Decorative Concrete Institute
Bob Harris
Consulting • Education • Installation • Training
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Picture a concrete coloring material that produces a unique look whenever it's applied. No two floors or walls look the same after being treated with this material. That's the magic of the acid-etch chemical staining process-an infinite number of possible results limited only by the creativity of the installer. It's why installers get so involved with the process, and it's why their customers must have realistic expectations about the outcome. They should expect variability, not uniformity. "We're trying to make concrete look interesting, not like some other material," says Mike Miller, president of The Concretist, a well-known consultant to the decorative-concrete industry. He often combines colors from acid-etch stains, dyes, and tints in his work, and extends the range even further by applying the stains to concrete already colored integrally or with color hardeners. The variegated and natural look of the end product is its most attractive feature, but this variability can also lead to disagreements among owners, specifiers, and installers. Because so many variables can affect the final color, contractors should provide an approval sample of stain applied to a small, concealed part of the concrete to be treated. How
stains work The
acid in chemical stains opens the top surface of the concrete, allowing
metallic salts to reach the free lime deposits. Water from the stain
solution then fuels the reaction, usually for about a month after
the stain has been applied. Other factors that affect the outcome
include: In general, cements that produce larger amounts of calcium hydroxide during hydration will show more stain color, and higher cement contents produce more intense colors. Air-entraining or water-reducing admixtures don't pose a problem. However, calcium-chloride accelerators can cause very mottled, darkened areas, and for this reason aren't recommended. Nonchloride accelerators don't cause this mottling effect. If they're near the surface, calcium-based aggregates, such as limestone, take stain readily and deepen the color of the concrete above them. Siliceous aggregates, such as gravel, don't react with the stain. Open finishes achieved by floating followed by minimal troweling take more stain and produce denser colors than do hard-troweled surfaces. However, open finishes lose color faster because the concrete wears away. Because of this, Gary Jones, president of CP Concrete Systems in Burnabv, British Columbia, Canada, prefers staining hard-troweled surfaces because the stain color lasts longer. "Colors on troweled surfaces also look richer than those on floated surfaces," says Jones. "But you have to sand the surface or use a higher acid concentration to ensure adequate stain penetration." Slabs placed in wet weather result in a richer stain color if the concrete is stained soon after it's placed. However, wet slabs are more likely to effloresce, lightening the color and causing a more mottled effect in areas where the stain doesn't take because efflorescing salts hinder penetration. On sunny days, the concrete can become hot and dry, and the stains won't penetrate as deeply into the concrete. The
continued presence of water will cause the reaction to continue
for a long time, and concrete stained blue green will gradually
turn brown or even black. Initially, this provides nice variation
to the appearance, but eventually, nearly all the blue-green color
may change to brown and black. Because of the possible color shifts,
some manufacturers advise against using these colors for exterior
concrete. Interior slabs must be placed on a well-drained base or
subgrade and have a low moisture content before stain is applied.
Jones believes the brown-colored "flowering" of blue-
green stains is caused by oxidation of a copper component resulting
from water vapor passing through the slab. Others believe the brown
color is caused by a fungus, which can be eliminated by using sealers
containing a fungicide. Acid-etch
stain finishes don’t require much equipment for application.
For surface preparation, you may need a grinder or a buffing machine
equipped with sanding pads. Power-washing equipment also is useful.
Any equipment that comes in contact with the staining liquid, such
as sprayers, must resist hydrochloric acid. Brushes used to apply
or spread the stain should have acid-resistant, uncolored bristles. Workers should wear the proper safety equipment including acid-proof gloves, goggles, boots, and facemasks to filter acid vapors. And good-quality wet vacuums are highly recommended for cleanup When
to stain Surface
preparation First, throw a little water on the surface in several locations to see if the concrete absorbs water. If it doesn't, curing agents or sealers may be blocking the entry of stains and must be removed. Also remove any grease and oil, paint drops, taping compounds, caulk, or other surface contaminants. Scraping, applying solvents or stripping agents, sanding, and grinding are the principal removal methods. To pick up contaminants more easily, use stripping agents that will mix with water. If you choose to grind the surface, avoid making grinding marks that will reflect through the colored finish by using either a cup grinding head with a fine-grit (diamond or black abrasive) or diamond pad. Use a light touch, laying the cup flat on the concrete and moving it in small-diameter circles until the blemish disappears. If a slab must be patched, use acrylic-modified, low-shrinkage materials that will accept stain. These patches will always show in the finished product, and the owner should be made aware of this. To open up the surface for stain penetration, many installers prefer to sand floors using floor buffing machines with #60, #80, or #100 paper or screen-mesh sandpaper that allows dust to pass the pad. This process can add its own pleasing effect to the final appearance by accentuating high and low areas on the surface. (More material is removed from high spots, giving them a richer color.)
Decorative cuts and sandblasted patterns achieved with stencils can enhance the appearance of stained surfaces. Timing of these operations, though, depends on the desired effect. When you want the overall stain finish to be as evenly colored as possible, cut lines and patterns after staining is complete. Stains penetrate differently around indentations. If there is to be a color change at a pattern line, cut the line first to form a barrier to stain movement. If sawed joints are to be grouted, complete the staining and sealing before grouting to help prevent grout accumulation on the unprotected stain. Pattern lines are generally laid out with pencil or chalk. Mark only where you cut, and don't use chalk colors that are difficult to remove, or adhere lines to the concrete surface using clear fixative sprays. Many tools are available for cutting pattern lines in concrete. Most installers use grinders or hand-held saws with tables that ride against guides. Dry-cutting diamond blades that do minimum damage to the edge of the cut are a good choice. Dust-collection devices that attach to grinders and saws capture almost all of the dust. A 1-1/2-inch extruded aluminum "L" angle, available in most hardware stores, makes a good saw guide. If you cut patterns before staining, cut them just before cleaning the surface in preparation for the stain. Sawing dust contains free time that can adhere to the surface, causing color distortion. If you cut after staining, do it after the first coat of sealer has been applied. Dramatic effects also can be achieved by applying stencils to surfaces after staining and then sandblasting to reveal plain or colored concrete in areas not covered by the stencils. These stencils are usually made from plastic materials and have adhesive backings that stick to the floor surface. One coat of sealer is recommended before sandblasting to improve stencil adhesion. Stain
application When
choosing how to apply stain, keep the following things in mind: There are many ways to apply stain, with each method providing a different final appearance. However, there are some general guidelines. Sprayers often are used to apply stains, but they should be rated for acid and have no metal parts. Acid will quickly destroy metal parts, which can affect the color of the stain. Miller advises using a spray tip with a circular pattern, spraying in a pattern that goes from left to right and then right to left, with someone scrubbing the stain into the surface using a medium-bristle brush in a circular motion just behind the spray. It's important to scrub in the stain and not just push it around. An additional spray pass just behind the scrubbing removes brush marks. This method ensures good penetration and minimal marking from either the sprayer or brush. Additional water can be used to create different concentrations of stain color. Wetting the concrete before stain is applied is one way to do this. Following the application with water from a spray bottle is another way. Stains applied by paintbrush will penetrate well, but care must be taken to minimize brush marks, which are not usually regarded as creative or desirable effects. Whatever stain application method is used, be sure to carefully mask surrounding areas to avoid accidental staining. Acid stains can be difficult, and in some cases impossible, to remove. The
increased interest in chemical-stain finishes is in the direction
of more subtle effects. Installers fre quently dilute stains with
water to produce less intense effects. For example, one contractor
often applies the stain the day after the concrete is placed, starting
with a 3% stain dilution (3 parts commer
Dyes are not chemically reactive with concrete, and their appearance is translucent. They can be organic or inorganic and diluted with either water or solvents. Jones says dyes are azochromium coloring agents fine enough to penetrate concrete surfaces, and they can create bright colors not possible with stain, such as reds and yellows. Some dyes are UV-resistant, but those that aren't can be coated with UV-resistant sealers to make them colorfast. Universal
tints, often available in paint stores, are opaque, and the colors
produced can mask deficiencies left by acid stains. Tints can also
lighten the color of the stained surface. To
the water-and-detergent cleaning mix, add a tablespoon of baking
soda per gallon of water to neutralize any remaining acid. Scrub
with a buffing machine using soft buffing pads (green or white)
or a scrub brush, and pick up the residue with a wet vacuum. Rinse
until the water is clear. Allow the floor to dry 1 or 2 days before
sealing. For interior slabs, three primary types of sealers are used: acrylics, urethanes, and epoxies. Acrylics are UV stable, inexpensive, and easy to apply or re-apply, as necessary. But they have the softest surface of the three and require the most maintenance. Solvent-based acrylic sealers are softer than water-based products. They also provide a wet look that greatly enhances the appearance of colored finishes. Epoxy sealers are much harder than acrylics. Water-based epoxies bond well to concrete and provide a clear finish, but they are nonporous and do not allow trapped moisture to escape. Epoxies are probably the best choice for concrete countertops and food-preparation areas. They are not UV-resistant, and reapplication involves more elaborate preparation than for acrylics. Urethane sealers, though the most costly, provide the most abrasive-resistant finish. However, they don't bond well to concrete, so they must be applied over water-based epoxy applications. They are not UV-stable, and reapplication is expensive. Pricing Concrete Construction asked several contractors what they charge to install acid-etch finishes on floors. Prices vary widely depending on the part of the country, whether the contractor is union or nonunion, and the contractor's reputation in the market. The following are some general price ranges: -- Simple stain applications that include cleanup and a final sealer coat and involve minimal slab preparation will cost about $2 to $4 per square foot. Larger projects typically have the lowest square-foot prices.
-- Stain applications with sawcut patterning and different colors
between sawed lines cost $4 to $10 per square foot. Owner
acceptance In
terms of color, owners seem to prefer light-tan finishes, which
make up about 60% of the market. Greens and browns are popular,
too, and often work well together. Black washes (1 part stain to
15 parts water) can be used to reduce the contrast between colors. If you are thinking of attempting your first acid-etch staining project, attend a training seminar. Most of the stain manufacturers and many retail outlets that sell staining products conduct educational seminars, often at no cost. Professional training opportunities are also available, for a fee. • |
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The
final preparation step involves carefully washing the surface with
water and detergent. Don't use acid to clean the surface because
it will diminish the effect of the stain. It's best to scrub with
a buffing machine using strip pads (preferably black) and to pick
up the effluent with a good-quality wet vacuum. The surface must
be clean and free of streak marks, footprints, and all residue.
Anything remaining on the surface will affect stain penetration.
Sometimes
a stained overlay is the best solution for concrete surfaces that
show damage or have been abused during construction. Commercially
available overlay materials can be integrally colored, textured,
and stained to provide a new range of decorative possibilities.
The overlays have high flexural strength and wear resistance. As
with everything involving stains, however, it's wise to create a
sample to ensure compatibility of the overlay cement with the stain
and to get owner approval for the result.