What is Architectural Precast?
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, etc.
HISTORY
Many suspect the precast concrete industry began in ancient Rome, as the extensive network of underground tunnels that exist to this day seem to indicate the use of precast building materials.
However, the documented history of the modern-day precast concrete industry began in the 1900s when an English engineer by the name of John Alexander Brodie discovered precast concrete components could come together to build a structure efficiently. Brodie was first to get a patent for the process of creating precast concrete paneled buildings.
In 1950, the first major precast concrete structure appeared in the United States — the Walnut Lane Memorial Bridge in Philadelphia. This bridge is recognized by many as the beginning of the precast concrete industry in the United States as we know it today. A few years later, the Precast Concrete Institute was formed to begin to set standards for this emerging industry.
PRECAST CONCRETE MANUFACTURING
Precast concrete is created off-site using a mold. That’s the main difference between precast concrete and site cast concrete, which is poured into its final destination on site. Here is a simplified overview of the precast concrete process:
Precast concrete is poured into a wooden or steel mold with wire mesh or rebar. This mold may also have prestressed cable, if needed.
It is cured in a controlled environment — usually at a plant.
Once finished, the precast concrete is transported to a construction site and put into place.
It’s important to note that not all precast concrete is prestressed with cable reinforcement. The addition of this reinforcement is particularly useful in many structures and buildings where maximizing the strength of the concrete is essential. The addition of the wire or rebar provides tension within the concrete, which is released once curing is complete. The release of the wire or rebar tension transfers strength to the concrete, creating an even stronger material.
Regardless of whether or not prestressing is a part of the equation, this process is faster, safer and more affordable than standard concrete. Precast concrete materials help you maximize your project’s potential while making sure it is completed on time. They are also among the most versatile products in construction, combining a strong structure with the ability to:
Choose any combination of color, form or texture
Integrate facades
Meet compatibility requirements for historic structures
Create everything from small sections to long open spans
Be recycled or reused upon removal or replacement
TYPICAL PRECAST PROJECTS
Perhaps the versatility is one of the reasons precast concrete structures are so diverse — ranging from parking garages, bridges and office buildings to stadiums, retail shops and housing. It’s clear any number of building types can benefit from the advantages of precast concrete products. Some of the most common construction projects that use precast concrete are listed below.
PRECAST CONCRETE STRUCTURES.
Since durability is one of the key characteristics of concrete construction, it’s no surprise that many precast concrete structures are used in applications that see a lot of wear and tear from everything from traffic to weather elements. Going hand-in-hand with durability is its strength — another reason it is especially popular for these applications.
Parking Structures: In parking structure design, durability, economy and installation are three key points of consideration, which is why precast concrete is usually the building material of choice. You’ll find several different precast concrete products in parking garages — columns, traffic barriers, stairs, paving slabs, architectural veneer and more. Precast concrete is useful for single-level parking structures as well as larger and more elaborate mid-rise structures.
Foundations: Precast concrete is used to create entire buildings — more about that below — but in cases where it isn’t utilized for the entire building, it may still be used for the foundation. Many residential homes and other buildings have precast concrete foundations, regardless of what is used for walls and floors in the rest of the building. Its reputation for providing a moisture-free, and energy-efficient basement is often what makes precast concrete the material of choice.
Bridges: The Walnut Lane Memorial Bridge began the precast concrete industry in the United States, and using precast concrete materials for bridges continues today. You’ll find precast concrete materials are used for beams, arches, girders, deck slabs, caps and more. Regardless of the size of the bridge, precast concrete gives engineers the ability to create a structure that blends in with the environment and is compatible with any historical surroundings.
Culverts: When you remember the underground tunnels of ancient Rome are suspected to be early signs of precast concrete, it’s easy to see how a section of modern-day underground infrastructure is the perfect application for precast concrete. Box and three-sided culverts are manufactured in all different shapes and sizes to aid in stormwater and wastewater drainage, create short bridges, retain rainwater and more. Many of them are built using precast concrete to ensure a high-quality and durable product that can be installed efficiently.
Curb Inlets and Catch Basins: Just like culverts are a part of the underground infrastructure, so are curb inlets and catch basins for wastewater management. Different states and local municipalities have different standards for these pieces, but precast concrete manufacturing can take all of them into consideration and create a product that helps stormwater runoff drain to the underground infrastructure in place.
Sound Walls: In urban areas, sound walls are erected as a noise barrier between highways and communities. Using precast concrete for these structures can cut noise pollution up to 50 percent. The versatility of design enables these sound wall structures to blend into their surroundings with a specific color, texture or design.
Retaining Walls: Many precast concrete retaining systems include segmental retaining wall (SRW) products, large precast modular blocks (PMB), mechanically stabilized earth (MSE) face panels, crib walls, cantilever walls and post-and-panel systems. Each of these elements specifications easily met in a timely fashion by precast concrete.
PRECAST CONCRETE BUILDINGS
The fire-resistant and sound-attenuating characteristics of precast concrete products make them ideal for a variety of building applications. Reducing moisture and creating an energy efficient environment are two other convincing factors when considering a precast concrete building. The diverse variety of buildings included below encompasses the versatility of precast concrete, as these materials come together to create an impressive result.
Office Buildings: The unique characteristics of precast concrete products allow for unique building designs that are attractive and functional. Take advantage of precast concrete columns paired with architectural panels to create large and open spaces.
Multi-Unit Housing: Precast concrete products have superior fire resistance — known to reduce fire insurance rates — and also act as a sound barrier. These characteristics make it a perfect choice for hotels, dormitories, apartment buildings and complexes, senior living communities and similar structures.
Hospitals and Medical Centers: For many of the same reasons precast concrete is preferred for multi-unit housing, it also provides a strong foundation for hospitals and medical centers.
Schools: Precast concrete makes school construction a breeze. With faster turnaround times from start to finish, precast concrete will keep your project on target. Whether you’re adding on to a university campus or an elementary school, you’ll get students moved in quicker without all the headaches of traditional building.
Retail Shopping Centers: Retail shopping centers vary — in rural areas, they may be built on a large plot of land, while urban areas tend to have smaller construction sites. They may or may not incorporate parking and can come in single stories, or a few stories high. Regardless of the application, precast concrete has the versatility to match, and its often used in constructing retail shopping centers. The Target Retail Center is an example of a precast retail shopping building.
Advantages:
Quality Control
Precast/prestressed concrete is manufactured in PCI Certified plants in a controlled environment.
Durability
The high-strength, high-performance concrete that is utilized in the production process resists weather, fire, corrosion, and vandalism.
Speed of Construction
Precast/prestressed concrete lends itself to compressed construction schedules. Components are manufactured off-site, allowing for a just-in-time delivery system. Speedy construction means earlier completion dates which equals earlier occupancy.
Design Flexibility
The combination of standard structural shapes and the capability of casting custom shapes gives designers maximum flexibility. Economies of scale can be achieved through repetition while the inherent plasticity of concrete allows for unique shapes.
Aesthetics
An infinite variety of sands, cements, aggregates and pigments combine to achieve endless colors, textures and finishes.
Green Products
Precast Concrete is a green product and can provide up to 26 LEED points.
IMPROVED EFFICIENCY
Precast concrete structural and architectural systems help save cost in a variety of ways, from the design phases through construction and throughout the building's service life.
A precast concrete structural system can create the building’s entire framing system. This design approach can take several forms, including precast columns or load-bearing precast walls with double tee beams or hollowcore flooring. It provides a significant number of advantages, especially when panels are included to create the entire building envelope. As a result, this approach is becoming the format of choice for many construction teams.
Among the benefits that a precast concrete structural system can provide are:
High Quality
PCI-Certified precast concrete fabricators must undergo two unannounced annual inspections that review more than 120 production and quality-assurance processes. The tight control ensures components are produced with uniform consistency, finish and size. This reduces site work required to achieve the final designer and owner approvals and ensures components need little field adjustment, speeding construction to complete the structure’s shell.
Safety
Plant casting keeps the site cleaner and eliminates trades from the construction zone, improving logistics and enhancing worker safety. The ability to provide a clean site is particularly vital on existing sites and in dense urban areas, where adjacent businesses can maintain near-normal activities.
Interior Design Flexibility
Precast concrete systems help buildings adapt to changing client needs. Double-tee spans of 45 to 50 feet match typical composite-steel framing and minimize the need for interior columns required with cast-in-place systems. Precast spans can reach as much as 70 feet, providing unique opportunities for challenging interior requirements. Precast also provides high floor-loading capability with little added cost.
Green Design
Precast concrete offers a number of environmental benefits. It can be produced locally and creates no jobsite waste. Cement reducers such as fly ash and other admixtures also aid environmental friendliness. And its high durability gives it a total service life that outpaces designs using other building materials.
Tight Floor-To-Floor Heights
Precast concrete systems sometimes fit within alternative system depths but shouldn’t add more than approximately 8 inches to each floor level, creating an approximate 5% increase in exterior wall material. This slight addition is easily overcome by working with the precaster to make effective use of the overall floor-plan shape and using the benefits precast provides in repetition of component fabrication.
REDUCES RISK
Precasters can be a single-source supplier for the total building solution. Using an integrated team approach precasters can work closely with the design team providing engineering and technical support. The design team to can make changes or adapt the design with less risk, fewer coordination issues because of a reduction of trades, while keeping costs to a minimum.
Precast concrete is sometimes viewed as a riskier form of construction by design professionals because it requires an early commitment during the design phase and many architects are unfamiliar with how precast buildings are designed and constructed. In reality, precast concrete construction is an inherently less risky form of construction for both owners and architects:
Budget and Schedule Compliance
For the owner and design professional, budget risk is one of the most significant factors they face in meeting the building’s program and aesthetic needs. Because precast is a plant-built project, precast producers can make schedule and cost assessments in the late schematic and preliminary design phases to help design professionals assess budget compliance. Total precast (where precast concrete components are used for both the structure and enclosure of the facility) can represent the dominant portion of the construction budget. When the intricacy of the exterior wall panels can be tied down early in the project, precasters can estimate fabrication and erection costs with a high degree of accuracy.
Detailing Risk Reduction
Building façades are often composed of multiple products with varying degrees of expansion and contraction. Even the best architectural documents can suffer from poor coordination and improper installation in the field, causing the design professional lost time in coordination and problem resolution, as well as potential liability if the products are installed improperly. Precast concrete allows the creation of multi-part façades without these detailing risks. Precast can affordably mimic most exterior finishes with a limitless variety of appearances.
Winter Conditions
Precast concrete is plant-produced and trucked to the jobsite when needed. Unlike poured-in-place concrete, masonry, or exterior insulation systems, the product can be installed in cold (or even freezing) weather, eliminating the cold-weather procedure delays, general conditions claims, and potential change orders associated with other products.
SAVE MONEY
variety of cost calculations are required on every project to determine what design approaches will generate the most advantages and allow budgets to be allocated most efficiently. Initial, in-ground costs are the most obvious expenses, but hidden and longer-term costs are becoming more significant as owners and designers study the budget impact of various specification choices.
The key to finding the most efficient design is to realize that every system and decision impacts others. The goal is to ensure all products and systems work together without creating redundancy or inefficiencies.
Spending more of the budget to add insulation and other energy efficiencies, for instance, may allow the installation of smaller HVAC equipment that will save equipment expenditures.
Because of precast concrete’s tightly controlled and shorter production process, costs can be more accurately estimated earlier in the process. Parallel effort by precast engineering ensures estimates remain stable, assuring the contractor, owner and design team that the budget is sound.
Using a design and materials that enclose the building quickly avoids winter slow downs and gets crews inside quicker, bringing the project on-line faster so revenues can be generated quicker.
Maintenance needs throughout the building's life also must be considered. These expenses come from the operating budget rather than the construction budget, so they sometimes have not been considered when evaluating the building's cost.
Durability, such that a building does not need to have its exterior refurbished or possibly replaced in 20 years, also has become more of a consideration. The entire life-cycle costs of a project are being determined, and each material choice must justify its value today, tomorrow and many years from now.
SAVE TIME
Precast concrete systems provide a variety of savings to a project schedule that are not always considered when looking at upfront costing versus other materials. These savings include, speed in the design, construction, and finishing processes.
Design
The repetitive nature of precast panels and components allows design work to move more quickly to the shop drawing stage. Precast components can also aid a fast-track design by completing designs while other design work is still underway.
Design economy through repetition, maximizing piece size and shape and other approaches that limit form requirements.
Flexibility of design that offers inherent aesthetic qualities, as well as the ability to mimic the appearance of materials such as granite, marble, limestone, sandstone, or slate.
Material reduction by designing integrated structural components with architectural finishes and by using hollowcore slabs as combined ceiling / flooring units.
Construction
Precast components can be installed quickly, often cutting weeks or months from the schedule. This allows construction to get into the dry quicker and allows interior trades to begin work earlier.
Construction efficiency, due to the precaster’s ability to cast and erect throughout the year because precast components are fabricated under factory-controlled conditions in a plant, harsh winter weather does not impact the production schedule or product quality. This eliminates added time to accommodate unforeseen schedule condition due to delays caused by weather or site requirements. Factory production also provides tight tolerances, minimizing the need for field adjustments.
Elimination of hidden costs, by reducing the time to carry financial bonds, lowering contractor overhead costs and risks, eliminating the expense of nonprecast-related equipment, and reducing subcontractor costs.
Interior Completion
Precast concrete insulated sandwich wall panels provide a finished interior wall that avoids the time and cost of furring and dry-walling while offering energy efficiency. Electrical conduit can be embedded in the panels. The entire wall assembly can be constructed with one trade, versus the six or seven for a typical wall assembly. Using hollow-core planking to combine ceiling and flooring units can speed construction even further.
Disadvantages of Precast
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.
If not properly handled, the precast units may be damaged during transport.
It becomes difficult to produce satisfactory connections between the precast members.
It is necessary to arrange for special equipment for lifting and moving of the precast units.
The economy achieved in precast construction is partially balanced by the amount to be spent in transport and handling of precast members. It becomes therefore necessary to locate the precast factory at such a place that transport and handling charges are brought down to the minimum possible extent.
Precast is very difficult to repair.
Precast Finishes
Precast concrete is a visually rich material that allows the architect to be innovative and obtain design objectives that cannot be achieved with other materials. The proper selection of color, form and texture is critical to the aesthetic appearance of architectural precast concrete components. The choice of appropriate aggregates and textures, combined with well-conceived production and erection details, can achieve a wide variety of design objectives.
Precast components also mesh well with other materials, including curtain wall, and they can provide any required penetrations. Special considerations will aid the installation of mechanical systems and vapor barriers, all of which can be accommodated easily.
Design flexibility is possible in both color and texture of precast concrete by varying aggregate and matrix color, size of aggregate, finish processes and depth of exposure. Combining color with texture accentuates the natural beauty of aggregates.
With the vast array of colors, textures and finishes available, designers can use precast concrete to achieve almost any desired effect.
COLOR
Color is a relative value, not an absolute. It is affected by light, shadow, density, time and other nearby colors. Selections should be made under lighting conditions similar to those under which the precast concrete will be used, such as the strong light and shadows of natural daylight or interior CFL lighting. Surface texture influences color. The building’s appearance is a function of the architect’s use of light, shadow, texture and color.
Cement plus a coloring agent exerts the primary color influence on a smooth finish, because it coats the exposed concrete surface. As the concrete surface is progressively removed and aggregates are exposed, the panel color increasingly exhibits the fine and then the coarse aggregate colors. The color of the cement always has an effect on the general tone of the panel. Cement may be gray, white, buff or a mixture. All cements have inherent color and shading differences, depending on their source.
Pigments and pigmented admixtures often are added to the matrix to obtain colors that cannot be created through combinations of cement and fine aggregate alone. White portland cement will produce cleaner, brighter colors and should be used in preference to gray cement with pigments, especially for the light pastels such as buff, cream, pink, rose and ivory.
Fine aggregates have a major effect on the color of white and light buff-colored concrete and can add color tones when the surface is given a shallow profile to increase the aggregate’s exposure. Coarse aggregate colors become dominant as the surface of the concrete is removed to obtain a medium or deep aggregate exposure profile.
Some finishing processes change the appearance of aggregates. Sandblasting will give the aggregates a matte finish, while acid-etching may increase their brightness. Exposure by retardation normally leaves the aggregates unchanged.
Maintaining consistency of color throughout the production run requires attention to detail and proper specification. Nine key factors should be closely watched when color consistency is critical:
Type and color of cement.
The quality and quantity of the coloring agent.
Facing characteristics of some coloring agents.
Proper batching and mixing techniques and the coloring agent’s effect on the concrete’s workability.
Quality (that is, freedom from impurities) of the fine and coarse aggregates.
Uniform quantities and gradation of the fine materials (capable of passing through a No. 50 sieve, including the cement) in the concrete mix.
Careful attention to curing and uniform duplication of curing cycles.
Constant water-cement ratio in the mix.
Consideration of the factors that can contribute to efflorescence. Its appearance on the concrete’s surface can mask the true color and give the appearance of color fading. The efflorescence can be washed off when its appearance on the panel is noticed.
Color should be judged from a full-sized sample that has the proper matrix and has been finished in accordance with planned production techniques. The sample should be assessed for appearance during both wet and dry weather.
TEXTURE
Texture allows the natural essence of the concrete ingredients to be expressed, provides some scale to the mass, expresses the plasticity of the concrete and improves the weathering characteristics.
A variety of textures can be achieved. Four major factors should be considered in choosing a texture:
The area of the surface. Coarse textures are difficult to use on small areas.
The viewing distance. From a greater distance, different textures will provide different tonal values. Determining the normal viewing distance in the final use will impact which textures and sizes of aggregates should be used.
The orientation of the building’s wall elevation. This siting determines the amount and the direction of light on the surface and how the panel will weather.
Aggregate particle shape and surface characteristics. Both the shape and surface characteristics determine how the surface will weather and reflect light.
Three key levels of exposure that are used in creating a finished appearance are:
Light exposure involves removing only the surface skin of cement and sand. This sufficiently exposes the tips of the closest coarse aggregate.
Medium exposure requires further removal of cement and sand to cause the coarse aggregates to visually appear approximately equal in area to the matrix.
Deep exposure requires cement and sand to be removed from the surface so the coarse aggregates become the major surface feature.
COMMON FINISHES
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
FACING PRODUCTS AND CAST STONE
Clay product-faced precast concrete panels combine the pleasing visual appearance of traditional masonry products with the strength, versatility and economy of precast concrete. Clay products that can be cast integrally with precast concrete panels include brick, ceramic tile and terra cotta.
The clay product can cover the entire exposed panel surface or only a portion, serving as an accent band or contrasting section. Marble, glass and ceramic mosaics also can be cast integrally (which is preferred) or applied to the hardened concrete.
The combination of precast concrete and clay products has several advantages over site laid-up masonry. By using precast concrete panelized construction, the need for on-site scaffolding is eliminated, which can be a significant cost savings over masonry construction.
Structural design, fabrication, handling and erection aspects of clay product-faced precast concrete units are addressed similarly to those for other precast concrete wall panels, except that special consideration must be given to the clay-product material and its bond to the concrete.
The physical properties of the clay products must be compatible with the properties of the concrete backup. The most significant property is the coefficient of thermal expansion, which causes volume change. It is best to select material with similar coefficients of expansion.
Reinforcement of the precast concrete backup should follow recommendations for precast concrete wall panels relative to design, cover and placement. Because of the difference in material properties between the facing and concrete, clay product-faced concrete panels are more susceptible to bowing than non-faced concrete units. However, panel manufacturers have developed design and production procedures to minimize bowing.
Natural stone has been widely used in building construction due to its strength, durability aesthetic effect, availability and inherent low-maintenance costs. Stone veneers for precast concrete facings are usually thinner than those used for conventionally set stone, with the maximum size generally determined by the stone strength.
As with clay product-faced panels, veneered panels are more susceptible to bowing than all-concrete units. The flat surfaces of cut stone will reveal any bowing more prominently than all-precast concrete panels. Again with these products, precasters have created procedures that minimize bowing.
Cast stone is manufactured to simulate natural cut stone. It is used in masonry work mostly as ornamentation and architectural trim for stone bands, sills, lintels, copings, balustrades and door and window trimming. It replaces natural cut stone or terra cotta in these applications.
ACCEPTANCE OF FINISH - WHAT IS UNACCEPTABLE
Contract documents should spell out who the accepting authority will be, typically the owner, architect, general contractor or site inspector. One person must have final authority on all issues of appearance. Acceptable ranges of color and shading should be determined when samples, mockups or initial production units are created.
Components should be assessed for appearance during both wet and dry weather.
Minor defects and blemishes will occur on occasion, and precasters can adequately repair minor problems quickly. There are key defects or problems that are considered unacceptable in the fabrication of high-quality, PCI-certified architectural precast concrete. These should be addressed as soon as they appear to ensure the problem is corrected early in the production sequence.
The unacceptable problems are:
Ragged or irregular edges.
Excessive air voids (“bug holes” or “blow holes”) in the exposed surface.
Adjacent flat and return surfaces with noticeable differences in exposure from the approved samples.
Casting lines evident from different concrete placement lifts and consolidation.
Visible form joints or irregular surfaces.
Rust stains on panel surfaces.
Panels not matching the approved sample or non-uniformity of color within a panel or in adjacent panels due to areas of variable aggregate concentration and variations in depth of exposure.
Blocking stains or acid stains evident on panel surface.
Non-uniformity of color or texture.
Areas of backup concrete bleeding through the face concrete.
Foreign material embedded in the face.
Repairs visible at 10- and 20-foot viewing distances.
Reinforcement shadow lines.
Visible cracks after wetting.
Precast concrete generally undergoes far less cracking than cast-in-place concrete. This resistance results in part from the high compressive and tensile strength of the prestressing activity.
A certain amount of cracking may occur without having any detrimental effect on the structural capacity of the member, and it is impractical to impose specifications that prohibit cracking. But cracks are unsightly and create potential locations for concrete deterioration, so any cracking should be avoided and inspected.
The best methods to minimize cracks are to provide proper reinforcement, prestressing and proper handling. Whether cracks are acceptable will depend on an evaluation of the cause of the cracking and the stress conditions that the crack will be under with the precast concrete unit in place.
The cement film on the concrete may develop surface crazing, which consists of fine and random hairline cracks. Crazing has no structural or durability significance, but it may become visually accented if dirt settles in these minute cracks. A relatively lean, properly consolidated concrete mix will show little crazing.
Tension cracks can be caused by temporary loads during production, transportation or erection of the products. The amount and location of reinforcing steel has a negligible effect on performance until a crack develops. While some of these cracks may be repaired and effectively sealed by pressure-injecting a low-viscosity epoxy, their acceptability should be governed by the importance and function of the panel.
Every effort should be made to promptly identify the cause of any cracking, particularly when several units display similar cracking. Such cracking often results from one problem, be it design, production or handling, and it can be corrected.
Erected panels not complying with range samples may require additional work. If the architectural precast concrete panels cannot be corrected to match the repair samples or repairs demonstrated on the mockup, they may be subject to rejection. A certain amount of repair is to be expected as a routine procedure. Repair and patching of precast concrete is an art requiring expert craftsmanship and careful selection and mixing of materials.
Repairs should be done only when conditions exist that ensure the repaired area will conform to the balance of the work’s appearance, structural adequacy and durability. Slight color variations can be expected between the repaired area and the original surface due to the different age and curing conditions of the repair. Time will tend to blend the repair into the rest of the component to make it less noticeable.
Should minor damage occur to clay-faced or veneer stone products during shipping, handling or erection, field remedial work can successfully be accomplished. Such repairs normally are done by the precaster with repair procedures developed in consultation with the product’s fabricator.
GFRC (Glass-Fiber Reinforced Gypsum)
Glass-fiber reinforced concrete, commonly known as GFRC, is a composite concrete product fabricated by many precast concrete manufacturers. It consists of a portland-cement-based composite that is reinforced with alkali-resistant glass fibers, which are randomly dispersed through the material. The fibers serve as reinforcement to enhance the concrete’s flexural, tensile and impact strength. The three images above are an example of precast GFRC.
The major benefit provided by GFRC is its light weight, which provides substantial economy resulting from reduced costs of product handling, transportation and erection. It also can lower seismic loads, requiring a lighter support system than typical precast concrete components would require. By casting it into a mold, GFRC also can create highly detailed, ornamental pieces.
The manufacture of GFRC products requires a greater degree of craftsmanship than other precast concrete products. Many combinations of shapes, sizes, colors and textures are demanded of this product. Typically, the fibers in a GFRC component make up at least 4 percent of the total weight, with a minimum thickness of 1/2 inch for the piece. The fibers are specially designed for use in these components, and no others should be used.
With GFRC, any change in face-mix materials or proportions will affect the surface appearance. If the face-mix surface is progressively removed by sandblasting, retarders or other means, the color becomes increasingly dependent on the fine and coarse aggregates. A change in aggregate proportions, color or gradation will affect the uniformity of the finish, particularly where the aggregate is exposed.
Different cements have different color characteristics that affect the desired GFRC face mix. The cement color exerts a considerable influence on the color of the finished product.
Normal production variables, such as changes in water content, curing, cycles, temperature, humidity and exposure to climatic conditions at varying strength levels, all will cause color variations. Color variations also will be greater in a gray-cement matrix than in white-cement matrices. If a gray color is desired, consideration should be given to using white cement with a black pigment or a blend of white and gray cement.
Rib formers may be used to produce supports that provide structural rigidity and stiffness for the GFRC panels. Expanded polystyrene foam and polyurethane foam are the most common materials used. Tests should be run to determine the allowable loads for any insert molded into the GFRC skin. Many inserts used in GFRC panels have been designed and tested by UP Ceilings.
GFRC components often are created by spraying the material into a mold to create a lightweight but highly detailed decorative piece. The appearance of the finished panel surface in these pieces is directly related to the choice of mold material and the quality of the mold. The in-service life of a mold also is a function of the mold materials.
Molds can be made of a range of materials, including plywood, concrete, steel, plastics, polyester resins reinforced with glass fibers and GFRC. For complicated details, molds of plaster, rubber, foam plastic or sculptured sand may be used.
FAQ
Which finishes provide the best consistency?
A wide variety of finishes are available in architectural precast concrete, ranging from a smooth form finish to deeply exposed aggregates. As a general rule, a textured surface provides more uniformity than a smooth surface because the natural variations in the aggregates will camouflage subtle differences in the texture and color of the concrete. A medium sandblast finish, for example, generally provides more uniformity and consistency than an acid wash finish. Dividing larger areas into smaller ones with reveals or rustications can also help lessen any variation in texture that might be visible.
What is the largest panel dimension I can design?
It is more economical to maximize panel size and minimize the number of precast units on a project. This results in fewer erected pieces, fewer connections and fewer crane picks. However, the maximum size of a precast panel depends on a variety of factors. For example, the size may be limited by site conditions or the reach of the crane that will be used to set the pieces. A site with limited access, or one where the maximum panel weights are set by the crane capacity could be the overriding factor in determining panel dimensions. Similarly, the size or weight of precast panels may be limited by shipping or fabrication considerations specific to a region or individual precast supplier. Usually panels should not exceed a width of approximately 12'-0", without consideration for a special permit or escort. Also, panels that exceed 40' in length may require the use of prestressing to reduce handling stresses and minimize cracking. The maximum size of panels is also a function of the design loads and locations of building supports. In general, it is best to work with a MAPA precast supplier to determine the most economical sizes and dimensions for your project.
What is the optimum joint size?
The recommended precast panel to panel joint width for architectural projects is 3/4". This is the minimum nominal joint width needed to adequately account for production and erection tolerances and still maintain an effective minimum joint width that can be caulked. A 30' long spandrel panel is allowed, per PCI tolerances up to a 1/4" variation in length. Keep in mind that of 3/8" is the minimum width that caulk suppliers will warrant. It is also important that the joint between precast panels and window frames also maintain the same nominal joint width.
Should sealers be used on precast?
Sealers are often specified to improve the weathering characteristics of precast panels, especially in urban areas where the building may be subjected to airborne industrial chemicals. Sealers can also help facilitate the cleaning and maintenance of the panels if they should become dirty. When sealers are used, they should be applied in the field, after all of the joints are caulked, repairs made and cleaning complete. Otherwise, it is likely that the panels will have to be recoated in spots, which could, in turn, lead to inconsistencies in color and finish.
What is recommended as a preferred distance from the architectural precast panel to the edge of the slab?
The slab edge location should be clearly defined on the contract documents. It is recommended that a 1-1/2 inch dimension be allowed between the edge of slab and the precast panel to account for tolerance both in the slab as well as the precast. Pay particular attention to slab edge conditions along skewed or curved building edges as these areas are often the areas that cause the most difficulty during layout.
What is recommended for interior dimensions?
It is important to consider tolerances when designing the interior wall finishes and locations. For example, if inadequate space is left between the back of the precast panel and the inside face of the interior finish, connections may become exposed to view. Allowing at least an extra 1/2-inch between the back of the drywall and the theoretical back edge of the connection hardware is strongly recommended. When the distance between the back of the precast and the interior finish does not accommodate this, connections may have to be recessed. It is also a good practice for the engineer to specify the allowable locations for slab recesses and to provide reinforcing details to account for this.
For more information regarding using molded products or GFRC on your next project, contact a representative at UP Ceilings today.