Getting the Architect into Architectural Precast: Finishes and Techniques

By Brian Miller

Architectural precast concrete is a versatile, durable, economical and practical solution to most building and construction needs. However, it’s underutilized in today’s $470 billion-plus above-ground construction market. Architectural precast represents only 0.3 percent of this market, which suggests huge growth opportunities.

Architectural precast offers the greatest degree of aesthetic versatility compared with other competitive materials. The “finish” of the product is the final surface that will be visible when the project is completed and encompasses three parts: color, texture and technique. Color and texture are interlaced to create the finish or look of the precast. This is conceptually determined by the architect and/or owner then finalized through samples provided by the precaster. The techniques described herein are used to achieve a given texture and color. Precasters should understand the advantages and disadvantages of each technique in order to assist the architect and owner in deciding which one is best suited for their application.

Color
The color of architectural precast is first achieved with a combination of the concrete constituents and textures and then by physical design of the precast. Several things will affect the perception and stability of color over time. For example, the perception of color may change throughout the day with the position of the sun. On the other hand, weathering, pollution and design will affect how the color changes over longer periods of time. These effects are more easily communicated to the customer by building a mock-up (large sample), which will be discussed later.

Concrete constituents that affect color include the cement and fine and coarse aggregate. Cement and fine aggregate (sand) combine to form the matrix. Cement usually comes in white, gray and buff and is sensitive to color variations from its raw materials and manufacturing process. Matrix color will change with different production runs of cement, mixture proportions, water content, etc. To reduce matrix color variations, purchase cement for the entire job from the same lot number when possible, and maintain a precise water-cementitious (w/c) ratio and batching procedure.

Pigments can be used to obtain almost any color of matrix desired, but they can be messy and difficult to work with. However, many admixture suppliers now offer liquid dispersion systems to simplify their use and improve their consistency. Check your admixture supplier for more information on these new systems.

Coarse aggregate comes in many natural colors that are fairly stable. The coarse aggregate’s contribution to color depends primarily on the degree to which it is visible in the final product, known as exposure. The greater the coarse aggregate exposure, the greater it contributes to the overall color.

Texture
Textures range from smooth to rough and refer to the feel of the concrete as your hand rubs across it. Examples of such textures are a hard-trowel finish and a deep-exposed aggregate finish, respectively. Exposure refers to the degree the coarse aggregate is visible. Exposures are typically classified into four categories: nonexposed, light, medium and deep. See Table 1 below for a brief description of each category and techniques used to achieve them.

The following are some general practices associated with different exposures regardless of which techniques are used to achieve them. They are presented in two groups: non-exposed to light, and medium to deep.

It should be noted that good concreting practices are essential for all precast production. For more information on good concreting practices, refer to NPCA’s “Quality Control Manual for Precast Concrete Plants.”

Nonexposed to light exposures, smooth textures
Use this group to create crisp, sharp details. Common finishes include ordinary concrete, limestone, sandstone and marble. Form liners are typically used for these finishes as well. Cast stone and certain veneers (marble, terra cotta, brick, etc.) also provide a smooth to light texture.

Emphasis must be placed on form preparation. Forms must be plane and level. The slightest defects will be mirrored to the finish and become easily visible due to the glassy smooth or mildly altered texture. All joints, form defects and fasteners must be filled and made smooth. Forms should be properly seasoned with a thin layer of a reactive-type release agent applied prior to casting. Reactive release agents won’t discolor concrete and will reduce bug holes that are more apparent on smooth surfaces.

Aesthetic-face mixes of a few inches are commonly used to reduce costs. This will also reduce the size and number of bug holes. The face mix contains the more expensive architectural mixture proportions and is backed with a secondary pour of standard concrete mix. This is more applicable to panels cast face down.

Color variation tends to be more noticeable with smoother finishes. This is related to variations in cement and w/c ratio. White cement is the most stable in creating uniform color. Mixture proportions should utilize a low w/c ratio with a high-range water-reducing admixture. This will help release entrapped air and improve finish details while maintaining a consistent color.

Medium to deep exposures, rough textures
Use this group to create the look of granite or exposed coarse aggregate or to produce a weathered appearance. These exposures tend to be more forgiving with respect to form defects. They camouflage small bug holes in the shadows of aggregates, and they weather better with time relative to smoother finishes. However, crisp corners and sharp details are difficult to achieve due to the rounded or jagged nature of the aggregate.

In general, color variations are reduced because the aggregate color is predominant. Aggregate color is unaffected by w/c ratio, curing, etc. Appearance variations can be further reduced by matching the coarse aggregate color to that of the matrix.

Techniques
There are several techniques used during production and processing to achieve the end finish. Each has different advantages, disadvantages and costs associated with it. These techniques can be combined to produce favorable results and reduce some of the difficulties associated with them. The precaster should be knowledgeable about these techniques so they may guide the architect and owner in the best direction to achieve the end goal. This should result in fewer problems, more satisfied customers and greater profits.

Form finish (as-cast)
All precast concrete is made in molds or forms. When stripped, the concrete has a relatively smooth finish. For underground products, this is typically the end finish. For architectural precast, sometimes the form finish is used as the end finish, which is very economical since no further processing is performed. However, there are several things to consider when producing architectural precast with a form finish.

Weathering. The surface skin is a smooth film of hardened cement paste. Therefore, it is highly susceptible to acid rain or regular weathering over time. Weathering will erode the surface, exposing more sand. These effects alter the finish and may not occur uniformly, depending on location and design. Designers should take into account the water flow over the precast surface during rain to avoid nonuniform weathering. For example, a scupper that does not extend beyond a parapet will typically erode the finish below.

Crazing cracks. Crazing cracks are a surface phenomenon of web-like cracking occurring within a thin layer of cement paste at the surface. They are more common on horizontal surfaces and become more apparent when the precast becomes wet or dirty. They can be reduced by slow, even curing using low w/c ratios and lower cement contents.

Shadowing. Shadowing is coarse aggregate transparency at the finished surface. It is more noticeable with form finishes. This is usually caused by overvibration or heavy consolidation requirements that cause aggregate segregation. This most often occurs when dark aggregate is used with light paste colors. Using light-colored aggregate and appropriate vibration should reduce this.

Design. Products should be designed with the proper form drafts (minimum 1:12, but 1:8 is better for a form finish) to reduce damage during stripping. The design of these products should include shading and depth effects to mask some of the possible defects in the finish. These include reliefs, subdivision of sections, incorporating ribs or sculpturing, etc.

Repairs. Repairs are difficult to blend due to the smooth texture. The color is also hard to match due to the small batch size of the repair material. When possible, repairs should be made with concrete from full scale batches during production of other pieces.

Acid etching
Dilute solutions of hydrochloric acid are used to dissolve cement and expose the fine and coarse aggregate. This procedure is typically used for light to medium exposures and to simulate limestone finishes. Acid etching should be performed only after adequate curing (approximately two weeks or a minimum compressive strength of 4,500 psi). Prior to applying acid, paint or seal all exposed metal surfaces to protect them. A zinc-rich compound applied at a thickness of 3 mils should be sufficient.

The concrete surface must be thoroughly wetted prior to applying acid to avoid streaking and overexposure. Acid is typically applied with a scrub brush. However, a nonmetal sprayer with a wide-angle tip provides a uniform application. Pay special attention to returns, flat areas or locations where acid may puddle or concentrate, as this may also cause overexposure. Exercise caution to avoid splashing the acid on surrounding products or items. In addition, use the proper safety gear, including face shield, gloves, respirator, boots and protective clothing. The surface should be flushed with clean water to remove all residue within 15 minutes of the original acid application. Here are some other considerations:

Materials. Use only acid-resistant aggregate such as granite and quartz in the concrete. Carbonate aggregate such as limestone, dolomite and marble will discolor or dissolve due to their high calcium content.

Color and uniformity. Acid tends to darken the finish and expose the fine aggregate. Pink, buff, yellow and brown concrete colors are more forgiving for piece-to-piece variations. Grays tend to be the most difficult. Matching fine and coarse aggregates to the cement will improve gray color matching.

Weathering. Acid-etched finishes tend to weather better than form finishes. Increased efflorescence can occur from acid washing. Sufficient curing, use of supplementary cementitious materials and thorough rinsing will reduce efflorescence.

Repair. Repairs are typically easy to perform and match to appearance. Acid etching is also used to clean up or bring a sparkle to the finish after abrasive blasting or removal of a surface retarder. Acid cleaning is also performed on precast concrete to remove dirt and environmental debris.

Abrasive blasting
Abrasive blasting is the removal of the form-finished surface with hard particles (usually sand) forced by air at a high velocity. The blast wears away paste as well as fine and coarse aggregate. The rate at which the coarse aggregate is worn away depends on the hardness of the aggregate and the blasting material. Silica sand is the most commonly used material. However, this has become an expensive option with respect to EPA and OSHA requirements for containment and safety. Other materials may be used for abrasive blasting such as “Black Beauty” or crushed walnut shells. To prevent inhaling fine silica particles, the proper safety gear must be used, including protective clothing, gloves, boots and a hood equipped with a breathing line.

Abrasive blasting usually flattens and smoothes the aggregate, resulting in a dull or matted finish. This technique is used for light to medium exposures. Abrasive blasting may also be used for deep exposures; however, the aggregate is worn down in the process, and therefore the color may still be influenced by the matrix. Surface retarders tend to provide best results for deep aggregate exposures. Blasting is usually performed within 24 to 72 hours after casting. As the concrete cures, blasting requires greater time and therefore greater expense. All precast should be blasted at the same age to help ensure consistency.

Color and uniformity. Blasting tends to lighten a finish as aggregates lose their edges and become rounded and frosted. Color and depth variations are easier to control with abrasive blasting, but they depend heavily on the skill of the operator. Deeper blasts appear more uniform.

Weathering. Greater exposure reduces the effects of weathering by channeling the water runoff more evenly.

Repairs. Repairs are relatively easy to make and blend in with abrasive blasting. However, repairs on site can be expensive due to the protective measures required.

Surface retarders
Surface retarders are chemical mixtures that slow the hydration of cement. Typically these are rolled or sprayed onto the form with the precast product being cast finished side down. After the concrete has set, usually overnight, the surface retarder is removed by water blasting or scrubbing. Surface retarders allow for the removal of the paste without damaging or altering the coarse aggregate. The aggregate maintains its shape and becomes more pronounced, resulting in a brighter and more natural-looking aggregate finish. Surface retarders are used when medium to deep exposures are required. Here are some considerations:

Color and uniformity. Surface retarders must be applied evenly and consistently to the form. If the surface retarder application is scraped, scoured or varies in thickness, the finished product will have lines or hard spots, or it will look splotchy in color due to different exposure depths. Vertically cast returns will differ from the face due to aggregate orientation. Rounded or cubical aggregate will minimize this. Also, after curing, the face and sides where sealant material will be applied may be washed with a dilute acid to remove any remaining retarder or retarded paste.

Placement. Keep concrete drop heights low during placement to reduce damage to the surface retarder. Also, vertical and curved sections may have surface retarders scoured during placement. Place the concrete from the lowest to the highest part of the form.

Weathering. Exposed aggregate tends to weather very well. Exposed aggregate lets water run more uniformly over a surface.

Repairs. This finish is one of the easiest to repair and blend. This is due to the color stability and deep exposure of the aggregate.

Form liners
Form liners are used to create images, shapes and patterns or to mimic other building materials. Some examples are natural stone, wood plank siding, ribs, brick or custom designs. Form liners help mask some defects associated with form finishes by providing relief in the visual plane. To further reduce concerns related to form finishes, other techniques such as a light abrasive blasting or acid etching may be performed. Form liners are typically made from elastomeric materials. However, wood, plastic, foam, plaster and steel are also used.

Bushhammered or tooling finish
The use of tooling techniques, hammers or equipment to abrade the surface of the precast is referred to as “bushhammering.” This technique fractures the surface concrete and coarse aggregate, thereby exposing their color. It is used for medium to deep exposures without the aggregate protrusion. Fractured rib designs are commonly used with this technique. Tooling typically removes approximately three-sixteenths of an inch of concrete cover. Due to the labor-intensive procedures, this finish is not common today. Form liners and abrasive blasting have been used to provide a similar finish.

Design. This technique is most applicable for flat or convex surfaces. All tooled surfaces must be accessible with equipment. The protective cover of the reinforcement should be increased to account for the removed concrete surface.

Repairs. Repairs are difficult due to the fracturing of the aggregates. The force from the impacts to fracture the repaired section may result in failure of the repair. To reduce this risk, the repair area should be oversized with reinforcement embedded into the existing piece.

Grinding
This technique is used to polish the concrete’s surface by means of wet or dry grinding. The grinding is performed in succession from very coarse to finer abrasive agents (grits) and can remove approximately one-eighth of an inch of the surface. The procedure produces a relatively smooth surface with coarse aggregate visible that resembles polished granite. The degree of aggregate exposure depends on the depth of grinding. These smooth, dense surfaces resist dirt and weathering very well.

Minimize matrix exposure by maximizing the aggregate density on the finished surface. A continuous-graded concrete mix will help achieve this. Also note that softer aggregate (marble, onyx) will grind easier, reducing processing costs. The compressive strength should be a minimum of 5,000 psi, with all repairs and filling of bug holes completed and cured prior to grinding. This expensive technique is usually performed on flat surfaces.

Veneers
Veneers are cast into the product to gain the look of another material with the benefits of precast. Some common veneers include clay brick, terra cotta, granite, marble and large stones. Usually veneers are used for buildings but can be used for sound and retaining walls, columns, signs, trim, etc. Veneers can reduce the liability of the precaster for the aesthetic acceptability of the project when the source has been approved. Veneers typically require fewer repairs, disputes and replaced pieces. When repairs are required, they are typically easier to perform, provided the veneer material is still available. Veneers are more expensive than many other finishes, but they usually cost less than the veneer material installed in the field by other means.

Connection details of the veneer to the precast are critical. Clay products may be cast into the concrete directly, while granite or marble facades must be connected by corrosion-resistant clips or other devices. There should also be a barrier preventing bond of the concrete to stone materials. The coefficient of thermal expansion is different between the veneer and concrete and must be taken into account during design. The difference in expansion and contraction may cause bowing of the precast. Some methods of adjusting for bowing include adding two layers of reinforcing steel, prestressing and cambered forms.

Cast stone
Cast stone is a predominantly dry-cast process. Cast stone looks like naturally cut stone such as limestone or sandstone. This creates products with crisp edges and no aggregate exposure. Cast stone is typically used for smaller pieces such as coping, headers, window sills, cap stones, address stones, etc. Cast stone, like other dry-cast products, allows for reuse of a form the same day. Dry-cast stone also requires moist curing at elevated temperatures after being stripped to properly develop early strength.

Wet casting is also used to make cast stone. The products typically are acid etched after curing to expose the sand.

Samples
There cannot be enough importance placed on samples and mock-ups. Architectural precast must be accepted aesthetically by the owner and architect. Communication of expectations is vital to a successful project.

Mock-ups allow you to set criteria and guidelines for acceptable color variations, finish variations, size and limit of defects (bug holes), tolerances, etc. They can be observed for weathering effects and color changes throughout the day (position of the sun, cloud cover, rain) as well as long-term weathering effects. Connection details and other material selections such as windows and roofing can also be evaluated. Mock-ups should be built to full scale, preferably on site. This allows for conditions that are similar to what the final product will be exposed to. As with any project, careful planning and clear communication of possibilities and expectations will produce better overall results.

Architectural precast concrete is a superior product and offers many opportunities for owners, architects and precasters. For those who are producing and using architectural precast already, hopefully you will increase its use and promotion by bidding on projects that may have other materials specified and to develop new creative applications. For those who are not as familiar with architectural precast, hopefully you will explore the possibilities further. Let’s get the Architect into Architectural Precast!