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.
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
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.
Precast/prestressed concrete is manufactured in PCI Certified plants in a controlled environment.
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.
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.
An infinite variety of sands, cements, aggregates and pigments combine to achieve endless colors, textures and finishes.
Precast Concrete is a green product and can provide up to 26 LEED points.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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 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 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.
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