Often because of age, abuse, 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 '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. These overlays may also then be topped with another type of thin cement overlay, called a "micro topping", that allows the first overlay to show through slightly. This system generally can add a more varied color effect than the use of a single overlay. Lastly, concrete overlays may be stenciled or stamped during placement to impart pattern and texture.
There are several techniques for applying decorative concrete overlays. Some overlays are troweled over the concrete slab. This is the most common type. Other overlays are self-leveling, meaning they are poured out on to the concrete and then spread to a uniform depth. Finally, some cement overlays are applied with a paint roller or hopper gun. The type of overlay used typically depends on the durability required by the project environment and the look being sought. For example, trowable overlays generally exhibit greater color mottling or "antiquing" through the use of concrete stain than do self-leveling ones, whereas self-leveling overlays provide great uniformity in color, commonly referred to as a "warehouse look". All overlays, regardless ofapplication method, require some type of 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 the overlay in a natural looking way. Concrete dyes and water-based stains are great for achieving a specific color but tend to show the artificial, circular spray patterns that were used to apply them. They also exhibit penetration and adhesion difficulties due to the increased density found with overlays.