engineer's level. The position of the screeds should be checked immediately before laying the finish.

When the surfacing material is placed MONOLITHICALLY with the base, grade-blocks cut from 4 X 4-in sticks, and of a length equal to the thickness of the slab including the finish, are nailed to the floor-forms. They are slightly beveled to facilitate removal, and placed on about 10-ft centers. The concrete of the structural slab is then graded to 34 or 1 in below the tops of the blocks, depending upon the thickness of the finish. The blocks are then removed, the holes filled with concrete, and screeds placed at the level desired for the finished surface. As the mortar of the finishing coat is deposited it is spread over the slab and worked to the required level by means of the strick-board which is

moved back and forth with a sawing motion and guided by the screeds. In order to be considered as monolithic, a wearing surface should be placed within one-half hour after the base course. Fig. 5 shows the PROPER CONSISTENCY of the mixture used for the finish. In exposed locations light frames and canvases, or other adequate rain protection, must be available. On jobs constructed during COLD WEATHER it is usually inadvisable to apply the floor-finish monolithically with the base.

When it is intended to lay the finish later, the GRADE-BLOCKS are cut of a length equal to the thickness of the structural slab, that is, usually 1 or 2 in less than the total thickness of slab including the finish. The concrete is then leveled and roughly smoothed by means of long-handled wood-floats, or DARBIES, see Fig. 6, which, of course, may be used equally well for a monolithic finish.

After the surface has been smoothed the grade-blocks are removed and the holes filled with mortar. When the concrete has become sufficiently hard the surface is roughened with long-handled wire PUSH-BROOMS, in that manner exposing the coarse aggregate. Before placing the surfacing materials the hardened concrete-slab is thoroughly cleaned by flushing with clean water and scrubbing with brooms. This should preferably be done the day before it is intended to place the finish, and at this time any laitance or loose pieces of mortar are removed. Immediately before the finish is placed the floor is again thoroughly wet, but not to the extent that puddles are formed. After all surfacewater is removed from the slab, A MIXTURE OF CEMENT AND WATER, having the consistency of rich cream, is spread over the surface and worked into the slab

by means of brooms. Following this application the finish is laid before the grout commences to dry.

The FINISHING OF THE SURFACE is accomplished by means of a wood-float, or steel-trowel, or both may be used to advantage. The Joint Committee, 1924, specifies that a wood-float shall be used. In any case two separate trowelings are required, the first occurring as soon as the surface has hardened sufficiently to support the knee-boards and the second, or last troweling, just before the INITIAL SET takes place. A woOD-FLOAT has the advantage of not exerting as much suction as a STEEL TROWEL and a surface so finished is less likely to craze or dust from the effect of overtroweling. The steel, however, is required for a smooth, hard, metallic surface. If excessive troweling is avoided excellent results can be obtained. The practice of sprinkling dry cement upon the surface

of the concrete in order to hasten the drying up process is not to be recommended. Fig. 7 illustrates the process of troweling. Granolithic floors should be carefully GUARDED and PROTECTED from rain or the direct rays of the sun, or the effect of drying winds, until sufficiently hard to permit the application of protective coverings. This period varies from eight to 24 hours, depending upon conditions. After such time has elapsed the surface is covered with 1 or 2 in of sand, or earth, which should be kept wet for at least ten days. SAWDUST has also been widely used, but is apparently capable, under some conditions, of injuring the green concrete by means of organic acids. When water is costly a CALCIUM-CHLORIDE solution may be used for a curing agent, as its hygroscopic quality draws moisture from the air. A solution is made by dissolving 4 lb of

commercial Calcium-chloride in 1 gallon of water. This solution mixed with four parts of water is then used to wet the protecting material such as sand. When ARTIFICIAL HEAT is required a uniform temperature should be obtained, avoiding excessive heat, which has the tendency to cause premature drying of the floor and results in a dusty surface. JOINTS are not usually required in finishes placed upon structural floors except at the fill-lines or constructionjoints. As such lines in the finished floors, made with a mason's floor-jointer, must follow the construction-joints in the structural slab, the latter is stopped along straight lines. Minwax, or some similar substance, is used to fill the joints if it is desired to make them waterproof.

10. Plain Concrete Floors. Finishes are placed upon PLAIN CONCRETE FLOORS in the same manner as described for structural slabs, except that additional care must be exercised in regard to jointing and the type of sub-grade

often presents special problems. When concrete slabs rest upon SAND, or GRAVEL no SUB-BASE is ordinarily required, but the surface should be well compacted and carefully graded. If placed upon soil it is customary to lay a course of CINDERS or GRAVEL, compacted by wetting and tamping or rolling. Such a sub-base varies in thickness, depending upon conditions, but should be at least 6 in thick. Where any appreciable degree of moisture is present, or likely to occur at certain seasons of the year, a system of DRAINAGE should be installed.

When no reinforcement is employed the surface is divided by means of CONTRACTION-JOINTS, cut through the base as well as through the finish, and dividing the area into blocks, the greatest area of which is not over 100 sq ft nor greatest dimension over 10 ft. Joints are also often left between floors and walls and between floors and columns. All exposed edges, such as curbs, should be protected by metal angle-guards or else rounded to a 1⁄2-in radius.

11. Integrally Colored Floor-Finish. Various pigments may be incorporated in the mixtures employed for surfacing concrete-floors. Only the best grades of MINERAL-PIGMENTS should be used and the amount under no circumstances should exceed 10% by weight of the cement. As the tendency of such admixtures is generally to reduce the strength and wearing qualities of the surface, such should be entirely avoided in floors subject to heavy traffic, or only small percentages employed. In practice the pigments are preferably purchased in POWDERED FORM, proportioned by weight, and MIXED DRY with the cement. Particularly good results are obtained by grinding the cement and pigment together in a small BALL-MILL. This, however, is a slow process, and mechanical sifters are more generally employed. Whatever means be used, it is of vital importance to have the color UNIFORMLY BLENDED with the other materials. As certain pigments, when used as admixtures, even in extremely small quantities, greatly reduce the strength of concrete and its resistance to abrasion, only thoroughly accredited materials should be used, the effect of which upon the cement has been established by test. The use of a solution of MAGNESIUM-FLUOSILICATE, OF SODIUM-SILICATE, is considered effective in setting the color in the concrete and rendering it more permanent.

12. Paints and Chemical Treatments. A number of manufacturers supply excellent paints specially prepared for use upon concrete surfaces. When not subject to heavy traffic such are generally satisfactory, but should be purchased only from responsible firms whose directions should be carefully followed. A number of so-called hardeners are employed in the form of solutions used with the object of overcoming the dusting, or excessive wear, sometimes occurring on granolithic floors which have been carelessly laid. Among those most widely used are: magnesium-fluosilicate, sodium-silicate, aluminum-sulphate, zinc-sulphate, paraffin, and soap-solutions. The following recommendations. are taken from the literature of the Portland Cement Association.

(a) The MAGNESIUM-FLUOSILICATE treatment consists usually in one or more applications. The solution for the first application is made by dissolving about 1⁄2 lb of the silicate in 1 gallon of water. For the second and subsequent applications about 2 lb of the silicate are used to each gallon of water. Any number of applications of the more concentrated solution may be given to the

floor, depending on its condition and the apparent penetration obtained. In no case should one application follow in less than three-quarters of an hour after the preceding one. The surface treated should be kept wet with the solution for at least three minutes at each application, so as to replace any of the solution absorbed at once by the concrete, the object being to secure as great a penetration as possible.

(b) SODIUM-SILICATE, as a 20% solution containing a small addition of an organic acid, is applied in two or more coats 24 hours apart. Ordinarily the sodium-silicate requires considerable time to dry before the floor can be used. The sodium-silicate treatment is, of course, inexpensive. Commercial sodiumsilicate varies in strength from a 30% to a 40% solution. It is quite viscous and requires thinning with water before it will penetrate the floor. Ordinarily it will be found satisfactory to dilute each gallon of the silicate with 3 gallons of water. Each gallon of the resulting solution is sufficient for one coat upon about 200 sq ft of floor-surface. The floor should be thoroughly cleaned of all grease, plaster, dirt, etc., and should be thoroughly dry before the first application of the silicate-solution.

(c) The ALUMINUM-SULPHATE treatment consists in one or more applications of solutions of aluminum-sulphate. The solution is made up in a wooden barrel or stoneware vessel, and the water should be acidulated with about 2 cc. of commercial sulphuric acid for each gallon of water. The sulphate does not readily dissolve and requires occasional stirring for a few days until a solution is formed. About 21⁄2 lb of the powdered sulphate are required for each gallon of water and 1 gallon of the solution should cover about 100 sq ft of floor-surface. For the first treatment the solution may be diluted with twice its volume of water. Twenty-four hours after this application the normal solution is used and 24 hours should elapse between each additional application.

(d) The zinc-sulphate treatment consists in the application of about a 16% solution of zinc-sulphate with 42% free sulphuric acid. It is applied in two coats, at an interval of four hours. The surface should be scrubbed with hot water and mopped dry just before the application of the second coat. This treatment gives the floor a darker appearance than that of the original concrete.