The Art of Applying Color Hardeners to Concrete

With dry shake color hardeners, beauty is more than skin deep. Not only do these powders offer a range of color options, including colors lighter than natural gray concrete, but they increase surface strength to as much as 8,000 psi or more. The vast majority of decorative concrete applications using color hardeners are imprinted, but the increased durability these materials impart also makes them attractive for large commercial and industrial projects such as warehouse or factory floors.

Concrete preparation
There is no extraordinary preparation required for concrete that will be colored with a color hardener, but as in all architectural applications, it is important to pay attention to details. Concrete for a successful color hardener job must strike a fine balance: There must be enough moisture at the surface to be absorbed by the hardener, but no standing bleed water. Too much air entrainment can prevent moisture from reaching the surface, but concrete with no air entrainment might have excessive bleed water. The color hardener must be applied during a narrow window of time, after the bleed water evaporates but before the surface starts to crust and lose plasticity. So contractors need to pay attention when preparing concrete for color hardeners.

The first detail to balance is the mix. Experts recommend a slump between four and five to achieve workability without excess bleed water. Admixtures typically used for air entrainment or for accelerating or retarding the set will not affect the performance of a color hardener. In areas of the country where air entrainment is necessary because the concrete will be subjected to freezing, Bob Harris of the Decorative Concrete Institute (DCI) in Douglasville, Ga., cautions that the entrained air should be reduced from 6 percent to 3.5-4 percent to allow moisture to reach the surface so it can react with the color hardener. He also suggests experimenting with the amount of sand in the mixture — reducing sand can allow the hardener to absorb more water.

"Working with your local ready mix producer is the best advice I can give," says Scott Thome, director of product services for L.M. Scofield Company. "Performing test pours with different mix designs will cost some money up front but will save money and time in the future."

Once the mix has been established, the second detail is placement. Concrete should be placed with the same form work and subgrade preparation as conventional concrete, following American Concrete Institute (ACI) guidelines. If the colored concrete will be imprinted, Thome recommends driving the form stakes flush with the top of the forms so they won't interfere with stamp placement.