Often because of age, degradation, cracking, or adhesive from previous floor treatments, such as carpet, tile or hardwood, an existing concrete slab cannot be effectively stained. In this instance, thin polymer-fortified cement overlays can be installed to provide a brand new concrete surface to work with. At depths as shallow as 1/8", these cement or "decorative concrete" overlays are extremely durable, permanent, and allow for great creativity in patterning and coloring. Because of their polymer modification, these cement overlays also have great adhesion to concrete, and installed properly will not break apart or delaminate.
Cement overlays normally come in white and gray, and may be stained as such, or integrally colored during installation and then stained to provide a unique, customized color pattern that may not be achieved through the use of concrete stain alone. Overlays may also be topped with another type of even thinner cement overlay, called a "micro topping", that allows the color of the first ( or "base") overlay to show through slightly. This type of system generally can add a more varied color effect than the use of a single overlay. Lastly, cement overlays may be stenciled or stamped during placement to impart pattern and texture and make them resemble natural stone or other building materials.
There are several techniques for applying decorative concrete overlays. Some overlays are troweled over the existing concrete slab. This is the most common type. Other overlays are self-leveling (see photo above), meaning they are poured out on the concrete and then spread to a uniform depth. Finally, some cement overlays can be applied with a paint roller or sprayed with a hopper gun. The type of overlay used typically depends on the durability required by the project environment and the look being sought. For example, trowelable overlays generally exhibit greater color "mottling" or "antiquing" through the use of concrete stain than self-leveling ones, whereas self-leveling overlays provide great uniformity in color (frequently referred to as "warehouse look"). All overlays, regardless of application method, require mechanical surface preparation to ensure proper adhesion.
As a rule, the best look from decorative concrete overlays comes through acid-staining. Acid-staining brings out the variation in the texture and finish of a cement overlay in a natural looking way. Concrete dyes and water-based stains can be used, also, and are great for achieving a specific color, but dyes and water-based stains tend to show the artificial, circular spray patterns that were used to apply them. They also exhibit penetration and adhesion difficulties because overlays are more dense than concrete and dyes and water-based stains form a film on the surface.