630. One of the most important uses of concrete is in the construction of street and. park walks. It has not only driven stone nagging almost out of use, but it is being employed to a large extent in towns and villages where board walks have formerly been used almost exclusively.
A concrete walk is made up of a sub-base or foundation, a base, and a wearing surface.
As in other structures, one of the most important essentials for success lies in the preparation of the foundation, and the care that must be bestowed on it will depend upon the character of the soil and the climate. In the higher latitudes of the United States, frost may soon destroy a walk the foundation of which is not well drained.
The excavation should be made to the sub-grade previously determined upon, any objectionable material such as loam or organic matter being removed, and the bottom of the excavation smoothed and well rammed. Upon this is laid the sub-base, its thickness varying from nothing to twelve inches. In a sandy soil with good natural drainage and little danger from frost, and where light traffic is expected, it may be unnecessary to provide any special sub-base, since the soil itself furnishes a good foundation for the concrete, but in clay soil in northern climates, twelve inches of sub-base may be required. The best material for this sub-base is broken stone varying in size from one-half inch to two and one-half inches. Usually broken stone is considered too expensive, and gravel, coarse sand, cinders, or broken brick is employed. A layer four inches thick is usually sufficient for good materials, but six to twelve inches of cinders are sometimes required. It should be well rammed to a level surface, and when completed should be firm but porous.
The most important point is that this course shall have good drainage, otherwise it may be a menace to the walk. If it is more porous than the retaining soil, it will naturally drain this soil, and if the water is not able to escape into the sewer or elsewhere, it may be frozen and heave the walk. An excellent plan sometimes adopted is to lay at intervals of twenty to twenty-five feet, a blind stone drain from the walk foundation to the foundation of the curb. In exceptional cases it may be necessary to lay a tile drain in the sub-base to lead the water away from the walk.
The base is the body of the walk giving stiffness to the structure. Its functions are to furnish a solid foundation for the wearing surface and to give transverse strength to the walk, transmitting the pressure uniformly to the sub-base. The base is of concrete, which need not be very rich for ordinary traffic. A proportion of one part packed Portland cement to two and one-half volumes of dry sand and six volumes broken stone is excellent, and proportions of one, three and seven parts cement, sand and stone, respectively, will usually be found sufficient, though the richer the concrete in the base the better will the top dressing adhere to it.
The broken stone for this concrete should be of a size not exceeding one and one-half inches in any dimension, some cities requiring three-quarters inch or less. Crushed granite and trap are excellent, though limestone or any other moderately hard rock may be used that is suited to making concrete for ordinary purposes. If of a hard rock, the screenings may well be left in the broken stone, and when this is done, the dose of sand should be diminished. (See Art. 37).
The thickness of the layer of concrete should not be less than three inches. Four inches is much better and is recommended for general use in sidewalks, while in exceptional cases six inches is required. The top of the concrete base should be finished to a plane parallel to the proposed surface of the walk and at a distance below it equal to the proposed thickness of the top dressing.