Production Considerations and Advantages
By Brian Miller
Architectural precast concrete is a versatile, durable, economical and practical solution to most building and construction needs – yet it is underutilized in today’s construction market. Overall, architectural precast products make up only 0.3 percent of above-ground construction each year. Considering that above-ground construction is a $470 billion-plus market, it suggests a huge growth opportunity for precasters.
Architectural precast concrete is generally defined as any precast product that contributes to the aesthetic and architectural value of the structure. Products include buildings, wall or panel systems, sound barriers, picnic tables, ornamental pieces, signs, support slabs, columns and so forth.
In most cases, besides versatility and durability, the use of architectural precast products results in cost and time savings for the project. So why aren’t more precasters taking advantage of these product lines?
Several reasons come to mind. Architectural precast requires greater quality control; more detailed form set-up, usually with less repetition of form use; indoor production facilities (depending on location); varied stockpiles; clear communication as to expected results and limitations; and full-time dedication to marketing and sales. Also, the final acceptance of projects is more subjective.
Although these are important factors to consider when producing quality architectural precast for a profit, some may be beyond an individual precaster’s immediate control, such as indoor production facilities or a full-time marketing staff. This article offers tips that any precaster can employ right away.
Quality Control and Production
Because the aesthetic qualities of architectural precast are critical, consistency in color and finish is also critical and is a direct result of good quality control. While this is always important when producing quality products, it is vital for architectural precast. Variations in color and finish may result from aggregate contamination, varying water-cementitious ratios, placement techniques, improper vibration or inconsistent finishing techniques. Here are some things to consider when manufacturing quality architectural precast:
Storage and Handling
Since architectural precast may require different aggregates and cements to achieve multiple finishes, a producer may need to incorporate storage and handling systems for multiples of both. An emphasis on proper cleanout of aggregates between mixtures, to avoid cross-contamination, is very important as the slightest contamination may show up in the finished surface.
Slight changes in water-cementitious ratio will change the color of the finished surface. This is especially significant when paste exposure is greatest as with form finish, light sandblasting and acid-etch finishes. Determining the moisture content of aggregates and making appropriate adjustments are essential.
Vibration and Consolidation
Thin panels (6 inches or less) should be consolidated with table vibration or a vibrating screed. Internal vibrators or stingers are inefficient for thin panels, and if they are dragged across the product they will often leave a trace on the finished surface that remains visible. Over-vibration will lead to segregation and lightened splotches on the finished surface. This is common where leakage occurs at joints. Under-vibration may result in larger bug holes and poor consolidation around reinforcing steel.
Keep drop heights under 4 feet to avoid mix segregation. Avoid casting exposed areas from multiple batches when possible. Lift or batch lines occur when cold joints develop between batches or where segregation occurs.
Forms must be square, and features like reveals, chamfers and blockouts must be set correctly. Typically, there is less tolerance due to alignment of patterns, connection details and/or abutting pieces, etc. The form joints must be correctly aligned and sealed to prevent leakage. Silicone is commonly used to seal joints and should be applied prior to the form release agent. All fasteners, such as screws or nail heads, must be properly covered so that their image is not transferred to the finished surface (one method is to use a sandable epoxy resin). Forms must be properly seasoned so that concrete does not bond to them. Reactive release agents should be used in accordance with the manufacturer’s recommendations. Petroleum-based products may change the color of the concrete finish and should be avoided. Moisture absorption by forms will also discolor the concrete surface and can be prevented with proper form preparation.
The reinforcing steel should not be set on chairs or supports that touch an exposed surface. These supports may become exposed over time, impairing the finish. Secure reinforcing cages by other means. Cages can be suspended from the top of the form. However, this needs to be taken into account when designing the form. Reinforcing steel should be installed after the form release agent is applied so that no release agent contaminates the steel, which would prevent it from bonding to the concrete.
For further information about good concreting practices refer to NPCA’s “Quality Control Manual for Precast Concrete Plants.”
NPCA quality control and plant certification programs offer a great advantage. Certified plants are required to employ controls that cannot be duplicated in the field. Quality control personnel inspect the entire manufacturing process, including raw materials, forms prior to casting, locations of hardware, batching and post-production processes, and they maintain detailed records as well. Comparatively, field inspectors usually make final inspections or partial inspections of random samples at the job site. This allows for many mistakes to go unnoticed until project completion, when repairs are costly and cause an inconvenience. Architectural precast is subjected to a more thorough quality control process, ensuring specifications are met and quality is maintained.
Communication of expectations is extremely important with architectural precast. One suggestion to better understand and communicate finish and color variations to owners and architects is to cast full-size samples or mockups. This will let owners and architects see what they can expect and provide a guideline for product acceptance. Small samples don’t encompass all the variations that may occur during full production and should be limited to preliminary use.
Since architectural precast products can be made to mimic almost any finish or shape, or blend harmoniously with other building materials, why aren’t more items made from architectural precast? First of all, owners and architects must also be willing to accept its use. Many are not familiar and/or educated on the uses, advantages and potential savings of architectural precast. Others may have had a bad experience from using a low-quality producer. Whatever the reason, we must educate them on the advantages of quality architectural precast in order to expand its use. The reality is that no other building material has as much value for the money or as much versatility in its use.
One proven marketing tool for educating the owners and architects is to arm yourself with knowledge. That is, know your own product as well as those made of competing materials, including cast-in-place or tilt-up concrete, masonry, stone, metal, glass, wood or exterior insulated finishing systems (EIFS). Knowing where architectural precast products are superior to these materials is indispensable knowledge.
Consider promoting these advantages when marketing your product:
Costs Savings and Site Impact
Overall labor costs can be reduced by using architectural precast. Typically, plant-based labor is less expensive than construction site labor. One reason for this is that many construction projects require union labor and/or prevailing wage rates, which increase costs through higher wages and extensive administrative procedures. Therefore, the more labor that can be performed off site, the greater the cost savings.
Other methods of cost reduction associated with architectural precast include distribution of fixed costs over multiple jobs; speed of erection or installation; purchase of bulk materials, instead of single-site use; and overall faster project completion. These savings may be passed on to the owner or used to increase the profit margin relative to alternative architectural products.
Off-site manufacturing reduces the impact at the construction site. Site impact includes storage space for materials and equipment; space required to install the products; the duration the space is needed; and the ease at which other contractors may work on site during the storage and installation period. Most products such as cast-in-place concrete or masonry have a high site impact, which can make it difficult for other trades to access and perform work.
On the other hand, precast products limit disruptions to the site, and other trades can begin work sooner, resulting in reduced completion times and costs.
Some products, such as masonry, EIFS or cast-in-place concrete, require protection from cold weather and rain to prevent freezing and washout, respectively. Hot climates can accelerate setting times, which reduce prime working time and strength, and increase cracking potential.
During the delicate phases of early cement hydration, architectural precast concrete is protected from rain, sunlight and temperature changes. Proper curing is easier to achieve in a plant environment and results in stronger and more durable products.
Durability and Maintenance
Service life and life-cycle costs are related to the durability and maintenance of structures. Service life refers to how long the product is expected to last. Life-cycle costs include the initial as well as the maintenance costs of the product over its expected service life. This is typically examined at a per-year cost. Ultimately, most owners want a product that has exceptional service life and minimal life-cycle costs.
The durability of above-ground structures includes resistance to weathering, corrosion, damage from impact, etc. Over time, weathering mechanisms such as rain, temperature changes and ultraviolet light (UV) exposure may change the appearance of materials and cause wear. Materials such as EIFS and painted metals often become stained or fade with time, requiring routine maintenance that increases life-cycle costs.
Architectural precast uses natural aggregates and cements producing stable colors and finishes that have exceptional resistance to weathering, thereby reducing life-cycle costs.
Moisture ingress is another weather-related issue that occurs through the material itself or through joints between units of the building material. Other materials, such as block, are highly permeable and require routine sealing, which is a maintenance expense. Also, materials that require a greater number of joints have a greater potential for leaks. Joint or joint filler material failure is a primary cause of moisture ingress and another maintenance expense.
Materials such as glass, metal facades and masonry that typically have a greater number of joints per unit area have a greater potential for moisture problems and higher maintenance costs. Furthermore, moisture that gets trapped behind finishing materials is a common problem with improperly installed brick and EIFS. Trapped moisture can lead to mold problems and degradation of structural and other finishing elements.
Finishing materials must also be corrosion-, impact- and fire-resistant. Metals will rust if not properly maintained. Impacts will often cause dents or cracks in EIFS, break glass panels or crack mortar joints in masonry, and fire can consume wood. In fact, intense heat causes steel to yield and may compromise structural integrity.
Architectural precast resists corrosion and impacts very well, is not combustible and offers a very low permeability.
Architectural precast can be made in any shape or form with a very high degree of detail, and its design flexibility allows precasters to reduce costs through repetition of products.
The uniqueness of architectural precast may require additional planning and scheduling, however. All pieces that utilize a common shape should be cast in succession regardless of shipping order to maximize the life of the form. Some forms may be saved and utilized in future jobs, although with architectural precast this becomes more unlikely.
It is also possible to incorporate studs, insulation, electrical, mechanical and plumbing components into architectural precast to reduce labor costs and time. This reduces the labor and time onsite for completion.
Strength refers to the material’s ability to withstand stresses generated from forces or loads. Most materials can carry their own loads. However, additional stresses generated from live, roof, seismic and wind loads may require special design, additional support or may not be carried at all.
Materials such as glass, metal or EIFS products are not as strong as architectural precast or as resistant to external forces. During the hurricanes that hit Florida in 2004, buildings with EIFS had huge sections ripped off, resulting in millions of dollars in damages. The superior strength of architectural precast provides greater protection against natural disasters by having a better resistance against high winds, fires and storms.
Precast can be designed as a structural component and therefore serve a duel purpose. Structural architectural precast can be used to eliminate exterior columns and beams. In other cases, architectural precast can be combined with other precast components to eliminate costly steel frames or cast-in-place structures altogether and increase available interior space. The use of precast for the entire structure can generate even greater savings.
Aesthetic versatility refers to the number and types of finishes and colors available with a material. Architectural precast has a distinct advantage over other finishing materials in that it can be made to resemble almost any finish such as brick, stone, wood, smooth concrete, various textures and patterns, exposed aggregate and many combinations. The possibilities are limited only by the imagination.
Several materials may be cast into precast as cladding, such as granite, marble, brick, stone or terra cotta. This option allows for the advantages of precast with a natural material finish. Architectural precast can also be made in an abundance of colors.
Other building materials are limited in their aesthetic versatility. For example, brick is available only in earth tones or shades or red. Glass, metal and EIFS have only one texture available. Architectural precast can be made to match existing architecture and older, weathered materials, making it the ultimate aesthetic choice. A more detailed discussion on aesthetic versatility will be presented in Part 2 of this series.
Precast plants typically retain experienced personnel who are familiar with the work and projects. Construction sites tend to have greater turnover, which brings in new labor unfamiliar with the project. This can lead to greater oversights or mistakes.
Precast is noncombustible and offers superior fire resistance. It requires no additional protection as with structural steel.
Many hotel chains use architectural precast to reduce noise pollution and provide quieter rooms since sound waves cannot travel easily through precast.
Architectural precast acts a barrier to sudden climatic changes. “Sandwich panels” (precast panels with a layer of insulation in between) offer greater R-values or thermal resistance. Concrete is a thermal mass in that its density allows for storage of large amounts of heat. Architectural precast doesn’t rapidly adjust in temperature. Therefore, in environments where temperatures differ greatly between night and day, it can reduce the load on heating and air conditioning systems by acting as a barrier to outside temperatures and by slowly conducting heat transfer.
Accessory items made from architectural precast are very heavy and therefore difficult to steal. Also, concrete is burglar proof. It requires incredible efforts to get through a precast wall, as opposed to wood, glass, EIFS or metal walls. Architectural precast resists insects and animal nesting as well.
Obviously, architectural precast can expand far and wide into commercial and industrial building applications – but where else can we go? Residential! Today, precast foundations are becoming more common. Precast foundations can be designed as ready-to-finish or installed as finished products. As noted earlier, with the use of many textures, finishes and colors, a precast foundation can be a one-shot deal for structural support, insulation and finished basement walls.
Concrete homes are increasing in popularity as well. Precast homes can take this to the next level by offering a variety of durable aesthetic finishes as well as all the benefits of precast concrete. Furthermore, architectural precast can be used for interior and exterior features such as fireplaces, countertops, sidewalks and driveways.
Overall, the advantages – from design and aesthetic versatility to up-front savings and reduced life-cycle costs – strongly conclude that architectural precast products are the best option for most above-ground construction applications. More precasters should explore these advantages and investigate projects early on to determine whether it can be used. By educating owners, architects, specifiers and engineers, we can expand into this underutilized market.