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 nail. ing 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 quarterround 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 about four days, but it is better to remain covered with the damp sand for a week. 640. Cost of Concrete Walk. The cost of concrete walks varies from ten cents to twenty-five cents per square foot. A fair price for a walk of average quality where there are no special difficulties is twelve to eighteen cents per square foot. As an instance of a walk built with special care, the one constructed about the top of the bank of the Forbes Hill Reservoir may be mentioned.1 The sub-base of this walk was of stone and twelve inches thick, the layer of concrete was five inches thick at the center of the walk and four inches at the sides. The top was of granolithic finish one inch in thickness. The walk was laid in separate blocks about six feet square. The average gang employed on the concrete consisted of six men and one team, while the finishing was done by two masons and one tender. The amount laid per day was about forty square yards. The cost per square yard was as follows: cu. yd. stone in foundation or sub-base, at $.40 per cu. yd. . $0.133 Labor, placing stone at $1.50 per day .502 1 C. M. Saville, M. Am. Soc. C. E., Engineering News, March 13, 1902. 641. The following is given as an estimate of cost of items in a walk built with six inch cinder sub-base, four inch concrete base and one inch top dressing. Preparation of foundation, excavation and ramming . 0.33 0.20 0.25 . cu. yd. concrete, at $3.00 per cu. yd. for materials alone cu. yd. concrete, placing, at $1.80 per cu. yd. Top dressing cu. yd. mortar, at $9.00 per cu. yd. Placing top dressing and finishing walk. Superintendence and molds. Totals Total cost per sq. yd., $1.44, or 16 cents per sq. ft. 642. As an example of a low priced walk, the concrete walks in San Francisco' are but three inches thick, two and one-half inches of concrete composed of one part Portland cement, two parts beach gravel, and six parts of crushed rock of size not exceeding one inch; the top dressing being one-half inch thick. of equal parts Portland cement and beach gravel. With cement $2.50 per bbl., crushed rock and gravel from $1.40 to $1.75 per cu. yd., and wages twenty cents an hour for laborers and forty cents for finishers, this walk is constructed at from nine to ten cents per square foot. It is stated that a gang of three or four men will lay 150 to 175 square feet per day. ART. 74. FLOORS OF BASEMENTS, STABLES AND FACTORIES 643. The principles governing the laying of walks apply also in a general way to the construction of floors that rest directly on the ground. For residences, basement floors may be laid with three inch base of concrete and one-half inch wearing surface. The thickness of sub-base will depend upon the character of the soil. Where natural conditions do not assure good drainage of the foundation, this should always be provided for by either a blind. stone or tile drain laid around the outer edge of the building and leading to the sewer or other outlet. The finished surface of the floor should always have a slight slope toward the center 1 Engineering News, March 4, 1897. or one corner of the basement, and a trapped sewer connection set at this lowest point in such a way that it is accessible for repairs and cleaning. 644. Wet Basements. Where much ground water is encountered, and especially where a basement is subjected to a head of water from without, special precautions must be taken in building the floor. The concrete must be made thick enough so that its weight and the arch action set up, shall be able to withstand the upward pressure of the water. In building such a floor it is necessary to keep a sump hole, preferably in the center, towards which the construction proceeds from the sides. A pipe placed in the sump hole permits pumping until the concrete is laid about the pipe, when the latter may be filled with rich cement mortar. In such cases the side walls of the basement should be plastered with Portland cement mortar on the outside and special care taken in joining the floor to the wall. 645. Size of Blocks. As the changes in temperature in a building are usually much less than in open air, the blocks of concrete may be of much larger size, say ten feet square, and many basement floors are laid without any joints, though sooner or later they will probably crack if so laid. In factories for certain purposes, however, the floors may be subjected to greater changes in temperature than walks laid in the open. air. In such cases the blocks should not be more than three or four feet on a side, and the joints may well be filled with asphalt, especially if water-tightness is desired. 646. Stable floors may be made of six inch cobble or broken stone sub-base, six inches of concrete made with mortar containing three parts sand to one cement, and one inch of top dressing containing three parts sand (mixed sizes) or crushed granite to two parts cement. Factories having heavy machinery with much vibration require strong floors. Such a floor may be made of six inches. of cobble stone sub-base covered by six inches of a lean concrete made with one-to-four mortar, and above this, three to five inches of rich concrete made with mortar containing two and one-half parts sand to one cement, and one inch of top dressing, equal parts cement and sand or cement and crushed granite. 647. Example and Cost. In the construction of the new printing building for the Government Printing Office at Wash |