High performance, or HP precast panels are made up of two wythes of concrete separated by a continuous, edge-to-edge wythe of insulation. These insulated wall panels provide a versatile and economical means to meet the structural, thermal, moisture, and architectural requirements of a structure. HP precast concrete sandwich wall panels can provide an aesthetically pleasing, durable exterior finish, as well as a paint-ready, durable interior surface. Since the panels are insulated, they also deliver effective thermal and moisture protection for a building. In addition to providing insulation and aesthetic value to the building, sandwich panels also serve a structural role, resisting lateral forces, gravity loads, and temperature effects. Lateral forces may include seismic, wind, soil, and blast effects. Gravity loads can include self-weight, as well as loads imposed by floor or roof structures.
HP precast concrete panels can be used either as bearing walls or as cladding. As load-bearing walls, HP precast panels can support gravity loads, as well as resist wind, seismic, and blast loads. Or, depending on the application, HP precast panels can be used as cladding, thereby transmitting wind, seismic, and blast loads to the structural frame and foundation.
Tilt-up wall panels are cast on site next to the project and are tilted into their final position. HP precast panels are cast in a PCI-certified manufacturing facility. Producing the panels in a controlled and monitored plant environment provides a much higher degree of quality control to ensure the product will meet project specifications and standards.
There are no restrictions on the use of HP precast concrete wall panels. They can be designed into, specified, and used for any type of project.
When architectural precast concrete is used in a cladding application, quality standard PCI MNL 117, Quality Control for Plants and Production of Architectural Precast Concrete Products is appropriate. When precast concrete panels are load bearing or being used in an industrial grade application, use PCI MNL 116, Quality Control for Plants and Production of Structural Precast Concrete Products.
To accomplish this connection, use a special non-conductive connector. Carbon reinforcement and E-glass connectors are common because they will not cause a thermal bridge between the insulation layers. Wythe connectors are used to transfer wind and seismic forces, and must also be capable of resisting fabrication and installation forces. The insulation should run edge to edge to provide the maximum R-value.
Thermal bridging is minimized by using low-conductivity wythe connectors. These composite material connectors, which enable edge-to-edge insulation coverage in precast concrete sandwich panels, can significantly reduce thermal bridging. They help an insulation layer retain up to 99.7 percent of its listed R-value.
There are three common insulations used in high performance insulated precast concrete walls: XPS, EPS, and polyISO. Expanded Polystyrene (XPS) is a closed cell structure. Of the three, XPS generally has a higher thermal resistance, higher compressive strength, and reduced moisture absorption. Its R-value is 5.0 per inch of thickness. Expanded polystyrene (EPS) has an R-value that varies from 3.85 to 4.35 per inch of thickness. Cellular polyurethane/polyisocyanurate (unfaced) has an R-value that varies from 5.56 to 6.25 per inch of thickness.
Edge-to-edge insulated precast concrete sandwich wall panels with minimal or no thermal bridging maintain the R-values for continuous insulation as defined by ASHRAE 90.1-2010. This level of thermal performance can lower energy costs. In some climates, increasing wall and roof R-values by as little as 5 can reduce energy costs by 5 to 20 percent. High performance precast concrete sandwich wall panels commonly have steady state R-values ranging from 12 to 20 hr ft2 °F/Btu. The thickness of the insulation is determined by the thermal characteristics of the insulating material and the thermal loads on the structure.
Heat Capacity (HC) is used in energy codes to determine when a wall has enough thermal mass to use the mass criteria or mass credit in those codes. HC refers to the wall’s ability to store heat per unit area of wall area. It includes all layers in a wall. Energy codes generally require a heat capacity greater than 6 Btu/ft2·°F in order to use the mass wall criteria. These criteria typically allow a lower R-value for the wall.
Thermal mass refers to a material’s ability to absorb and release heat. This property is a key benefit of concrete structures. Because concrete has a high specific heat, high density, and low conductivity, it can absorb a large amount of heat energy with little change in temperature. High thermal mass provides thermal storage, which reduces daily and seasonal temperature swings. In the summer, concrete absorbs heat during the day and cools the building by storing that heat over the surface of the building rather than allowing it to flow into the interior. This cycle reverses at night. During the cooler time of the day, heat is released back out into the atmosphere. By damping and shifting peak loads to a later time, thermal mass reduces peak energy requirements for building operations. ASHRAE Standard 90.1 acknowledges the thermal mass benefits of concrete walls in specifying lower minimum insulation R-value and higher maximum wall U-factors for mass (concrete) wall construction.
12. What is the effective whole wall R-value of an HP precast concrete insulated sandwich wall panel?
Due to the thermal mass effect of high performance precast concrete insulated sandwich wall panels, the effective whole wall R-value of an HP precast concrete wall system can be up to two times greater than that of the material (steady state) R-value. This translates into energy cost savings. The degree of improvement in R-value in a given structure is greatly dependent upon its location and local climate, the occupancy type, the building orientation, and other features of the design.
Normal weight, quality concrete in thicknesses of 3 inches or more can be considered a semi-impermeable vapor retarder. Published values of concrete permeability are approximately 3 perm·inches. This means that 3 inches of concrete has a permeance of approximately 1 perm, provided it remains relatively crack-free. Permeance is a function of the water-cement ratio of the concrete. A low water-to-cement ratio, such as that used in most precast concrete members, results in concrete with low permeance. Where climatic conditions demand, insulation, sufficient concrete, or the addition of a vapor retarder may be necessary to prevent condensation. Thicknesses of 1 inch or more of rigid extruded polystyrene board (XPS), or 2 to 3 inches of expanded polystyrene (EPS), if properly applied, will serve as their own vapor retarder. The International Energy Conservation Code (IECC) 14 requires a vapor retarder with 1 perm or less on the inside of insulation in cold climates. The code does allow exceptions where moisture or the freezing of that moisture will not damage the materials, or where other means are provided to prevent condensation. This requirement is workable for concrete since 3 inches of concrete has a permeance of approximately 1 perm. The important questions to consider are whether condensation will occur and, if it does, will it damage the materials?
Materials such as precast concrete panels, polyethylene, gypsum board, metal sheeting, or glass qualify as air barriers because they are low air-permeable materials when joints are properly sealed. Precast concrete panels have negligible air infiltration. Minimizing air infiltration between panels and at floors and ceilings will provide a building with low air infiltration.
The typical sizes of precast concrete wall panels are 8 feet/10 feet/12 feet, with joint sizes of 3/4 inches. On larger projects, special trailer arrangements may be needed, based on road restrictions. Depending on internal shipping requirements, they may be shipped flat, or on their side with a U-rack or A-frame system.
First, determine the corner conditions and design. Does the project call for MITRE, lapped, or L-shaped corner panels? Next, determine the module size for infill panels and place the locations for doors and windows.
The panels can be designed vertically or horizontally.
The recommend joint size is 3/4 inches.
Spider plate connections can be embedded on the backside. Threaded in Inserts (PSA straps) also can be used. Base connections can be plate to embed within foundation or Wilson sleeve design? Connections for erection Lifting loops/ burkes/. Consideration for fixed vs. Lateral connections.
This is a plate connection with a slot provided to ensure that adjustments can be made within field. The concept is based on engineering criteria for lateral movement. Shim strips fortify a 3/4-inch or 1-inch joint.
Traditional concrete finishes, such as exposing the matrix of architectural concrete mix designs, are available by various etching procedures. However, additional finishes such as embedded stone facing and thin brick tiles are becoming very popular.
Thin brick and stone can be used as facing materials incorporated into the exposed architectural concrete. Designers should contact their local manufacturers regarding lead times and availability of materials, as well as the manufacturers’ experience working with these materials and their recommendations regarding detailing assistance.
Thin brick is a manufactured clay product produced in a way similar to tile. It is designed to simulate traditional masonry full-depth brick. The thickness of thin brick varies depending on the supplier’s manufacturing process. It is much denser and less absorbent than traditional brick to prevent water infiltration into the tile. Specifying these products to meet the requirements established in the PCI Design Handbook will ensure a mechanical bond within the concrete mix design, thus locking the tiles into the consolidated concrete.
Approved manufacturers demonstrate through testing that their products have the low water absorption properties critical to the performance of the system. This is true both for traditional architectural precast concrete cladding, as well as for insulated precast wall panels. To ensure no moisture can compromise sealants and enter the building, a double layer of sealant application is recommended.
A value-added feature available with insulated precast concrete wall panels is the option to use the durable concrete interior surface as the final finish. Surfaces can be left untreated for an industrial application, or prepared to receive paint as one would for traditional concrete surfaces.
A minimum of four architectural concrete samples should be reviewed to establish an acceptable range of color and texture that will occur for each.
The project location usually dictates restrictions on transportation for panel sizing. Consult your local manufacturer to discuss the specifics for your project.
Depending on site access and the orientation of panels on a building, between 3,000 to 4,500 square feet of wall area can be installed each day.
Yes. Unlike many other materials, high performance (HP) precast concrete walls are not susceptible to temperature restrictions for installation. This quality makes the product ideal for constructing through the winter months.
Please consult the PCI Approved manufacturer’s list found at www.pci.org/find/manufacturer/index.cfm for a list of producers servicing region surrounding your project.
Typically, both the transportation and installation of precast concrete is handled under the precast producers contract. If subcontracted separately, it is recommended that a PCI-certified erector be chosen for the job. These erectors have a high degree of familiarity with the product being used, thereby ensuring quality workmanship.
Insulated precast concrete walls provide structural integrity and security, and an added measure of fire safety. The panels are inherently non-combustible and do not serve as fuel or contribute to the fire load. Precast concrete panels can be designed to meet any degree of fire resistance required by building codes or insurance companies. The fire resistance of insulated sandwich wall panels is conservatively equivalent to the fire resistance of a solid panel with a thickness equal to the sum of the thickness of the two wythes. Fire ratings of one, two, three, or more hours can be achieved by varying the thickness of the wythes. Precast concrete eliminates the need for and cost of additional fireproofing measures. The insulation is protected by the concrete and also does not contribute to the fire load. In addition, the danger of toxic fumes caused by burning of cellular plastics is essentially eliminated when the plastics are completely encased within concrete sandwich panels.
Yes. Items such as electrical conduits and boxes can be cast into the panels during fabrication. Careful coordination with design team and producer is essential to achieving the best results.
When designing for blast resistance, the inner wythe of concrete of the HP precast concrete wall is designed for the blast forces provided by the blast consultant, then is connected to the building structure. The exterior wythe and insulation are treated as a veneer and typically are not included in the design calculations for blast resistance.
Precasters will perform component and connection design of the members they produce when it is required by the contract documents. Precast concrete reinforcement is determined by building codes, industry standards, and the design criteria defined by the contract documents. All drawings and specifications that convey the requirements for the precast concrete should be provided to the precaster. Depending on the size and scope of the project, pertinent drawings might include: architectural, structural, electrical, plumbing, and mechanical drawings; approved shop drawings from other trades; and site plans that show available erection access and storage areas.
The architect develops the project design concept, establishes overall structure geometry, selects the cladding material for appearance and function, provides details and tolerances for proper material interfacing and weatherproofing, and specifies performance characteristics. Inspection parameters and testing requirements are also spelled out in the contract documents.
The EOR has responsibility for specifying the design criteria for the design of the precast concrete elements and for describing the intended load paths. In addition, the EOR should anticipate the loadings in the structural design and provide a structural system adequate to support these loads. When applicable, the EOR should also define the type of loading to be applied to the panels and the structure, as well as provide information regarding applicable codes and design criteria, including wind, seismic, or blast design. Finally, the EOR should consider any consequences of the eccentricities of the weight of the precast concrete panels when designing the supporting structure.