Color is a relative value, not an absolute. It is affected by light, shadow, density, time and other nearby colors. Selections should be made under lighting conditions similar to those under which the precast concrete will be used, such as the strong light and shadows of natural daylight or interior CFL lighting. Surface texture influences color. The building’s appearance is a function of the architect’s use of light, shadow, texture and color.
Cement plus a coloring agent exerts the primary color influence on a smooth finish, because it coats the exposed concrete surface. As the concrete surface is progressively removed and aggregates are exposed, the panel color increasingly exhibits the fine and then the coarse aggregate colors. The color of the cement always has an effect on the general tone of the panel. Cement may be gray, white, buff or a mixture. All cements have inherent color and shading differences, depending on their source.
Pigments and pigmented admixtures often are added to the matrix to obtain colors that cannot be created through combinations of cement and fine aggregate alone. White portland cement will produce cleaner, brighter colors and should be used in preference to gray cement with pigments, especially for the light pastels such as buff, cream, pink, rose and ivory.
Fine aggregates have a major effect on the color of white and light buff-colored concrete and can add color tones when the surface is given a shallow profile to increase the aggregate’s exposure. Coarse aggregate colors become dominant as the surface of the concrete is removed to obtain a medium or deep aggregate exposure profile.
Some finishing processes change the appearance of aggregates. Sandblasting will give the aggregates a matte finish, while acid-etching may increase their brightness. Exposure by retardation normally leaves the aggregates unchanged.
Maintaining consistency of color throughout the production run requires attention to detail and proper specification. Nine key factors should be closely watched when color consistency is critical:
- Type and color of cement.
- The quality and quantity of the coloring agent.
- Facing characteristics of some coloring agents.
- Proper batching and mixing techniques and the coloring agent’s effect on the concrete’s workability.
- Quality (that is, freedom from impurities) of the fine and coarse aggregates.
- Uniform quantities and gradation of the fine materials (capable of passing through a No. 50 sieve, including the cement) in the concrete mix.
- Careful attention to curing and uniform duplication of curing cycles.
- Constant water-cement ratio in the mix.
- Consideration of the factors that can contribute to efflorescence. Its appearance on the concrete’s surface can mask the true color and give the appearance of color fading. The efflorescence can be washed off when its appearance on the panel is noticed.
Color should be judged from a full-sized sample that has the proper matrix and has been finished in accordance with planned production techniques. The sample should be assessed for appearance during both wet and dry weather.
For more information, contact a PCI Certified Manufacturer.