PHILIP L. WORMELEY, JR
A useful and comparatively simple application of concrete is in the construction of sidewalks, for which purpose it has been used with marked success for a number of years.
The ground is excavated to subgrade and well consolidated by ramming to prepare it for the subfounda-tion of stone, gravel or cinders. The depth of excavation will depend upon the climate and nature of the ground, being deeper in localities where heavy frosts occur or where the ground is soft than in climates where there are no frosts. In the former case the ex-
cavation should be carried to a depth of 12 in., whereas in the latter from 4 to 6 in. will be sufficient. No roots of trees should be left above subgrade.
The subfonndation consists of a layer of loose material, such as broken stone, gravel or cinders, spread over the subgrade and well tamped to secure a firm base for the main foundation of concrete which is placed on top. It is most important that the sub-foundation be well drained to prevent the accumulation of water, which, upon freezing, would lift and crack the walk. For this purpose it is well to provide drain tile at suitable points to carry off any water which may collect under the concrete. An average thickness for subfoundation is 4 to 6 in., although in warm climates, if the ground is firm and well drained, the subfoundation may be only 2 to 3 in. thick or omitted altogether.
The foundation consists of a layer of concrete deposited on the subfoundation and carrying a surface layer or wearing cont of cement mortar. If the ground is firm and the subfoundation well rammed in place and properly drained, great strength will not be required of the concrete, which may, in such cases, be mixed in about the proportions 1-3-6, and a depth of only 3 to 4 in. will be required. Portland cement should be used and stone or gravel under 1 in. in size, the concrete being mixed of medium consistency, so that moisture will show on the surface without excessive tamping.
To give a neat appearance to the finished walk a top dressing of cement mortar is spread over the concrete, well worked in, and brought to a perfectly smooth surface with straightedge and float. This mortar should be mixed in the proportion 1 part cement to 2 parts sand, sharp coarse sand or screenings below 1/4 in. of some hard, tough rock being used. The practice of making the concrete of natural cement and the wearing surface of Portland is not to becommended, owing to a tendency of the two to separate.
A cord stretched between stakes will serve as a guide in excavating, after which the bottom of the trench is well consolidated by ramming, any loose material be-low subgrade being replaced by sand or gravel. The material to form the subgrade is then spread over the bottom of the trench to the desired thickness and thoroughly compacted. Next, stakes are driven along the sides of the walk, spaced 4 to 6 ft. apart, and their tops made even with the finished surface of the walk, which should have a transverse slope of 1/4 in. to the foot for drainage. Wooden strips at least 1 1/2 in- thick and of suitable depth are nailed to these stakes to serve as a mold for the concrete. By carefully adjusting these strips to the exact height of the stakes they may be used as guides for the straightedge in leveling off the concrete and wearing surface. The subfoun-dation is well sprinkled to receive the concrete, which is deposited in the usual manner, well tamped behind a board set vertically across the trench, and levelod off with a straightedge as shown in Fig. 2, leaving 1/2 to 1 in. for the wearing surface, 3/8 in. sand joints are provided at intervals of 6 to 8 ft. to prevent expansion cracks, or, in case of settlement, to confine the cracks to these joints. This is done either by depositing the concrete in sections, or by dividing it into such sections with a spade when soft and filling the joints with sand. The location of each joint is marked on the wooden frame for future reference.
Care must be exercised to prevent sand or any other material from being dropped on the concrete, and thus preventing a proper union with the wearing surface. No section should be left partially completed to be finished with the next batch or left until the following day. Any concrete left after the completion of a section should be mixed with the next batch.
It is of the utmost importance to follow up closely the concrete work with the top dressing in order that the two may set together. This top dressing should be worked well over the concrete with a trowel, and leveled with a straightedge (Fig.2) to secure an even surface. Upon the thoroughness of this operation often depends the success or failure of the walk, since a good bond between the wearing surface and concrete
base is absolutely essential. The mortar should be mixed rather stiff. As soon as the film of water begins to leave the surface, a wooden float is used, followed up by a plasterer's trowel, the operation being similar to that of plastering a wall. The floating, though necessary to give a smooth surface, will, if continued too long, bring a thin layer of neat cement to the surface and probably cause the walk to crack.
The surface is now divided into sections by cutting entirely through, exactly over the joints in the concrete. This is done with a trowel guided by a straight-edge, after which the edges are rounded off with a special tool called a jointer, having a thin shallow tongue (Fig. 3). These sections may be subdivided in any manner desired for the sake of appearance.
A special tool called an edger (Fig. 4), is ran around the outside of the walk next to the mold, giving it a neat rounded edge. A toothed roller (Fig. 5) having small projections on its face is frequently used to produce slight indentations on the surface, adding somewhat to the appearance of the walk. The completed work must be protected from the sun and kept moist by sprinkling for several days. In freezing weather the same precautions should be taken as in other classes of concrete work.
Basement floors in dwelling houses as a rule require only a moderate degree of strength, although in cases of very wet basements, where water pressure from beneath has to be resisted, greater strength is required than would otherwise be necessary. The subfounda-tion should be well drained, sometimes requiring the use of tile for carrying off the water. The rules given for constructing concrete sidewalks apply equally well to basement floors. The thickness of the concrete foundation is usually from 3 to 5 in., according to strength desired, and for average work a 1-3-6mixture is sufficiently rich. Expansion joints are frequently omitted, since the temperature variation is less than in outside work, but since this omission not infrequently gives rise to unsightly cracks, their use is recommended. It will usually be sufficient to divide a room of moderate size into four equal sections, separated by 1/2 in. sand joints. The floor should be given a slight slope toward the center or one corner, with provision at the lowest point for carrying off any water that may accumulate.
Concrete stable floors and driveways are constructed in the same general way as basement floors and sidewalks, but with a thioker foundation, on account of the greater strength required. The foundation may well be 6 in. thick, with a 1-in. wearing surface. An objection sometimes raised against concrete driveways is that they become slippery when wet; but this fault is in a great measure overcome by dividing the wearing surface into small squares about 4 in. on the side, by means of triangular grooves 3/8 in. deep. This gives a very neat appearance and furnishes a good foothold for horses.
Concrete may be advantageously used in the construction of steps, particularly in damp places, such as areaways and cellars of houses; and in the open, where the ground is terraced, concrete steps and walks can be made exceedingly attractive. Where the ground is firm it may be cut away as nearly as possible in the form of steps, with each step left 2 or 3 in. below its finished level. The steps are formed, beginning at the top, by depositing the concrete behind vertical boards so placed as to give the necessary thickness to the risers and projecting high enough to serve as a guide in leveling off the tread. Such steps may be reinforced where greater strength is desired or where there is danger of cracking, due to settlement of the ground.
Where the nature of the ground will not admit of its being cut away in the form of steps, the risers are moulded between two vertical forms. The front one may be smooth board, but the other should be a piece of thin sheet metal, which is more easily removed after the earth has been tamped in behiud it. A simple method of reinforcing steps is to place a 1/2 in. steel rod in each corner, and thread these with 1/4 in. rods bent to the shape of the steps, as shown in Fig. 6, the latter being placed about 2 ft. apart. For this class of work a rich Portland cement concrete is recommended, with the use of stone or gravel, under 1/2 in. in size. Steps may be given a 1/2 in. wearing surface of cement mortar mixed in the proportion 1 part cement to 2 parts sand. This system, as well as many others, is well adapted for stairways in houses.