Stakes for grade and line having been given, a maitre cord is stretched along the line stakes to mark the sides of the excavation. After the material has been excavated to the proper sub-grade and all soft material in the bottom removed, the bottom of the trench is well rammed. If tile drain is necessary, it is laid with open joints on this foundation. The material to form the sub-base is now wheeled in and rammed to the proper thickness, water being used freely if it facilitates the packing. The top of the sub-base is brought to a level plane at the proper distance below the grade stakes.
The molds for the walk are now to be laid. These are made of two by four or two by six inch scantling, sized and dressed on at least one side and one edge. Stakes are first securely driven, about five or six feet apart, with their faces two inches back from the side lines of the proposed walk, and their tops at grade. Against these stakes the scantlings are placed on edge with dressed side toward the walk, and smooth edge level with the grade stakes. These molds are held in place by nailing through the supporting stakes into the scantling, and if these nails are not driven "home," they may easily be pulled to release the mold when the work is completed. On the upper edges of the mold are then marked off the sizes of blocks desired, being careful that the marks defining a joint are exactly opposite each other on the two scantlings.
635. The concrete materials having been previously delivered near the work, the concrete is mixed, either by hand or machine, according to the methods already given, and rammed in place after the sub-base has been well wet down to receive the concrete. The concrete should be just short of quaking, and in ramming care must be taken not to disturb the molds. For tamping next the molds, the makers of cement working tools offer a light rammer with square face at one end and blunt, chisel shaped tamper at the other. The surface of the base is brought to a plane parallel to the proposed finished surface of the walk, and at a distance below it equal to the thickness of the top dressing. A straight edge, long enough to span the walk and notched out at the ends so that when placed on the molds the straight edge will define the correct grade of the base, is a convenience here.
636. The concrete is now cut into blocks exactly corresponding to the proposed blocks in the top dressing. For this purpose a straight edge is laid across the walk in line with marks previously made on the molds to define the joints, and with a spade or special tool the concrete base is cut entirely through to the sub-base. This division is necessary to allow for expansion and contraction, and prevent cracks in the top dressing elsewhere than at the joints. This joint in the base should then be filled with clean sand. If preferred, these joints in the base may be made by placing thin steel strips across the molds to be removed after the concrete for the next block is in place.
The end block made from a given batch of concrete should be limited by a cross mold set exactly on line of a proposed joint. When the base is continued, this cross mold is removed. A part of a block should never be molded and then built on after having stood long enough to begin to set. Any concrete left over from finishing a block should either be mixed in with the next batch, if this is to follow in a very short time, or it should be wasted. A disregard of this rule will probably result in a crack in the top dressing above the line of division between adjacent batches.
637. When a block of base is finished, the top dressing or wearing surface should be applied immediately. The lack of adhesion between the base and wearing surface is one of the most frequent causes of failure in cement walks. The mortar should not merely be laid on in a thick layer and then struck off to grade, but it should be worked and beaten into close contact with the concrete at every point. The mortar should be tamped with a light rammer and beaten with a wooden batten, and to accomplish this properly the mortar must not be very wet. The surface is then to be struck off with a straight edge bearing on the top of the mold planks. Some hollows or rough places will remain, and the straight edge should be run over a second or perhaps a third time, a small amount of rather moist mortar, made from thoroughly screened sand, having been first applied to such places.
When the surface film of water is being absorbed, the surface is worked with a wooden float. The exact time when the work should be floated will soon be known by experience. After the floating is completed, the trowel may be used to give a smoother surface, but this makes the walk so slippery that it is not usually desirable.
638. If the top dressing is worked too long, the cement is brought to the surface, robbing the next lower layer of its cement and resulting in scaling. The top dressing is now cut entirely through on exact line above the joints in the base. This may be done by a trowel working against a straight edge, but special tools are made for cutting through the mortar and rounding the edges of the joint at one operation. A quarter-round tool is also run along the edges of the mold to give a neat finish. When desired, an imprint roller run over the walk gives it the appearance of having been bushhammered.
It is important that the top dressing be applied before the concrete has begun to set, and it must not be applied to a portion of a block and then some time allowed to elapse before applying the remainder. The edge of the top dressing must be cut off squarely at the end of the block. If desired, the wearing surface may be colored by the use of lamp black in the mortar, giving a uniform gray color to the walk. (§ 535).
639. When the walk is completed, it should be fenced off so that animals may not walk over it while still fresh, and it should be protected from a hot sun. The surface should be kept moist, and this may be done after the first twenty-four hours by spreading a layer of damp sand over the walk and wetting the sand with a rose nozzle as often as may be needed. The walk may be opened to light travel after aboutfour days, but it is better to remain covered with the damp sand for a week.