325. Mouldings and Ornamental Shapes. Concrete is now in demand in ornamental shapes for buildings and bridges. They may be either constructed in place, or moulded in sections and placed the same as cut stone. Plain cornices or panels are usually con structed in place, and complicated mould ing or balusters (Fig 120) are usually made in sections and erected in separate pieces.
The moulds may be constructed of wood, metal, or plaster of Paris, or moulded in sand. The operation of casting concrete in sand is similar to that of casting iron. The pattern is made of wood the exact size required. It is then moulded in flasks exactly as done in casting iron. The ingredients for concrete consist of cement and sand or fine crushed stone; the mixture, with a consistence about that of cream, is poured into the mould with the aid of a funnel and a T-pipe. Generally the casting is left in the sand for three or four days, and, after being taken out of the sand, should harden in the air a week or ten days before being placed. Balusters are very often made in this manner.
326. Colors for Concrete Finish. Coloring matter has not been used very extensively in concrete work, except in ornamental work. It has not been very definitely determined what coloring matters are detrimental to concrete. Lampblack (boneblack) has been used more extensively than any other coloring matter. It gives different shades of gray, depending on the amount used. Common lampblack and Venetian red should not be used, as they are apt to run or fade. Dry mineral colors, mixed in proportions of two to ten per cent of the cement, give shades approaching the color used. Red lead should never be used; even one per cent is injurious to the con crete. Variations in the color of cement and character of the sand used will affect the results obtained in using coloring matter.
327. Painting Concrete Surface. Special paints are made for painting concrete surfaces. Ordinary paints, as a general rule, are not satisfactory. Before the paint is applied, the surface of the wall should be washed with dilute sulphuric acid, 1 part acid to 100 parts wa ter.
328. Finish for Floors. Floors in manufacturing buildings are often finished with a 1-inch coat of cement and sand, which is usually mixed in the proportions of 1 part cement to 1 part sand, or 1 part cement to 2 parts sand. This finishing coat must be put on before the concrete base sets, or it will break up and shell off, unless it is made very thick, 1 to 2 inches. A more satisfactory method of finishing such floors is to put 2 inches of cinder concrete on the concrete base, and then put the finishing coat on the cinder concrete. The finish coat and cinder concrete bond together, making a thickness of three inches. The cinder concrete may consist of a mixture of 1 part cement, 2 parts sand, and 6 parts cinders, and may be put down at any time; that is, this method of finishing a floor can be used as satisfactorily on an old concrete floor as on one just constructed.
In office buildings, and generally in factory buildings, a wooden floor is laid over the concrete. Wooden stringers are first laid on the concrete, about 1 to 2 feet apart. The stringers are 2 inches thick and 3 inches wide on top, with sloping edges. The space between the stringers is filled with cinder concrete, as shown in Fig. 121, usually mixed 1: 4: 8. When the concrete has set, the flooring is nailed to the stringers.
329. Efflorescence. The white deposit found on the surface of concrete, brick, and stone masonry is called efflorescence. It is caused by the leaching of certain lime compounds, which are deposited on the surface by the evaporation of the water. This is believed to be due primarily to the variation in the amount of water used in mixing the mortar. An excess of water will cause a segregation of the coarse and fine materials, resulting in a difference of color. In a very wet mixture, more lime will be set free from the cement and brought to the surface. When great care is used as to the amount of water, and care is taken to prevent the separation of the stone from the mortar when deposited, the concrete will present a fairly uni form color when the forms are removed. There is greater danger of the efflorescence at joints than at any other point, unless special care is taken. If the work is to be continued within 24 hours, and care is taken to scrape and remove the laitance, and then, before the next layer is deposited, the scraped surface is coated with a thin cement mortar, the joint should be impervious to moisture, and no trouble with efflorescence should be experienced.
A very successful method of removing efflorescence from a con crete surface, consists in applying a wash of diluted hydrochloric acid. The wash consists of 1 part acid to 5 parts water, and is applied with scrubbing brushes. Water is kept constantly played on the work, by means of a hose, to prevent the penetration of the acid. The cleaning is very satisfactory, and for plain surfaces costs about 20 cents per square yard.
330. Laitance. Laitance is whitish, spoiigy material that is washed out of the concrete when it is deposited in water. Before settling on the concrete, it gives the water a milky appearance. It is a semi-fluid mass, composed of a very fine, flocculent matter in the cement; generally contains hydrate of lime; stays in a semi-fluid state for a long time; and acquires very little hardness at its best. Laitance interferes with the bonding of the layers of concrete, and should always be thoroughly cleaned from the surface before another layer of concrete is placed.