The best base for general use under pavements is without doubt that formed of hydraulic cement con crete. A bed of concrete made of good hydraulic cement, well rammed and allowed to set and harden, becomes a practically monolithic structure, nearly im pervious to water and possessing a high degree of strength against crushing.
The concrete is formed of a mixture of cement, sand, and broken stone or gravel. The proportions vary for different work and with the character of the materials. With good Portland cement the most common proportions for ordinary work are about one part cement, 3 parts sand, and 5 to 7 parts broken stone. With the various natural cements the pro portions vary somewhat, but are usually about i part cement, 2 parts sand, and 4 or 5 parts of stone or gravel.
Natural cement is usually employed for this pur pose as being cheaper and possessing ample strength for the work, and concrete of the ordinary propor tions with natural cement is to be preferred to that made with meager proportions of Portland cement giving about the same strength and cost. Proper tests should always be imposed for the purpose of securing good cement.* Sand for use in mortar should be as clean and as free from loam, mud, or organic matter as possible. In general the presence of any foreign matter is to be avoided. Coarse sand is usually preferable to that which is very fine, provided it be fine enough to give a smooth mortar, as it affords better strength. The use of a mixture of grains of various sizes is usually desirable as giving less voids to be filled by the cement.
The aggregate used for concrete should be as hard and durable as possible, and that of angular form is preferable to rounded. The materials should be uni form in quality. When gravel is used which varies in quality, it should be blended by mixing in order to obtain uniform strength in the concrete. The best concrete will usually be made from the stone contain ing the smallest percentage of voids, provided the material be uniform. In a mass of ordinary broken stone the voids are usually from 4o per cent to 55 per cent of the volume. This may be considerably reduced by careful adjustment of the sizes. The broken stone is commonly limited in size to 2 or 21 inches, and the whole output of the crusher is used, with the dust screened out. The quantity of sand needed is such as
will fill the voids in the aggregate.
In preparing the concrete, the cement and sand should first be thoroughly mixed while dry, then the proper quantity of water be added all at once, and the mortar be vigorously worked with hoe or shovel for 2 or 3 minutes, until it comes to a smooth and uni form condition.
The quantity of water should be such as under energetic working will reduce the mortar to a soft, plastic condition, and should be determined by meas ure. The application of the water from a hose during the mixing is objectionable on account of the diffi culty of regulating the quantity to produce mortar of proper consistency.
When the mixing of the mortar is complete, the stone or gravel may be added, and the whole mass turned several times with shovels until the mortar is evenly distributed through the aggregate. The stone should be wet by sprinkling before it is mixed with the mortar, in order to clean the surfaces of dust and to prevent the absorption of water from the mortar before it sets.
The concrete, when ready, is placed in position and tamped to surface. For this use it is preferable that the concrete be of jelly-like consistency, such that it will quake under light ramming. The rammer com monly employed consists of a block of wood, or of cast iron, 6 to 8 inches square, flat on the bottom, and weighing zo to 30 pounds. The tamping should cause the mortar to flush to the surface.
After completion the foundation should be allowed to stand several days before the pavement is placed upon it,— 3 to 6 days are usually required,— in order that the mortar may become entirely set. During setting the concrete should, be protected from the drying action of the sun and wind, and should be kept damp to prevent the formation of drying cracks.
The quantity of material necessary to make a cubic yard of concrete varies with the density of the broken stone. For materials measured loose, to make a cubic yard of I, 2, 4 concrete will require I-I to i- barrels of natural cement, cubic yard of sand, and to I cubic yard of broken stone. To make I cubic yard of I, 3, 6 concrete requires is to I barrel of Portland cement, is to cubic yard of sand, and T to I cubic yard of broken stone.