The potentials of concrete as a finish material in residential and commercial applications have only recently begun to be explored; however, an exposed concrete floor is often an energy- and cost-efficient alternative to other finish solutions such as wood flooring and carpet. Virtually every new residential construction project in the Austin area is slab-on-grade, and many commercial projects utilize site-cast concrete floor slabs. Typically, the concrete slab has acted as a substrate for a finish material such as hardwood flooring, vinyl or ceramic tile, or carpet. The emergence of a variety of concrete coloring, staining, and finishing techniques has made interior exposed concrete floors and surfaces an additional finish option with a unique aesthetic.

There are many benefits to choosing to leave interior concrete floors exposed. Economic advantages include not having to purchase and install an additional finish material on top of the floor slab; additionally, the inherent durability of concrete also saves long-term maintenance and replacement costs that are associated with conventional finish flooring such as carpet, hardwood, and the like. Concrete is a thermally comfortable surface, slow to heat up and cool off, which helps to moderate indoor climate. It is also a good choice for radiant floor applications. Health benefits include better indoor air quality as the need for flooring adhesives is eliminated, and elimination of indoor allergens that typically accompany carpeting. Exposed concrete floors are very low-maintenance, requiring only a wet-mop for cleaning.

Much of the more specific information in the "Considerations" and "Guidelines" sections was excerpted from the Concrete Network website, www.concretenetwork.com .

Also see the Green Building Factsheets for introductions to this and other green building topics.

There exists a wide variety of concrete finish and design options. They can be divided into two groups according to when in the construction process a finish material is added or incorporated.

Pre-Curing Finish Techniques

Colored Concrete

Pigment and colors can be added to wet concrete prior to pouring (at the plant) or directly after pouring and before curing commences. Generally, colored concrete renders a more consistently saturated, regular appearance than stains applied after curing.

• Integrally colored concrete is the term for an entire batch of concrete that has had color added to it, either at the plant or at the jobsite as the slab is poured. The entire mix, and thus the entire thickness of the slab, is colored. This method allows earth tones and lighter pastel coloring.
• Dry-Shake colored concrete is concrete that has had a dry-shake color hardener broadcast onto a freshly poured slab; this is then troweled into the surface, producing more intense colors. The concrete is not colored all the way through. Brighter colors and blues are possible with this method.
• After curing, sealers can be applied to the surface of colored concrete to enhance colors and to provide moisture protection. Integrally colored concrete may involve additional costs, as the trucks used to deliver colored concrete to site must be cleaned after each pour.

Aggregate Seeding

Stamped Concrete

Concrete Finish Floors, Fig. 1

Two examples of stamped concrete

Post-Curing Finish Techniques

Acid-Etch Staining

(Excerpted from "How Stains Work" and "When to Stain" by Joe Nasvik on www.concretenetwork.com )

Chemical stains can be applied to new or old, plain or colored concrete surfaces. Although they are often called acid stains, acid isn't the ingredient that colors the concrete. Metallic salts in an acidic, water-based solution react with hydrated lime (calcium hydroxide) in hardened concrete to yield insoluble, colored compounds that become a permanent part of the concrete. Several companies manufacture chemical stains that are variations of three basic color groups: black, brown, and blue-green. 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:

• Cement properties and amount
• Admixtures used
• Type of aggregate used
• Concrete finishing methods
• Concrete age and moisture content when stain is applied
• Weather conditions when stain is applied
• Efflorescence

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. Non-chloride 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 stains. 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. 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. Exterior concrete that is exposed to the sun, once it becomes hot and dry, is not penetrated as deeply by stains. The continued presence of water will cause the acid reaction to continue for a long time, and concrete stained blue-green will gradually turn brown or even black. Initially, this provides a 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 sub-grade and have a low moisture content before stain is applied.

A major environmental drawback to this method is the presence of heavy metals in many stains. This makes disposal of any washout rinse substances a matter of hazardous materials disposal, which is sometimes not covered in the stain's application and clean-up instructions. Check with the stain manufacturer about the presence of heavy metals in the stain and to verify proper disposal techniques. Though less color variety and intensity is possible, integrally colored concrete is generally a more environmentally friendly concrete coloring option.

Concrete stains achieve a variety of appearances.

Sawcutting

Sawcutting cured concrete is another method to add depth and texture to concrete finish floors. ADA standards allow sawcuts up to 1/2 inch deep; these grooves allow stains to accumulate, taking on richer and more intense tones.

Concrete Finish Floors, Fig. 3

Polishing

(Excerpted from "Polishing Concrete" on www.concretenetwork.com )

Polishing concrete is similar to sanding wood. Heavy-duty polishing machines equipped with progressively finer grits of diamond-impregnated segments or disks (akin to sandpaper) are used to gradually grind down surfaces to the desired degree of shine and smoothness.

The process begins with the use of coarse diamond segments bonded in a metallic matrix. These segments are coarse enough to remove minor pits, blemishes, stains, or light coatings from the floor in preparation for final smoothing. Depending on the condition of the concrete, this initial rough grinding is generally a three- to four-step process. The next steps involve fine grinding of the concrete surface using diamond abrasives embedded in a plastic or resin matrix. Crews use ever-finer grits of polishing disks (a process called "lapping") until the floor has the desired sheen. For an extremely high-gloss finish, a final grit of 1500 or finer may be used. Experienced polishing crews know when to switch to the next-finer grit by observing the floor surface and the amount of material being removed. During the final polishing step, some contractors spread a commercial polishing compound onto the surface to give the floor a bit more sheen. These compounds also help clean any residue remaining on the surface from the polishing process and leave a dirt-resistant finish. During the polishing process an internal impregnating sealer is applied. The sealer sinks into the concrete and is invisible to the naked eye. It not only protects the concrete from the inside out, it also makes the concrete harder and denser. This eliminates the need for a topical coating, which reduces maintenance significantly.

If the decision is made to polish concrete in advance of the pour, additional finish options exist:

• Aggregate can be applied to the concrete mix or "seeded" into the top layer of the mix. The polishing process will reveal these aggregates.
• Integrally colored concrete can be used.

Kept clean and dry, polished concrete floors are generally no slicker than plain concrete surfaces. And they tend to be less slippery than waxed linoleum or polished marble. But public facilities that want to provide extra protection against slip-and-fall accidents can treat polished floors with anti-slip conditioners. These products contain special additives designed to improve traction and make wet surfaces safer. They must be reapplied periodically, but they can simply be mopped on during routine cleaning.

Concrete Finish Floors, Fig. 4

Plain polished concrete

Care should be taken during construction and finish-out to protect slabs that will be exposed. Damage to the slabósuch as nail holes, chemical spills, or pocks and nicksóis usually concealed by typical floor coverings. Making sure all trades know that the slab is not a substrate for another finish veneer is important. Whether employing pre-curing or post-curing finish techniques, project managers and contractors should devise a method for protecting the slab before work starts, or factor into the project time and expenses involved in cleaning and preparing the slab for finish treatment. This is especially pertinent if you are considering staining a new concrete slab. For instance, drywall dust on a surface to be stained will react with the stain, coloring the surface differently wherever it is present. And spills of grease and oil, other lime-based materials, paint, or caulk before or after staining will produce unwanted color variations. A good relationship with the owner or project manager helps to prevent such mishaps. Project management should ensure that the floor isn't damaged before or after staining and keep other trades away from areas where surface preparation and staining are in progress. It's often best to install chemical-stain finishes and a first coat of sealer, and then protect the floor surface with a cover before allowing other trades on the floor. Unfortunately, any protective material will affect the final appearance of the stained floor to some degree, usually by leaving an outline of its shape on the floor and by darkening the surface a bit. Don't use any cover material that doesn't allow water vapor to escape. Breathable cloth tarps are perhaps the best covers for preventing discoloration caused by the work of other trades. Another alternative is to pour a thin overlayment over the concrete floor after all other trades are finished with the space, and to stain that. This technique is often used in renovation and remodeling projects.

American Concrete Institute
38800 Country Club Dr.
Farmington Hills, Michigan 48331 USA
(248) 848-3700
Fax: (248) 848-3701
www.concrete.org
Professional concrete trade organization

The Concrete Network
www.concretenetwork.com
Informational website, mainly residential, with articles and links to contractors

Applied Color Concrete
PO Box 770
Wimberley, TX 78676
(512) 847-9271
concrete@starband.net
www.appliedcolorconcrete.com
Concrete contractor specializing in colorized concrete floors and concrete counter tops

Colormaker Floors Ltd.
2080 Enterprise Blvd.
West Sacramento, CA 95798
(888) 875-9425
www.colormakerfloors.com
Concrete overlayments, coloring, textures, and stains

Cornerstone Concrete Stain
7010 Burnett Road
Austin TX 78757
512-467-8246

Evans Industrial Coatings, Inc.
P.O. Box 2611
Russellville, AR 72811
(501) 967-7005
Fax: (501) 967-3837
www.evansindustrial.com
Acid stained concrete

Lambert Southwest
P.O. Box 1111
Henderson, TX 75653
(903) 657-4680
www.lambertsw.com
Concrete colors, "Deco-Etch" acid stains, sealers, and dust-on hardeners and colors

Peyton & Associates
Houston, TX
(281) 363-7908
www.stainedconcrete.org
Acid stained concrete contractors

Texas Etch & Score
(830) 798-2717
Fax: (830) 798-0917
www.texasetchandscore.com
Concrete floor and countertop scoring, stencil sandblasting, staining and sealing