| 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 4½ 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.
Sidewalks and Porch Floors
Concrete Sidewalk
It is generally recognized that one-course concrete sidewalks are the most successful when built by the average workman, for the slab is of one uniform body and not two layers, which might not have knitted together properly. For porch floors and walks these slabs should be 5 inches thick and laid on a good foundation. It is best to excavate 4 inches for the depth of the walk, tamp the ground, and pour water over it, to note whether it is absorbed or stays on top. If it is not readily drained off, it ought not to be used as the foundation of the walk, but should be excavated to a depth of 10 inches to 12 inches. In this excavation should then be tamped gravel or cinders, and some provision should be made by which any water that would seep through this gravel may be drained off. The timbers used for the forms along the edges of the walk may be 2 by 6’s, held in position with pegs. Slabs should then be determined for length. Usually they should not be in excess of 6 feet in any one direction and ¼-inch expansion joints should be placed in the walks every 25 feet. If alternate slabs are laid, the forms can be removed, so that the intermediate slabs can be poured between them. Of course, a partial bond will be developed between slabs in this way, but these joints will be the weakest point in the walk, and if settlement takes place unequally and one slab breaks from the other, the crack will develop at this joint and not appear on the face. The expansion joints should, however, be real separations, made with strips of asphaltic felt set between slabs. The usual mixture for concrete walks should be 1 part cement to 2 parts sand to 3 parts of gravel. The mixture should not have too much water in it, and when poured into the forms the top should be levelled off with a straight stick stretched across from one side of the form to the other. Too much trowelling should be avoided, since this is apt to draw excess water to the surface and also cement, which will show hair cracks when hardened. It is best not to use a metal trowel but a wooden one, so that a partial sandy surface is made. After the walk has been laid it should be protected from drying out too quickly by laying over it 4 inches of earth or two or three layers of burlap, which should be wet down about twice a day for a week. All walks and porch floors should have graded tops, so that water will run off of them. This is usually ¼ inch to the foot.