Colored, stamped concrete all the rage

With stamping techniques and colorization, interior concrete flooring can imitate marble, limestone, cobblestone, stained wood planking and various imaginative decorator looks to enhance your décor. According to the Portland Cement Association, the three most satisfactory methods of producing colored concrete finishes are the one-course or integral method; the two-course method; and the dry-shake method. In applying color to concrete, the manufacturer’s specifications must be followed closely.

Adding the appropriate amount of color pigment to the concrete in the mixer to produce a uniform color through the entire slab is the one-course method. The pigment can be pure mineral oxide or a natural or a synthetic iron-oxide colorant especially prepared for use in concrete. Both the natural and synthetic pigments are satisfactory if they are insoluble in water, free from soluble salts and acids, fast to sunlight, fast to alkali and weak acids, limited to small amounts of calcium sulfate, and ground fine enough so that 90 percent passes a 45 micron screen.

Full-strength pigments will normally produce a good color when 7 pounds is mixed with one bag of cement; 1 1/2 pounds per bag normally produces a pleasing pastel color. White Portland cement will produce cleaner, brighter colors and should be used in preference to normal gray Portland cement, except for black or dark gray colors. To obtain a uniform color throughout the slab, all materials in the mix must be carefully proportioned by weight. Mixing time should be longer than normal to ensure uniformity. To prevent streaking, the dry cement and color compound should be thoroughly blended before they are added to the mix.

In the two-course method, the base slab is placed in the usual manner, except the surface is left in a rough texture to provide a better mechanical bond with the colored topping. The colored topping course can be placed on the base slab of concrete as soon as it is firm enough to support a cement mason’s weight. If the concrete has hardened, the topping course can be bonded to the base concrete by either a cement-water grout or a cement-sand-water grout. The topping mix is normally  1/2- to 1-inch thick, with a ratio of cement to sand of 1:3 or 1:4. Color pigments are added to the medium-consistency mixture according to the manufacturer’s specifications. The mix is floated and toweled in the prescribed manner. Allowance must be made in the base slab thickness to accommodate the topping course and bring it to the proper grade. The two-course color finish is often used in place of the one-course method because of its economy. The savings in materials normally more than offset the higher labor costs.

The dry-shake method consists of applying a prepared, packaged dry color material that can be purchased from various manufacturers in ready-to-use quantities. The basic ingredients are a pigment, white Portland cement, and specially graded silica sand or fine aggregate. Proportioning and mixing a dry-shake material on the job site is not as satisfactory as using a prepared mix.

After the concrete has been poured, screed and bull floated, and excess moisture has evaporated from the surface, the slab should be power- or hand-floated. This preliminary floating should be done before the dry-shake material is applied so that enough moisture will rise to the surface to combine with the dry material. Floating also removes any ridges or depressions that might cause variations in color intensity. If too much color is applied in one spot, nonuniformity in color and possible surface peeling will result. Colored dry-shake material is applied in a two-step color application process after floating, with two or three toweling sequences following while the concrete is in different stages of setting.

Colored slabs, as with other types of freshly placed concrete, must be cured thoroughly. After thorough curing and surface drying, interior surfaces may be given at least two coats of special concrete floor wax containing the same pigment used in dry-shake material. Nonwax polymeric sealers are also available. Sealing or waxing accentuates the color and makes cleaning easier.

The description of methods described here are taken from materials produced by the Portland Cement Association. They are informational, but not meant for a technical guide on how to do-it-yourself. The Portland Cement Association recommends using a professional installer for decorative concrete floors. For more information, visit the Portland Cement Association Web site at www.cement.org.