CONCRETE WALKS 247. Drainage of Foundations. The excavation should be made to a sufficient depth so as to get below the frost line. The ground should he tamped thoroughly, and the excavation filled with cinders, broken stone, gravel, or brickbat, to within four inches (or whatever thickness of slab is to be used) of the top of the grade. The foundation should be thoroughly rammed, and by using gravel or cinders to make the foundation, a very firm surface can be secured Side drains should be put in at convenient intervals where outlets can be secured. The foundation is sometimes omitted, even in cold cli mates, if the soil is porous. Walks laid on the natural soils have proven in many cases to be very satisfactory.
At the Convention of the National Cement Users' As sociation, held at Buffalo, N. Y., January 21 to 23, 190S, the Committee on Side .
walks, Streets, and Floors presented the following speci fications for sidewalk founda tions: "The ground base shall be made as solid and permanent as possible. Where excavations or fills are made, all wood or other materials which will decompose shall be removed, and replaced with earth or other filling like the rest of the foundation. Fills of clay or other material which will settle after heavy rains or deep frost, should be tamped, and laid in layers not more than six inches in thickness, so as to insure a solid embankment which will remain firm after the walk is laid. Embankments should not be less than 21 feet wider than the walk which is to be laid. When porous materials, such as coal ashes, granulated slag, or 'gravel, are used, under drains of tile should be laid to the curb drains or gutters, so as to prevent water accumulating and freezing under the walk and breaking the block." 248. Concrete Base. The concrete for the base of walks is usually composed of 1 part Portland cement, 3 parts sand, and 5 parts stone or gravel. Sometimes, however, a richer mixture is used, consisting of 1 part cement, 2 parts sand, and 4 parts broken stone; but this mixture seems to be richer than what is generally required. The concrete should be thoroughly mixed and rammed, and cut into uniform blocks. See Fig. 78. The size of the broken stone or ()Tavel should not be larger than one inch, vary ing in size down to f inch, and free from fine screenings or soft stone. All stone or gravel under * inch is considered sand.
The thickness of the concrete base will de pend upon the location, the amount of travel, or the danger of being broken by frost. The usual thickness in residence districts is 3 inches, with a wearing thickness of 1 inch, making a total of 4 inches. In business sections, the
walks vary from four to six inches in total thickness, in which the finishing coat should not be less then 1+ inches thick. The concrete base is cut into uniform blocks.
The lines and grades given for walks by the Engineer, should be carefully followed. The mould strips should be firmly blocked and kept perfectly straight to the height of the grade given. The walks usually are laid with a slope of inch to the foot toward the curb.
The blocks are usually from four to six feet square, but some times they are made much larger than these dimensions. The joints made by cutting the concrete should be filled with dry sand, and the exact location of these joints should be marked on the forms. The cleaver or spud that is used in making the joints should not be less than f, of an inch or over of an inch in thickness.
249. Top Surface. The ing surface usually consists of 1 part Portland cement and 2 parts crushed stone or good, coarse sand, all of which will pass through a 1-inch mesh thoroughly mixed so as to secure a uniform color. This mixture is then spread over the concrete base to a thickness of one inch, this being done before the crete of the base has set or come covered with dust. The mortar is leveled off with a straight edge, and smoothed down with a float or trowel after the face water has been absorbed. The exact time at which the surface should be floated depends upon the setting of the cement, and must be determined by the workmen; but the final floating is not usually performed until the mortar has been in place from two to five hours and is partially set. This final floating is done first with a wooden float, and afterwards with a metal float or trowel. The top surface is then cut directly over the cuts made in the base, care being taken to cut entirely through the top and base all around each block. The joint is then finished with a jointer, Fig. SO, and all edges rounded or beveled. Care should be taken in the final floating or finishing, not to overdo it, as too much working will draw the cement to the surface, leaving a thin layer of neat cement, which is likely to peel off. Just before the floating, a very thin layer of dryer consisting of dry cement and sand mixed in the proportion of one to one, or even richer, is frequently spread over the surface; but this is generally undesirable, as it tends to make a glossy walk. A dot roller or line roller, Figs. 81 and 82, may be employed to relieve the smoothness.