FINISH ON CONCRETE WORK Floors.—The most common finish for floors is the ordinary cement finish. This is a cement mortar composed of one part Portland cement and two parts clean, sharp sand. It is preferably laid at the same time as the main body of the concrete work in order to procure adhesion to the same. If for any reason this cannot be done, the old concrete should be thoroughly cleaned before the finish is laid, and the finish should be made at least one inch in thickness, preferably more. A less thickness will crack off. The cement finish should be marked off in squares, the lines of the marking being so arranged as to bring them over all beams and girders.
Another method of finishing old floors which is sometimes used is as follows: A 3-inch layer of 1:2:6 cinder concrete is laid on the old floor, and then the 1:2 coating of ce ment and sand laid on that. The cinder con crete provides a good bonding surface for the finishing coat and is said to prevent cracking in same.
In factories, hotels, office buildings, etc., where finished wood floors are laid on concrete, beveled wood sleepers are used as nailing strips. These sleepers are about 2 by 3 inches in size, and are placed usually 16 inches on centers. Be tween the sleepers a filling of weak cinder con crete is used to hold them in place.
Marble, tile, mosaic, and similar floors are laid on concrete construction by imbedding them in a cement mortar.
Walls.—Where a cement finish is desired on concrete walls, the finish should be placed while the wall is being built. The rough concrete is spaded back from the forms, and the rich mortar placed in front of it. A cement finish plastered on concrete after the wall is built will usually crack and not give the best results. After the forms are removed, the concrete should be rub bed smooth and given a coat of cement wash mixed and applied as a paint.
In cases where it is necessary to plaster con crete walls, precaution should be taken to make the plaster stick. In many cases it is well to wash or scrub the surface, or to pick it to make it rough, before applying the plaster. A rich mortar of 1 part cement to from 1 to 2 parts sand should be used for such work. Lime paste added to the mortar is advisable in some cases. This increases the adhesion and lessens the lia bility of cracking. If a hard surface is desired, only a small amount of the paste should be used. In plaster work of this kind it is customary to brush over the surface after troweling, to re move the tension in the cement.
An inexpensive manner of plastering is what is called a splatter-dash coat. The mortar is
thrown against the surface with a stick or pad dle, making a very effective rough surface. A rough surface is generally better in appearance and less liable to crack than a smooth surface.
A method recommended as providing a good bond is to first wash the surface thoroughly with water, and then brush on a coat of neat cement. While this is wet, a coat of plaster about 1/4 inch thick is put on. This is followed by succeeding coats applied about an hour apart until the de sired thickness is reached. If desired, the last coat may be thrown on in order to produce a rough surface.
Mr. Ernest McCullough advises the follow ing: "If the appearance of the work requires a coat of plaster, clean the surface with steam, afterwards using wire brushes and then the steam again. Wet it with water, paint it with neat cement, and immediately follow with two coats of 1:3 mortar, the lower coat scratched and the top coat floated to a sand surface." Plastering should as a general rule be re sorted to only to fill holes and to smooth over rough places. Godfrey states that a plaster coat should either be very thin—that is, just enough to fill irregularities—or it should be 1 to 3 inches thick, so that it will have some strength in itself.
Many types and methods of finish are used on walls—such as placing a rich mortar in front of the concrete when the forms are being filled; or a granolithic surfacing composed of 1 part cement, 2 parts coarse sand or gravel, and 2 parts granolithic grit made into a stiff mortar and placed in a layer about 1 inch thick in front of the concrete in the moulds. The face of the form is removed in this latter form as soon as the concrete has become rigid enough—generally on the following day—and the wall scrubbed with water until the grit shows up. The wall should then be protected and kept moist for three or four days.
Strips of wood are often nailed into the forms to give the effect of cut stone to the sur face when the forms are removed. Dry surface finish is produced by using a fine stone in the aggregate, mixing fairly dry and not spading the concrete next to the forms. Facings of brick, tile, stone, and even cast slabs of concrete, are used in walls for a finish, backed up by the rougher poured work.
There are many other ways of obtaining pleasing appearances to finished concrete work, such as bush-hammering, pebble dash, Quimby process, and a large variety of patented pro cesses, all of which have been used with more or less success.