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.
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 "Seeding" the top layer of a freshly poured slab with any of a number of types of aggregate allows that aggregate to be exposed by polishing, which is described further in the "Post-Curing Finish Techniques" section below. Aggregate choices range from glass pieces to computer chipsóvirtually anything relatively durable can be cast into concrete as an aggregate. Aggregates in the polished surface of concrete lend a variegated, textured appearance to the floor.
Wet concrete can be stamped with a pattern resembling stone, wood, tile, or a custom pattern. This is more commonly seen in exterior concrete applications such as patios, stairs, and the like; however, interior spaces can also benefit from such textured floor effects. Making sure the concrete is adequately jointed and of the proper consistency are important considerations when stamping.
Concrete Finish Floors, Fig. 1
Two examples of stamped concrete
Post-Curing Finish Techniques
(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
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 Finish Floors, Fig. 2
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
Sawcut checkerboard pattern
(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