Often, because of age, degradation of the concrete (like erosion, crumbling or cracking) or adhesive residue from previous floor treatments, (for e.g., carpet, tile or hardwood), an existing concrete slab cannot be effectively or successfully stained. In these instances, the existing concrete can be resurfaced with a thin, polymer-modified cement overlay, commonly referred to as a "decorative concrete overlay", to creates a brand new concrete surface. At depths as shallow as 1/8", decorative concrete overlays are extremely durable, permanent, and allow for great creativity in patterning and coloring. Because of the polymer modification of the cement, the overlay material used to resurface the concrete also has great adhesion. Installed properly, a decorative concrete overlay will not break apart or delaminate (come loose) from the original concrete.
Decorative concrete overlays come in white and gray versions, depending on whether they are manufactured with white or Portland gray cement, and the may be stained as such, or integrally colored with pigment (liquid or powder) during installation and then stained, so as to provide a unique, customized color pattern not achievable through the use of concrete stain alone. Decorative concrete overlays may also be overlaid with another type of even thinner cement overlay, at a depth of an additional 1/16", called a "micro topping", that will allow the color of the first overlay - the base coat - to show or reflect through slightly. This type of combined decorative concrete overlay system adds a more varied color effect than the use of a single, uniformly-colored overlay. It is a great option if a plain white or plain gray concrete look, with just a little bit of distress, or differentiation, is desired Lastly, decorative concrete overlays may be stenciled or stamped during placement to impart pattern and texture and make them resemble natural stone and other building materials, like brick or cobblestone.
There are several techniques for applying "decorative" concrete overlays. Some overlays are troweled down over the existing concrete slab. This is the most common type. Other overlays are "self-leveling" (see photo above), meaning they are poured out onto the concrete and spread to a uniform depth, with what is called a "gauge rake". Finally, some overlays, for example, micro-toppings, are 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, trowel-down overlays generally exhibit greater color "mottling" or "antiquing" through the use of concrete stain than self-leveling ones, whereas self-leveling overlays provide greater smoothness and uniformity in color. "Spray-texture" overlays also provide uniformity in color, but they are best for outdoors, especially pool decks and patios, because the sandpaper-type finish provides built-in slip-resistance. All overlays, regardless of application method, require mechanical surface preparation to ensure proper adhesion.
For further information about the resurfacing of existing concrete, check out our recent, in-depth article about "The Expanding Role of Self-Leveling Overlays in Decorative Concrete", in the What's New section of this website.