Concrete pavements, when well constructed of good materials, make the most satisfactory of foot ways. They form an even surface, quite agreeable in service, and are durable and economical where exposed to considerable travel.
In the construction of a concrete sidewalk a base of cinders is usually employed, supporting a layer of rather meager concrete and a thin surface layer of cement mortar. The cinders are commonly 4 to 8 inches thick, 6 inches being ample for most walks, and 4 inches being sufficient for walks in residence districts of small travel, where the soil is firm. Care should be taken to insure the proper drainage of the base, so that water may not remain in the soil immediately under the walk, or stand in the cinders. The cinders should be placed to proper depth and well tamped with the upper surface parallel to the finished top of the pavement.
The concrete base is usually 3 or 4 inches thick, and sometimes on streets of heavy traffic it is made 5 inches. The wearing coat is from # inch to i inch in thickness, depending upon the wear to which it is to be subjected. A concrete base 31 inches thick, with a wearing sur face inch thick, makes a very satisfactory walk for residence streets carrying moderate travel.
The composition of the concrete base must depend largely upon the materials available in the locality. Either gravel or broken stone may be used, with or without sand, according to the character of the materials. A mixture of one part Portland cement, three parts sand, and six parts broken stone is com monly used. When good limestone is available, a mixture of one part Portland cement to four parts broken stone, without sand, is found very satisfactory, the stone being broken to pass a one inch screen and with only the fine dust removed. When sand or gravel
is used, it is important that it be clean, as any dirt is likely to work to the surface in tamping and prevent the proper adhesion of the surface layer. For the same reason, the concrete must net be mixed too wet, and it should be well compacted by ramming.
The wearing coat is composed of Portland cement mortar, one part cement to one or two parts sand or screenings. The amount of cement used should be sufficient to fill the voids in the sand but not greatly in excess, as the resistance to abrasion is lessened by excess of cement. The material for wearing coat should be either clean, hard sand, or screenings from the crushed stone. The screenings should have the very fine dust removed, and when from a good quality of rock are superior to most natural sands. The mortar is brought to a uniform surface by drawing a straight edge along the tops of the forms at the sides of the walk. The surface is then worked smooth and uniform with a float and finished with a plastering trowel.
Joints should be left at intervals of 4 or 5 feet. to prevent irregular cracks through contraction of the concrete. These joints need not all extend through the base of the walk, but at intervals of 3 or 4 joints one should extend through. The surface of a concrete walk should have a transverse slope of about I inch to i foot to provide for proper surface drainage. For residence streets of moderate travel a width of four to six feet is commonly required. Four feet is incon veniently narrow unless the street is very little used, while six feet is sufficient for a very considerable amount of travel.