Precast concrete comes in almost an unlimited array of colors, forms and textures including exposed concrete, veneers of traditional materials, or a combination of both. This section provides basic information on finishing options. More detailed information can be found in the Architectural Precast Concrete Manual (3rd edition).
Concrete Finishes and Textures
The following are the most commonly used finishing techniques when the precast concrete will remain exposed as part of the aesthetic finish.
As-Cast (i.e. form finish) – This requires no additional finishing. The surface is left as is, as it comes out of the form.
Acid-Etched – This process uses acid with high pressure water to etch the surface of the concrete. This usually darkens the finish and leaves a sparkle or "sugar-cube" effect.
Abrasive Blasted (e.g. sand-blasted) – This process uses a blasting material (commonly sand) to abrade away the surface. This typically frosts, and lightens the surface.
Exposed Aggregate (surface retarder) – This process uses a chemical surface retarder which is applied onto the forms prior to casting. The chemical retards the set of the cement so that it may be removed (usually by water blasting) the next day. This results in exposure of the aggregate while preserving its natural beauty and texture.
Polishing – This process uses a series of diamond grinding wheels to polish the surface. This results in exposure of the aggregate with a smooth polished finish similar to polished granite.
The color of concrete is made up of the paste (cement and water), matrix (paste and sand), and stone (coarse aggregate). Which components contribute mostly, or dominate the color contribution, depends on the finish selected and degree of exposure (The term "exposure" refers to the matrix and coarse aggregate). Form finishes to light exposures receive most of their color from the paste and matrix; whereas with deeper exposures, the coarse aggregate becomes the primary contributor to color.
Paste can be colored by pigments, which come in a variety of colors. However, paste is the binder in concrete resulting from the mixing of cement and water. This is a chemical reaction, and as such, can produce different results when variables are changed slightly. For example, if more water is added to a concrete mix, typically a lighter color of paste will result. This is one reason it is important to specify PCI Certification to help ensure that producers have all the appropriate equipment and quality control procedures in-place. Paste dominate finishes are usually more susceptible to color variation and changes in color with time.
Aggregates come in an array of colors. Other than natural variation of color throughout a particular quarry, once incorporated into precast concrete, aggregates provide great color stability.
Precast can be veneered or embedded with many traditional materials. These include brick, tile, and terra cotta which are usually embedded or cast into precast; and granite, limestone, and marble which are typically veneered via pins or connectors. This allows for differential movement between these materials which are often large sections. Precast offers a faster, more durable, and sustainable method to build with these materials. For specific information on clay and stone veneer, see these Designer's Notebooks:
Designer's Notebook: Clay Products-Faced Precast Concrete
Designer's Notebook: Stone Veneer-Faced Precast
Forms and Formliners
Since precast concrete is cast into forms, a variety of shapes and details can be accomplished in precast concrete. Formliners can also be used, especially when repetitive patterns or unique textures are needed. To learn more about forms and design, see the following Designer's Notebooks.
Designer's Notebook: Sculptural Forms
Designer's Notebook: Cornices
Designer's Notebook: Reveals
Designer's Notebook: Radiused Precast
Designer's Notebook: Bullnoses
Painting and Staining
Precast concrete can also be painted or stained. This is most common when precast is left exposed to serve as the interior finish for a project. Often these surfaces receive smooth-trowel finish. In some application, the exterior walls have been painted or stained as well, but this approach will require maintenance in the future.