Texture, materials, design and exposure can all affect your color.
By Brian Miller
Brian Miller was formerly a Technical Services Engineer with NPCA and a member of the NPCA TechTeam.
When people think of concrete, they usually think of the color gray. That’s because the concrete in driveways, sidewalks and patios typically have gray hues. However, concrete can be so much more than just plain gray; in fact, a whole world of color is possible. We usually refer to precast concrete that will remain exposed when in its final position as architectural precast concrete, and it can be made in just about any color, texture and finish imaginable.
Repeatable results with colored concrete require great skill and precision. Hence, precasters are capable of providing the quality control procedures, personnel and equipment to produce colored precast concrete consistently.
Concrete color and texture come from the constituents used to make the concrete and/or a veneer material. A veneer is another material such as clay brick or stone that is used as facing for the precast concrete. This article will not discuss veneers, but rather focuses on using the constituents of concrete to achieve the desired color and finish. These constituents include course and fine aggregates, cement, water, admixtures and pigments.
When concrete color comes from the constituents, it is developed from the paste (hydrated cement), matrix (sand and paste) and in some cases the coarse aggregate. When trying to achieve a desired color, it is important to understand which finish will be used. The finish determines which constituent will predominantly contribute to the color and uniformity. The term “finish” usually refers to the overall look of the precast and includes its texture (rough, smooth) color, design and exposure. The term “exposure” usually describes how much coarse aggregate will be visible.
All the constituents can affect the color, but depending on finish and exposure, some constituents will dominate. For discussion, we can break these into two groups: paste dominant and aggregate dominant.
PASTE DOMINANT COLOR
Paste is the primary contributor to color when smooth and light exposure finishes are used. Some of these include form finishes, light acid-etched, abrasive blasted finishes and many form liner finishes. The color of paste comes from the cement, water and pigments when used. Since the hydration of cement, which creates paste, is a chemical process, the color generated is sensitive to many things. Some of the critical ones include water-cementitous (w/c) ratio, curing procedures, cement type, temperature and form materials. It is crucial to minimize fluctuations in these variables from batch to batch to minimize color variations.
It is likewise important to use the same type and manufacturer of cement throughout a project; this will help to minimize color variations. Cement typically is available in gray and white. Gray cement receives most of its color from tetracalcium aluminoferrite (C4AF), which is a component in cement that helps reduce the temperature of clinker during manufacturing. Gray cement can have color variations from one lot to the next since color control is not a monitored parameter when producing gray cement. Most of the gray cement produced ends up in structures, bridges, and those driveways and sidewalks mentioned earlier.
When using gray cement, it is a good idea to maintain a dry powder sample from each lot or shipment, which can be compared visually for color differences. While purchasing enough cement for a project is the best approach, noticeable variations may require adjustments in mix design or pigment dosage. This has been done successfully to maintain color consistency of the final product but requires prior experimentation.
White cement, on the other hand, has very little ferric (iron) components and is monitored for color variations during production. White cement is manufactured for use in colored concrete. It is available in several types and is fairly consistent from one lot to the next. Therefore, precast products made with white cement have less color variation. Also, white cement has been used to create more brilliant colors than gray cement and usually requires less pigment. Sometimes white and gray cement are blended in order to improve uniformity of the final product.
Pigments have been used to color concrete for about a century. Pigments actually tint the color of the paste portion of concrete. The most common pigments are made from iron oxides. Iron oxides consist of 325 mesh particle sizes that provide stable colors that do not fade or leach out of concrete. The primary iron oxide colors come in yellow, red and black, with the reds having either a blue or orange tint to them. Combinations of these basic colors are used to create a multitude of other colors or shades of color. Other colors such as greens are commonly created from chromium oxides and blues from cobalt. These pigments work well but are used more sparingly, since they are more expensive.
In general, pigments are inert and do not react with cement, therefore they are not considered as part of the cementitous material content. Instead, pigments bond with the cement via the hydration process, masking the color of cement grains. Raw pigmentary materials should meet ASTM C 979, “Standard Specification for Pigments for Integrally Colored Concrete.” Pigments are typically dosed by weight of cement, usually in the 1 percent to 7 percent range, and should never exceed 10 percent by weight of cement. Higher doses of pigments usually do not improve color but may reduce strengths.
Pigments come in powdered, granular and liquid forms. All three forms work fine but require different handling, dosing and mixing procedures. The oldest form – powders – have been described as somewhat messy and labor intensive when weighing, since this is often done by hand.
Some pigments are available in preweighed, dissolving bags that can be tossed into a mixer, which helps reduce handling problems.
Storage of powder pigments should be done carefully to avoid any moisture or high levels of humidity. Usually powdered pigments are added with the cement in the batching process, but always follow the manufacturer’s recommendations.
Granular forms were developed as an improvement for handling and dosing pigments. Some systems automate the dosing of granulated pigments, which allows for easier storage and varied batch sizes without having to use a partial bag. Again, these types of pigments must be protected from moisture until use.
More recently, liquid dispersions have been developed that work quite well. Pigment particles are suspended in a liquid, usually water, and can be disbursed like any other liquid admixture. This allows for easy handling, accurate dosing (usually done automatically) and complete distribution of the color throughout the concrete with minimal mixing time.
Precasters should always adjust the batch water for any water added from an admixture or any other source to maintain the water-cementitous ratio.
Pigment dispersions should be agitated often to maintain a uniform concentration of the pigment. While liquid dispersion coloring admixtures are typically batched with the mix water, always follow the manufacturer’s instructions in regards to use, dosage, storage, etc., of any pigment.
It is always important to maintain a low and consistent water-cementitous ratio. This is even more important with regards to color. Lower water-cementitous (w/c) ratios will improve color consistency. High w/c ratios may cause color streaking and increase color variation. Slight fluctuations in water-cementitous ratio, curing procedures and temperature may result in color variations of the paste.
Casting and production
During placement of concrete, mix and place all the concrete for the exposed area of a piece at one time. This helps to reduce any variations within a piece. Many architectural producers will use a “face mix” to help reduce the volume of concrete needed to accomplish this. Face mixes also reduce costs of expensive aggregates and materials commonly used in architectural precast concrete. Face mixes should be at least 1.5 times as thick as the maximum nominal aggregate size, but never less than 1 inch after consolidation.
Also, avoid cold joints. Place all the concrete quickly and maintain low drop heights. Do not use internal vibrators on thin panels, since these typically get dragged and cause segregation, which may leave marks on the finished surface. For more information on good concreting practices, refer to NPCA’s “Quality Control Manual for Precast Concrete Plants.”
AGGREGATE DOMINANT COLOR
Aggregates are the primary contributor to color when medium to deep exposure finishes are used. These are common finishes produced by abrasive blasting, grinding, deep acid etching and chemical surface retarders, the latter of which are used to expose the aggregate. Aggregates provide natural color, which is not as affected by the batching, mixing and curing process since there is no chemical reaction taking place within the aggregate. The greater the aggregate exposure, the more important the color and uniformity become.
Aggregates come in a variety of colors, including whites, grays, reds, blacks, greens, browns, tans, etc. The colors are very stable and do not change with time. As always, it is recommended to purchase all the aggregate for a project from one source to minimize any natural variations.
When using a finish with exposed aggregate, it is a good idea to match the matrix or go slightly darker than the aggregate. This creates a more uniform finish and typically minimizes changes with time and weathering. Also, gap-graded aggregates are commonly used to create more uniform deep exposure finishes.
Design and location
It is recommended that color and finish selection be made under similar conditions in which the project will be viewed. Many products will not be seen close up after installation. For example, a wall panel on the fifth floor will be viewed from probably 60 feet away and looking up at it from underneath. Therefore, the finish and color may have a different look than when standing directly in front of it. When selecting a finish, samples should be viewed from different angles, far away as well as close up.
The design of a precast piece and its final installed location will also contribute to how its color is perceived. Sometimes referred to as color tone, or the perception of the color’s shade, is affected by weather, light, density and other surrounding materials. For example, a light-colored precast finish such as white will appear differently if gold or bronze window glazing is used by it. Likewise, deep exposure of aggregates may create shadows that darken lighter colors; however, this may not be as evident on a cloudy day. The use of larger samples and multiple viewings under different conditions helps to achieve the desired results of the architectural finish and color.
Usually during the bidding process, precasters supply 12-by-12-inch concrete samples for color and finish. It is suggested that prior to production of the job, a mock-up be made that incorporates half- to full-scale pieces made from actual production lines. This can assist in setting realistic acceptance criteria and final product expectations. Mock-ups can also help identify any production or constructability concerns prior to full production of the project.
Weathering can play a large role in how a project looks and how colors change over time. Environmental factors such as pollution and dirt can accumulate on structures and alter their colors, typically darkening them and reducing their luster. Most of this can be cleaned off with soap and water, mild cleaning products or similar means as were used in developing the finish, such as acid-etching or abrasive blasting. For aggregate dominant finishes, the use of rounded aggregates will reduce the amount of dirt that is collected relative to angular aggregates. Angular aggregates provide greater surface areas and angles in which to trap dirt.
Paste dominant finishes are usually more susceptible to efflorescence, weathering and discoloration from organic and inorganic pollution. Since paste is porous and reactive, contaminants can be pulled into the concrete pores, making it more difficult to remove them by cleaning. Similar means as described above may be used; however, note that acid etching typically darkens paste, and abrasive blasting typically lightens a finish. Also notable is that form finishes and light exposures are more susceptible to acid rains relative to deeper exposures. Over time these finishes may darken permanently, because more sand is exposed.
Overall, a combination of constituents and finishes can provide a wide variety of long-lasting colors for precast concrete.