It's estimated that U.S. buildings consume 65% of the electricity generated and produce 30% of the national output of greenhouse gases. This has shifted design and construction practices toward methods that reduce the negative impact building operation has on the environment. One such design area addresses the building's energy efficiency and the thermal resistance (R-value) of the building envelope. Today, most model energy codes require a building to exceed minimum thermal performance requirements. One way to meet and exceed such minimum requirements is with insulated sandwich wall panels.
Tilt-up sandwich wall panels have been used extensively for years, but given current code requirements and the shift toward panelized construction and sustainable systems, sandwich walls have grown in popularity and frequency of use. Whether it's schools, offices, correctional facilities, or religious buildings, tilt-up sandwich panels can provide the durability, speed of construction, and design flexibility of tilt-up while providing significant R-value and moisture protection. In general, sandwich wall panels comprise two layers, or wythes, of concrete separated by a layer of rigid insulation, and are tied together with a series of connectors or fasteners. The performance of the tilt-up sandwich panel, both structurally and thermally, depends on the capacity of the connector and the detailing of the insulation.
A sandwich wall panel is formed up to the total wall thickness, which includes the exterior and interior layers of concrete, and the integral insulation. The panels themselves are prepared similarly to that of an uninsulated tilt-up wall panel. Architectural features, such as reveals and rustications, are laid out and a bond breaker is applied.
The minimum exterior wythe thickness of a sandwich wall panel is 2 inches plus the depth of any reveal or rustication. Therefore, if incorporating a ¾-inch architectural reveal, the minimum exterior concrete thickness is 2¾ inches. The exterior concrete layer of a sandwich wall panel is typically nonstructural and the connectors are installed in a plastic mix, as described in ACI 551R. The concrete typically is reinforced with a 6x6 W2.9/W2.9 mesh positioned on plastic bolster strips or chairs. A working slump of 5 to 7 inches is recommended to ensure proper consolidation around the connector. Additionally, minimum size aggregate is ¾ inch and a 4000 psi minimum concrete is typical.
The insulation thickness is a function of the desired R-value and interior condition (ambient, cooler, freezer). Ambient facilities, where the interior space will remain at “room temperature,” typically require 2 inches of extruded polystyrene insulation, while coolers require 3 inches, and freezers use 6 inches or more. In all cases, an isothermal analysis and dew point analysis should be performed to confirm R-value and condensation control respectively.
To achieve a high R-value and consistent thermal and moisture protection, the edges of the insulation layers must remain in contact along their entire length, separating the two layers of concrete. If the insulation is not continuous, thermal bridges occur, resulting in a loss of thermal integrity and increasing the likelihood for moisture migration.
The casting of a sandwich wall panel typically requires two pours. The exterior wythe pour and insulation/connector placement occur concurrently. The second step involves installing the interior concrete reinforcement, lifting and bracing hardware, and any embeds isolated to the interior wythe of concrete. Before installation, however, a pull out test on the connector should occur once the exterior concrete achieves 25% of its 28-day strength. Depending on air temperature, this could be 12 to 36 hours. In colder climates, this could require a cylinder test (see chart).
When the anchorages of the connectors are confirmed and the interior concrete components are in-place, concrete is placed and finished, erected and braced, much like uninsulated tilt-up wall panels. The finishing and sealing of a sandwich wall panel varies by climate and interior conditions. Panel joints are sealed both inside and out with a closed cell backer rod and caulk. Joints also may be foamed in-place with an expandable urethane when deemed necessary by the designer.
Once in place, sandwich wall panels can provide a viable solution by combining concrete's thermal mass and the performance of integral insulation, all while maintaining the architectural and functional integrity of tilt-up concrete walls.
Darryl Dixon is director of technical services for Composite Technologies Corp. He can be reached at 800-232-1748 or email@example.com.