Fancy Flatwork 2: more trade secrets for decorative concrete

Tools of the Trade, March-April, 2004 by David Frane

Most decorative concrete techniques involve changing the color and texture of a concrete surface to make it look like a more expensive material such as brick, flagstone, and slate. Skilled finishers can produce stone- and tile-like surfaces that are hard to distinguish from the real thing.

INTEGRAL COLORING

The easiest way to color concrete is to have your supplier add color pigment to the mix at the concrete plant. You also can add pigment after the truck arrives on your site, but either way, the concrete is colored before it's poured, and placing and finishing it isn't much different than usual.

Pigments are usually powdered and come in a wide range of colors, including reds, greens, yellows, blues, browns, tans, grays, and black. Integral colors mix all the way through the concrete, so chips and breaks in the surface will not be that noticeable.

While adding integral color is a simple way to color concrete, there are some things you have to be careful of. The most important thing is to be consistent, especially for jobs that require more than one load of concrete. Each batch should contain the same type of materials with the same amount of pigment, mixed consistently before pouring it.

Pigment should be measured by weight and added in proportion to the amount of cement that's in the mix. Intense colors require more pigment to achieve than subtle colors and are therefore more expensive. It might take 3 pounds of pigment to color a cubic yard of light ran concrete and as much as 24 pounds to produce a deep brick red.

Watch Out for Cold Weather. It's common practice to use calcium chloride to reduce setting time in cold weather, but don't add it to colored concrete because chloride-based accelerators can discolor the finished slab. If you need to pour concrete in cold temperatures, ask your supplier to use hot water, or increase the cement content of the mix. Other options include using more expensive non-chloride accelerators or concrete containing fast-setting type III cement.

Hot Days. Dark colors absorb heat, so a dark mix may set faster than you expect when the weather is sunny and dry. That can make the concrete hard to finish and may lead to thermal cracking. If you can't avoid pouring dark colors on hot sunny days, you should tent the slab or use extra curing compound to keep the concrete from drying out too quickly.

COLOR HARDENERS

Another way to color concrete is to add pigment after it's placed. This is done by heavily dusting the surface with a powdered color hardener after you float the concrete surface and the bleed water has disappeared. The moisture in the slab activates the hardener, which is incorporated into the surface by floating, troweling, or stamping with textured mats. The color typically goes about 1/8 inch into the slab.

The main ingredients in a color hardener are pigment and portland cement. The pigment provides color, and the portland enriches the surface and makes the surface harder than the rest of the concrete below it. The extra-hard surface helps the slab wear better and makes it more resistant to freeze-thaw cycles.

Skilled Labor Needed. Using color hardener takes more skill and labor than using integrally colored concrete. Color hardener is applied in stages: The powdered hardener is spread onto the fresh slab and worked into the surface with a trowel or float. More hardener is then worked into areas that didn't get enough color the first time. The slab is then ready for finishing.

Color hardener comes in a wide range of colors. There's more design flexibility than with integral color because you can control the process; you decide how much color to apply and you're able to create highlights by using more than one color. It takes between 60 and 120 pounds of color hardener to color 100 square feet of concrete. Lighter colors typically require more. Most color hardener costs between 35 cents and $1 per square foot. Blues and greens are more, costing between $1.20 and $6.50 per square foot. As with integral coloring, you should avoid using chloride-based accelerators.

ACID STAIN

Concrete also can be colored after it cures by treating it with an acid stain. The stain, which is made from water, acid, and inorganic salts, can be applied with a roller, brush, or garden sprayer. It's applied to the surface but does not form a coating like paint. Instead, it soaks in and reacts with the free lime that's in the concrete. Free lime is not evenly distributed, so the treatment produces an attractive mottled effect. You can use a single color, multiple colors, or mix colors on the surface. Geometric patterns can be created by using a power saw to cut notches or "kerfs" into the slab prior to staining. The saw kerfs can be grouted later to create a surface that looks like tile.

Manufacturers recommend allowing new concrete to cure for two to four weeks before staining. The slab should be carefully cleaned before treating it because surface contamination from oil or dust can keep the stain from soaking in evenly. Check with the stain manufacturer before using any cleaning solution on the slab. And whatever you do, don't use acid to clean the slab. Acid cleaners will react with the free lime in the slab and will keep the acid stain from working.