­crete job, as many people have trouble with their concrete, and it is a relatively simple matter to get a good job, if you are careful and observe these few precautions.

Do not forget to put the bolts in the top of the wall while the concrete is still soft. They are usually spaced from four to six feet apart, and additional ones are added at the corners and at the sides of doors or at other places where there might be a loose end.

As far as possible, the concrete should all be poured in one day. If it is necessary to join new con­crete to that which has already set, or to join new concrete to old, thoroughly clean the old concrete, and rough it up if possible, make a thin paste of ce­ment and water and brush it on the old concrete to make a thin coating over the entire area; work this paste thoroughly into the surface with an old broom or suitable brush, then immediately pour the new concrete while the paste is still wet. This should bond the new concrete to the old.

Concrete is the universally accepted material for foundations. Formerly large stones were used. If you have them on the property, they may still be used if you take care to bed them properly into the ground; or they may be used in combination with the con­crete.

When ready-mixed concrete is available it is often much better than trying to mix your own, and as cheap if labor is considered to be worth anything. By the time you buy the cement, sand and gravel, and do the mixing you have had a lot of trouble and mess for nothing. Use ready-mixed concrete if you can get it.

Ordinarily the grade known as 2,000 lb. concrete is good enough for ordinary work. For floors that are to be troweled without an additional finish coat, it would be better to get a mixture containing five sacks per cubic yard. This latter mixture would also be preferred for reinforced concrete work.

The pressure on a container can easily be calcu­lated by the following simple formula; pressure per square foot at a distance h below the surface is equal to the weight of the liquid per cubic foot, 150 lbs. for concrete times the h distance below the surface. At 7 ft. this would be 1050 lbs. per square foot at the bottom of the wall. To get the total pressure on a wall use this formula:
house blue print

which for a wall 7 ft. high and 20 ft. long would give150 x 7 x 7 x 20= 73,500 lbs.

pressure against the forms on a section of wall only 20 ft. long. This is somewhat reduced by the fact that concrete is not exactly a liquid. That is why it is almost impossible to brace a wall enough to hold it in line. You must have some ties through the wall, either wires or bolts, or other metal ties.

Of course if you do not pour more than a section a foot high at a time, going around and around, let­ting the previous pour set slightly before you pour more concrete on top of it, the pressure is greatly lessened. If your form starts to give a bit, move on to another section and pour in other places until the concrete at the weak place sets up a little. Then you can safely pour the rest of the concrete. Build your forms as strong as you can. It is very embarrass­ing to have your forms give and let the concrete bulge in the wrong places, or run out on the ground, and it is hard to repair once things have given way.

Pouring the Slab for a House

The simplest way to build a house is to build it on a concrete slab. Dig a trench around the outside of the house, making the overall dimensions of the trench measuring from the outside the same as the house, and extend it at least one foot into the solid undisturbed earth, and more if necessary. The trench must be at least one foot wide. Set up forms of 2" lumber exactly on the lines of the outside walls of the house, and to the exact level of the floor. These forms should coincide with the outside wall of the trench just dug. Then stretch strings over the area of the house and fill with sand to a point just 4" below the tops of the forms. This will leave room for a 4" floor. Be sure the sand is well compacted by sprinkling with water. Dig trenches 12" deep and 12" wide for the bearing partitions. Waterproof the entire area with a large sheet of plastic waterproofing membrane, or with roofing paper, or in some satis­factory manner, and then you are ready to pour the concrete floor. You may need to put a straight 2" x 4" across the house to help in keeping the floor level. See detailed drawing on page 105.

Pour the foundations up to a point about 4" from the top of the floor first, then begin and pour the floor from one wall to your straight edge called a screed, and rod off the top with a long straight 2" x 6", going over it several times with the straight edge until it is level, and proceed over the entire area. Then you are ready to start finishing.

Take the float you have made and rub hard on the surface to work it down. Follow immediately with the steel trowel, trying to get it as smooth as you can. Work rapidly, not letting the concrete get too hard, as it becomes more difficult to work as it gets harder. You can do more with concrete during the first half-hour after it is poured than you can ever do later. What you do not do within two or three hours, you will never do. Don't forget the bolts for the outside walls.

The surface should be level and smooth for the application of floor coverings later. If you plan to put ceramic tile on the floors in certain places, leave these places 2" lower by surrounding them with 2" x 4"s laid flat. You do not need to finish or worry about the level of the parts to be tiled later.