Prefabrication—Integrating Systems Boosts Efficiency

Below is an excerpt of an article originally published in its entirety in the summer 2015 issue of Ascent Magazine, written by Craig A. Shutt.

“Designers find integrating materials—masonry, insulation, windows, electrical, plumbing, and more—into precast concrete panels early in the process results in efficiency and economy.”

Designers long have integrated stone veneers and thin bricks into precast concrete panels to gain aesthetic versatility and cost savings. Insulation also has been cast into panels at the plant to provide energy efficiency and construction savings. Now, more materials, including windows, electrical service and HVAC ducts, are being installed in the plant to reduce trades and general conditions, cut construction time, boost quality, and reduce site congestion and liability. These concepts, while not new, are growing in popularity as designers recognize fabricators’ capabilities and the flexibility of precast concrete to create more complete units under factory-controlled conditions while site work continues. This approach also speeds construction and alleviates congestion and coordination of trades.

Many Systems Installed

Integrating insulation into the panels can create a continuous thermal barrier which provides the most efficiency. Typically the exterior and interior wythes of concrete can be connected with rigid ties, carbon fiber, longitudinal welded-wire trusses, or solid-concrete connections. However, greater energy efficiency is typically achieved using special, nonconductive ties or carbon fiber essentially eliminating thermal bridging. Windows and doors also are gaining recognition as systems that can be integrated seamlessly into precast concrete panels. Door and window frames, properly braced to prevent bowing during concrete placement, can be cast into the panels and then the glazing or door panels are installed prior to or after delivery to the site. Efficiency is gained through the capability of precast concrete molds to create repetition that makes installation move smoothly while providing a secure, protected, open space in which crews can work quickly. Prefabrication eliminates need for site measurements and delays in ordering often long-lead window framing elements. HVAC requirements are being designed into panels to avoid having to make any penetrations on-site. Return-air vents and other ventilation needs are worked out in advance, with blockouts created in the panels to allow fast installation once the panels are erected. Special attention to waterproofing must be paid and alignment is critical to ensure ideal performance without field adjustments. Installing electrical conduit and boxes requires coordination with the electrical contractor to ensure everyone’s needs are met. The metal or plastic conduit is typically pre-bent to the desired shape and delivered to the casting bed already connected to the electrical boxes. The joints and connections are thoroughly sealed to prevent clogging the system. The wires then are pulled through at the jobsite. Plumbing units also can be connected and assembled prior to delivery to the site. Bathroom and kitchen modules for residential projects, such as hotels or condominiums, can consist of molded plastic units or be fabricated from drywall components. To eliminate a double floor, the module can be plant-built on the structure member, or the walls of the unit can be designed strongly enough for all fixtures to be wall-hung.

Pre-Assembled Panels

Some precasters are extending their capabilities beyond traditional insulated, sandwich wall panels to provide additional efficiencies and cost savings. Clark Pacific, for instance, offers a composite architectural precast concrete panel (a thin-shell type of panel). The panels consist of an exterior wythe of concrete, typically 2 ¼ to 3 inches thick, with 2 inches of NCFI spray-foam insulation coating the rear face. Four-inch, light-gauge tube steel is laid over that with galvanized pins to connect the skin to the frame and provide support for window units. The window assemblies are installed with the designer’s choice of energy-efficient glazing. HVAC vents, LED lighting connections, and other systems also can be installed. Performance mock-up tests conducted at Construction Consulting Lab West in Ontario, Calif., showed that the panels meet all ASTM and AAMA tests for air infiltration, structural wind loading requirements, and elastic and inelastic in-plane seismic movement needs. The panels were used recently to clad the 23-story LA Live Marriott hotel, a 295,750-square-foot hotel with 393 units of various sizes. It also features ground-level retail, a rooftop pool, a home theater, and fitness rooms. “I was unfamiliar with the precast panels until Clark Pacific showed us the concept, but I was interested once I understood them,” says Edward Kirk, project manager for Holland Construction, who served at the time as project manager at SODO Builders, the general contractor for the project . “The ability to combine so many systems into one panel in the yard under controlled conditions had a lot of appeal.

 “Installing the systems ahead of time while other work was underway at the site reduced field-installation time greatly, aiding the schedule.”

The panels spanned floor to floor with punched window openings of varying widths and spacing. Vertical elements between windows feature returns at either edge that form the window jambs. Gray sill pieces were cast below the window openings, set back 2 inches from the face, while charcoal colored lintels were set back 3 inches from the face.

Foam Insulation Added

The spray-foam insulation was added at Clark Pacific’s Fontana, Calif., plant and represented the first use of the system there, says Marshall Stearns, project manager. “It required special training to get the foam operation up and running.” An efficient operation was created to apply 2 inches of NCFI Insulbloc spray insulation to the panels. “By far, the biggest challenge with the insulation was not in the application itself but in keeping it out of areas where it was not required,” he says. Forms were built to produce sheets of insulation, which were then cut into strips and used to form dams at blockout locations. At the peak of production, five panels were insulated per day. Casting window openings and installing the units also created challenges. “One of our biggest challenges was in forming,” he says. “Not only were there eight different window sizes, three colors, and two different steps in the panel faces, but the spacing of the windows was constantly changing.” Instead of using multiple pans to form the window steps, requiring constant changes and relocations, the precaster cast the recessed sill and lintel pieces separately and assembled then onto the frame. “This allowed production to set up long lines of formwork and cast multiple assembly pieces each day, saving form setup costs and minimizing wasted concrete.” The panels’ joints did not receive foam, as the project is located in a high seismic zone. “We didn’t want to restrict the panel movements in any way if a seismic event were to occur,” says Mike Ryan, director of architectural systems for Clark Pacific. “There was a minimal thermal penalty for not foaming them.” The foam insulation is more costly than typical insulation, he notes, but cost is saved because there’s no additional interior stud wall or insulation required. Hat channel and drywall can be applied directly to the back of the panel frame, Ryan says. That also frees up room at the perimeter of the structure. Another challenge came in preparing the windows for installation. The windows were installed just prior to shipping, but the glazing contractor, Architectural Glass & Aluminum (AGA), still needed to install window clips to ensure quick installation at shipment time.

Three-Step Process Used

A three-day production cycle was established, with the panels cast on the first day. They were stripped and moved to the insulation area for foam application on the second day. They then moved to the sandblasting area, where AGA installed its clips. The panels were then sandblasted and transported to storage. “Integrating AGA into the production cycle, ensured they could maintain a minimal crew and still keep up with our casting schedule,” Stearns explains. When the panels were prepared for shipping, AGA crews installed as many as 36 windows in an eight-hour shift. Installing the windows at the plant ensured the crews worked under controlled conditions with all tools readily available.” For some applications on projects requiring integration of electrical, Clark Pacific has also delivered panels to locations where electrical crews’ can perform their work. “By taking the panels to them, we can cut the time requirement for union electricians by half.” Says Stearns. Shipping the panels also took a three-part process. On the first day, the panels were taken from storage, cleaned, and prepared. On the second day, the windows were installed. On the third day, they were shipped to the jobsite and erected. In all, 484 preinsulated, preglazed panels, encompassing 119,000 square feet, were erected.

 “More than $1.1 million was saved on the façade alone.”

The process was efficient and economical, and it will continue to produce savings due to the highly energy efficient design. “Marriott was very risk-averse on this project and wanted no possibility for water penetration or other lapses,” Ryan explains. “But their initial design to achieve these goals was way over budget. We showed that by combining the panels, glass, and insulation in a thin, lightweight panel, we could save them more than $1.1 million on the façade. Then we looked at other options to cut costs.” More insulation was considered, he notes, but a cost/benefit analysis showed additional inches resulted in more than a 50-year payback term. “Mock-up testing showed that we’d reached the efficient level, so they cut out some of the redundancies. And the additional jobsite savings in having everything installed ahead of time was huge.” The panels, the largest of which were about 15 by 35 feet, weigh only about 35 pounds per sq. ft., considerably less than a full concrete panel at 65 pounds per sq. ft., Ryan notes. “Over 30 stories of panels, that’s a big deal.” It resulted in less framing, as well as smaller foundations and lower shipping costs. The weight reductions alone saved about $2 million in construction costs. The savings throughout the process were considerable, agrees Holland’s Kirk. “The system works well if there isn’t a lot of articulation required in the skin, due to its thin profile. The panels went on so quickly and so well, there wasn’t much pain involved with the installation.”

Tips To Consider

When considering integrating materials into precast concrete panels, architects should consider various factors. “The biggest concern is that designers wait too long in the process to bring in the subcontractors,” Ryan says. “The total enclosure and energy system needs to be considered early and as one system, as each part affects others. A full energy strategy needs to be considered. Too often, concepts are mixed and matched and it creates a busy, inefficient wall system.” That was the case with the initial Marriott plan, he notes. “It was too complex and busy, adding cost and time that wasn’t necessary.” A close communication system, including meetings with the various involved subcontractors, also is critical. This allows each participant to present its most efficient techniques and work to integrate or adapt them to meet others’ needs. Although more time is spent in upfront communication, planning, and drawing, the work pays off. “The jobsite savings on the Marriott project was huge due to preinstalling so much equipment,” says Ryan. The erection was done at night, as the project is directly across from the Staples Center in a congested area. “Preinstalling the windows made the process much faster and easier.” “It’s a very effective system,” says Kirk.

 “The ability to combine all systems into one panel in the yard under factory conditions provides multiple benefits. We get high quality, more complete and larger pieces to erect, and a much faster schedule overall.”

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