January 1, 2010
By Moffette Tharpe, Robert Bouchard, CPCI,
and Michael Chusid, RA, FCSI
There was more than half a metre of snow on the ground—the kind of weather when construction crews hate jockeying truckloads of precast concrete panels into position under a crane. However, on the jobsite of Montreal’s Hilton Garden Inn, it was worth the effort because every truckload covered a large expanse of wall. There were more panels per load than a truck usually carries, and some were 9.8 x 3.2 m (32 x 10.5 ft)—a size generally impractical with conventional precast architectural concrete. However, even these surprisingly large panels weighed less than 4600 kg (10,141 lb).
These panels were studcast precast architectural concrete. With 50 mm (2 in.) of concrete thickness, mated to a panelized frame of heavy-gauge galvanized steel studs, such products could enclose a wall two to four times as quickly as conventional precast, sealing out freezing weather and thereby speeding up construction on the building interior by weeks.
In the eternal struggle between building cost and quality, these studcast systems can be potent tools. The thin-panel system provides benefits of architectural precast concrete, but reduces weight by more than 50 per cent. The light weight leads to cost reduction that multiplies through every level of the superstructure, into the foundation. Building with studcast panels saves time, tends to be lighter on the environment than conventional panels, and offers performance advantages.
Weighing the options
Architectural precast is known for esthetic versatility, durability, affordability, speed of erection, and, unfortunately, its great weight. Typical 150-mm (6-in.) thick panels tip scales at about 350 kg/m2 (70 lb/sf), and carry none of their own load. Some precast is even thicker and heavier, hanging on building frames, needing a weighty superstructure for support, which then demands a heavy foundation. The resulting building can also be an environmental burden with respect to materials consumption and greenhouse gas (GHG) emission; about 1 kg (2.2 lb) of portland cement production will emit approximately 1 kg of carbon dioxide (CO2) into the atmosphere.
During the past two decades, several proprietary systems have been developed with the goal of preserving the great virtues of architectural precast, but losing the weight. These different systems are composed of a thin slab of concrete, each with a different integrally cast connection to panelized steel stud framing.
Such studcast panels, made of the same concrete type as a conventional precast panel, can cut the cladding’s weight by about 60 per cent. For the Hilton Garden Inn, this difference alone saved about 1.7 million kg (3.8 million lb), plus further consequent weight reductions in the superstructure and substructure.
The significant thinness is made possible by re-envisioning the interaction between concrete and steel. Conventional precast concrete must be heavily reinforced so the panels can span from floor to floor and transfer wind, seismic, and other horizontal loads into the building structure. The steel reinforcing bars, at the panel’s centre, are not efficiently located to provide tensile forces required to hold together a panel. Consequently, conventional architectural precast panels are usually 152 to 203 mm (6 to 8 in.) or more thick.
In a studcast panel, by contrast, the steel studs are efficiently deployed to resist horizontal loads. If studs can be considered the ‘bones’ of the panel, the relatively thin precast surfaces are the weather-resistant ‘skin.’ The precast requires only a modest amount of reinforcing (typically, welded wire mats) to strengthen short spans between steel studs. The two materials work together for an efficient wall.
Studcast panels are typically made with 50-mm (2-in.) minimum concrete thickness. The concrete can be full-density ‘hard rock’ with compressive strength in the range of 35 MPa min (5000 psi). (‘Hard rock’ refers to the kind of concrete normally used in buildings and sidewalks. It is made with crushed rock and sand for coarse and fine aggregate.) It can be cast utilizing any possible options with conventional precast panels (e.g. multiple integral pigments, exposed aggregate, form liners for cast-in textures such as brick or stone, returns, and reveals).
The panelized steel stud framing is integrally cast with concrete, making it possible to achieve composite action. The panelized frame is easy to properly position for casting, allowing the process to become fast and efficient. This integral framing also eliminates need for applying additional furring to the panel’s interior; as cast, it is ready to receive interior finishes. Moreover, the frame provides built-in cavities for utilities and insulation. By eliminating the need for additional furring, 50 to 100 mm (2 to 4 in.) of floor space is gained around the entire building perimeter. At least one system mounts out-board of the floor edge, providing even more floor space. (Depending on the project’s size, this extra space could be worth a ‘free’ office or more.)
There are several proprietary types of studcast panels. The main distinguishing factor between them is the method of achieving connections between concrete and steel. In some systems, specially designed connectors are attached to the steel frame and embedded in concrete during casting. Another method involves embedding part of the steel frame itself in the concrete.
The choice of attachment technique can impact several aspects of panel performance. Thermal isolation of the steel frame from concrete affects the panel’s insulating ability. The smaller the surface area of steel/concrete contact, the better thermal isolation. Use of a connector creating a small air gap between the framing and interior concrete surface is ideal, eliminating ‘hot spots’ and ‘cold spots’ that would telegraph stud positions on the concrete’s exterior surface.
A connector with slight flexure and a thermal coating gives the concrete skin a small degree of movement freedom relative to the framing. Flexure of the connector isolates the building’s concrete skin from movement of the frame, allowing the panel to respond better to thermal expansion and contraction, wind loads, and seismic movements. It minimizes tensile stress in the concrete at connection points—reducing the possibility of cracking—and helps maintain integrity of seals in panel joints.
Perhaps the greatest advantage is reduced dead load. It results in a lighter superstructure, which in turn allows a smaller investment in foundation and footings. This saves cost in terms of reduced material consumption, excavation, and construction time. In locations where seismic activity is a design concern, the lower mass of thin panels reduces the level of bracing required.
Lighter panels make possible certain designs that would be impractical with conventional precast concrete. For example, the Marriott ExecuStay in New York City was built next to a low-rise building, and ‘air rights’ over the low-rise were secured. Cantilevered floor slabs were designed over the top of the lower building without column supports for the floor slabs, creating extra leasable floor space. This design required lightweight cladding, so the architect specified a studcast panel system.
Less weight per square metre of wall area makes it practical to cast larger panel sizes. Numerous benefits flow from this ability:
• more wall surface area can be carried on a single truck, speeding up delivery and minimizing associated transportation-related costs and energy consumption;
• larger panels mean fewer ones, so more expanse of wall gets enclosed with each panel erected;
• fewer panels minimizes the number of joints that must be sealed, and reduces locations for air or water infiltration in the event of sealant failure;
• larger panels give architects more freedom to design joint spacing in a logical and esthetically attractive manner; and
• faster enclosure of the building allows other trades to begin work sooner on the interior, minimizing potential weather delays.
Alternatively, if larger panel sizes are impractical due to building design, the lightweight system means regular-sized panels weigh less. Lightweight panels can be erected with a lighter-duty crane, which can create significant savings or simplify construction operations.
The basic cost difference between designing with studcast versus conventional concrete panels varies, depending on the particular studcast system and the specific project conditions. Generally, the overall cost of installed studcast panels—materials plus delivery costs, labour, other installation expenses, and the contractor’s overhead for the significant time difference—make them lower in comparison to traditional methods. In addition, the reduced cost of other structural elements and the minimized foundation excavation increases savings.
Light on the environment
Reducing the concrete in cladding panels can have important environmental value. Concrete has a high carbon footprint due to large CO2 emissions from the process of portland cement production. Portland cement accounts for approximately 15 to 18 per cent of solid materials in concrete. Every kilogram of portland cement produced releases about a kilogram of carbon dioxide into the atmosphere. The cement industry is attempting to increase efficiency of its manufacturing energy consumption, but that has a limited impact on the problem—almost half the CO2 comes from the chemical reaction of limestone calcining, so it is inherent in the nature of the material.
The key to more sustainable concrete is reducing the quantity of material needed. Studcast panels made of conventional concrete can cut the cement-related carbon footprint by almost 70 per cent. Thinner panels also consume less aggregate. Recycled content in the steel and inclusion of recycled cementitious products, such as fly ash, can also contribute to a greener building. Lighter panels also reduce transportation-related energy consumption and air pollution.
The light weight of the panels has inspired innovative erection methods. It is possible to do a lift-and-release technique, positioning the panel by crane and depositing it on landing hooks that temporarily hold it in position prior to the final connection. This allows the crane to be disconnected from the panel quickly to bring the next one without waiting for the first to be fully welded into place. Numerous panels can be temporarily positioned this way, enabling more perfect alignment before any sort of permanent connection occurs.
A specially designed, proprietary multi-layer joint-sealing system can keep water out of the interior. It includes a built-in pathway to channel out moisture that penetrates the outermost sealant layer. The channelling device also acts as a leak detection system, producing a small but visible wet spot on the exterior surface that pinpoints leak locations to within a few metres.
Remarkably, the advantages of studcast come with virtually no trade-offs. Studcast panels can be made with high-strength concrete to equal durability of conventional precast. With integral water repellant products, the thin concrete can achieve moisture-proof properties of full thickness precast. All the options of cast-in colour, texture details, and architectural veneers are possible. The thickness of concrete can also vary to allow for reveals or other architectural details.
How to specify
The concrete components can be specified in accordance with the revised Best Practice Guide: Architectural Precast Concrete: Walls and Structure by the Canada Mortgage and Housing Corporation (CMHC), the Design Manual (4th ed.), CSC’s Tek-Aids, and various other technical documents by the Canadian Precast/Prestressed Concrete Institute (CPCI). CPCI also has a plant-certification program to assure certified precast fabricators meet acceptable quality assurance programs. Steel framing, on the other hand, should comply with the parameters of Canadian Standards Association (CSA) S136-07, North American Specifications for the Design of Cold-formed Steel Structural Members.
Established studcast producers can provide more than 15 years of experience and test data to establish the best way to integrate the panel’s concrete and steel elements. An experienced precast producer can make valuable contributions to the design team by suggesting the most effective means to produce desired ends.
Whenever possible, the decision to use studcast panels should be made early in the project to take advantage of reduced dead load when engineering the structure. On complex projects (and where appearance is crucial), a mockup should be specified to allow all parties to determine acceptable standards of performance.
The re-thinking of tried-and-true materials allow for improved construction methods and results, while utilizing the supply-and-delivery infrastructure on which the industry is built. It allows these time-tested materials to function more efficiently, and better meet the economic, environmental, and esthetic demands of the changing industry and society.
Moffette Tharpe is managing director of Easi-Set Industries, developers of the SlenderWall architectural precast system. He has more than 20 years of experience advancing innovative precast products and is active with the National Precast Concrete Association (NPCA) and the Prestressed Concrete Institute (PCI). Tharpe can be reached via www.slenderwall.com.
Robert Bouchard, CPCI, is CEO of Bétons Préfabriqués Du Lac (BPDL) in Alma, Que., a concrete precaster and producer of studcast panels, glass fiber reinforced concrete (GFRC), and ornamental masonry units. He has 20 years experience in concrete/cast stone. Bouchard has been a member of the board of directors of the Canadian Precast Concrete Institute (CPCI) since 2002, and is an active member of the Quebec Chapter. He can be contacted at www.bpdl.com.
Michael Chusid, RA, FCSI, CCS is principal of Chusid Associates, a technical and marketing consulting firm specializing in building products. He can be reached via www.chusid.com.
For more on thin precast/steel stud hybrids, click here.
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