Before a specific finish is specified, sample panels should be created to ensure the finish achieves the desired aesthetic and functional goals.
Samples may be provided by competing producers initially in small, 12-inch squares. The selected producer should then provide three four- or five-foot-square samples to determine color range. The range of acceptable variations in color, texture and uniformity should be determined when these mockup units are approved. Full-scale mockups should be specified by the architect to arrive at a final approval of the desired design.
The most common types of finishes available include:
Smooth or off-the-form finishes show the natural look of the concrete without trying to simulate any other building product. Fine surface details and sharp arrises can be achieved. This finish is one of the most economical.
Exposed-aggregate finishes, via chemical retarders or water washing, are achieved with a non-abrasive process that effectively brings out the full color, texture and beauty of the coarse aggregate. The aggregate is not damaged or changed by this exposure method.
Form liners provide an almost unlimited variety of patterns, shapes and surface textures. The concrete is cast against liners made of a range of materials, including wood, steel, plaster, elastomeric, plastic or foam plastic.
Sand or abrasive blasting provides all three degrees of exposure noted above. This process is suitable for exposure of either large or small aggregates and is used when a light exposure is desired, as costs increase with depth.
Acid etching dissolves the surface cement paste to reveal the sand, with only a small percentage of coarse aggregate being visible. It is most commonly used for light or light to medium exposure.
Tooling, usually called bushhammering, mechanically spalls or chips the concrete using any of a number of hand or power tools, exposing the aggregate textures. Each tool produces a distinctive surface effect and a unique shade of concrete color.
Hammered-rib or fractured-fin designs are created by casting ribs onto the surface of the panels and then using a hammer or bushhammer tool to randomly break the ribs and expose the aggregate. The effect is a bold, deeply textured surface.
Sand embedment creates a bold and massive visual appearance for the panel, using 1- to 8-inch-diameter stones or flagstones. The stones typically are placed in a sand bed at the bottom of the mold, and finishing reveals the stone face, resulting in the appearance of a mortar joint.
Honing or polished finishes are achieved by grinding the surface to produce smooth, exposed-aggregate appearances. Polished exposed aggregate concrete finishes compare favorably with polished natural stone façades, such as granite.
Painting is used purely for decorative purposes, due to the high-strength, durable nature of precast concrete panels. There is a vast difference in paint types, brands, prices and performance, and knowledge of composition and performance standards is necessary to obtain a satisfactory result. In some cases, the precast concrete surface can be so smooth that it makes adhesion difficult to obtain, so a decision on painting should be made prior to casting if possible.
Two or more finishes can be readily achieved using the same concrete mix. This procedure will raise the cost of the product, but it will be less expensive than producing a separate unit that must be attached to the primary panel, as with an accent sill. The first mix is placed within an area bounded by a raised demarcation strip that is the thickness of the face mix. The second mix can be placed and vibrated within 1-1/2 hours of pouring the first.
Combinations of various finishes on the same unit depend on the shape of the unit. Some finishes, such as acid etching, can’t be easily applied to only one portion of a unit. The combination of a polished or honed surface and acid etching provides a surface that exposes a very high percentage of stone.
For more information, contact a PCI Certified Manufacturer.