Variety Of Finishes Possible

One of the many advantages which concrete possesses is that by various kinds of manipulation a great variety of surface finish can be secured. Some of these finishes are given entirely after the work has been completed, while others are partly arranged for when the materials are being selected.

In small ornamental objects, colored sands and selected aggregates, such as marble chips, granite screenings, etc., are often used in place of the ordinary aggregates. After forms have been removed, the surface is treated in some one of a number of different ways to take advantage of color effects that can be secured by using such selected materials.

An example of rubbed finish showing a surface in which fine aggregate was used in the concrete surfacing mixture.

Leaving Surface As Produced By Forms

The simplest form of surface finish that can be given to concrete is that secured by properly placing the material in well-made forms and spading the concrete next to form faces so that coarse particles will not be exposed on the surface. Such a surface finish, however, is rather monotonous. The only color it has is due almost entirely to the cement. The sand does not affect the color of the surface to any great extent because the surface of the sand particles is covered with a film of cement. If forms have been well made, the only treatment that is necessary to give such a surface is to patch up small spots or imperfections due to not having everywhere thoroughly forced back the coarse aggregate from the form face. These imperfections should be filled with a mortar of the same proportions of cement and sand as the mortar of the concrete, and rubbed smooth and even with the general surface of the object by using a wood float. If the whole surface of the object is gone over in this manner, a fairly good finish can be secured at little expense. The surface will be as smooth as the forms against which the concrete is placed, and if the concrete was properly proportioned, the surface will also be dense and watertight. Dressed lumber will produce a smooth surface finish if forms are tightly jointed so that the sand-cement mortar cannot flow into joints, thus molding ridges or seams on the surface. If surface finish is to depend on forms alone, then metal or metal-lined forms will give the best surface.

A combination of the rubbed and tooled surfaces. This is a little piece of concrete bridge ornamentation.

Cement-Water Wash

Another method of finishing a concrete surface consists of painting the concrete with a cement wash, usually consisting of 1 part cement and 1 part fine sand, mixed with water to form a paint. This is applied with a brush after forms are removed. It fills up the pores and covers small inequalities and grain marks of wood forms and gives a smooth surface of even finish and uniform color. However, it is likely to check and hair crack and sometimes scales off in spots, if not entirely. For this reason it is not a reliable surface finish.

"Floating" With Stone or Wood Float.-There are several other methods of after treatment which in general are as follows: Forms are removed as soon as the concrete has hardened sufficiently to be self-sustaining, or to permit handling the object if reasonable care is used. Ridges caused by joints in forms are carefully chipped off, the concrete surface is thoroughly wet, and rubbed with a carborundum block, the surface being kept wet constantly while so rubbed. Another method of rubbing the surface is to use a wood float or trowel. The surface must be kept well flushed with water during the rubbing and thoroughly washed after rubbing has been completed. The earlier the surface is rubbed after form removal, the better will be the results. In other words, the concrete should be rather "green" (not hard). Rubbing removes most of the inequalities, fills the pores and small cavities and gives to the surface a uniform finish and appearance. Such treatment is proof against the pitting and scaling that frequently result from painting on a cement and water paint, or grout, as it is called.