Precast System Panel
Precast concrete is available in many shapes and sizes, including structural elements and unreinforced pieces. Panels are the most common members used in building construction, and may serve as structure or cladding (finish) or both. As structural-architectural panels, they provide a cost effective solution to building enclosures. Panels can be full- or half-sandwich or hollow core construction. Full-sandwich panels are popular because they provide 2 durable faces and allow a space between that is filled with insulation. The half-sandwich panels contain just one concrete face and therefore require additional insulation and finishing after erection. Hollow core panels can be used for walls, but are especially well suited to floors. The cores can be used for utility chases or sometimes as ducts for HVAC.
The following points describe precast concrete panels for low-rise buildings:
More information on precast is available from the Precast/Prestressed Concrete Institute and from the Canadian Precast/Prestressed Concrete Institute. From codes and standards development to education and marketing, these organizations support the success of precast concrete products and systems.
Concrete sandwich panels are highly engineered to provide rugged, durable surfaces for walls and can be used for floors, as well. Precast concrete panels (and other items) are manufactured in factory settings. Not only is this conducive to improved quality control, it’s also possible to manufacture year round during any type of weather.
By designing buildings to have one predominant panel size, panel manufacturing becomes a repetitive process and gains efficiency. Following a two-step casting process, the resulting void between panels can be filled with insulation.
The ability to quickly erect panels at the site is another advantage of precast. Once footings are prepared, panels can be placed quickly to form the enclosure. This requires some lead time to order the panels, but is an advantage at the site. It also means that construction can be completed within a relatively small footprint and may allow building in weather that would negatively affect other types of construction. Solid, high-quality concrete construction built with an efficient use of labor makes for cost effective structures.
Precast, like all concrete construction, provides fire, rot, termite, and mold resistance. Exterior walls can withstand tornado-force winds. It is solid construction that protects occupants from weather and from natural disasters.
The casting process itself is not limited to any particular size, but because the panels have to be shipped on trucks and erected at the building site, there is a practical limit to their dimensions. When the precasting facility and site are located relatively close to each other, panels have been as long as 50 ft. Panels can be just about any size, as long as the trucks and streets can accommodate them and the equipment can place them.
Panel size should be suited to the building design. For instance, for a single-story structure, the panel may be 8 to 10 ft tall, and 20 or 30 ft long. For a two-story building, the panel might be 15 to 20 ft tall. Again, as long as the panel can be trucked to the site without causing problems on roads or under bridge overpasses, the size is dictated by the building itself.
Installation, Connections, and Finishes
Concrete weighs about 150 lb per cu ft, so precast items are heavy. For instance, a full-sandwich panel that is 10 ft by 20 ft by 8 in. thick (with a 3-in. void for insulation) weighs more than 6 tons. Cranes and other lifting equipment should be appropriately sized to handle the panels. Tight quality control of dimensions allows panels to be erected quickly at the site with only a minimum of field adjustments.
Sandwich panels are bolted to the footing or slab. The panel must be placed, leveled, and plumbed before the attachment is made. It must be braced during this time. Generally, the brace remains in place until the panel is connected to neighboring panels which complete the structure and provide support. Hollow core panels are attached to dowels in the footing or slab and the core is grouted to create a column that ties the wall structurally to the base.
Connections between panels are made either by grout columns (hollow core slabs) or by bolting or welding plates cast with the concrete. Bolted joints should be finished by caulking with a sealant to seal the building enclosure and protect the steel connections from rusting.
Architectural finishes for precast are varied. Concrete can be integrally colored with mineral pigments. The range of colors is quite wide if white cement is used. One of the most common surface treatments is exposed aggregate. Form faces can be treated with surface retarders prior to casting, or, following hardening, the surface of the concrete can be textured with abrasive or water blasting. Exposed aggregate finishes are usually sealed to provide added resistance to water penetration. Alternately, panels may be cast with a form liner and painted with a masonry-type paint. This allows the simulation of many other finishes such as hand laid masonry (brick or stone). Some people prefer casting a thin brick or concrete unit into the face of the panel. Still others prefer attaching the finish at the site to create either a cavity wall or a barrier wall. Portland cement plaster is also a suitable finish for precast, and can be directly applied to panel faces.
Interior finishes may be given the same treatment as exterior faces, may receive a direct application of plaster or paint, or they can be finished with drywall for a more conventional appearance.
Sustainability and Energy Considerations
In terms of sustainability, precast has something to offer both as a material and in its performance. On the material side, it can contain recycled materials like fly ash and rebar, which may help contribute to credits in LEED® or other green rating systems.
From a performance perspective, the system creates an energy efficient envelope. Precast panels are erected on site and joined together. The size of panels dictates joint spacing, but often the number of joints is reduced over other types of construction, which helps create tight building envelopes. This protects against unwanted air losses and may even require that air changes be provided. It is common to downsize HVAC equipment and increase mechanical ventilation in precast concrete buildings.
Insulated precast panels may reduce energy consumption by 20% to 30% in smaller buildings. This is because the panels are so energy efficient, having insulation combined with the thermal mass of concrete. A related benefit of mass walls is that they are great for soundproofing. Next-door neighbors may like it warmer or cooler, louder or quieter than each other, but the insulated concrete panels means they can both have it just the way they like it.
Depending on the location of manufacturing relative to the project site, precast may also contribute to local materials credits in some green building rating systems. Most concrete materials are locally sourced. Also, precast producers are located throughout North America, and it is common to source the panels from a relatively close producer to minimize the time and expense of shipping.
Manufacturing and Physical Properties
Often, the production of panels is a highly automated process. Forms are built, openings laid out, reinforcement is placed, and perhaps even conduit is placed for mechanical or plumbing needs. Fresh concrete is deposited into these forms in a factory setting. This greatly improves the control over concrete mixing and temperature. Tight quality control is exercised at every step in the process, including adjustments to thickness and other dimensions for precise panel sizes.
Panels can be cast vertically or horizontally. Vertical casting is similar to cast-in-place concrete. The panels are cast in the same orientation which they will be placed in service. Horizontal casting is perhaps more common because it is easier and faster to lay out the panels, openings, and reinforcement, and cast the concrete. Following casting, panels are placed into curing chambers to develop strength and durability.
Casting beds create the panel faces. Horizontal casting also simplifies sandwich type fabricating. This allows for casting 2 separate panels—one interior, one exterior—so that a void is created between them. The void can then be filled with insulation to improve the panels’ performance as part of a building envelope. This also permits quite a bit of flexibility for the aesthetics of each finished face.
Panel manufacturers usually provide engineered, stamped plans for single-family residential homes they supply. They also provide a resource to answer questions from building departments. On larger buildings, it’s common to custom engineer the building. The panel producer provides engineering data and specifications for their panels. Designers use this information in their analysis.
Precast is designed following the International Building Code, which has provisions based on ACI 318, the Building Code Requirements for Reinforced Concrete.
Precast is a premium construction method that offers many performance benefits. It becomes increasingly cost effective and economical when repetition can be built into the process. As such, it’s more common on medium and larger sized buildings.
Photos courtesy of Dukane Precast.
Premium Project: Melrose Places Precast Condos
A 48-unit condominium building in Melrose Park, Ill. showcases just how versatile precast construction can be. Nearly the entire building is made from precast components: beams, columns, double-tees, sandwich panels, slabs with integral steel joists, and cornices. Inside and out, this building exemplifies what can be done with precast.
The 5-story structure has 4 stories of condos sitting atop an at-grade parking garage. The exterior precast walls are made using highly insulated panels (R-value 22) that are furred out on the inside and finished conventionally with drywall. The interior hallway precast walls, however, are constructed with a double-wall precast system, which provides a completely finished wall on both sides (and contains electrical conduit)—so no extra finishing is necessary. This allowed the contractor to build out the units faster and with less cost, since only a prime and paint finish was needed on walls throughout the corridors at every level. And tenants will enjoy quiet living spaces since interior precast walls provide excellent sound attenuation for reducing hallway noise.
What may be most important, however, is how livable and safe the space is. The strong concrete construction stands up to all sorts of rough weather. Although the Midwest experiences quite a range of high and low temperatures throughout the year, the building envelope is energy-efficient to keep occupants comfortable and minimize heating and cooling expenses. The units have high ceilings and open floor plans. Developers selected an innovative precast flooring system: the individual precast floor panels contain integral steel joists that provide open undersides, so mechanicals can be routed through this area and enclosed by drywall attached to the bottom of the joists. The floor panels are 40 ft long and have variable widths—up to 12 ft—to help create the gently curving floor plan. This footprint nicely fits the somewhat tight urban site and adds curb appeal to the building. Other amenities include exterior patios and balconies.
Precast concrete really allows this building to be a place of comfort, a home to all its occupants.
Precast Provides Affordable Housing Solution
Most people would agree that two of the most desirable characteristics of real estate are location and affordability. One recent Chicago project has both, with a 2010 PCI Award in the multifamily category to show for it. These precast apartment buildings demonstrate that it’s possible to build attractive, quality housing economically.
In any economy, and perhaps especially in today’s uncertain economic times, many people who may want to purchase a home may not have the means to do that, so the rental market fills an important need for housing. Rosa Parks Apartments is a new development that provides 94 affordable rental units. As an infill project, these 8 buildings are located near downtown Chicago, giving residents easy access to jobs. And in part thanks to the economy of building with precast, the units can be offered at an affordable rate.
Structural precast panels were a good fit for this project because they carry the loads of these 3- to 4-story apartment buildings and they form the exterior walls. There’s no need for a separate steel or wood frame or for any other façade. And because precasting involves repetition, panels can be cast economically, especially when 8 buildings are being constructed as part of the same project.
It was critical that the structures not just be economical to build, but also, sustainable for long-term operation and maintenance. The insulated wall panels feature cast-in brick on the street facades and a painted brick pattern on the rear. These are low maintenance exteriors. With the finish cast into the panels, construction is expedited but quality can still be closely controlled. The different colors of the brick panels help the new buildings blend in well with the surrounding neighborhood at the different locations and also contrast nicely with the bay windows.
Several factors contribute to the energy efficiency of this building system. Panels have built-in insulation, create tight envelopes, largely due to the size of the panels, allowing for reduced exterior joints, and provide thermal mass of concrete to help minimize temperature swings and reduce heating and cooling demands.
As a result of being spread over many city blocks, one challenge for this project was maintaining security during construction. Precast allowed the builders to place the panels, install the windows and doors, and secure the building quickly with minimal weather setbacks.
For this and other projects, visit 2010 PCI Design Award winners.
Photos courtesy of Mark Ballogg, Ballogg Photography
As the name implies, wall panels, including door and window openings, are cast in forms on the floor slab and simply tilted into position. Once the walls are up, they are tied together with cast-in-place concrete columns or other fastening devices. This system is cost-effective as long as the home features mostly right angles ... curves and intricate wall configurations are best left to other methods.
Tilt-Up Concrete Association
P.O. Box 204
Mount Vernon, IA 52314
(319) 895-6911 / Fax: (319) 895-8830
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source: cement online