As-Cast

A smooth as-cast finish showcases the natural look of the concrete without trying to simulate any other building product. Fine surface details and sharp arrises can be achieved with a smooth finish. An as-cast, or form, finish requires no additional finishing after being removed from the form and is therefore one of the most economical finishes to produce. However, given the nature of a paste finish, panel-to-panel will mostly likely have less color uniformity and often a mottled look. There also is the potential for surface change when exposed to the weather. Smooth surfaces tend to weather unevenly and become discolored from rainwater and airborne particles. While a rough concrete surface will scatter reflected light and soften the impact of blemishes, a smooth surface will make variations more conspicuous.

Of all precast concrete finishes, this finish is the most misunderstood when it comes to acceptability. An acceptable smooth finish can be very difficult and expensive to achieve if the architect or owner anticipates a high degree of uniformity. However, if the surface is to be painted or stained, this finish will provide an excellent surface and keep costs to a minimum.

Many of the aesthetic limitations of smooth concrete can be minimized by the shadowing and depth provided by profiled surfaces, such as fluted, sculptured, or board finishes; by subdividing the panels into smaller surface areas, by means of vertical and horizontal reveals or rustications; or by using white cement. Any introduction of shapes to provide shadow effects will also enhance the final finish.

Abrasive Blast and Acid Etch

Both abrasive blast and acid etch methods provide exposure of the aggregate and matrix. Abrasive blasting typically mattes, or dulls, the color, while lightening the finish. This typically is good for color matching, hiding imperfections, and producing good weathering characteristics. Acid etch usually provides a light to medium exposure of the matrix (sand and paste). Typically, acid etching darkens the color and provides a sparkle, or sugar cube-type finish. This can be a good finish for acid-rain environments.

Overall, deeper exposures minimize panel-to-panel variations. The mixture proportions, aggregate gradation and physical characteristics of the aggregate and matrix/aggregate color compatibility are important. It is advisable to align the color, or tone, of the matrix wherever possible to match or blend with the color of the aggregate. This match can be achieved through careful selection of cement, sand, and pigment colors. A good matrix-to-coarse-aggregate color match will minimize noticeable mottled effects.

Acid etching is most commonly used for light to medium exposures and retention of detail. An acid-etched finish typically produces a fine sandy texture closely resembling natural stones, such as limestone or sandstone. It is often substituted for a light sandblast texturing. Aggregates on an acid-etched surface present a clean or bright look. However, after normal weathering, the aggregates lose their brightness and will closely resemble their original condition. Because of variations in surface color and texture possible over large fields, the panel surface should be broken with rustications or other details to minimize the visual effect of the variations.

A minimum depth of etch is required to obtain a uniform surface. This depth will expose sand and only the very tip of the coarse aggregate. Attempts to go any lighter than this often result in a blotchy panel finish. It is difficult to achieve a totally uniform light exposure on a highly sculptured panel. This is due to acid spray being deflected to other areas of the panel, particularly at inside corners. This may, however, be acceptable if the sculpturing creates differential shadowing.

Exposed Aggregate

If the bright, natural colors of the aggregate are the prime concern, exposed aggregate from retarded surfaces is the best way to achieve that result. Exposed aggregate uses a chemical surface retarder that is usually sprayed or rolled into the forms. This retards the set of the cement, which is then removed the next day, usually by water blasting. That results in exposing the aggregate without altering the aggregate.

This textural finish is typically used for medium to deep exposures, and is good for color matching, hiding imperfections, and offering excellent weathering characteristics.

While abrasive blasting (AB) creates a smoother texture and wears down the aggregate, typically frosting it, exposed aggregate (EA) via a retarder allows the aggregate to protrude and avoids wearing down the surface, thus maintaining color.

Polishing

Grinding and polishing can be used to mimic surfaces such as polished granite. While polishing can create a deeper exposure, its ultimate texture is flat, smooth, and higher gloss surface.

Typically, polishing is a more expensive and labor-intensive texture relative to other concrete finishes, but it is less expensive than using granite veneer.

Cast Stone

Cast stone is the method by which precast concrete can be used to simulate natural stones, such as limestone or sandstone. It can be wet cast or dry cast. Dry cast usually means the concrete mix is very granular, with a very low water-to-cement, or hydration, ratio. It is tamped into the forms. Normally, the dry cast method avoids air or bug holes at the surface.