FINISHING SURFACES OF CONCRETE 316. Imperfections. To give a satisfactory finish to exposed surfaces of concrete is a rather difficult problem. Usually, when the forms are taken down, the surface of the concrete shows the joints, knots, and grain of the wood. It has more the appearance of a piece of rough carpentry work than that of finished masonry. Or, failure to tamp or flat-spade the surfaces next to the forms, will result in rough places or stone pockets. Lack of homogeneity in the concrete will cause a variation in the surface texture of the concrete. Varia tion of color, or discoloration, is one of the most common imperfec- • tions. Old concrete adhering to the forms will leave pits in the surface; or the pulling-off of the concrete in spots, as a result of it adhering to the forms when they are removed, will cause a roughness.
To guard against these imperfections, the forins must be well constructed of dressed lumber, and the pores should be well filled with soap or parafflne. The concrete should be thoroughly mixed, and, when placed, care should be taken to compact the concrete thoroughly, next to the forms. The variation in color is usually due to the leaching-out of lime, which is deposited in the form of an efflorescence on the surface; or to the use of different cements in adja cent parts of the same work. The latter case usually can be avoided by using the same brand of cement on the entire work, and the former will be treated under the heading of (Article 329).
317. Plastering. Plastering is not usually successful, although there are cases where a mixture of equal parts of cement and sand has apparently been successful; and when finished rough, it did not show any cracks. It is generally considered impossible to apply mortar in thin layers to a concrete surface, and make it adhere for any length of time. When the plastering begins to scale off, it looks worse than the unfinished surface. This paragraph is intended more as a warning against this manner of finishing concrete surfaces than as a description of it as an approved method of finish.
31S. Mortar Facing. The following method has been adopted by the New York Central Railroad for giving a good finish to exposed concrete surfaces: The forms of 2-inch tongued-and-grooved pine were coated with soft soap, all openings in the joints of the forms being filled with hard soap. The concrete was then deposited, and, as it progressed, was drawn back from the face with a square-pointed shovel, and 1: 2 mor tar poured in along the forms. When the forms were removed, and
while the concrete was green, the surface was rubbed, with a circular motion, with pieces of white firebrick, or brick composed of one part cement and one part sand. The surface was then dampened and painted with a 1: 1 grout, rubbed in, and finished with a wooden float, leaving a smooth and hard surface when dry.
The following method of placing mortar facing has been found very satisfactory, and has been adopted very extensively in the last few years: A sheet-iron plate 6 or S inches wide and about 5 or 6 feet long, has riveted across it on one side angles of 1-inch size, or such other size as may be necessary to give the desired thickness of mortar facing, these angles being spaced about two feet apart (Fig 116). In operation, the ribs of the angles are placed against the forms; and the space between the plate and forms is filled with mortar, which is mixed in small batch es, and thoroughly tamped. The concrete back-filling is then placed; the mould is withdrawn; and the facing and back filling arc rammed together. The mortar facing is mixed in the pro portion of one part cement, to 1, 2, or 3 parts sand; usually a 1:2 mixture is employed, mixed wet and in small batches as used. As mortar facing shows the roughness of the forms more readily than concrete does, care is required in constructing, to secure a smooth finish. When the forms are removed, the face may be treated either in the manner already described, or according to the following method taken from the "Proceedings" of the American Railway Engineering and Maintenance of Way Association: "After the forms are removed, any small cavities or openings in the conc,ete shall be filled with mortar, if necessary. Any ridges due to cracks or joints in the lumber shall be rubbed down; the entire face shall be washed with a thin grout of the consistency of whitewash, mixed in the proportion of 1 part cement to 2 parts of sand. The wash shall be applied with a brush." 319. Masonry Facing. Concrete surfaces may be finished to represent ashlar masonry. The process is similar to stone-dressing; and any of the forms of finish employed for cut stone can be used for concrete. Very often, when the surface is finished to represent ashlar masonry, verticaland horizontal three sided pieces of wood are fastened to the forms • to make V-shaped depressions in the concrete, as shown in Fig. 117.