The strength of the slab is not always governed by its thickness. The greater strength is obtained by properly proportioning the gravel or crushed stone, sand and Portland cement, so that all the spaces between the stone are filled with sand and cement.
The Portland cement, sand and gravel or crushed stone should be mixed in proportions, if the sand is not very coarse, of 1: 2: 4—which means, 1 part Portland cement, 2 parts sand, 4 parts gravel or crushed stone, all passing a ¾-inch mesh and all collected on a ¼-inch mesh. If the sand is coarse and the crushed stone or gravel well graded in size of particles, it may be mixed in proportions of 1 part Portland cement, 2½ parts sand, 5 parts gravel or broken stone. All proportions are measured by volume.
Bank run gravel is often used for sidewalk work, particularly where a good bank can be found on the farm. It is safer, if this material be used, to screen out the pebbles, using them as stone, measuring the quantities of stone and sand as described above. Concrete should not be laid in freezing weather.
Consistency of Concrete
Mix the concrete as described on [page 15] to a consistency that when tamped, it will not quake, but it should be sufficiently wet so that some moisture will rise to the surface under tamping.
Placing
Divide the walk by setting forms at right angles to the side forms. The cross forms can be made of 2 by 4’s. These provide for expansion and contraction joints. Hold these forms in place by driving stakes through the foundation into the ground on the opposite side from where the concrete is to be placed. Spread the concrete over the drainage foundation to the thickness of the walk or floor, and in slabs not over 6 feet square. The thickness of a walk should be 4 inches, a driveway 6 inches, a floor over which a wagon may be driven 6 inches, and all other floors 4 inches.
Fill in every other slab, placing enough forms to use up all the concrete mixed in one batch. No batch should stand longer than one half hour before being placed.