A few simple tips will help you collaborate with your precast producer to get the results you want.
By Nick Paris and Aaron Chusid
Color adds permanent beauty and value to precast concrete. It can make a building blend in with or stand out from its environment to fulfill the artistic visions of the designer. Long known as a dependable and economical alternative to paint and other surface-applied coatings, integral color is part of what makes precast concrete attractive.
Consistency is key to producing high-quality precast. Variations in materials, workmanship and production techniques can result in noticeable differences from one precast element to the next. It is important to be as consistent as possible in every step of the process, from material selection to mixing to curing. Precast concrete has a noticeable advantage in this regard over cast-in-place concrete, as the members are made in an environment that is more controlled than even the best of work sites.
Begin by selecting the right precast manufacturer. The National Precast Concrete Association (NPCA) has a plant certification program that can help with this by ensuring an acceptable standard for production, processes, plant facilities and quality control operations. It is advisable to contact local producers early in the design process so they can help you optimize your design for precast and can provide you with samples for color selection. On large projects or structures that will be completed in phases, it may be necessary for your producer to stockpile enough materials to see the project through to completion; running out of sand or cement midway through construction can lead to inconsistent coloration if a suitable replacement cannot be found.
Here are some key points to bear in mind when specifying colored concrete to minimize unwanted variation.
Controlling the cementitious materials and aggregates is the first step toward producing a wide range of colors. Portland cement is produced in a variety of shades, from white to gray to tan. Changing the tint of the cement has a similar effect on the color of the concrete’s tone. Adding colors to light or white cement produces brighter, more pastel concrete; adding colors to gray cement produces more muted hues.
Most concrete is now mixed with supplementary cementitious materials in addition to portland cement. Since these come in a range of colors, they must be considered when determining the final appearance of precast concrete. Fly ash and silica fume are usually in shades of gray, while high-reactivity metakaolin and ground granulated blast furnace slag are white and can brighten a mixture.
A uniform water-to-cementitious material (w/cm) ratio should also be maintained in a mixture to produce consistent results.
As concrete is mixed, a paste of portland cement coats each grain of sand and aggregate. Although the base color of the cement paste is the primary determinant of the concrete’s color, the color and grading of sand and aggregate can show through the thin coating of cement paste and influence the concrete’s appearance.
The color of the aggregate has a greater impact when the surface layer of cement paste is removed. This occurs when the concrete is given an exposed-aggregate finish. Many different effects can be produced by selecting aggregate colors that either complement or contrast with its surrounding concrete color.
Mineral oxide pigments are the most common colorants in precast concrete; they are chemically stable; resist ultraviolet light, weather and the alkalinity of concrete; and become a permanent part of the concrete, ensuring that the color lasts as long as the concrete. Leading color pigment manufacturers carefully produce additives to ensure uniform color and tinting strength, and to comply with ASTM C979, “Pigments for Integrally Colored Concrete.”
Pigments are available in three forms: powder, granular and liquid. Many precast producers now use automated color mixing systems, allowing greater flexibility in custom colors. These systems are capable of fine adjustments to meet designer-specified requirements. The customized color mix data can then be saved to ensure that later batches will have the same pigmentation.
Variations in the dosage rate of color additives affect concrete’s color. The pigment dosage rate is based on the weight of all cementitious materials in a mix. Dosage rates as low as one-half percent can impart a hint of color that can give concrete a warmer or cooler impression, and increasing the dosage rate produces more intense colors. Color saturation generally occurs at dosage rates beyond 6 percent; adding pigment will not intensify the concrete color beyond this point. ASTM C979 stipulates that dosage rates above 10 percent should not be used.
If an as-cast surface finish is required, careful attention must be given to the forms used by a precaster. The form-release compound should be tested before use to make sure it does not stain the concrete. Smooth-surfaced form materials such as clean steel or plastic-faced plywood can produce concrete surfaces that are equally smooth and shiny. This effect can be especially enhanced when self-consolidating concrete is used.
Smooth as-cast concrete surfaces, however, will show every blemish in the concrete or formwork. Where this is not acceptable, it is best to use a rough or textured concrete surface that will scatter reflected light and soften the impact of blemishes. A variety of textured form liners are available and can be used to create attractive architectural effects.
A chemical retarder can be used to create an exposed-aggregate finish; after the concrete cures, its surface can be scrubbed to expose the aggregate that would otherwise be concealed beneath a surface of cement past. Another alternative is to use mechanical methods such as sandblasting, grinding or bush hammering to roughen the surface.
Variations in the depth of aggregate exposure also affect appearance. As more cement paste is removed, more of the aggregate becomes visible and has a greater impact on the overall appearance of the concrete. Mechanical abrasion methods that fracture, grind or polish the aggregate can reveal the crystalline grain of the aggregate, producing a finish that can sparkle in the sunlight.
Efflorescence occurs when soluble salts in the concrete are carried to the surface where they are deposited by evaporation. Since the powdery efflorescence deposits are white, they are more visible on dark-colored concrete than on light, uncolored concrete. Efflorescence can be reduced by using a low water/cement ratio, concrete with low permeability and thorough curing of the concrete. Pozzolanic admixtures like fly ash and metakaolin can also reduce the potential for efflorescence. As always, design the structure to prevent water penetration and leaks.
On large or demanding projects, specify a mock-up to be erected before the rest of the precast members are produced. This is an essential step, as it ensures the finished product meets designer expectations. Require the mock-up to use the actual production techniques, and view it in the location it will be used. As the concrete cures, expect the color to lighten. Full color development in concrete takes about 28 days. Some precasters use steam to accelerate this process. In either case, be sure to inspect the color after it is fully cured so that the color you see will be the color you get.
The list of project types for which colored precast is used continues to expand. It is used for architectural building panels, site furniture and amenities, concrete standards for street lights, concrete roofing tiles, unit pavers and precast veneers. In infrastructure projects, the life-cycle value of colored precast makes it increasingly popular for use in bridges and other transportation projects. Sound walls, median strips and guide rails all can be colored to match their surroundings.
Inside buildings, colored precast is also seeing increased use for flooring tiles, countertops, wall cladding and even furniture. This trend will likely increase as new, ultra-high strength types of concretes make it possible to reduce the thickness of precast members so they are lighter weight and offer new design versatility.
Affordable and sustainable
The cost of integral color is at least partially offset by the elimination of coating materials. More significantly, pigments for integral colors are added at the same time as other concrete ingredients and thus have only minimal impact on labor required for placing and finishing. Applying paint or other surface coatings often requires at least three visits to the work site during construction to sandblast, apply primer and finally apply the surface coating. By contrast, integrally colored precast arrives on the job site already colored; at most it requires a controlled sandblasting.
These savings become even greater over the life cycle of the building. Surface coatings need to be reapplied throughout the structure’s service life. By contrast, the pigments in concrete are locked into the cement matrix and last as long as the concrete itself, significantly reducing life-cycle costs.
Permanent concrete colors also have a positive impact on a building’s ecological profile. They eliminate the environmental burden of multiple coats of paint, including the solvents and volatile organic compounds associated with coatings.
Despite a producer’s best quality assurance efforts, all concrete – whether tinted or not – is subject to mottling and visual variation. This can be a desirable trait, giving the concrete a more natural, artistic quality. Still, designers must have realistic expectations about the range of finish variation that may occur in colored concrete and then specify acceptable tolerances.
Visit projects built by your proposed precaster, and discuss the benefits and limitations of colored concrete with your client or the building owner. Successful colored precast projects begin with understanding your customer’s expectations and then following best industry practices.
Following these guidelines will help you get a structure that will continue to look its best for generations to come.
Colored Concrete on Large Surfaces
Colored concrete can be surprisingly affordable, even on large precast structures like bridges. On these jobs, the volume of concrete is relatively small in comparison with the structure’s surface area.
Box girders and other hollow sections typically have thin walls. This minimizes the amount of concrete and pigment required.
Surfaces with a higher volume-to-surface area ratio – including many recessed areas within the structure’s shadow line – can be left with an as-formed finish. These areas will be slightly darker than sandblasted areas, giving an illusion of greater depth to the architectural recesses. Using as-formed surfaces at selected areas eliminates the cost for controlled sandblasting across these areas of an integrally colored structure.
Nick Paris is vice president of marketing for Davis Colors, a leading producer of concrete colorants. He can be reached at www.daviscolors.com. Aaron Chusid, CSI, is with Chusid Associates, consultants to Davis Colors. He can be reached at (818) 774-0003 or www.chusid.com. This article is copyrighted and printed with permission by Davis Colors.