Vertical polymer cement mixes have been available for more than 15 years but interest has recently increased. These overlays were developed primarily for contractors who hand-carved artificial rockwork, which led to polymer mixes that could be plastered on walls and hand-carved to look like unit masonry. Mixes made with cement and sand for that kind of work often developed shrinkage cracks, marring the appearance. This evolved into the development of small, hand-held stamps for creating rock masonry patterns rather than carving them. The material and the process is currently being used for “theme” concrete, insulating concrete form (ICF) wall surfaces, artificial rock work and trees, and swimming pool amenities.
Stan Pace, owner of FossilCrete, Oklahoma City, says that mixes for stampable overlays must be flexible enough to move when stamps are being impressed. Otherwise cracks develop, or the impression won't reach the required depth.
An advantage of these mixes is that, because of the high level of polymer additives, they don't require curing. After initial set, workers don't have to cut control joints or do anything special until coloring or final sealers are applied. But the amount of water added to the mix is critical—too much or too little can make an installation difficult. Too much water will also compromise the strength. Pace also warns that you shouldn't wet the surface after the material is applied, otherwise the durability of the surface can be reduced.
Vertical mixes for stamping impressions usually include portland cement, lightweight aggregates of different sizes, fine aggregate, admixtures, and sometimes fibers. Once the mix is applied, initial set occurs fairly soon—often within 5 minutes. So the material must be applied, textured, and stamped in that time.
Steps in the application process
Unlike many other types of bonded topping concretes, stampable overlay mixes require little surface preparation, and they can be applied over many different materials, including wood. Providing a clean surface, free of oils and greases, is all that's required; sandblasting, bead-blasting, or other preparation is unnecessary.
The first step is usually the application of a bond coat—simply applying the undiluted polymer bonding agent liberally to the surface of the existing material. After it becomes tacky, the first scratch coat of polymer cement is applied. When initial set occurs, the “texture coat” can be added, up to 2 inches if desired. Stamping impressions immediately follows. The entire process can take place, under average temperature conditions, in as little as one hour. Generally, the thicker the application, the more working time there is.
Stamps and textures
Because the use of stamps and texture mats provides much more detail than hand-carving, their use is increasing. Stamping is much faster too. Unlike stamps for patterning horizontal slabs, these stamps are small with only one or two pattern units, allowing for a more randomized masonry unit appearance.