Concrete for pavement foundations should not be less than six inches in thickness, and a greater thickness will be required where the ground is insecure. The excavation having been made to the required sub-grade, and all loose soil removed and the places refilled with broken stone, the earth is thoroughly rolled to a smooth surface parallel to the surface of the proposed pavement. Drainage for the foundation should be provided where necessary by broken stone or tile drains beneath the curb. Before beginning the placing of concrete, stakes may be driven in the foundation, with their tops at grade, at intervals of five to ten feet over the entire pavement, to assist in securing the proper grade of concrete surface.
649. The stone for the concrete should be broken so that no piece is larger than two and one-half inches in its greatest dimension. If the stone is of good quality, it need not be screened except to remove the finest dust, if this is present in considerable quantities. Sufficient mortar should be used to fill the voids in the stone, this mortar being composed of about two parts sand to one of natural cement, or better, two and one-half or three parts sand to one of Portland cement. This concrete is thoroughly rammed in place, care being taken that adjacent batches as laid in the street mingle with each other so as to show no line of demarcation. In stopping work for the night, the concrete should cut off sharply on a straight line parallel to the direction of the proposed joints in the wearing surface. Joints extending across the street should be left at intervals of thirty to forty feet to allow for expansion and contraction.
1 Report of Capt. John S. Sewall, Report Chief of Engineers, 1896.
650. The concrete is finished to a surface parallel with the proposed street surface, a templet being employed to secure this. The concrete should be kept damp for a few days, and no traffic allowed upon it until the wearing surface is laid. If the wearing surface is of brick or wooden blocks, a layer of sand about one inch thick is first spread over the concrete.
The advantages of a concrete foundation for street pavements are its strength and durability and water-tightness.
Concrete has not been a popular material for a street surface except for short driveways and in courts where both vehicles and pedestrians must be accommodated. One reason for this is that concrete is slippery, and another, that owing probably to carelessness or ignorance, the wearing qualities have not been good. The first objection may be largely removed by cutting the surface into blocks, four by eight inches, by deep grooves, or by the use of a deep imprint roller on the wearing surface. As to wearing qualities, there seems to be no good reason why a concrete cannot be made tough enough to withstand heavy traffic. It will of course be necessary to divide the work into blocks of twenty to twenty-five square feet, with expansion joints of sand, asphalt, or tarred paper between. A third objection is the glare of the surface in summer. A partial remedy for this may be had by placing some coloring matter, such as lamp black, in the top dressing.
652. The sub-base may consist of a six inch layer of broken stone, or twelve inches of cinders, well drained and thoroughly compacted by rolling. For exceptionally heavy wear it may be advisable to use a five inch layer of lean concrete for the sub-base, after rolling the bottom of the excavation and providing drainage.
Upon the sub-base should be laid a base, composed of four inches of concrete made with first class stone, such as granite, trap or hard limestone crushed to pass a ring one and one-half inches in diameter, and containing enough mortar, one part Portland cement to two or three parts sand, to fill the voids in the stone. The top dressing, a layer of granolithic one and one-half or two inches thick, should then be immediately applied. This mortar should be made with one or two parts granite, trap, or other hard rock crushed to pass a five-eighths inch screen, to one part Portland cement.
These two layers are placed in much the same manner as that described for laying concrete sidewalks, but the joints in base and top dressing should run at angles of forty-five degrees with the curb to prevent ruts following the lines of the joints. A roller making deep imprints is then run over the finished surface to furnish a foothold for horses, or, for this purpose a special roller may be used to mark the top dressing into blocks approximately four by eight inches, with deep (one-half inch) grooves.
When completed, the pavement should be kept moist, preferably by a layer of damp sand, and no traffic should be allowed upon it for at least a week or ten days.
653. Concrete pavement laid in Bellefontaine, Ohio, was found to be in good condition after ten years' service;1 the only serious defect apparent being that, since the blocks were marked off parallel to the curb, ruts have sometimes formed along these joints. This pavement was made with four inches base concrete, laid directly on sub-grade where foundation is gravel, sand or porous soil; or if soil is impervious, the base was laid on four inches of broken stone or cinders. The top layer was two inches thick, equal parts cement and sand or pea granite. Sub-drains of three inch tile were laid inside each curb line, and the curb is formed as part of the outer blocks. Both the base and top dressing were cut through in squares, five feet on a side. The Cost of the pavement is said to have been $2.15 per square yard, and very few repairs have been found necessary.
In Germany a cement macadam, made with six inch subbase of broken stone or gravel, with a wearing surface of hard macadam stone mixed with cement, has been successfully used.
1 Municipal Engineering, December, 1900, and Engineering News, Jan. 7, 1904.