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Dry-shake hardeners come in a wider array of hues than integral colors, including various shades of blue and green. L.M. Scofield Company in Los Angeles, CA.

Dry-Shake HardenersMost shake-on color hardener manufacturers make blend pigments, portland cement, finely graded silica sand, and wetting agents. They come in powdered form, packaged in bags or pails, and are tossed or hand broadcast onto the fresh concrete. After the hardener wets up, a wood bull float is used to float the hardener into the surface before the concrete hardens. Unlike integral pigments, which color the entire concrete matrix, hardeners color only the top 1/8 to 3/16 inch of the slab. Decorative contractors often use dry shakes to color stamped concrete flatwork or concrete overlays, because the rich surface paste helps to produce sharper imprints.

AdvantagesDry-shake hardeners come in a wider array of hues than integral colors, including various shades of blue and green. And because the color is concentrated on the surface, it tends to be more intense.

As the name implies, color hardeners also densify the concrete surface because they contain hard mineral aggregates and portland cement. The result is a surface that's stronger, more wear resistant, and less permeable to moisture and deicing chemicals than standard concrete.

LimitationsThe two main disadvantages of color hardener are the labor and the mess, according to Jeff Potvin of Architectural Concrete Consultants. Most manufacturers of color hardener recommend applying color hardener in two separate applications, or "shakes," instead of applying the entire recommended dosage all at once. This gives the hardener a chance to "wet out," or absorb water. After each application, the hardener must be worked into the surface with a float. Potvin says that it usually takes six man-hours to spread color hardener on approximately 500 square feet of concrete.

Because of their shake-on application, dry shakes generally are limited to use on horizontal surfaces. However, Harris says that you can apply them to step faces by combining the hardener with a bonding agent and water and then plastering this mixture onto the vertical faces with a trowel.

What to consider before buying

  • As with integral pigments, make sure the color hardener meets ASTM C 979 standards for color stability. If abrasion resistance is a priority, look for products that produce surfaces meeting the abrasion-resistance standards of ASTM C 944.

  • Always follow the manufacturer's recommendations for color hardener dosage rates. Most suppliers package hardener in 60-pound buckets or bags that will cover approximately 100 square feet of surface area (or about 2/3 pound of material per square foot). But lighter colors often require a heavier application, ranging from 90 to 120 pounds (or two containers) of hardener per 100 square feet.

  • Consult with your ready-mix producer and explain that you will need a mix design appropriate for use with a dry-shake hardener. Some admixtures, such as air-entraining agents and water reducers, can reduce or minimize the amount of bleed water available for absorption by the color hardener. Some manufacturers recommend limiting the air content of the concrete to 4%.

  • If you want to use a dry-shake hardener to color an overlay, ask the manufacturer of the overlay system if hardeners are a viable coloring method. A dry-shake hardener must absorb some moisture from the overlay so you can work it in properly. Some overlay systems, however, don't contain enough moisture to wet out the hardener.

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