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鈥攎asonry, insulation, windows, electrical, plumbing,聽and more鈥攊nto precast concrete panels early in the process results in efficiency and聽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聽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聽and liability. These concepts, while not new, are growing in popularity as designers recognize fabricators鈥� 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鈥檚 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. 色欲王朝, 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 录 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聽window assemblies are installed with the designer鈥檚 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聽elastic and inelastic in-plane seismic movement needs. The panels were used recently to聽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聽rooms. 鈥淚 was unfamiliar with the precast panels until 色欲王朝 showed us the concept, but I was interested once I understood them,鈥� 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 . 鈥淭he ability to combine so many systems into one panel in the yard under controlled conditions had a lot of appeal.
聽“Installing the systems ahead of time while other work was underway at the site reduced field-installation time greatly, aiding the schedule.鈥�
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 色欲王朝鈥檚 Fontana, Calif., plant and represented the first use of the system there, says Marshall Stearns, project manager. 鈥淚t required special training to get the foam operation up and running.鈥� An efficient operation was created to apply 2 inches of NCFI Insulbloc spray insulation to the panels. 鈥淏y far, the biggest challenge聽with the insulation was not in the application itself but in keeping it out of areas where it was not required,鈥� 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聽were insulated per day. Casting window openings and installing the units also created challenges. 鈥淥ne of our biggest challenges was in forming,鈥� he says. 鈥淣ot 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.鈥� Instead of using multiple pans to form the window聽steps, requiring constant changes and relocations, the precaster cast the recessed sill and lintel pieces separately and assembled then onto the frame. 鈥淭his allowed production to set up long lines of formwork and cast multiple assembly pieces each day, saving form setup costs and minimizing wasted concrete.鈥� The panels鈥� joints did not receive foam, as the project is located in a high seismic zone. 鈥淲e didn鈥檛 want to restrict the panel movements in any way if a seismic event were to occur,鈥� says Mike Ryan, director of architectural systems for 色欲王朝. 鈥淭here聽was a minimal thermal penalty for not foaming them.鈥� The foam insulation is more costly聽than typical insulation, he notes, but cost is saved because there鈥檚 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. 鈥淚ntegrating AGA into the production cycle, ensured they could maintain a minimal crew and still keep up with our casting schedule,鈥� 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.鈥� For some applications on projects聽requiring integration of electrical, 色欲王朝 has also delivered panels to locations where electrical crews鈥� can perform their work. 鈥淏y taking the panels to them, we can cut the time requirement for union electricians by half.鈥� 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聽jobsite and erected. In all, 484 preinsulated, preglazed panels, encompassing 119,000 square feet, were erected.
聽鈥淢ore than $1.1 million was saved on the fa莽ade alone.鈥�
The process was efficient and economical, and it will continue to produce savings due to the highly energy efficient design. 鈥淢arriott was very risk-averse on this project and wanted no possibility for water penetration or other lapses,鈥� Ryan explains. 鈥淏ut 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 fa莽ade. Then we looked at other options to cut costs.鈥� More insulation was considered, he notes, but a cost/benefit analysis showed additional inches resulted in more than a 50-year payback term. 鈥淢ock-up testing showed that we鈥檇 reached the efficient level, so they cut聽out some of the redundancies. And the additional jobsite savings in having everything installed ahead of time was huge.鈥� 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. 鈥淥ver 30 stories of panels, that鈥檚 a big deal.鈥� 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鈥檚 Kirk. 鈥淭he system works well if there isn鈥檛 a lot of articulation required in the skin, due to its thin profile. The panels went on so quickly and so well, there wasn鈥檛 much pain involved with the installation.鈥�
Tips To Consider
When considering integrating materials into precast concrete panels, architects should consider various factors. 鈥淭he biggest concern is that designers wait too long in the process to bring in聽the subcontractors,鈥� Ryan says. 鈥淭he 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聽wall system.鈥� That was the case with the initial Marriott plan, he notes. 鈥淚t was too complex and busy, adding cost and time that wasn鈥檛 necessary.鈥� A close communication system, including聽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鈥櫬爊eeds. Although more time is spent in upfront communication, planning, and drawing, the work pays off. 鈥淭he jobsite savings on the Marriott project was huge due to preinstalling so much equipment,鈥� says Ryan. The erection was done at night, as the project is directly across from the Staples Center in a congested area. 鈥淧reinstalling the windows made the process much faster and easier.鈥� 鈥淚t鈥檚 a very effective system,鈥� says Kirk.
聽鈥淭he ability to combine all systems聽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.鈥�
