Concrete has become such an excellent servant to the needs of various objects built around the house that no apology will be offered for devoting a chapter to its use. Of course, one is familiar with the artistic flagstone walk with open joints through which the grass is allowed to grow, and one cannot deny the beauty of brick pavements; but in spite of these the concrete walk is found about more houses wherever one goes than any other type, and, although in most cases very ugly, yet it cannot be relegated to the past even by the most fastidious, for its existence depends upon very fundamental qualities of practical serviceability. And likewise, although we may not have seen concrete walks that had the charm of rubble-stone or brick, yet they are coming to be used more and more, for they can be made to appear very beautiful if properly made. Concrete garden furniture, concrete pools, fountains, garden ornaments, tennis-courts, and other familiar adjuncts to the lawn about the house, are making themselves evident on all sides. There is something about the material that lends itself to such uses, for even the owner of the house can get out and work in it, and need not call in a contractor.
However, much of the prejudice that exists against concrete is due to its usual ugly appearance, which is no fault of the material but of the one who built with it. We see too much concrete that is dull, pasty, and gray, and marred on the surface with cobweb lines of cracks; but this need not be. Concrete surfaces can be made as brilliant as any other material by properly treating it. All that is needed to do this is to carefully study the methods of producing textures, and texture is nothing more than breaking up the surface into small patches of light and dark, so intermingled that they give interest. For example, after the forms have been removed, the outside of the concrete can be covered with cement mortar, thrown onto it with a whisk-broom, which will make the mortar stick to the surface in little lumps and hills. The light playing over such a surface will cast shadows in the hollows between the lumps and light up the tops of the lumps. This will give a texture of interest that is pleasing to the eye. On the other hand, the cement mortar may be plastered over the surface of the concrete and used as a sticking bed to hold small pebbles of different colors and shades thrown against it. These pebbles will be colorful, some dark and dull and some light or sparkling like glass. Thus a play of broken light will be thrown back from the surface to the eye, and the observer will be pleased. Then, too, the outer layer of the cement, which was next to the forms, may be composed of white cement and some aggregate like small chips of marble. When the forms are removed it will be found that this beautiful aggregate will not show, but the entire surface will partake of the monotonous white or gray of the cement. However, if this thin coating of cement is removed, then the variety and sparkle of the aggregate below will be revealed. This might be done by striking the surface all over with a stone-cutting tool which is used to surface stones, or it might be done by a scrubbing or rubbing with carborundum blocks. There are innumerable ways by which texture can be developed on anything made of concrete, and experimenting in this line is a most fascinating employment. For this reason, if properly handled, concrete is particularly adapted to the making of all kinds of house accessories, since it is also easily shaped in moulds.
Rough Coat Finish or Splinter Dash.
Finish made by the Pointer.
Finish made by the Bush Hammer.
The materials used for this concrete work have much to do with its success. Ordinarily there is no need of inspecting the cement, for most of the well-known brands of cement on the market are about as reliable as human effort can make them. The materials which do need consideration, however, are sand and gravel. The one essential of sand is that it be free from loam, mica, clay, and organic matter. No sand should contain more than 3 per cent by weight of loam or clay or 1 per cent of mica. The quantity of loam or other fine impurities can be determined by shaking the sand up with water in a bottle, and allowing it to settle. The fine impurities will settle on the top and its proportional relation to the sand estimated. To determine whether the sand has much organic matter in it, a 12-ounce prescription bottle can be filled with sand to 4 1/2 inches and then added to this should be added a 3-per-cent solution of caustic soda until this solution and the sand fill seven ounces. The contents should be shaken well and allowed to stand for twenty-four hours. If the liquid which settles on top shows a dark color, then the sand has too much organic matter in it, but if it is clear or slightly yellow it may be used without washing. The size of sand particles should be such that they will pass through a quarter-inch screen.
The usual size of aggregates should range from one-quarter inch to an inch and a half in diameter, and the various sizes should be so graded that they will make the most compact mass. The common run of bank gravel must be screened and washed. To make really good concrete that is water-tight, the grading of the aggregate is most important.
In fact, to determine the various quantities that should be used of the materials on hand, some method must be adopted to give the quantity of cement necessary to fill the voids in the sand and the quantity of cement and sand necessary to fill the voids in the aggregate. A rather crude way of doing this is to employ water as the measure of the voids. Fill a pail with sand, and then pour water into it until the water, which is absorbed by the sand, comes to the same level as the sand. Note the quantity of water used up. If it represented 45 per cent of the volume of the sand, then it is known roughly that about 50 per cent of the volume of the sand ought to be the quantity of cement needed to fill in the voids of the sand. Thus, one part of cement to two parts of sand. If now the gravel is measured in the same way and it is found that the voids show about 40 per cent of the volume of the aggregate, then, assuming a little more than the water shows, about 50 per cent of sand and cement will be required to fill up these voids. That is, there should be just twice as much stone as there is cement and sand. We finally, then, arrive at the proportion for the concrete as follows: 1 part of cement to 2 parts of sand to 4 parts of gravel.
The amount of water which is added to make the mixture of concrete should not be too much. It should be of such a quantity that the mix is mushy but not watery, even when it is to be poured into forms.