The four pigment colors offered in granular form. (Photo courtesy Solomon Colors)
By Debbie Sniderman
Of all the adjectives that come to mind when you think of precast concrete, beautiful is probably not high on the list. However, adding color to concrete, when done correctly, can add depth, meaning and beauty to a product. Architects specify color in pieces such as exterior façades for a variety of reasons such as looks or to help a project become safer. Color can also affect the solar reflectivity, solar reflectance index, and heat effects of a piece, and some reflective pigments can help builders attain LEED points and sustainable credits for structures.
Integral color, applying color pigment throughout the concrete, is the most popular method of coloring precast structures. The coloring process involves obtaining the color pigments, adding pigments to the concrete mix, curing, setting the product in place at the job site, and cleaning or finishing. Larger pieces such as architectural panels may be cleaned by the manufacturer before installation, and smaller pieces like ornamental fills or keystones are often cleaned after they are installed.
Several precast manufacturers agreed the most important factor in coloring concrete is achieving consistency from panel to panel, batch to batch and month to month. Most of their recommendations and practices revolve around reaching this goal.
Determining the exact color mix depends on many factors. Precasters spend a fair amount of time working with a color lab to specify the pigment colors, how much to use, delivery and storage. Precasters most commonly use a dry powder pigment. These are the least expensive and are available in sized bags that are emptied or added into a mixer. Powders also have an unlimited shelf life as long as they are kept dry.
Liquid pigments have been used to color concrete for more than two decades. They are a slurry made from powdered pigments suspended in a liquid and are used with automated dispensers for larger jobs. Liquid is more expensive than powders but the color develops faster in the mix, and automated dispensing increases consistency.
Granular pigments are powders that go through a dry spray process and are clumped together in small balls. Pigment granules are mainly used in concrete paver plants and are rarely used for architectural precast. Automated granular dispensers are fairly expensive, and the granules need to be broken down into powder form before the color develops, so it takes longer to mix.
Dale Keller, director of marketing at Solomon Colors Inc., recommends using dry pigments if less than 40,000 pounds of pigment will be used per year. Users approaching the 40,000 pounds-per-year volume should evaluate liquid pigments and automated dispensing to see if the costs and advantages make sense. Whereas dry pigment must be ordered for every project, only the four primary colors – yellow, light red, dark red and black – are ordered in liquid form. Then a computer-controlled system with a customized formula creates the required color. Keller said a producer would need to use 80,000-100,000 pounds of pigment per year for the granular form to be a cost-effective option.
How much pigment to use?
The amount of color is specified by weight and batch size. “The amount of color pigment to use is based on the weight of the cement, not the sand, or water, or any other admixtures going into the concrete,” Keller explains. “To color a one-yard concrete job that has 564 pounds of Portland cement, typical loading is 2% of 564 pounds of cement. So 11.28 pounds of pigment are needed to color that yard of concrete.”
Cathy Higgins is vice president of sales at Dynamic Color Solutions, another color pigment supplier for the construction industry. She suggests an industry rule of thumb of using 1-to-5%.
“Don’t use less than 1% of the weight of the cement,” she said. “Under that amount, it is harder to control the color, get a consistent blend or be sure the color is dispersed evenly throughout the mix. Adding more color from 1-5% increases the intensity, but don’t use more than 5%.”
“It’s a waste of money and problems with strength and set times will be on the higher additions over 5%. More than 10% can affect the concrete’s compressive strength.”
How much pigment is needed also depends on the color of the cement. Typically, less pigment is needed to color white cement than grey Portland cement. Higgins said her customers who are most interested in color consistency only use white cement. They request a pigment to add to white since they are not going to use grey.
“Even though white cement is twice as expensive as grey, this makes sense after looking at the cost of inconsistency versus the cost of materials,” said Higgins.
Aggregate color also matters
The color of aggregates affects the final color, so a best practice is to routinely check the base color of all of the cementitious materials. Color labs will make samples and match colors of all of the materials that will be used in the final mix. Another best practice that both color suppliers recommend is to keep track of the cement and aggregates over time. Create and retain small samples of each and perform a visual color check to make sure nothing has changed. Even small color changes affect the final color. Higgins also recommends keeping small volumes of pigment samples as well.
Follow consistent mixing procedures
Producers using dry color pigment typically add bags of pigment manually into the mixer. Some bags are water soluble and pre-weighed into batch-sized bags, so one entire bag can be added into the batch without having to open it. With liquid pigment and an automated delivery system, users enter the batch size and the system computes how much pigment is needed. It weighs and pumps the pigment directly into the mixer.
The best mixing practice is to have a mixing order and follow it. Keller recommends adding aggregates to the mixer, then a little water, then add the pigment and let it disperse before adding the cement. Whatever the mixing procedure is, it should be consistent. The ingredients should always be added in the same order. If the procedure changes from batch to batch, the color will change.
“Those that work with color daily have mixing procedures based around color,” he said. “For those that don’t work with color often, it’s important to discuss the mixing process used with the color supplier to make sure the process may not cause any trouble.”
Plan time for curing and cleaning
Higgins said hardly anyone waits until concrete samples are fully dry to submit them for approval, but it’s important to do so when using grey cement or pigments that use synthetic yellow oxides. It takes longer for grey cement to cure and its appearance changes as it does. Precast concrete is darker when it is first poured because of high amounts of moisture.
“A wait time is definitely needed to see the yellows, especially when used with overpowering grey concrete,” she explains. “Yellow iron oxides aren’t visible until the grey is cured.”
Another best practice is to wait to clean colored concrete products. Typical precast structures are poured and pulled from a form or mold. When released, the bond breaker will leave a residue and needs to be cleaned off the piece. The longer the cement cures, the harder the concrete becomes and the less damage cleaning will do to the surface. Also, both color vendors recommend not finishing the surface of colored precast structures because most of the color variation in form finished pieces is on the surface. Higgins said removing the surface reveals a much more consistent color underneath, but it’s not possible to make a form-finished piece look consistent in color on the surface because of how the product dries.
Best practices for achieving consistency
Procedural consistency is the most effective way to achieve color consistency on both large and small jobs. Olympian Precast in Redmond, Wash., is an architectural precast plant that creates exterior building façades and building components and has been coloring concrete for 80 years. Kevin Jewell, Olympian’s semi-retired operations manager, estimates that they use around 10,000-20,000 pounds of color per year.
Jewell said the key to Olympian’s consistency is that they manually weigh the color with precision to make sure the color-to-cement ratio is accurate. They use a gram scale to weigh out pigment and put it in a Ziploc bag for the next day.
“Pre-weighed, pre-formulated bags from the vendor that dissolve in the mix are great, but one day we may need eight yards and the next day nine yards,” Jewell said. “If we used the same size bag each day, the color would be different. This way our ratio is consistent. We’re coloring the cement, not the coarse aggregates or the sand.”
Olympian also looks at the color of the base and never switches cement types. Jewell said he stocks enough pigment so if the supplier did make a change for some reason, he’d have enough to finish existing jobs.
On high-volume jobs, automated delivery systems increase consistency and offer many other advantages. Increte Systems, a precaster in Nothern California, completed a colored architectural precast panel project for a high-end fitness resort. Rich Nagler, West Coast Integral Color Specialist for Euclid Chemical, introduced a granular color dispensing system to create the two colors specified by the architect. Nagler set up an on-site lab, found color formulas that matched the architectural samples and married color with a high-performance concrete mix containing admixtures.
“It reduced labor costs since the colors didn’t need to be weighed and added by hand,” he said. “And, it saved money and storage space since fewer primary colors needed to be held in inventory instead of a large stock of pre-blended powders.”
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.
For More Information:
Dynamic Color Solutions Inc.
Increte Systems – The Euclid