Precast Loses Weight, Gains Value

Welcome to thin-wall precast systems.

By Brian Miller
Brian Miller was previously a Technical Services Engineer with NPCA and a member of the NPCA TechTeam.

It used to be that when people thought of precast concrete wall systems, they thought of 5- to 10-inch-thick panels that weighed between 60 and 100 pounds per square foot. But with today’s new systems, precast has slimmed down. These systems are commonly called “thin-wall” or “thin-section” systems. This generic term describes several proprietary versions of these systems. While each system has some unique designs and features, they all share some common features and advantages.

What are thin-wall systems?
Essentially, thin-wall panel systems incorporate a stud and cavity design. The cavity sections help reduce the mass of the panels and provide space for additional insulation and utility connections. The studs have a load-carrying capacity, and in some applications connect to a structural frame. The skin, or continuous wall portion of the panel, typically varies in thickness from about 1.5 inches to 3.0 inches. The skin provides the barrier between the outside and inside environments and contains the architectural finish on the exterior of the panel. Usually the skin portion of the wall is reinforced with some form of mesh and/or fiber reinforcement.

The interior side of a panel typically contains some form of stud system. These studs are often made of heavy gauge steel or concrete. The stud system reinforces the panel to resist lateral loads, carries vertical loads and can serve as a frame for the attachment of interior finishing materials. For example, when thin-wall panels are used as foundations, the studs can be used to attach drywall, making it easy to finish the basement. This eliminates the need for additional basement wall framing or furring.

Thin-wall systems can be designed as load bearing or non-load bearing. All systems must resist their own weight and environmental forces such as wind. Non-load bearing systems are commonly used as cladding for retail centers, offices, commercial structures including high-rise buildings, parking garages, hospitals and many other structures.

Load bearing systems are designed to support other structural loads as well, such as floor and roof loads. Load bearing systems can serve a dual purpose as both the cladding and primary structural system. They are commonly used for foundations and walls of above-ground construction such as single- and multi-family homes as well as low-rise buildings.

Finishes and more
Thin-wall systems can receive most of the surface finishes that standard precast can, such as acid etch, abrasive blast, exposed aggregate, as-cast (smooth off the form), veneer thin brick and polished. Form liners can be used to create a multitude of finishes as well. Also, trim elements such as window sills, lintels, banding and column bases
can be cast into one panel, or different finishes can be combined to create a unique-looking project.

Many systems can incorporate other items such as insulation, utilities and window frames. These can be cast in or installed at the plant, thereby reducing the amount of time and labor required on site.

Connections
When thin-wall systems are used as load bearing units, they make up the structural system of the building. They are typically connected to each other with bolts and welds. When thin-wall systems are used as cladding, the interior stud system of the panels is usually attached to the structural frame system of the building by conventional means such as welding. Connections should be designed by an engineer familiar with precast concrete in conjunction with the precaster.

Thermal conductance
Some systems use low thermal conducting materials, such as carbon fiber or epoxy-coated anchors to attach the stud frame to the skin. This reduces thermal bridging, thereby decreasing “cold spots” in the walls. Also, it is possible to design these systems so that the interior frame reacts almost independently from the skin section, providing a thermal break and preserving the structural integrity and watertightness of the wall system.

Summary
Thin-wall systems can reduce the weight of the precast by up to 70 percent from precast concrete walls of years past. This weight reduction can assist in reducing shipping costs, the size of the crane necessary for erection and reduced dead loads. The reduced weights of the panels enable structural members of the building and its foundation to be made smaller, thereby reducing costs.

An example of this is an Extended Stay high-rise hotel project in New York City. Due to the reduced weight of the precast, the upper floors could be extended out over older buildings below. This did not require additional columns or structural supports, but it does increase the usable floor space of the project, which increases revenue potential. The thin-wall precast panels also helped decrease the foundation and structural member size, which saved enough money on the project to pay for most of the exterior precast cladding.

Several companies make and/or license proprietary thin-wall systems. To find out more about thin-wall systems, visit NPCA’s website.